- Email: [email protected]
Ice damage zone around the Molikpaq: Implications for evacuation systems
Cold Regions Science and Technology 44 (2006) 67 – 85 www.elsevier.com/locate/coldregions
Ice damage zone around the Molikpaq: Implications for evacuation systems G.W. Timco a,*, B.D. Wright b, A. Barker a, J.P. Poplin c a
Canadian Hydraulics Centre, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6 b B. Wright and Associates, Canmore, Alberta, Canada T1W 2R6 c Imperial Oil Resources, Calgary, Alberta, Canada T2P 3M9 Received 2 June 2005; accepted 6 August 2005
Abstract Effective design of an emergency evacuation system from an offshore structure in sea ice conditions must take into account all aspects of the anticipated ice conditions at the site. Evacuation systems are typically comprised of several different components or subsystems. These subsystems allow evacuees a choice in response to the emergency incident onboard the installation, as well as to environmental conditions off the installation. This paper discusses four possible evacuation means—preferred, primary, secondary and tertiary. Of these four techniques, only the preferred means of evacuation is not strongly influenced by the ice regime around the structure. This paper examines these ice regimes and quantifies the size of the broken ice region around a structure for different ice conditions. A large number of parameters affect the size of this broken ice zone including the general ice regime, the existence of a grounded rubble field, the failure mode of the ice, the ice thickness, and the ice roughness. Typical damage zone extent is extremely variable and can be in the order of 25 m from the structure for ice thickness of 1.5 m. This has significant consequences for evacuation systems deployed from the structure. D 2005 Elsevier B.V. All rights reserved. Keywords: Evacuation; Ice; Offshore structures; EER systems; Molikpaq
1. Introduction Evacuation from an offshore structure in ice conditions is not straightforward. Evacuation systems need to be suitable for both the ice conditions that they are designed to operate in, as well as for open water or partial ice covers associated with the melt season. To date, there is no single evacuation system that can be used in all ice conditions. The design of an evacuation * Corresponding author. E-mail addresses: [email protected] (G.W. Timco), [email protected] (B.D. Wright), [email protected] (A. Barker), [email protected] (J.P. Poplin). 0165-232X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.coldregions.2005.08.001
system must be carefully done by analyzing the type of ice conditions that the structure would likely encounter throughout the year (Timco and Dickins, in press). Reliable systems (potentially comprised of multiple means of evacuation) would be required for all conditions, and the most reliable means of evacuation may change throughout the season. During an emergency on an offshore platform in ice, one abandonment concept could be to evacuate the personnel with some type of life craft. This evacuation system would likely have to be launched perpendicular to the ice movement direction as the loaded side of the installation would be inaccessible due to the ice, and the lee side may, for instance, be unattainable due to
68
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
smoke. To have confidence in developing this concept as a reliable system, information is required on the dimensions of the zone of broken ice along the sides of the structure, in the direction perpendicular to the ice movement. For wide offshore structures, the focus for research is usually with respect to their stability due to ice loads. Consequently, to date there is little published information on the width of the damage zone along the sides of offshore structures. The only direct study in this regard is the recent publication by Poplin and Timco (2003) who investigated the width of the broken ice zone around conical structures and its impact on emergency evacuation. This paper summarizes the results of an analysis of the size of the broken ice zone around the Molikpaq structure over a range of ice conditions and the implications on emergency evacuation. Both pictorial and quantitative information on the extent of the damage zone and rubble height were obtained from the field observations. The results are related to the ice macrostructure (roughness), thickness, and failure mode and are discussed in terms of the deployment issues for evacuation systems. Note that although the detailed results are specific to the Molikpaq structure, they should be applicable to other vertical-sided structures in similar ice regimes. 2. Evacuation concepts The means of evacuation selected for any offshore platform is a very important consideration. Clearly, an evacuation system must be comprised of components which help ensure that safe and reliable methods are always available to allow personnel to abandon an offshore installation in response to an onboard emergency. Means of evacuation must be available across the range of environmental conditions anticipated. Evacuation methods for any offshore installation in a dynamic pack ice environment are not straightforward, given the high degree of variability in ice conditions that may be encountered around a structure, at any particular point in time. There are a number of factors that can influence the type of ice conditions found immediately adjacent to any platform, all of which are time varying. These include ice concentration, floe size, ice thickness and roughness, the ambient ice movement regime, the shape and size of the structure, and the nature of the ice/ structure interaction itself. In developing a platform evacuation system, the credible emergency incident scenarios that could
occur are developed. Details of those which could escalate to the point of necessitating installation abandonment are assessed further to help identify impairment time requirements for the temporary refuge (a location on an offshore installation in which all personnel can remain without harm for a specified time under any accident scenario). Evacuation timelines are developed to assess the time available for personnel to evacuate an installation before escalation renders an evacuation system ineffective or the incident imposes an undue risk to personnel using the evacuation system. In conjunction with developing emergency scenarios and establishing temporary refuge impairment time and evacuation timelines, the adequacy of different evacuation systems under the range of anticipated ice scenarios that should be expected around a platform are assessed. This approach provides a basis for systematically defining a platform evacuation system that can be effectively used to accommodate personnel escape over all credible ice situations. For most of the platforms that are now being considered for use in Arctic or sub-Arctic pack ice environments, a typical portfolio of evacuation options includes (according to the level of risk): ! Preferred—Helicopters are typically considered as the preferred means of evacuation. That is, they are the lowest risk option available to evacuate personnel and are used where possible in precautionary demanning (where time permits and the incident does not preclude their use). A number of factors impact the response times and/or availability of helicopters including the time required to complete the evacuation process and access constraints due to weather, visibility and the incident itself (e.g. fire, smoke, unignited gas release), etc. Support vessels may also constitute a preferred means of evacuation, for instances when the incident does not preclude the vessel from approaching the installation and a means (such as personnel basket) is available to transfer platform personnel to the vessel in a safe and timely manner. ! Primary—The primary means of evacuation is available under most emergency scenarios and provides personnel the most protection from the effects of the incident. Methods include devices that enable direct transfer to an ice-capable stand-by vessel including retractable stairways or gangways, chutes and slide systems, launch of a lifeboat to the vessel deck, launch of marine survival craft, etc. One of the key issues that is associated with this option is the ability for a standby vessel to reliably bapproach, closely
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
access and stationkeepQ at the platform in all of the ice conditions and incident conditions that can be expected. Another issue is the ability of the marine survival craft to get away from the incident and protect evacuees until rescue can be achieved. ! Secondary—The secondary means of evacuation comprises systems that offer evacuees less protection than the primary evacuation means and may transfer personnel from the platform to the ice surface or sea. To be both safe and effective, these types of evacuation methods must be able to blandQ escape craft away from any dangerous broken ice areas, where the ice is actively failing and/or clearing around the structure. ! Tertiary—The tertiary means of evacuation relies on higher risk methods. These include the use of individual descent methods, scramble nets, knotted ropes, stairs or ladders, etc. These options tend to be used as a last resort when lower risk alternatives are not available. Personnel using tertiary means will likely have to access ice or mixed ice/open water conditions in close proximity to the sides of a structure. Since the evacuation strategy must take into account the non-availability of lower risk methods, a range of evacuation alternatives is typically provided for any platform deployed. With the exception of the helicopter option, all of these evacuation methods will be affected by the ambient ice conditions, the nature of the ice action against the offshore platform, and the width of the broken ice zone that is seen around it. Consequently, information on the size and extent of the damage zone is critical in designing these systems to operate effectively in an Arctic environment. 3. Molikpaq structure The Molikpaq structure was owned by Gulf Canada Resources Ltd. and operated in Canada by Beaudril, a subsidiary of Gulf. The Molikpaq is a mobile arctic caisson (MAC) that was first deployed in the Canadian Beaufort Sea in 1984. The structure, which is a hybrid steel/sand platform consisting of an octagonal steel caisson annulus with dredged sand fill placed in its core, has 4 long sides approximately 60 m wide and 4 shorter corners approximately 22 m wide at the waterline. The total width of the structure is approximately 90 m at the waterline and 111 m at its base. The steel caisson supports a deck that carries a drilling rig, accommodation modules and other facilities. The sand core and steel caisson have sufficient mass to resist
69
horizontal ice loads. Depending upon the deployment design, the Molikpaq can be deployed with a sliding resistance in excess of 1000 MN. The Molikpaq, at its Tarsiut P-45 and Amauligak I-65 deployment locations, was placed on a submarine berm which resulted in a 20m local water depth. Since grounded rubble did not form around it at these locations, it was directly exposed to moving ice throughout the winter period. Thus, observations from these sites would provide important information on the size of the damage zone for different moving ice conditions. Information from these sites is used extensively in this paper. The Molikpaq was also deployed at two locations in which a grounded rubble field formed during the winter months. The Molikpaq at the Amauligak F-24 site was set on a 16-m high berm in 32 m water depth, giving a local effective water depth of 16 m. The Molikpaq was also used at the Isserk I-15 site in 11.5 m of water depth without a berm. Ice conditions at this site were essentially landfast shortly after freeze-up. A wedge-shaped, partially grounded rubble field extended over 700 m to the east of the structure at this site, but ice on the other sides of the platform was not so nearly deformed. 4. General ice environment and ice regimes Experience and photographs of the Beaufort Sea offshore structures show that there can be a wide variation in the type of ice conditions surrounding the structure—from solid level ice, to broken ice, to open water. This wide range of ice regimes is one of the most important factors to consider in the development of an offshore installation evacuation system. It makes it difficult to design an evacuation deployment system that can be used in all circumstances. The extent of the variations in the ice regimes surrounding the structure is a direct function of the location of the structure and the local ice conditions. There can be a wide variation in the type of ice conditions encountered for situations with moving ice. In general, the size of the ice damage zone around a structure is a function of several different parameters including the water depth, the existence of a grounded rubble field, the roughness of the ice, the floe size, the failure mode of the ice, the ice thickness, and the clearing behaviour of the broken ice. The following sections discuss their influence on evacuation systems. 5. Grounded rubble field If the depth of the water is fairly shallow, and the ice is relatively thick, a grounded ice rubble field can form
70
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
around the structure. In the Beaufort Sea, this was quite common for structures close to shore. In deeper waters, up to 20 m, grounded rubble fields did form on occasion, but they sometimes moved with changes in ice conditions. For effective water depths greater than 20 m, no permanent grounding occurred. Experience has shown that when a grounded rubble field forms, the active failure zone moves from the structure to the edge of the ice rubble (Croasdale et al., 1995). Grounded ice rubble features can often extend for large distances, depending upon the water depth and ice thickness. In the Beaufort Sea, for instance, grounded rubble piles could extend up to several hundreds of meters from the structure. However, there is a great deal of variability not only between structure locations, but the same location, from year-to-year. The rubble extent, roughness, growth, decay and stability are all dependent upon location and annual conditions. For example, as shown in Fig. 1, the shape of the rubble pile is not symmetrical. Rather, the shape usually reflects the predominant ice movement direction. Canatec (1994) and Timco et al. (2005) have documented the details of the rubble fields and rubble generation at a number of structure locations in the Beaufort Sea. Given the variability that exists, an example of a grounded rubble field is presented—the Molikpaq in the Beaufort Sea at the Amauligak F-24 site. Fig. 1 shows a large grounded rubble field surrounding the Molikpaq. A rubble field at this location began to develop at the end of November 1987. However, most of this rubble subsequently drifted away. It was not until the end of December that ice rubble formed which ultimately stayed and provided a stable core.
Most of the ice rubble field formed in January. The field continued to grow through the end of February 1988. By March, the rubble field extended 100 m from the structure, with a maximum sail height of approximately 10 m. The field was elongated in shape, extending along the East–West axis of the Molikpaq. Fig. 2 shows a timeline of the rubble field growth and decay at this site. The existence of a grounded rubble field has implications with regard to the evacuation of personnel from the structure. If a grounded rubble field forms, it may not be possible to access the installation with an icebreaker or effectively utilize lower risk platformbased evacuation systems. Depending upon the region and the specific circumstances, a range of potential strategies must be considered. For example, in certain circumstances, a platform-based Temporary Refuge (TR), designed with blast and fire protection, may be the best solution since it still provides access to a maximum range of evacuation alternatives. On the other hand, it might be possible to evacuate personnel directly onto the rubble field, provided the ice rubble is well grounded and stable. In this case, it would be necessary to pre-establish a Temporary Refuge on the rubble. Evacuation is inherently risky, so personnel would not abandon the platform until required to do so. Once personnel have abandoned the installation, they have committed to surviving in the off-platform environment. If off-platform evacuation is used, there are inherent risks. For example the topography of the rubble is quite rough (see Fig. 3), and movement across a rubble pile is usually very slow and arduous. This would have direct implications on the quick
Fig. 1. Grounded rubble field surrounding the Molikpaq at the Amauligak F-24 site.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
Amauligak F-24 (1987-1988)
December 20 through February 22 build-up of the rubble pile (nearly elliptical shape with the major axis in the E-W direction, highest point ~14m above WL (E of caisson)). tidal crack along the N boundary of the rubble pile separates the rubble from surrounding ice
October 30 new ice began to form at Amauligak F24 (slightly later than average) Until early November Open water
November 22 1st rubble build-up along the E end of the N face. At its maximum: 5-7m high, 1520m out. Was thought to be grounded but broke-up and drifted away by November 25
December 20 25-30m diameter rubble pile developed off the N end of the E face. Covered the complete face (70-80m long, 70m wide, 1015m high) by December 27
After December 31 ice rubble developing past that date was stable and survived until spring breakup
71
The initial rubble pile (December 22 event) continued to grow, but its lateral extent did not increase appreciably. January 9-January 11: rubble began to buildup on the N caisson face, until it covered it completely. January 23: build-up on the W and NW caisson faces. A rubble pile did develop to the S caisson face; however, it never matched the size or stability of those on the E-NE or W faces.
By March a stable grounded rubble field had already formed. Boundary of the rubble field had an oval shape and extended away from the caisson to 60m to 100m. Sail heights up to 10m above WL.
through April maximum thickness of the ice (1.7m)
January portions of the rubble piles were grounded and stable
March and May field trips FY ice with numerous leads and areas of open water surrounded rubble field.
April 26 break-up of the rubble started at the S side.
from May 26 to June 7 the rubble gradually disintegrated and broke away due to ablation, wave erosion, swell movements and ice impacts.
June 1 open water (ice in concentrations < 2/10ths)
June 9 last piece floated away
Fig. 2. Timeline of the ice rubble field generation and decay at the Amauligak F-24 site in 1987/1988.
evacuation across a grounded rubble pile. If this evacuation system is designed into the evacuation plan, it would be necessary to maintain a path to the TR site,
either for example by using a bulldozer or by using spray ice to even out the escape route across the rubble field.
Fig. 3. Photograph showing the roughness of a grounded rubble field.
72
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
During the spring break-up season, the ice surrounding the rubble field will melt, but the grounded rubble may remain into the open water season. During spring, the rubble field can have large melt holes throughout it, and traversing it would be extremely difficult. The use of a Temporary Refuge would not be possible at this time of year. The decaying rubble field would restrict the use of a standby vessel for evacuation since the vessel would not be able to closely approach the structure. Furthermore, the edge conditions can be quite slippery and unsafe for personnel (see Fig. 4). This would make it difficult to evacuate to a standby vessel at the edge of the decaying rubble field. Allowing grounded ice rubble to form and then overcoming the obstacles it creates is one approach. In some circumstances, an alternative pro-active strategy would be to remove the grounded rubble shortly after it forms. Rubble forms in a series of ice movement events. Consequently, the time interval between ice movement events can be used to remove the recently formed grounded ice rubble. Note that all of the rubble does not have to be removed, but rather only that portion that adversely affects the ability to evacuate the installation and to affect rescue. Experience with propeller wash in the Alaskan Beaufort and Bering Seas has shown that vessels can be used to remove grounded ice rubble. Azimuthal stern drive icebreakers in the North Caspian Sea were even more efficient at removing grounded rubble surrounding exploration facilities. The key to this strategy is to remove the ice rubble soon after it has formed and not allowing it to consolidate (W. Spring, personal communication).
6. Intact ice sheet An ice sheet moving past the structure presents a different situation than that of a grounded rubble field. Large ice floes moving past the offshore structure usually fail along the leading edge of the ice close to the structure and most of the ice clears around the structure. Timco and Dickins (in press) have defined three regions around an offshore structure in moving ice conditions: updrift direction (the direction from which the ice is approaching), the alongside direction (along the side of the structure perpendicular to the direction of the moving ice) and the downdrift (wake) direction (the region behind the structure (see Fig. 5)). These regions are clearly shown in Fig. 6 which shows an aerial view of ice moving past the Molikpaq. Note that because the ice movement direction varies over time, the preferential side of the installation from an evacuation standpoint will also change. In front of the structure in the Updrift Direction, dynamic ice conditions with different ice failure processes take place. In this area, relatively large ice pileups can occur (see e.g. Fig. 7). This region is not safe for either lowered systems or vessel stationkeeping but may be the preferred side from an incident perspective (being upwind). On the side of the structure, in the Alongside Direction, the ice conditions are also dynamic with the broken ice pieces moving along the structure and clearing around it. This region can be periodically safe for lowered systems and/or vessel stationkeeping. However, a lowered system must be sufficiently far from the structure so that it does not enter the ice
Fig. 4. Photograph showing the edge of a grounded rubble field (foreground) with the ice sheet decaying and breaking up in the spring.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
73
Alongside Direction
Downdrift (Wake) Direction USUALLY SAFE
Updrift Direction NOT SAFE
Fig. 5. Illustration of the updrift direction, alongside direction and the downdrift (wake) direction around an offshore structure in moving ice conditions. The safety of the regions is based on an ice perspective. This may not have the same safety aspects as the incident perspective.
damage zone and/or be sufficiently robust that the ice will not damage it. Behind the structure in the Wake, the broken ice often fills-in, providing a broken brash ice regime. This region is usually safe for lowered systems and for vessel stationkeeping. However, it may not be possible to evacuate from this side of the installation (due to smoke, unignited gas, etc.) because it is often downwind. As an illustrative example of the dynamics of these regions, a 5-h time period was monitored to show the variations in the rubble height and width. Videos recorded on February 17, 1986, from 7 p.m. to midnight were examined to extract this time-line information. During this time period, a first-year ice sheet with a nominal thickness of 0.7–0.9 m thickness moved past the Molikpaq at a speed on the order of 0.05–0.09 m/s.
The ice was predominately level, but there were a number of ridges within it. The sail heights of the ridges were from 0.5 m to 2 m high. Fig. 8 shows a time-series plot of the observed variation of the damage distance both in the updrift direction and alongside direction of the Molikpaq. There is a large variation in the damage width with time, even for ice conditions that are nominally similar. In the updrift direction, the damage length varied from 1 m to 18 m in front of the structure. Alongside the Molikpaq, the maximum value and variation are less, with maximum damage widths on the order of 8 m. Fig. 9 shows a time-series plot of the observed rubble height in both the updrift and the alongside directions. In this case, the rubble height is quite
Fig. 6. Aerial view of the Molikpaq showing the updrift direction, alongside direction and the downdrift (wake) direction. The ice is failing along the front of the structure (updrift), moving along the side of the structure (alongside), and clearing into the wake. Note that the wake is largely filled with ice blocks. The ice movement is from right-to-left in the photograph.
74
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
Fig. 7. Photograph showing the ice crushing and piling up in front of the Molikpaq. The direction of ice movement is from right to left. Note the ice bturning backQ on itself as it interacts with the structure.
comparable in the two directions. Typical values varied between 1 and 3.5 m in both regions. Figs. 8 and 9 clearly illustrate the dynamic behaviour of the ice in the damage zone surrounding an offshore structure. The Beaufort Sea experience has shown that there are a number of important observations that can be made with regard to the width and behaviour of the ice damage zones as it relates to the evacuation of personnel: ! In conditions of high concentration of moving pack ice, a wake forms downdrift of the structure. There
can be various degrees of in-fill of broken ice in this region with a range of essentially open water to heavily congested brash. ! The orientation and width of the wake is a function of the angle of ice attack. Thus, the ice movement direction and the relative placement of evacuation locations becomes an important consideration for the evacuation system (Barker et al., 2001; Barker and Timco, 2003). ! The widths of the wake and the damage zone along the side and in front of the structure appear to be a function of several parameters including the ice thickness and ice failure mode, and to a much lesser
20 Updrift Direction
Damage Distance (m)
Alongside Direction
15
10
5
0 0
50
100
150
200
250
300
Time (minutes) Fig. 8. Time variation of the length of the damage zone for both the updrift and alongside directions during a 5-h time period.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
75
5 Updrift Direction Alongside Direction
Rubble Height (m)
4
3
2
1
0 0
50
100
150
200
250
300
Time (minutes) Fig. 9. Time variation of the rubble height in the damage zone for both the updrift and alongside directions during a 5-h time period.
extent, the surface roughness. These factors are explored in the next sections. 6.1. Influence of ice failure modes The failure mode of the ice has direct implications on the width of the damaged zone along the side of an offshore structure. The manner in which the oncoming ice cover fails against, and in turn, clears around the structure, has an important influence on the width of the broken ice zone. In general, the failure behaviour of the
ice depends on the thickness, speed, size, confinement and roughness of the ice feature. Essentially, it integrates all of these factors. The ice failure modes against the Molikpaq have been discussed by Wright and Timco (1994) and Timco and Johnston (2003, 2004). In general, buckling failure occurs with thinner ice, and crushing and mixed mode behaviours occur with thicker ice. To develop a quantitative description of the width of the damage zone, an examination was made of information contained in the NRC Centre of Ice/Structure Interaction (Timco, 1996). Several reports published by Gulf Canada Resources Ltd. and videos were used to provide information in this area. This information was examined to get quantitative information on both the width of the damage zone and the rubble height in the updrift and alongside directions. In all cases, there was a high ice concentration surrounding the structure; i.e. it does not include data where there were small ice floes or large fractured areas in the ice sheet. 6.2. Ice crushing
Fig. 10. Ice crushing along the face and side of the Molikpaq.
Ice crushing occurred on the Molikpaq, but it was not a predominant failure mode for the ice. Fig. 10 shows a photograph of ice crushing along one face of the structure. Note that the ice pieces are quite small (almost powder-like) and that the damage zone is very close to the structure. Evaluation of the pictorial information provided some information on the size of the damage zone and the height of the rubble around the Molikpaq. Figs. 11 and 12 present the data on the maximum damage distance (D max) for different ice
76
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
20
Damage Distance (m)
Ice Crushing Updrift Direction
15
10 Dmax = 5.11 hice0.64
5
0 0
0.5
1
1.5
2
2.5
3
3.5
4
Ice Thickness (m) Fig. 11. Damage distance (D max) as a function of ice thickness (h ice) for the ice crushing failure mode in the updrift direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
thickness (h ice) for ice crushing in both the updrift and alongside directions respectively. In all of these graphs, a multiplicative power law has been fit to the data. Each graph shows the best fit line along with the 95% predictive limits (light dashed lines). The heavier dashed line represents the upper limit of the predictive value based on the available data. There are not many data points for ice crushing and there is considerable scatter in the data. Regression power-law curves fit to the data give the following
relationships for the width of the damage zone for ice crushing: Dmax ¼ 5:11h0:64 ice
ð1Þ 2
for the updrift direction (with r = 0.31), and Dmax ¼ 3:89h0:85 ice
ð2Þ 2
for the alongside direction (with r = 0.34). Figs. 13 and 14 provide information on the maximum rubble height (H max) for different ice thickness
20
Damage Distance (m)
Ice Crushing Alongside Direction
15
10 Dmax = 3.89 hice0.85
5
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 12. Damage distance (D max) as a function of ice thickness (h ice) for the ice crushing failure mode in the alongside direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
77
10
Rubble Height (m)
Ice Crushing Updrift Direction
5 Hmax = 2.92 hice0.86
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 13. Rubble height (H max) as a function of ice thickness (h ice) for the ice crushing failure mode in the updrift direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
(h ice) for the updrift and alongside directions, respectively. Regression power law relationships for the height of the rubble gave Hmax ¼ 2:92h0:86 ice
ð3Þ
for the updrift direction (with r 2 = 0.62), and Hmax ¼ 2:09h0:51 ice
ð4Þ
for the alongside direction (with r 2 = 0.36).
When ice was crushing against the Molikpaq, the following observations can be made: ! The broken ice zone widths that were seen around the Molikpaq were smallest when pack ice was crushing against the caisson, in comparison to the broken ice zone widths that were associated with other ice failure modes. Typical damage zone sizes were in the order of 5–10 m, with little difference in the updrift and alongside direction (Figs. 11 and 12).
10
Rubble Height (m)
Ice Crushing Alongside Direction
5
Hmax = 2.09 hice0.51
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 14. Rubble height (H max) as a function of ice thickness (h ice) for the ice crushing failure mode in the alongside direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
78
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
! Variations in ice drift speed had a minor influence on the dimensions of the broken ice zone when the ice was crushing, but higher drift speeds did tend to favour cleaner and bless brashyQ downdrift wake conditions, for a given ice thickness and roughness situation. ! When ice crushed against the Molikpaq, granulated ice debris accumulated on its updrift face where the ice was actively failing, and against its alongside faces where the ice was clearing. ! The dimensions of the broken ice zones were fairly consistent regardless of the angle of ice attack, although the broken ice widths along the structure’s sides where the ice was clearing tended to be slightly wider for oblique ice interactions. Fig. 15 shows a scaled drawing with guidelines suggesting the maximum extent of the damaged ice for various ice thicknesses along all sides of the structure. The figure also includes information on the general ice conditions in the wake for different ice thickness interactions. Note that this information is applicable to ice crushing failure behaviour on a vertical-sided structure. Fig. 15. Size of the damage zone in the updrift and alongside directions for ice crushing failure. Information on general ice conditions in the wake is also indicated.
! With continuous ice crushing, a downstream wake was always seen behind the structure, with the bproportion and heavinessQ of the brash ice in the downstream wake typically increasing with increasing ice thickness and roughness.
6.3. Mixed mode failure The mixed mode failure was the most predominant failure mode for the ice, when it was actively failing against the Molikpaq. This mixed mode was a combination of different failure behaviours. Fig. 16 shows a photograph of mixed mode failure. In this case, there is ice crushing along the structure and ice riding up on the
Fig. 16. Photograph showing mixed mode failure with ice crushing, flexural failure with ride-up, and ice fracture.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
79
50
Damage Distance (m)
Mixed Mode Failure Updrift Direction
40
30
20
10
Dmax = 12.91hice0.52
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 17. Damage distance (D max) as a function of ice thickness (h ice) for mixed mode failure of the ice in the updrift direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
rubble through a flexural failure. Further, a large crack has formed in the ice. Damage zone distances were generally larger than those observed with ice crushing failure. As such, this failure mode is more important than ice crushing from the point of view of personnel evacuation. Figs. 17 and 18 show the maximum damage distance (D max) for different ice thickness (h ice) for both the updrift and alongside directions for mixed mode failure. Note that the scales of the axes are not the same as those of the ice crushing graphs. There is considerably
more scatter in the data with the mixed mode failure compared to the ice crushing. Regression power-law (multiplicative) curves fit to the data give the following relationships for the width of the damage zone: Dmax ¼ 12:91h0:52 ice
ð5Þ
for the updrift direction (with r 2 = 0.17), and Dmax ¼ 6:84h0:76 ice
ð6Þ
for the alongside direction (with r 2 = 0.38).
50 Mixed Mode Failure Alongside Direction
Damage Distance (m)
45 40 35 30 25 20 15 10 5
Dmax = 6.84 hice 0.76
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 18. Damage distance (D max) as a function of ice thickness (h ice) for mixed mode failure of the ice in the alongside direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
80
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
Mixed Mode Failure Updrift Direction
Rubble Height (m)
10
5
Hmax = 2.98 hice0.28
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 19. Rubble height (H max) as a function of ice thickness (h ice) for ice mixed mode failure of the ice in the updrift direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
Figs. 19 and 20 show the rubble height (H max) for different ice thickness (h ice) for both the updrift and alongside directions for mixed mode failure. Note that the scales of the axes are not the same as those of the ice crushing graphs. Regression power law relationships for the height of the rubble gave Hmax ¼ 2:98h0:28 ice
for the alongside direction (with r 2 = 0.17). The graphs show very little dependence on ice thickness. The low r 2 values illustrate the wide range of observed values for both the extent of the damage zone and the rubble height. Note that this scatter is less with ice crushing. With regard to mixed mode failure, the following comments can be made:
ð7Þ
for the updrift direction (with r 2 = 0.10), and Hmax ¼ 1:69h0:47 ice
ð8Þ
! The predominant ice failure behaviour that was seen at the Molikpaq involved mixed modal and bending ice failures immediately adjacent to the caisson’s updrift faces. This mixed mode failure occurred for
Mixed Mode Failure Alongside Direction
Rubble Height (m)
10
5
Hmax = 1.69 hice0.47
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 20. Rubble height (H max) as a function of ice thickness (h ice) for ice mixed mode failure of the ice in the alongside direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
81
situation with mixed mode failure. Also, in comparing this figure to Fig. 15 for crushing failure, it is clear that there is a much larger extent of damaged ice with mixed mode ice failure. 6.4. Ice roughness Large ice floes can be uniform in thickness, but more often than not, there is a wide range in the ice thickness. Ice floes can contain level ice with ridges and hummock fields intermixed within it. A ridge is a linear feature containing a large accumulation of broken ice, whereas a hummock field contains very rough ice over a large aerial extent. It might be expected that the width of the failure zone would be substantially wider for the failure of the more extreme features, but photographic evidence from the Canadian Beaufort Sea indicates that this is not the case (see e.g. Fig. 22). The failure of a ridge and rubble tended to be a shear failure along the sides of the structure (Wright and Timco, 2000). Photographic evidence indicates that ice roughness doesn’t have a strong influence on the width of the damage zone. This result is quantified later in this paper, but some general comments from observations follow: Fig. 21. Size of the damage zone in the updrift and alongside directions for mixed failure mode of the ice. Information on general ice conditions in the wake is also indicated.
! Along the sides of the structure, the width of the broken ice zone was largely unaffected by the presence of rough ice conditions, because these features
more than about 50% of the time that the ice was failing against the structure. ! For the case of mixed modal ice failures against the Molikpaq structure, broken ice fragments and floating ice rubble accumulated on both its updrift and alongside faces, with broken ice dimensions that were fairly consistent regardless of the ice attack angle. ! The width of the broken ice zone that was seen around the Molikpaq was generally greater with mixed failure modes than for ice crushing failures. Thus, mixed mode failure represents a more severe situation regarding bacceptable evacuation system deployment distancesQ. ! Similar to the continuous ice crushing case, a downstream wake was always present behind the structure, with its brash ice content generally increasing with ice thickness and roughness, and with decreasing ice drift speeds. Fig. 21 shows a scaled drawing with guidelines suggesting the maximum extent of the damaged ice for various ice thicknesses for a Molikpaq-shaped structure. Note that this information is applicable to the
Fig. 22. Photograph showing the wake behind the Molikpaq. Note that the width of the wake is relatively independent of the ice roughness.
82
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
would tend to bshear pastQ the Molikpaq and remain intact as they moved by, without any breakout or rotation. ! In the updrift region, the active ice failure zone that was associated with these rough ice feature interactions was often further off the structure. In some instances, the failure of the feature occurred as far as 100 m updrift of the caisson. ! During oblique interactions between the Molikpaq and large ridges or heavily rubbled areas, fractures often propagated out from the caisson, resulting in large, thick ice platelets that were several tens of metres in extent, moving past the caisson. ! The effect of ice drift speed as well as the angle of ice attack had a secondary effect on the width of the broken ice zones that were seen around the Molikpaq with rough ice conditions. 6.5. Ice fracture Large-scale fractures through the ice cover were also common in the immediate vicinity of the Molikpaq. This type of failure tended to occur under the following conditions: 1. When leads and/or open water areas were present nearby, in high ice concentration situations. 2. When moderate and low pack ice concentrations were present, typically involving small floes sizes. 3. When pack ice interactions occurred in low, moderate and high ice concentrations, typically at drift speeds in excess of 0.5 m/s.
These situations tended to create fractures in the ice cover that quickly formed and opened, often resulting in small leads near the structure and very locally, a mixed ice and open water situation. Cracks often propagated tens to hundreds of metres out from the Molikpaq (normally towards the updrift side and spreading out to 458 from its corners), and opened to widths from a few meters to tens-of-metres wide. The size of the ice pieces produced by the ice fracture varied considerably. In some cases, very large pieces were produced which could support a small evacuation capsule or personnel, whereas in other cases, smaller, more broken pieces were formed. These smaller pieces might be safe for a small number of evacuated personnel. However, it is possible that further ice fracture could develop with additional load, depending upon the temperature and drift conditions of the ice. Also, evacuation onto small ice pieces might be very troubling for some people who are unaccustomed to going out on the ice. Quantitative measurements were made from photos and videos. However, there were limited data so they will not be presented in detail here. However, they, along with data on ridges, are shown in comparison to the failure mode data in the next section. 7. Summary of results The field information from the Molikpaq showed a very wide range in the characteristics of the damage zone, depending upon the failure mode of the ice. It was clear that ice crushing failure usually resulted in a relatively narrow damage width, and that large-scale
Updrift Damage Length (m)
70 mixed mode failure crushing failure Ridge ice fracture
60 50 40 30 20 10 0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 23. Updrift damage length for different failure modes and ice roughness as a function of ice thickness. The fracture data indicate a lower bound for the value.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
83
30
Alongside Damage Width (m)
mixed mode failure crushing failure
25
Ridge ice fracture
20 15
ice fracture Dmin = 18.27 hice0.16
10 Dmax = 5.68 hice0.72
5 0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 24. Alongside damage width for different failure modes and ice roughness as a function of ice thickness. The fracture data indicate a lower bound for the value. The solid curves are the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
fracture of the ice could result in very wide regions of broken ice. The mixed mode failure, including the failure of ridges, showed damage zones between these two extremes. Fig. 23 shows a comparison of all data for the length of the broken ice in the updrift direction for various ice failure modes including ice roughness and fracture. There is a wide range in the updrift length of the broken zone, as might be expected. Often, ice rubble forms in front of the structure and it remains stationary until the environmental driving forces change direction and clear the rubble away. With stationary rubble, the bactiveQ interaction zone is usu-
ally at the end of the rubble, away from the structure. The recorded data in the figure shows that typical rubble lengths can be up to 70 m. Evacuation of personnel onto it should be avoided at all costs since the rubble is unstable and not grounded, and therefore it is not a suitable platform from an evacuation point of view. If, however, the rubble does ground and become stable, it may be a region where personnel can evacuate from the platform. Fig. 24 shows a comparison of all of the data on the width of the damage zone alongside the Molikpaq for various ice failure modes (crushing, mixed mode, fracture) and ice roughness (ridges). There is a wide range
16 mixed mode failure crushing failure Ridge ice fracture
Updrift Rubble Height (m)
14 12 10 8 6 4 2 0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 25. Rubble height in the updrift direction for different failure modes and ice roughness as a function of ice thickness.
84
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
Alongside Rubble Height (m)
5 mixed mode failure crushing failure Ridge ice fracture
4
3
2
1
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 26. Rubble height in the alongside direction for different failure modes and ice roughness as a function of ice thickness.
in the width of the damage zone, with values on the order of 5 m for ice crushing up to over 30 m for largescale ice fracture. For both the ice crushing and mixedmode failure, there is a general trend of a larger damage zone with increasing ice thickness. A power law regression through the crushing and mixed mode failures (including ridges) gave Dmax ¼ 5:68h0:72 ice
ð9Þ
with r 2 = 0.28. A separate power law regression through the ice fracture data gave Dmin ¼ 18:27h0:16 ice
ð10Þ
with r 2 = 0.07. The fracture data is essentially independent of ice thickness. Note that this curve represents the lower bound damage distance for ice fracture. Fig. 25 shows a comparison of the height of the ice rubble in the updrift direction for various ice failure modes (crushing, mixed mode, fracture) and ice roughness (ridges). For the Molikpaq, rubble heights up to 15 m have been observed in the region where the rubble is forming. Fig. 26 shows a comparison of all data for the height of the broken ice in the damage zone alongside the Molikpaq for various ice failure modes and ice roughness. Typical values range up to 4 m in height. For ice fracture, the rubble height is low, since the failure is mainly in the plane of the ice cover. Since there is no apparent trend with ice thickness, or failure mode, a power-law regression was not performed on this data. The height of the rubble in the alongside region is significantly lower that than in the updrift direction (see Fig. 25).
8. Conclusions This paper has summarized four different evacuation concepts that could be utilized from an offshore structure in ice-covered waters. Three of the four means of evacuation (primary, secondary and tertiary) are directly influenced by the size of the ice damage zone around the structure. Direct observations indicate that there is a large variation in the extent of the damage zone and the height of the ice rubble. It was found that a large number of parameters affect the size of this broken ice zone. In order of importance, these are the general ice regime, the failure mode of the ice, the ice thickness and the ice roughness. This paper has provided quantitative information from a wide range of ice conditions. Typical damage zone extent in moving ice which act directly on the structure can be on the order of 25 m from it for ice thickness of 1.5 m. When grounded rubble is present and stable, it affords the opportunity to evacuate directly onto its surface. Acknowledgements The authors would like thank Gulf Canada Resources Ltd. (ConocoPhillips), especially Dennis Seidlitz, for access to the Molikpaq data through the NRC Centre of Ice-Structure Interaction. They would also like to acknowledge Isabelle Morin for assisting in extracting the data from the Molikpaq photos, Anne Collins for data analysis, and Noe´mie Durand, who compiled the data in order to produce the Amauligak F-24 timeline. Financial assistance from Exxon Production Research (currently ExxonMobil Upstream Research Company), the Canadian Program on Energy
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
Research and Development (MTS POL) and the Climate Change Technology and Innovation Initiative (Unconventional Gas Supply) is gratefully acknowledged. All photographs are courtesy Gulf Canada Resources except for Fig. 3 (G. Timco) and Fig. 6 (K. Croasdale). References Barker, A., Timco, G.W., 2003. The effect of structure shape on the broken ice zone surrounding offshore structures. Proceedings 17th International Conference on Port and Ocean Engineering under Arctic Conditions, POAC’03, vol. 2. Norwegian University of Science and Technology, Trondheim, Norway, pp. 787 – 796. Barker, A., Timco, G., Sayed, M., 2001. Numerical simulation of the broken ice zone around the Molikpaq: implications for safe evacuation. Proceedings 16th International Conference on Port and Ocean Engineering under Arctic Conditions, POAC’01. Canadian Hydraulics Centre, National Research Council of Canada, Ottawa, Canada, pp. 505 – 515. Canatec, 1994. Study of Ice Rubble Formation in the Beaufort Sea. Report prepared for the National Energy Board of Canada, Mobil Research and Development Corporation and Gulf Canada Resources Ltd., Calgary, Canada. Croasdale, K.R., Marcellus, R., Metge, M., Timco, G.W., Wright, B.D., 1995. Overview of load resistance and stability of grounded ice rubble around structures in shallow water. Proceedings 2nd Conference on Development of the Russian Arctic Offshore
85
(RAO’95). Krylov Shipbuilding Research Institute, St. Petersburg, Russia, pp. 255 – 260. Poplin, J.P., Timco, G.W., 2003. Ice damage zone around conical structures: implications for evacuation. Proceedings 17th International Conference on Port and Ocean Engineering under Arctic Conditions, POAC’03, vol. 2. Norwegian University of Science and Technology, Trondheim, Norway, pp. 797 – 806. Timco, G.W., 1996. NRC centre of ice/structure interaction: archiving Beaufort Sea data. Proceedings 13th IAHR Symposium on Ice, vol. 1. Chinese Hydraulic Engineering Society, Beijing, China, pp. 142 – 149. Timco, G.W., Dickins, D.F., in press. Environment guidelines for EER systems in ice-covered waters. Cold Regions Science and Technology. Timco, G.W., Johnston, M., 2003. Ice loads on the Molikpaq in the Canadian Beaufort Sea. Cold Regions Science and Technology 37 (1), 51 – 68. Timco, G.W., Johnston, M., 2004. Ice loads on the Caisson structures in the Canadian Beaufort Sea. Cold Regions Science and Technology 38, 185 – 209. Timco, G.W., Barker, A., Durand, N., 2005. Ice Rubble Fields around Offshore Structures in the Beaufort Sea. National Research Council of Canada Report (in press), Ottawa, Ont., Canada. Wright, B.D., Timco, G.W., 1994. A review of ice forces and failure modes on the Molikpaq. Proceedings of the 12th IAHR Ice Symposium, IAHR’94, vol. 2. Norwegian University of Science and Technology, Trondheim, Norway, pp. 816 – 825. Wright, B.D., Timco, G.W., 2000. First-year ridge interaction with the Molikpaq in the Beaufort Sea. Cold Regions Science and Technology 32, 27 – 44.
Ice damage zone around the Molikpaq: Implications for evacuation systems G.W. Timco a,*, B.D. Wright b, A. Barker a, J.P. Poplin c a
Canadian Hydraulics Centre, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6 b B. Wright and Associates, Canmore, Alberta, Canada T1W 2R6 c Imperial Oil Resources, Calgary, Alberta, Canada T2P 3M9 Received 2 June 2005; accepted 6 August 2005
Abstract Effective design of an emergency evacuation system from an offshore structure in sea ice conditions must take into account all aspects of the anticipated ice conditions at the site. Evacuation systems are typically comprised of several different components or subsystems. These subsystems allow evacuees a choice in response to the emergency incident onboard the installation, as well as to environmental conditions off the installation. This paper discusses four possible evacuation means—preferred, primary, secondary and tertiary. Of these four techniques, only the preferred means of evacuation is not strongly influenced by the ice regime around the structure. This paper examines these ice regimes and quantifies the size of the broken ice region around a structure for different ice conditions. A large number of parameters affect the size of this broken ice zone including the general ice regime, the existence of a grounded rubble field, the failure mode of the ice, the ice thickness, and the ice roughness. Typical damage zone extent is extremely variable and can be in the order of 25 m from the structure for ice thickness of 1.5 m. This has significant consequences for evacuation systems deployed from the structure. D 2005 Elsevier B.V. All rights reserved. Keywords: Evacuation; Ice; Offshore structures; EER systems; Molikpaq
1. Introduction Evacuation from an offshore structure in ice conditions is not straightforward. Evacuation systems need to be suitable for both the ice conditions that they are designed to operate in, as well as for open water or partial ice covers associated with the melt season. To date, there is no single evacuation system that can be used in all ice conditions. The design of an evacuation * Corresponding author. E-mail addresses: [email protected] (G.W. Timco), [email protected] (B.D. Wright), [email protected] (A. Barker), [email protected] (J.P. Poplin). 0165-232X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.coldregions.2005.08.001
system must be carefully done by analyzing the type of ice conditions that the structure would likely encounter throughout the year (Timco and Dickins, in press). Reliable systems (potentially comprised of multiple means of evacuation) would be required for all conditions, and the most reliable means of evacuation may change throughout the season. During an emergency on an offshore platform in ice, one abandonment concept could be to evacuate the personnel with some type of life craft. This evacuation system would likely have to be launched perpendicular to the ice movement direction as the loaded side of the installation would be inaccessible due to the ice, and the lee side may, for instance, be unattainable due to
68
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
smoke. To have confidence in developing this concept as a reliable system, information is required on the dimensions of the zone of broken ice along the sides of the structure, in the direction perpendicular to the ice movement. For wide offshore structures, the focus for research is usually with respect to their stability due to ice loads. Consequently, to date there is little published information on the width of the damage zone along the sides of offshore structures. The only direct study in this regard is the recent publication by Poplin and Timco (2003) who investigated the width of the broken ice zone around conical structures and its impact on emergency evacuation. This paper summarizes the results of an analysis of the size of the broken ice zone around the Molikpaq structure over a range of ice conditions and the implications on emergency evacuation. Both pictorial and quantitative information on the extent of the damage zone and rubble height were obtained from the field observations. The results are related to the ice macrostructure (roughness), thickness, and failure mode and are discussed in terms of the deployment issues for evacuation systems. Note that although the detailed results are specific to the Molikpaq structure, they should be applicable to other vertical-sided structures in similar ice regimes. 2. Evacuation concepts The means of evacuation selected for any offshore platform is a very important consideration. Clearly, an evacuation system must be comprised of components which help ensure that safe and reliable methods are always available to allow personnel to abandon an offshore installation in response to an onboard emergency. Means of evacuation must be available across the range of environmental conditions anticipated. Evacuation methods for any offshore installation in a dynamic pack ice environment are not straightforward, given the high degree of variability in ice conditions that may be encountered around a structure, at any particular point in time. There are a number of factors that can influence the type of ice conditions found immediately adjacent to any platform, all of which are time varying. These include ice concentration, floe size, ice thickness and roughness, the ambient ice movement regime, the shape and size of the structure, and the nature of the ice/ structure interaction itself. In developing a platform evacuation system, the credible emergency incident scenarios that could
occur are developed. Details of those which could escalate to the point of necessitating installation abandonment are assessed further to help identify impairment time requirements for the temporary refuge (a location on an offshore installation in which all personnel can remain without harm for a specified time under any accident scenario). Evacuation timelines are developed to assess the time available for personnel to evacuate an installation before escalation renders an evacuation system ineffective or the incident imposes an undue risk to personnel using the evacuation system. In conjunction with developing emergency scenarios and establishing temporary refuge impairment time and evacuation timelines, the adequacy of different evacuation systems under the range of anticipated ice scenarios that should be expected around a platform are assessed. This approach provides a basis for systematically defining a platform evacuation system that can be effectively used to accommodate personnel escape over all credible ice situations. For most of the platforms that are now being considered for use in Arctic or sub-Arctic pack ice environments, a typical portfolio of evacuation options includes (according to the level of risk): ! Preferred—Helicopters are typically considered as the preferred means of evacuation. That is, they are the lowest risk option available to evacuate personnel and are used where possible in precautionary demanning (where time permits and the incident does not preclude their use). A number of factors impact the response times and/or availability of helicopters including the time required to complete the evacuation process and access constraints due to weather, visibility and the incident itself (e.g. fire, smoke, unignited gas release), etc. Support vessels may also constitute a preferred means of evacuation, for instances when the incident does not preclude the vessel from approaching the installation and a means (such as personnel basket) is available to transfer platform personnel to the vessel in a safe and timely manner. ! Primary—The primary means of evacuation is available under most emergency scenarios and provides personnel the most protection from the effects of the incident. Methods include devices that enable direct transfer to an ice-capable stand-by vessel including retractable stairways or gangways, chutes and slide systems, launch of a lifeboat to the vessel deck, launch of marine survival craft, etc. One of the key issues that is associated with this option is the ability for a standby vessel to reliably bapproach, closely
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
access and stationkeepQ at the platform in all of the ice conditions and incident conditions that can be expected. Another issue is the ability of the marine survival craft to get away from the incident and protect evacuees until rescue can be achieved. ! Secondary—The secondary means of evacuation comprises systems that offer evacuees less protection than the primary evacuation means and may transfer personnel from the platform to the ice surface or sea. To be both safe and effective, these types of evacuation methods must be able to blandQ escape craft away from any dangerous broken ice areas, where the ice is actively failing and/or clearing around the structure. ! Tertiary—The tertiary means of evacuation relies on higher risk methods. These include the use of individual descent methods, scramble nets, knotted ropes, stairs or ladders, etc. These options tend to be used as a last resort when lower risk alternatives are not available. Personnel using tertiary means will likely have to access ice or mixed ice/open water conditions in close proximity to the sides of a structure. Since the evacuation strategy must take into account the non-availability of lower risk methods, a range of evacuation alternatives is typically provided for any platform deployed. With the exception of the helicopter option, all of these evacuation methods will be affected by the ambient ice conditions, the nature of the ice action against the offshore platform, and the width of the broken ice zone that is seen around it. Consequently, information on the size and extent of the damage zone is critical in designing these systems to operate effectively in an Arctic environment. 3. Molikpaq structure The Molikpaq structure was owned by Gulf Canada Resources Ltd. and operated in Canada by Beaudril, a subsidiary of Gulf. The Molikpaq is a mobile arctic caisson (MAC) that was first deployed in the Canadian Beaufort Sea in 1984. The structure, which is a hybrid steel/sand platform consisting of an octagonal steel caisson annulus with dredged sand fill placed in its core, has 4 long sides approximately 60 m wide and 4 shorter corners approximately 22 m wide at the waterline. The total width of the structure is approximately 90 m at the waterline and 111 m at its base. The steel caisson supports a deck that carries a drilling rig, accommodation modules and other facilities. The sand core and steel caisson have sufficient mass to resist
69
horizontal ice loads. Depending upon the deployment design, the Molikpaq can be deployed with a sliding resistance in excess of 1000 MN. The Molikpaq, at its Tarsiut P-45 and Amauligak I-65 deployment locations, was placed on a submarine berm which resulted in a 20m local water depth. Since grounded rubble did not form around it at these locations, it was directly exposed to moving ice throughout the winter period. Thus, observations from these sites would provide important information on the size of the damage zone for different moving ice conditions. Information from these sites is used extensively in this paper. The Molikpaq was also deployed at two locations in which a grounded rubble field formed during the winter months. The Molikpaq at the Amauligak F-24 site was set on a 16-m high berm in 32 m water depth, giving a local effective water depth of 16 m. The Molikpaq was also used at the Isserk I-15 site in 11.5 m of water depth without a berm. Ice conditions at this site were essentially landfast shortly after freeze-up. A wedge-shaped, partially grounded rubble field extended over 700 m to the east of the structure at this site, but ice on the other sides of the platform was not so nearly deformed. 4. General ice environment and ice regimes Experience and photographs of the Beaufort Sea offshore structures show that there can be a wide variation in the type of ice conditions surrounding the structure—from solid level ice, to broken ice, to open water. This wide range of ice regimes is one of the most important factors to consider in the development of an offshore installation evacuation system. It makes it difficult to design an evacuation deployment system that can be used in all circumstances. The extent of the variations in the ice regimes surrounding the structure is a direct function of the location of the structure and the local ice conditions. There can be a wide variation in the type of ice conditions encountered for situations with moving ice. In general, the size of the ice damage zone around a structure is a function of several different parameters including the water depth, the existence of a grounded rubble field, the roughness of the ice, the floe size, the failure mode of the ice, the ice thickness, and the clearing behaviour of the broken ice. The following sections discuss their influence on evacuation systems. 5. Grounded rubble field If the depth of the water is fairly shallow, and the ice is relatively thick, a grounded ice rubble field can form
70
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
around the structure. In the Beaufort Sea, this was quite common for structures close to shore. In deeper waters, up to 20 m, grounded rubble fields did form on occasion, but they sometimes moved with changes in ice conditions. For effective water depths greater than 20 m, no permanent grounding occurred. Experience has shown that when a grounded rubble field forms, the active failure zone moves from the structure to the edge of the ice rubble (Croasdale et al., 1995). Grounded ice rubble features can often extend for large distances, depending upon the water depth and ice thickness. In the Beaufort Sea, for instance, grounded rubble piles could extend up to several hundreds of meters from the structure. However, there is a great deal of variability not only between structure locations, but the same location, from year-to-year. The rubble extent, roughness, growth, decay and stability are all dependent upon location and annual conditions. For example, as shown in Fig. 1, the shape of the rubble pile is not symmetrical. Rather, the shape usually reflects the predominant ice movement direction. Canatec (1994) and Timco et al. (2005) have documented the details of the rubble fields and rubble generation at a number of structure locations in the Beaufort Sea. Given the variability that exists, an example of a grounded rubble field is presented—the Molikpaq in the Beaufort Sea at the Amauligak F-24 site. Fig. 1 shows a large grounded rubble field surrounding the Molikpaq. A rubble field at this location began to develop at the end of November 1987. However, most of this rubble subsequently drifted away. It was not until the end of December that ice rubble formed which ultimately stayed and provided a stable core.
Most of the ice rubble field formed in January. The field continued to grow through the end of February 1988. By March, the rubble field extended 100 m from the structure, with a maximum sail height of approximately 10 m. The field was elongated in shape, extending along the East–West axis of the Molikpaq. Fig. 2 shows a timeline of the rubble field growth and decay at this site. The existence of a grounded rubble field has implications with regard to the evacuation of personnel from the structure. If a grounded rubble field forms, it may not be possible to access the installation with an icebreaker or effectively utilize lower risk platformbased evacuation systems. Depending upon the region and the specific circumstances, a range of potential strategies must be considered. For example, in certain circumstances, a platform-based Temporary Refuge (TR), designed with blast and fire protection, may be the best solution since it still provides access to a maximum range of evacuation alternatives. On the other hand, it might be possible to evacuate personnel directly onto the rubble field, provided the ice rubble is well grounded and stable. In this case, it would be necessary to pre-establish a Temporary Refuge on the rubble. Evacuation is inherently risky, so personnel would not abandon the platform until required to do so. Once personnel have abandoned the installation, they have committed to surviving in the off-platform environment. If off-platform evacuation is used, there are inherent risks. For example the topography of the rubble is quite rough (see Fig. 3), and movement across a rubble pile is usually very slow and arduous. This would have direct implications on the quick
Fig. 1. Grounded rubble field surrounding the Molikpaq at the Amauligak F-24 site.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
Amauligak F-24 (1987-1988)
December 20 through February 22 build-up of the rubble pile (nearly elliptical shape with the major axis in the E-W direction, highest point ~14m above WL (E of caisson)). tidal crack along the N boundary of the rubble pile separates the rubble from surrounding ice
October 30 new ice began to form at Amauligak F24 (slightly later than average) Until early November Open water
November 22 1st rubble build-up along the E end of the N face. At its maximum: 5-7m high, 1520m out. Was thought to be grounded but broke-up and drifted away by November 25
December 20 25-30m diameter rubble pile developed off the N end of the E face. Covered the complete face (70-80m long, 70m wide, 1015m high) by December 27
After December 31 ice rubble developing past that date was stable and survived until spring breakup
71
The initial rubble pile (December 22 event) continued to grow, but its lateral extent did not increase appreciably. January 9-January 11: rubble began to buildup on the N caisson face, until it covered it completely. January 23: build-up on the W and NW caisson faces. A rubble pile did develop to the S caisson face; however, it never matched the size or stability of those on the E-NE or W faces.
By March a stable grounded rubble field had already formed. Boundary of the rubble field had an oval shape and extended away from the caisson to 60m to 100m. Sail heights up to 10m above WL.
through April maximum thickness of the ice (1.7m)
January portions of the rubble piles were grounded and stable
March and May field trips FY ice with numerous leads and areas of open water surrounded rubble field.
April 26 break-up of the rubble started at the S side.
from May 26 to June 7 the rubble gradually disintegrated and broke away due to ablation, wave erosion, swell movements and ice impacts.
June 1 open water (ice in concentrations < 2/10ths)
June 9 last piece floated away
Fig. 2. Timeline of the ice rubble field generation and decay at the Amauligak F-24 site in 1987/1988.
evacuation across a grounded rubble pile. If this evacuation system is designed into the evacuation plan, it would be necessary to maintain a path to the TR site,
either for example by using a bulldozer or by using spray ice to even out the escape route across the rubble field.
Fig. 3. Photograph showing the roughness of a grounded rubble field.
72
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
During the spring break-up season, the ice surrounding the rubble field will melt, but the grounded rubble may remain into the open water season. During spring, the rubble field can have large melt holes throughout it, and traversing it would be extremely difficult. The use of a Temporary Refuge would not be possible at this time of year. The decaying rubble field would restrict the use of a standby vessel for evacuation since the vessel would not be able to closely approach the structure. Furthermore, the edge conditions can be quite slippery and unsafe for personnel (see Fig. 4). This would make it difficult to evacuate to a standby vessel at the edge of the decaying rubble field. Allowing grounded ice rubble to form and then overcoming the obstacles it creates is one approach. In some circumstances, an alternative pro-active strategy would be to remove the grounded rubble shortly after it forms. Rubble forms in a series of ice movement events. Consequently, the time interval between ice movement events can be used to remove the recently formed grounded ice rubble. Note that all of the rubble does not have to be removed, but rather only that portion that adversely affects the ability to evacuate the installation and to affect rescue. Experience with propeller wash in the Alaskan Beaufort and Bering Seas has shown that vessels can be used to remove grounded ice rubble. Azimuthal stern drive icebreakers in the North Caspian Sea were even more efficient at removing grounded rubble surrounding exploration facilities. The key to this strategy is to remove the ice rubble soon after it has formed and not allowing it to consolidate (W. Spring, personal communication).
6. Intact ice sheet An ice sheet moving past the structure presents a different situation than that of a grounded rubble field. Large ice floes moving past the offshore structure usually fail along the leading edge of the ice close to the structure and most of the ice clears around the structure. Timco and Dickins (in press) have defined three regions around an offshore structure in moving ice conditions: updrift direction (the direction from which the ice is approaching), the alongside direction (along the side of the structure perpendicular to the direction of the moving ice) and the downdrift (wake) direction (the region behind the structure (see Fig. 5)). These regions are clearly shown in Fig. 6 which shows an aerial view of ice moving past the Molikpaq. Note that because the ice movement direction varies over time, the preferential side of the installation from an evacuation standpoint will also change. In front of the structure in the Updrift Direction, dynamic ice conditions with different ice failure processes take place. In this area, relatively large ice pileups can occur (see e.g. Fig. 7). This region is not safe for either lowered systems or vessel stationkeeping but may be the preferred side from an incident perspective (being upwind). On the side of the structure, in the Alongside Direction, the ice conditions are also dynamic with the broken ice pieces moving along the structure and clearing around it. This region can be periodically safe for lowered systems and/or vessel stationkeeping. However, a lowered system must be sufficiently far from the structure so that it does not enter the ice
Fig. 4. Photograph showing the edge of a grounded rubble field (foreground) with the ice sheet decaying and breaking up in the spring.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
73
Alongside Direction
Downdrift (Wake) Direction USUALLY SAFE
Updrift Direction NOT SAFE
Fig. 5. Illustration of the updrift direction, alongside direction and the downdrift (wake) direction around an offshore structure in moving ice conditions. The safety of the regions is based on an ice perspective. This may not have the same safety aspects as the incident perspective.
damage zone and/or be sufficiently robust that the ice will not damage it. Behind the structure in the Wake, the broken ice often fills-in, providing a broken brash ice regime. This region is usually safe for lowered systems and for vessel stationkeeping. However, it may not be possible to evacuate from this side of the installation (due to smoke, unignited gas, etc.) because it is often downwind. As an illustrative example of the dynamics of these regions, a 5-h time period was monitored to show the variations in the rubble height and width. Videos recorded on February 17, 1986, from 7 p.m. to midnight were examined to extract this time-line information. During this time period, a first-year ice sheet with a nominal thickness of 0.7–0.9 m thickness moved past the Molikpaq at a speed on the order of 0.05–0.09 m/s.
The ice was predominately level, but there were a number of ridges within it. The sail heights of the ridges were from 0.5 m to 2 m high. Fig. 8 shows a time-series plot of the observed variation of the damage distance both in the updrift direction and alongside direction of the Molikpaq. There is a large variation in the damage width with time, even for ice conditions that are nominally similar. In the updrift direction, the damage length varied from 1 m to 18 m in front of the structure. Alongside the Molikpaq, the maximum value and variation are less, with maximum damage widths on the order of 8 m. Fig. 9 shows a time-series plot of the observed rubble height in both the updrift and the alongside directions. In this case, the rubble height is quite
Fig. 6. Aerial view of the Molikpaq showing the updrift direction, alongside direction and the downdrift (wake) direction. The ice is failing along the front of the structure (updrift), moving along the side of the structure (alongside), and clearing into the wake. Note that the wake is largely filled with ice blocks. The ice movement is from right-to-left in the photograph.
74
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
Fig. 7. Photograph showing the ice crushing and piling up in front of the Molikpaq. The direction of ice movement is from right to left. Note the ice bturning backQ on itself as it interacts with the structure.
comparable in the two directions. Typical values varied between 1 and 3.5 m in both regions. Figs. 8 and 9 clearly illustrate the dynamic behaviour of the ice in the damage zone surrounding an offshore structure. The Beaufort Sea experience has shown that there are a number of important observations that can be made with regard to the width and behaviour of the ice damage zones as it relates to the evacuation of personnel: ! In conditions of high concentration of moving pack ice, a wake forms downdrift of the structure. There
can be various degrees of in-fill of broken ice in this region with a range of essentially open water to heavily congested brash. ! The orientation and width of the wake is a function of the angle of ice attack. Thus, the ice movement direction and the relative placement of evacuation locations becomes an important consideration for the evacuation system (Barker et al., 2001; Barker and Timco, 2003). ! The widths of the wake and the damage zone along the side and in front of the structure appear to be a function of several parameters including the ice thickness and ice failure mode, and to a much lesser
20 Updrift Direction
Damage Distance (m)
Alongside Direction
15
10
5
0 0
50
100
150
200
250
300
Time (minutes) Fig. 8. Time variation of the length of the damage zone for both the updrift and alongside directions during a 5-h time period.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
75
5 Updrift Direction Alongside Direction
Rubble Height (m)
4
3
2
1
0 0
50
100
150
200
250
300
Time (minutes) Fig. 9. Time variation of the rubble height in the damage zone for both the updrift and alongside directions during a 5-h time period.
extent, the surface roughness. These factors are explored in the next sections. 6.1. Influence of ice failure modes The failure mode of the ice has direct implications on the width of the damaged zone along the side of an offshore structure. The manner in which the oncoming ice cover fails against, and in turn, clears around the structure, has an important influence on the width of the broken ice zone. In general, the failure behaviour of the
ice depends on the thickness, speed, size, confinement and roughness of the ice feature. Essentially, it integrates all of these factors. The ice failure modes against the Molikpaq have been discussed by Wright and Timco (1994) and Timco and Johnston (2003, 2004). In general, buckling failure occurs with thinner ice, and crushing and mixed mode behaviours occur with thicker ice. To develop a quantitative description of the width of the damage zone, an examination was made of information contained in the NRC Centre of Ice/Structure Interaction (Timco, 1996). Several reports published by Gulf Canada Resources Ltd. and videos were used to provide information in this area. This information was examined to get quantitative information on both the width of the damage zone and the rubble height in the updrift and alongside directions. In all cases, there was a high ice concentration surrounding the structure; i.e. it does not include data where there were small ice floes or large fractured areas in the ice sheet. 6.2. Ice crushing
Fig. 10. Ice crushing along the face and side of the Molikpaq.
Ice crushing occurred on the Molikpaq, but it was not a predominant failure mode for the ice. Fig. 10 shows a photograph of ice crushing along one face of the structure. Note that the ice pieces are quite small (almost powder-like) and that the damage zone is very close to the structure. Evaluation of the pictorial information provided some information on the size of the damage zone and the height of the rubble around the Molikpaq. Figs. 11 and 12 present the data on the maximum damage distance (D max) for different ice
76
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
20
Damage Distance (m)
Ice Crushing Updrift Direction
15
10 Dmax = 5.11 hice0.64
5
0 0
0.5
1
1.5
2
2.5
3
3.5
4
Ice Thickness (m) Fig. 11. Damage distance (D max) as a function of ice thickness (h ice) for the ice crushing failure mode in the updrift direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
thickness (h ice) for ice crushing in both the updrift and alongside directions respectively. In all of these graphs, a multiplicative power law has been fit to the data. Each graph shows the best fit line along with the 95% predictive limits (light dashed lines). The heavier dashed line represents the upper limit of the predictive value based on the available data. There are not many data points for ice crushing and there is considerable scatter in the data. Regression power-law curves fit to the data give the following
relationships for the width of the damage zone for ice crushing: Dmax ¼ 5:11h0:64 ice
ð1Þ 2
for the updrift direction (with r = 0.31), and Dmax ¼ 3:89h0:85 ice
ð2Þ 2
for the alongside direction (with r = 0.34). Figs. 13 and 14 provide information on the maximum rubble height (H max) for different ice thickness
20
Damage Distance (m)
Ice Crushing Alongside Direction
15
10 Dmax = 3.89 hice0.85
5
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 12. Damage distance (D max) as a function of ice thickness (h ice) for the ice crushing failure mode in the alongside direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
77
10
Rubble Height (m)
Ice Crushing Updrift Direction
5 Hmax = 2.92 hice0.86
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 13. Rubble height (H max) as a function of ice thickness (h ice) for the ice crushing failure mode in the updrift direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
(h ice) for the updrift and alongside directions, respectively. Regression power law relationships for the height of the rubble gave Hmax ¼ 2:92h0:86 ice
ð3Þ
for the updrift direction (with r 2 = 0.62), and Hmax ¼ 2:09h0:51 ice
ð4Þ
for the alongside direction (with r 2 = 0.36).
When ice was crushing against the Molikpaq, the following observations can be made: ! The broken ice zone widths that were seen around the Molikpaq were smallest when pack ice was crushing against the caisson, in comparison to the broken ice zone widths that were associated with other ice failure modes. Typical damage zone sizes were in the order of 5–10 m, with little difference in the updrift and alongside direction (Figs. 11 and 12).
10
Rubble Height (m)
Ice Crushing Alongside Direction
5
Hmax = 2.09 hice0.51
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 14. Rubble height (H max) as a function of ice thickness (h ice) for the ice crushing failure mode in the alongside direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
78
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
! Variations in ice drift speed had a minor influence on the dimensions of the broken ice zone when the ice was crushing, but higher drift speeds did tend to favour cleaner and bless brashyQ downdrift wake conditions, for a given ice thickness and roughness situation. ! When ice crushed against the Molikpaq, granulated ice debris accumulated on its updrift face where the ice was actively failing, and against its alongside faces where the ice was clearing. ! The dimensions of the broken ice zones were fairly consistent regardless of the angle of ice attack, although the broken ice widths along the structure’s sides where the ice was clearing tended to be slightly wider for oblique ice interactions. Fig. 15 shows a scaled drawing with guidelines suggesting the maximum extent of the damaged ice for various ice thicknesses along all sides of the structure. The figure also includes information on the general ice conditions in the wake for different ice thickness interactions. Note that this information is applicable to ice crushing failure behaviour on a vertical-sided structure. Fig. 15. Size of the damage zone in the updrift and alongside directions for ice crushing failure. Information on general ice conditions in the wake is also indicated.
! With continuous ice crushing, a downstream wake was always seen behind the structure, with the bproportion and heavinessQ of the brash ice in the downstream wake typically increasing with increasing ice thickness and roughness.
6.3. Mixed mode failure The mixed mode failure was the most predominant failure mode for the ice, when it was actively failing against the Molikpaq. This mixed mode was a combination of different failure behaviours. Fig. 16 shows a photograph of mixed mode failure. In this case, there is ice crushing along the structure and ice riding up on the
Fig. 16. Photograph showing mixed mode failure with ice crushing, flexural failure with ride-up, and ice fracture.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
79
50
Damage Distance (m)
Mixed Mode Failure Updrift Direction
40
30
20
10
Dmax = 12.91hice0.52
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 17. Damage distance (D max) as a function of ice thickness (h ice) for mixed mode failure of the ice in the updrift direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
rubble through a flexural failure. Further, a large crack has formed in the ice. Damage zone distances were generally larger than those observed with ice crushing failure. As such, this failure mode is more important than ice crushing from the point of view of personnel evacuation. Figs. 17 and 18 show the maximum damage distance (D max) for different ice thickness (h ice) for both the updrift and alongside directions for mixed mode failure. Note that the scales of the axes are not the same as those of the ice crushing graphs. There is considerably
more scatter in the data with the mixed mode failure compared to the ice crushing. Regression power-law (multiplicative) curves fit to the data give the following relationships for the width of the damage zone: Dmax ¼ 12:91h0:52 ice
ð5Þ
for the updrift direction (with r 2 = 0.17), and Dmax ¼ 6:84h0:76 ice
ð6Þ
for the alongside direction (with r 2 = 0.38).
50 Mixed Mode Failure Alongside Direction
Damage Distance (m)
45 40 35 30 25 20 15 10 5
Dmax = 6.84 hice 0.76
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 18. Damage distance (D max) as a function of ice thickness (h ice) for mixed mode failure of the ice in the alongside direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
80
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
Mixed Mode Failure Updrift Direction
Rubble Height (m)
10
5
Hmax = 2.98 hice0.28
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 19. Rubble height (H max) as a function of ice thickness (h ice) for ice mixed mode failure of the ice in the updrift direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
Figs. 19 and 20 show the rubble height (H max) for different ice thickness (h ice) for both the updrift and alongside directions for mixed mode failure. Note that the scales of the axes are not the same as those of the ice crushing graphs. Regression power law relationships for the height of the rubble gave Hmax ¼ 2:98h0:28 ice
for the alongside direction (with r 2 = 0.17). The graphs show very little dependence on ice thickness. The low r 2 values illustrate the wide range of observed values for both the extent of the damage zone and the rubble height. Note that this scatter is less with ice crushing. With regard to mixed mode failure, the following comments can be made:
ð7Þ
for the updrift direction (with r 2 = 0.10), and Hmax ¼ 1:69h0:47 ice
ð8Þ
! The predominant ice failure behaviour that was seen at the Molikpaq involved mixed modal and bending ice failures immediately adjacent to the caisson’s updrift faces. This mixed mode failure occurred for
Mixed Mode Failure Alongside Direction
Rubble Height (m)
10
5
Hmax = 1.69 hice0.47
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 20. Rubble height (H max) as a function of ice thickness (h ice) for ice mixed mode failure of the ice in the alongside direction. The solid curve is the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
81
situation with mixed mode failure. Also, in comparing this figure to Fig. 15 for crushing failure, it is clear that there is a much larger extent of damaged ice with mixed mode ice failure. 6.4. Ice roughness Large ice floes can be uniform in thickness, but more often than not, there is a wide range in the ice thickness. Ice floes can contain level ice with ridges and hummock fields intermixed within it. A ridge is a linear feature containing a large accumulation of broken ice, whereas a hummock field contains very rough ice over a large aerial extent. It might be expected that the width of the failure zone would be substantially wider for the failure of the more extreme features, but photographic evidence from the Canadian Beaufort Sea indicates that this is not the case (see e.g. Fig. 22). The failure of a ridge and rubble tended to be a shear failure along the sides of the structure (Wright and Timco, 2000). Photographic evidence indicates that ice roughness doesn’t have a strong influence on the width of the damage zone. This result is quantified later in this paper, but some general comments from observations follow: Fig. 21. Size of the damage zone in the updrift and alongside directions for mixed failure mode of the ice. Information on general ice conditions in the wake is also indicated.
! Along the sides of the structure, the width of the broken ice zone was largely unaffected by the presence of rough ice conditions, because these features
more than about 50% of the time that the ice was failing against the structure. ! For the case of mixed modal ice failures against the Molikpaq structure, broken ice fragments and floating ice rubble accumulated on both its updrift and alongside faces, with broken ice dimensions that were fairly consistent regardless of the ice attack angle. ! The width of the broken ice zone that was seen around the Molikpaq was generally greater with mixed failure modes than for ice crushing failures. Thus, mixed mode failure represents a more severe situation regarding bacceptable evacuation system deployment distancesQ. ! Similar to the continuous ice crushing case, a downstream wake was always present behind the structure, with its brash ice content generally increasing with ice thickness and roughness, and with decreasing ice drift speeds. Fig. 21 shows a scaled drawing with guidelines suggesting the maximum extent of the damaged ice for various ice thicknesses for a Molikpaq-shaped structure. Note that this information is applicable to the
Fig. 22. Photograph showing the wake behind the Molikpaq. Note that the width of the wake is relatively independent of the ice roughness.
82
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
would tend to bshear pastQ the Molikpaq and remain intact as they moved by, without any breakout or rotation. ! In the updrift region, the active ice failure zone that was associated with these rough ice feature interactions was often further off the structure. In some instances, the failure of the feature occurred as far as 100 m updrift of the caisson. ! During oblique interactions between the Molikpaq and large ridges or heavily rubbled areas, fractures often propagated out from the caisson, resulting in large, thick ice platelets that were several tens of metres in extent, moving past the caisson. ! The effect of ice drift speed as well as the angle of ice attack had a secondary effect on the width of the broken ice zones that were seen around the Molikpaq with rough ice conditions. 6.5. Ice fracture Large-scale fractures through the ice cover were also common in the immediate vicinity of the Molikpaq. This type of failure tended to occur under the following conditions: 1. When leads and/or open water areas were present nearby, in high ice concentration situations. 2. When moderate and low pack ice concentrations were present, typically involving small floes sizes. 3. When pack ice interactions occurred in low, moderate and high ice concentrations, typically at drift speeds in excess of 0.5 m/s.
These situations tended to create fractures in the ice cover that quickly formed and opened, often resulting in small leads near the structure and very locally, a mixed ice and open water situation. Cracks often propagated tens to hundreds of metres out from the Molikpaq (normally towards the updrift side and spreading out to 458 from its corners), and opened to widths from a few meters to tens-of-metres wide. The size of the ice pieces produced by the ice fracture varied considerably. In some cases, very large pieces were produced which could support a small evacuation capsule or personnel, whereas in other cases, smaller, more broken pieces were formed. These smaller pieces might be safe for a small number of evacuated personnel. However, it is possible that further ice fracture could develop with additional load, depending upon the temperature and drift conditions of the ice. Also, evacuation onto small ice pieces might be very troubling for some people who are unaccustomed to going out on the ice. Quantitative measurements were made from photos and videos. However, there were limited data so they will not be presented in detail here. However, they, along with data on ridges, are shown in comparison to the failure mode data in the next section. 7. Summary of results The field information from the Molikpaq showed a very wide range in the characteristics of the damage zone, depending upon the failure mode of the ice. It was clear that ice crushing failure usually resulted in a relatively narrow damage width, and that large-scale
Updrift Damage Length (m)
70 mixed mode failure crushing failure Ridge ice fracture
60 50 40 30 20 10 0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 23. Updrift damage length for different failure modes and ice roughness as a function of ice thickness. The fracture data indicate a lower bound for the value.
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
83
30
Alongside Damage Width (m)
mixed mode failure crushing failure
25
Ridge ice fracture
20 15
ice fracture Dmin = 18.27 hice0.16
10 Dmax = 5.68 hice0.72
5 0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 24. Alongside damage width for different failure modes and ice roughness as a function of ice thickness. The fracture data indicate a lower bound for the value. The solid curves are the best fit multiplicative equation, and the light dotted lines are the 95% confidence limits for new data. The heavy dashed line represents the upper limit curve predicted by the available data.
fracture of the ice could result in very wide regions of broken ice. The mixed mode failure, including the failure of ridges, showed damage zones between these two extremes. Fig. 23 shows a comparison of all data for the length of the broken ice in the updrift direction for various ice failure modes including ice roughness and fracture. There is a wide range in the updrift length of the broken zone, as might be expected. Often, ice rubble forms in front of the structure and it remains stationary until the environmental driving forces change direction and clear the rubble away. With stationary rubble, the bactiveQ interaction zone is usu-
ally at the end of the rubble, away from the structure. The recorded data in the figure shows that typical rubble lengths can be up to 70 m. Evacuation of personnel onto it should be avoided at all costs since the rubble is unstable and not grounded, and therefore it is not a suitable platform from an evacuation point of view. If, however, the rubble does ground and become stable, it may be a region where personnel can evacuate from the platform. Fig. 24 shows a comparison of all of the data on the width of the damage zone alongside the Molikpaq for various ice failure modes (crushing, mixed mode, fracture) and ice roughness (ridges). There is a wide range
16 mixed mode failure crushing failure Ridge ice fracture
Updrift Rubble Height (m)
14 12 10 8 6 4 2 0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 25. Rubble height in the updrift direction for different failure modes and ice roughness as a function of ice thickness.
84
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
Alongside Rubble Height (m)
5 mixed mode failure crushing failure Ridge ice fracture
4
3
2
1
0 0
0.5
1
1.5
2
2.5
Ice Thickness (m) Fig. 26. Rubble height in the alongside direction for different failure modes and ice roughness as a function of ice thickness.
in the width of the damage zone, with values on the order of 5 m for ice crushing up to over 30 m for largescale ice fracture. For both the ice crushing and mixedmode failure, there is a general trend of a larger damage zone with increasing ice thickness. A power law regression through the crushing and mixed mode failures (including ridges) gave Dmax ¼ 5:68h0:72 ice
ð9Þ
with r 2 = 0.28. A separate power law regression through the ice fracture data gave Dmin ¼ 18:27h0:16 ice
ð10Þ
with r 2 = 0.07. The fracture data is essentially independent of ice thickness. Note that this curve represents the lower bound damage distance for ice fracture. Fig. 25 shows a comparison of the height of the ice rubble in the updrift direction for various ice failure modes (crushing, mixed mode, fracture) and ice roughness (ridges). For the Molikpaq, rubble heights up to 15 m have been observed in the region where the rubble is forming. Fig. 26 shows a comparison of all data for the height of the broken ice in the damage zone alongside the Molikpaq for various ice failure modes and ice roughness. Typical values range up to 4 m in height. For ice fracture, the rubble height is low, since the failure is mainly in the plane of the ice cover. Since there is no apparent trend with ice thickness, or failure mode, a power-law regression was not performed on this data. The height of the rubble in the alongside region is significantly lower that than in the updrift direction (see Fig. 25).
8. Conclusions This paper has summarized four different evacuation concepts that could be utilized from an offshore structure in ice-covered waters. Three of the four means of evacuation (primary, secondary and tertiary) are directly influenced by the size of the ice damage zone around the structure. Direct observations indicate that there is a large variation in the extent of the damage zone and the height of the ice rubble. It was found that a large number of parameters affect the size of this broken ice zone. In order of importance, these are the general ice regime, the failure mode of the ice, the ice thickness and the ice roughness. This paper has provided quantitative information from a wide range of ice conditions. Typical damage zone extent in moving ice which act directly on the structure can be on the order of 25 m from it for ice thickness of 1.5 m. When grounded rubble is present and stable, it affords the opportunity to evacuate directly onto its surface. Acknowledgements The authors would like thank Gulf Canada Resources Ltd. (ConocoPhillips), especially Dennis Seidlitz, for access to the Molikpaq data through the NRC Centre of Ice-Structure Interaction. They would also like to acknowledge Isabelle Morin for assisting in extracting the data from the Molikpaq photos, Anne Collins for data analysis, and Noe´mie Durand, who compiled the data in order to produce the Amauligak F-24 timeline. Financial assistance from Exxon Production Research (currently ExxonMobil Upstream Research Company), the Canadian Program on Energy
G.W. Timco et al. / Cold Regions Science and Technology 44 (2006) 67–85
Research and Development (MTS POL) and the Climate Change Technology and Innovation Initiative (Unconventional Gas Supply) is gratefully acknowledged. All photographs are courtesy Gulf Canada Resources except for Fig. 3 (G. Timco) and Fig. 6 (K. Croasdale). References Barker, A., Timco, G.W., 2003. The effect of structure shape on the broken ice zone surrounding offshore structures. Proceedings 17th International Conference on Port and Ocean Engineering under Arctic Conditions, POAC’03, vol. 2. Norwegian University of Science and Technology, Trondheim, Norway, pp. 787 – 796. Barker, A., Timco, G., Sayed, M., 2001. Numerical simulation of the broken ice zone around the Molikpaq: implications for safe evacuation. Proceedings 16th International Conference on Port and Ocean Engineering under Arctic Conditions, POAC’01. Canadian Hydraulics Centre, National Research Council of Canada, Ottawa, Canada, pp. 505 – 515. Canatec, 1994. Study of Ice Rubble Formation in the Beaufort Sea. Report prepared for the National Energy Board of Canada, Mobil Research and Development Corporation and Gulf Canada Resources Ltd., Calgary, Canada. Croasdale, K.R., Marcellus, R., Metge, M., Timco, G.W., Wright, B.D., 1995. Overview of load resistance and stability of grounded ice rubble around structures in shallow water. Proceedings 2nd Conference on Development of the Russian Arctic Offshore
85
(RAO’95). Krylov Shipbuilding Research Institute, St. Petersburg, Russia, pp. 255 – 260. Poplin, J.P., Timco, G.W., 2003. Ice damage zone around conical structures: implications for evacuation. Proceedings 17th International Conference on Port and Ocean Engineering under Arctic Conditions, POAC’03, vol. 2. Norwegian University of Science and Technology, Trondheim, Norway, pp. 797 – 806. Timco, G.W., 1996. NRC centre of ice/structure interaction: archiving Beaufort Sea data. Proceedings 13th IAHR Symposium on Ice, vol. 1. Chinese Hydraulic Engineering Society, Beijing, China, pp. 142 – 149. Timco, G.W., Dickins, D.F., in press. Environment guidelines for EER systems in ice-covered waters. Cold Regions Science and Technology. Timco, G.W., Johnston, M., 2003. Ice loads on the Molikpaq in the Canadian Beaufort Sea. Cold Regions Science and Technology 37 (1), 51 – 68. Timco, G.W., Johnston, M., 2004. Ice loads on the Caisson structures in the Canadian Beaufort Sea. Cold Regions Science and Technology 38, 185 – 209. Timco, G.W., Barker, A., Durand, N., 2005. Ice Rubble Fields around Offshore Structures in the Beaufort Sea. National Research Council of Canada Report (in press), Ottawa, Ont., Canada. Wright, B.D., Timco, G.W., 1994. A review of ice forces and failure modes on the Molikpaq. Proceedings of the 12th IAHR Ice Symposium, IAHR’94, vol. 2. Norwegian University of Science and Technology, Trondheim, Norway, pp. 816 – 825. Wright, B.D., Timco, G.W., 2000. First-year ridge interaction with the Molikpaq in the Beaufort Sea. Cold Regions Science and Technology 32, 27 – 44.