Explosive removals of offshore structures in the Gulf of Mexico

Ocean & Coastal Management 45 (2002) 459–483

Explosive removals of offshore structures in the Gulf of Mexico Mark J. Kaiser*, Dmitry V. Mesyanzhinov, Allan G. Pulsipher Center for Energy Studies, Louisiana State University, Baton Rouge, LA 70803-0301, USA

Abstract A statistical description of the explosive removal of offshore structures in the federally regulated Outer Continental Shelf of the Gulf of Mexico are presented based on data collected by the US Minerals Management Service. The influence of factors such as water depth, planning area, configuration type, and structure age upon the application of explosive removal methods are investigated. The number of structures expected to be removed from the Gulf of Mexico using explosive methods is also forecast over a short-term time horizon according to structure, configuration type, water depth, and a planning area categorization. r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction The offshore Outer Continental Shelf (OCS) of the US Gulf of Mexico (GOM) is one of the most highly developed and mature basins in the world (Fig. 1). Over the last 50 years the oil and gas industry has installed over 6000 structures and 15,000 miles of interconnecting pipelines in the gulf waters. A wide variety of structures are used in the GOM to develop hydrocarbons ranging from simple vertical caissons supporting one well in shallow water to tension-leg platforms and spars in deep water that support multiple wells drilled directionally from the platform to bottomhole targets thousands of feet away (Figs. 2, 3). Today on the Gulf of Mexico OCS there are about 4000 active structures installed in federal1 water depths ranging from less than 10 ft to over 6000 ft: There are also about 1000 structures in state waters off *Corresponding author. Tel.: +1-504-578-4554; fax: +1-504-578-4541. E-mail address: [email protected] (M.J. Kaiser). 1 Federal jurisdiction in the OCS varies with the Gulf state: Florida and Texas have an extended 9 n: miles state jurisdiction, while Alabama, Louisiana, and Mississippi have the standard 3 n: miles state jurisdiction. 0964-5691/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 4 - 5 6 9 1 ( 0 2 ) 0 0 0 8 1 - 9

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Fig. 1. The Gulf of Mexico.

Fig. 2. Offshore development systems—bottom supported and vertically moored structures. Source: Minerals Management Service.

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461

Fig. 3. Offshore development systems—floating production and subsea systems. Source: Minerals Management Service.

the coast of Louisiana and Texas, almost all of which are small and installed in less than 35 ft of water. Structures need to be constructed, delivered, installed, and equipped prior to production, operated and serviced during production, and then eventually decommissioned and removed after production. Each of these activities has both a direct and indirect impact on the communities in which the service facilities and manufacturing operations are located, and hence induce a ‘‘spill-over’’ effect on the economic growth of regions which serve the development. An entire industry has been built in the GOM around installing production equipment and structures, servicing those structures (maintenance, repairs, supply), and then removing the structures when production ceases. During the life of the OCS lease, the leaseholders apply for permits to place structures on the seafloor to aid in drilling, development, and production operations. Near the end of the economic life of the lease when the structure has been fully depreciated and reserves depleted, the structure represents a financial and operational liability, and at this point in time a decision is made to abandon the structure. Within 1 year of lease termination, the Minerals Management Service (MMS)—the federal agency that manages the leasing, exploration, development, and production of oil and gas on the OCS—requires that the lessees remove all structures to a depth of 15 ft below the mudline and that the structure site be returned to prelease conditions. Although multiple techniques may be used to sever the structural components, they are generally categorized as either explosive or nonexplosive methods.

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Structure removal requests are reviewed by the MMS on a case-by-case basis, and at present, all structure removals require an environmental assessment. During the removal process, permits are required to plug and abandon wells and pipelines, remove structures, and verify site clearance. If explosives are to be used in the removal process to cut bracing, conductors, or piles, an Incidental Take Statement will need to be acquired. If an operator requires the use of more than 50 lb of explosives for a particular detonation, then a special consultation with the National Marine Fisheries Service (NMFS) is required. The NMFS specifies criteria pertaining to the size of explosive charge used, detonation depth, and number of blasts per structure grouping. The use of explosives to cut conductors and piling could cause injury or death to protected marine mammals and endangered turtles, but since 1986 when the NMFS observer program began, only one sea turtle is known to have been harmed from the use of explosives. The NMFS sends observers to every structure removal where explosives are used. A pre-blast aerial survey is conducted immediately prior to the explosive detonation using a helicopter with a NMFS observer on board. If marine mammals or sea turtles are found within 1000 yards of the structure, the detonation is delayed until the area is clear. A post-blast aerial survey is then conducted after the explosives are detonated to assess the impact of the explosive charges to the marine life. The purpose of this paper is to provide a statistical description of structures that have been removed in the Gulf of Mexico and the manner of their removal. The influence of factors such as water depth, planning area, configuration type, and structure age will be examined, and the relationship of these factors with explosive removals will be discussed. Brief background material is provided in Section 2 followed by a statistical analysis of structure removals in Section 3. In Section 4 a short-term 5-year forecast of structure removals is developed based on historic trends and in Section 5 conclusions are presented.

2. Background information 2.1. Decommissioning options Offshore oil and gas structures vary widely according to function and configuration type. Since 1947, 5981 structures have been installed in the Gulf of Mexico and 2004 structures have been removed. The vast majority (96%) of the structures removed have been in shallow water (0–60 m) with caissons the most frequently removed configuration type (46%), followed by fixed platforms (42%) and well-protector jackets (12%). Caissons and well-protector jackets are installed to protect wells from damage, while fixed platforms refer to the familiar conventionally piled structures. Fixed platforms with wells hold the drilling and processing equipment necessary for hydrocarbon production, while platforms without wells are used to house personnel and to support gas compressor stations, production equipment, oil storage tanks, etc. Floating production systems like tension leg

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Offshore oil and gas facility decommissioning

Oil and gas processing equipment and piping

Deck and jacket structure

Send to shore

Refurbish and reuse

Sell for scrap

Onshore scraping

Waste to landfill

Recycle steel

Move to new location and reinstall

Waste to landfill

Dispose of in deep water

Partial removal

Convert to an artificial reef

Topple in place

Placement offsite

Fig. 4. Offshore oil and gas facility decommissioning decision tree.

platforms and spars are also employed in the deep water GOM but there are only a handful of such structures and their numbers are negligible relative to the traditional fixed structure. Structures are installed to produce hydrocarbons and when the oil reservoir is depleted, decommissioning is inevitable. When the time arrives that the cost to operate a structure (maintenance, operating personnel, transportation, fuel, etc.) outstrips the income from the hydrocarbons under production, the structure exists as a liability instead of an asset. Current regulations in the OCS require that all structures on a lease to be completely removed within 1 year of the termination of the lease. The topsides removal and disposal options available in decommissioning projects is shown in Fig. 4 as a decision tree. Oil and gas processing equipment and piping is sent to shore refurbished and reused, sold for scrap, or sent as waste to the landfill. Because many components are designed for a specific set of functional requirements, the opportunity for wholesale reuse of topside equipment are somewhat limited [1,2]. Deck and jacket structures have more options for disposal. The deck and jacket structure may be scrapped onshore, moved to a new location and reinstalled, disposed of in deep waters,2 or converted to an artificial reef site. The complete removal of the jacket is the most frequently used technique comprising roughly 90% of the total number of decommissions to date. The remaining 10% of structures that have been decommissioned have been toppledin-place within an artificial reef or towed to an approved reef site3 [3,4]. Partial removal of structures have occurred in only a handful of cases for large, heavy structures.

2

Deep water disposal requires a special ruling or exemption and the practice is rare in the GOM. The greater the water depth the more likely decommissioned structures are converted to artificial reefs; e.g., 40% of decommissioned structures located in 100–200 ft of water and 85% of the structures located in 200–400 ft of water have been converted to artificial reefs. 3

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2.2. General description of platform installation and removal processes The function, size, and configuration of platforms on the OCS vary widely, and although no general description can encompass all types, most fixed structures are composed of four major components: the superstructure or deck, the jacket, the conductors, and the piling. All structures are designed under specific environmental conditions and operator loads.4 The jacket rests on the ocean floor and extends above the water surface typically 10–20 ft to avoid being inundated by waves. The jacket has open pipe columns, or legs, interconnected by bracing members making the jacket a rigid frame structure. Piling is inserted into the legs of the jacket and driven into the ocean floor 100–400 ft to support the weight of the platform and resist the horizontal forces (wind, waves and current) at the site. The deck is installed on top of the jacket to accommodate the equipment and personnel required for the platform’s function (drilling, producing, storage, etc.). Conductors are then installed through a central conductor bay on the deck for the wells which will be drilled and completed to produce oil and gas. To remove a fixed platform the installation steps are essentially reversed. Topside equipment such as living quarters, generators, and processing equipment are returned to shore for scrap or reuse. The deck section is then detached, lifted from the platform, and placed on cargo barges for transportation. Removal of the conductors, piles, and jacket then follow. The conductors and piles are severed 15 ft below the mudline and removed. The jacket is then disconnected from the seabed and lifted and set on a cargo barge. Depending on equipment availability and the technical success of removing the piles, the jacket may need to be sectioned. The deck and jacket are typically transported to shore and offloaded at commercial salvage yards. The primary procedure for removing fixed steel platforms is to cut the piling and conductors below the mudline, remove by lifting, and then cut the platform into sections and remove each individual section. The number and size of the components that is lifted is determined by the capacity of the lifting equipment at the site. Piling and conductors can be cut by divers directly—through the application of mechanical or abrasive cutting techniques—or explosive methods can be employed. The easiest and most reliable procedure to cut piling is to wash the soil out from inside the piling and detonate a charge inside the pile to sever it. The explosion may expand the pile, however, so that it cannot be lifted out of the jacket leg separate from the jacket. Another major concern surrounding the use of explosives is the environmental impact that explosives can have on marine life, especially marine mammals. Cutting conductors and piling is considered a speciality service and several alternative methods can be used. Mechanical, abrasive waterjet, and explosive methods are the most common, and each procedure has its advantages, limitations, and physical/ environmental/safety/operational impact. Mechanical and abrasive cutting is not as 4 For example, data for wind, waves, and currents associated with both normal and worst-case storm conditions are calculated and the platform size and weight are designed for the anticipated drilling and production equipment required for the field.

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reliable as removal, and the risk of failure associated with these techniques is considerably higher than explosive methods, making removal the generally preferred method of operators [5]. 2.3. Factors involved in selecting a removal method A number of factors are involved in selecting a removal method, with cost, safety, risk of failure, and technical feasibility the main factors that are considered when alternative decommissioning options are available. Unfortunately, cost, safety, failure risk, and technical feasibility are not generally observable quantities. The cost of removal and estimates of the cost of alternative removals is well defined but such data is not publicly available nor reported to the MMS. Safety issues are well known but difficult to quantify in a consistent manner. The risk of failure and technical feasibility is company, time, and site specific. Variables that drive the cost and risk associated with a given structure removal are numerous and involve factors such as the water depth, structure configuration, salvage/reuse decision of the company, equipment availability, operator experience, the age of the structure, the weather at the time of removal, and the terms of the contract [6]. Some of these variables are observable, but unfortunately, the degree of correlation between the observable proxy variables and primary decision variables are weak and noisy, and so the extent to which structure removals can be accurately predicted based on these factors is uncertain. The economics of decommissioning are usually considered in terms of ‘‘least cost liability’’ as opposed to ‘‘return on investment’’. Decision criteria thus favor minimum cost alternatives as the generally preferred means of most disposals. In shallow waters nonexplosive removal methods carry less financial and operational risk than in deep waters. Nonexplosive techniques typically involve divers removing the structure directly through mechanical and abrasive cutting and torch cutting. As water depth increases, however, the reliability of mechanical and abrasive cutters decrease while the cost and risk of diving operations increase [7]. Abrasive and mechanical cutters have been used effectively on simple configurations such as caissons and well-protector jackets, but as the complexity and size of the structure increases, so does the risk associated with employing nonexplosive removal methods. The age of the structure may also be a factor in selecting the method of removal since older structures are less likely to have accurate records and drawings and this additional uncertainty may lead to increased risk for the diver and add to the cost of the operation.

3. Statistical description of structure removals 3.1. Notation The GOM is partitioned by the MMS according to protraction area, water depth, and planning area categories. The three large planning areas which divide the GOM

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are denoted the Western, Central, and Eastern Gulf of Mexico planning area (WGOM, CGOM, EGOM). Each planning area is subdivided into smaller regions, called protraction5 areas, which in turn are divided further into numbered blocks. A block is designated by a number and is normally a nine square mile area ð3 mile  3 milesÞ consisting of 5760 acres. A single block is the smallest unit that can be leased for oil and gas exploration on the OCS. The water depth categorization applied in MMS resource assessment evaluations, W ¼ fW1 ; y; W5 g ¼ f02200 m; 2012800 m; 80121600 m; 160122400 m; 2400þ mg; is too broadly defined for platform removal studies since nearly all (96%) structures removed in the GOM to date have been within a 0–60 m water depth range. To examine the use of explosive methods as a function of water depth a finer level of disaggregation therefore needs to be employed. A partition is selected that decomposes the 0–200 m category into a 0–60 and 61–200 m category, and then a further partition of the 0–60 m category into subcategories is adopted. We found it convenient to use 11 subcategories within the 0–60 m water depth range classified according to feet, and we shall transition between the measures as convenience dictates. The first five subcategories employ a 10 ft range, and then from 50 to 200 ft; a 25 ft range is employed: fW1 ; y; W14 g ¼ f0210; 11220; 21230; 31240; 41250; 51275; 762100; 1012125; 1262150; 1512175; 1762200; 2012656; 65722624; 2624þ ftg: Note that the 0–60 m water depth category is equivalent to 0–200 ft; while the 61– 200 m category is equivalent to 201–656 ft: The Gulf of Mexico planning areas are denoted by P ¼ fP1 ; P2 ; P3 g ¼ fWGOM; CGOM; EGOMg; but since the Eastern GOM has seen only a very small level of activity this planning area will not be considered. Further, since the water depth and planning area schemes are disjoint, the two categories can be combined using a Cartesian product as follows: W  P ¼ fGi;j ¼ ðWi ; Pj Þ j i ¼ 1; y; 14; j ¼ 1; 2g; where Gi;j denotes the water depth and planning area category indexed by i and j; e.g., G4;2 denotes the 31–40 ft water depth range in the Central GOM. Structures can be classified through their attributes. The main attributes required in this study include the configuration type and age of the structure upon removal. Configuration type is described in four categories as fT1 ; T2 ; T3 ; T4 g ¼ fcaissons; well protectors; fixed; floatingg; 5

The protraction areas of the GOM are shown in Fig. 1.

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while the age of the structure upon removal is grouped according to the categorization fA1 ; A2 ; A3 ; A4 g ¼ f0210; 11220; 21230; 30þ yearg: The number of structures removed from the water depth and planning area region Gi;j over the time interval ðt 1; tÞ is specified in terms of configuration type and age as follows:

RðGi;j ; Tk ; tÞ ¼

number of structures removed from region Gi;j of type Tk in year t; RðGi;j ; Al ; tÞ ¼ number of structures removed from region Gi;j that fall within age group Al in year t: RðGi;j ; Tk ; Al ; tÞ ¼ number of structures removed from region Gi;j of type Tk that fall within age group Al in year t: The number of structures removed using explosive methods is denoted by the subscript E; e.g.,

RE ðGi;j ; Tk ; tÞ ¼ number of structures removed from region Gi;j of configuration type Tk using explosive techniques in year t: The percentage of structures of a given classification that are removed through explosive technology is then computed as the ratio of RE ð Þ to Rð Þ; e.g., the percentage of structures of configuration type Tk removed through explosive technology in year t is computed as pE ðGi;j ; Tk ; tÞ ¼

RE ðGi;j ; Tk ; tÞ ; RðGi;j ; Tk ; tÞ

while in most cases time will be ‘‘integrated out’’ of the data set using a summation across time: P RE ðGi;j ; Tk ; tÞ : RðGi;j ; Tk Þ ¼ Pt t RðGi;j ; Tk ; tÞ Percentage applications must always be employed cautiously, however, since if the number of elements in the set Rð Þ or RE ð Þ is ‘‘small’’ then pE ð Þ cannot be considered a reliable statistic. For instance, if there are less than a dozen elements in a given set, then one cannot assign much confidence to the values as being ‘‘representative’’ of conditions in the region. It is for this reason that the tables of summary statistics present raw data as well as the percentage values to convey this information. The total number of structures removed from the water depth and planning area region Gi;j is denoted by RðGi;j Þ and is equal to any complete summation of the

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decomposed data over the universe of the partition: RðGi;j Þ ¼

4 X X t

¼

4 X X t

¼

RðGi;j ; Tk ; tÞ

k¼1

RðGi;j ; Al ; tÞ

l¼1

4 4 X X X t

k¼1

RðGi;j ; Tk ; Al ; tÞ:

l¼1

3.2. Structure installation and removals by water depth Information on offshore structures in federal waters was obtained from the US Minerals Management Service, and whenever possible, this data was checked for consistency and accuracy. The MMS updates its master database on a periodic basis as new information is made available, and so there is always a small time lag between when the data is reported and entered into the database. The structure data employed in this study was current through November 2001. The total number of structures installed and removed in the OCS of the GOM since 1947 as a function of water depth and planning area is depicted in Table 1. Structures are defined to include all caissons, well-protector jackets, fixed, and floating configurations. Nearly 6000 structures have been installed in the GOM through the year 2001 and one-third of these structures have now been removed. The vast majority of installations and removals have been in shallow water: 90% of all structures installed in the Gulf of Mexico and 96% of all the removals have been in less than 200 ft (60 m) of water. Within the 0–200 ft category, 36% of all the structures that have been installed through the year 2001 have been removed, while only 14% of structures within 200 ft or more have been removed. Activity levels vary widely as a function of water depth and no trends are observed. The average annual number of structures installed and removed per water depth and planning area category over a 5-year (1996–2001) and 10-year (1991–2001) time horizon is depicted in Tables 2 and 3, respectively. The value of the average annual number of installations and removals is surprisingly robust over the 5- and 10-year horizon in the sense that the mean and standard deviation of the installation and removal rates do not change appreciably. On the other hand, observe that activity levels are highly uncertain throughout most of the water depth categories,6 and so the normal statistical interpretation bounding the mean through a confidence interval employing one- or two-standard deviations should be approached rather cautiously.

6

The standard deviation represents a substantial percentage of the average value (and in some cases exceeds the mean) and hence the mean should be used carefully as a statistical measure.

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Table 1 Total number of structures installed and removed by water depth and planning area in the Gulf of Mexico (1947–2001) Water depth range (ft)

Installed

Removed

WGOM

CGOM

GOM

WGOM

CGOM

GOM

0–10 11–20 21–30 31–40 41–50 51–75 76–100 101–125 126–150 151–175 176–200 Subtotal

2 0 2 20 67 216 123 50 52 48 51 631

103 527 695 660 597 834 439 282 242 170 190 4739

105 527 697 680 664 1050 562 332 294 218 241 5370

1 0 1 5 33 84 40 20 20 19 19 242

37 263 291 190 216 305 140 86 64 37 45 1674

38 263 292 195 249 389 180 106 84 56 64 1916

201–656 657–2624 2624þ Subtotal

123 14 2 139

447 19 6 472

570 33 8 611

22 2 0 24

63 1 0 64

85 3 0 88

Total

770

5211

5981

266

1738

2004

Structures are defined to include all caissons, well-protector jackets, fixed and floating configurations located within the federal offshore waters of the Gulf of Mexico.

The CGOM and WGOM planning areas exhibit significantly different activity levels. The number of structures installed in the CGOM is roughly five times WGOM activity, and a similar level of activity governs the removal rates. In the shallow waters structure removal rates are comparable to installation rates across planning area. In deeper waters structure installations dominate removals. The historic magnitude of installation and removal activity is also clearly dominated by shallow water activity. To date 82% of all WGOM structures and 91% of all CGOM structures have been installed in less than 200 ft of water. In terms of structure removals, 91% of all WGOM removals and 96% of all CGOM removals have been in 200 ft of water or less. 3.3. Age distribution of active and removed structures The age distribution of active structures in the GOM is shown in Figs. 5 and 6 according to planning area and configuration type. In Table 4 the percentage of active structures that fall within each age category is depicted. Infrastructure in the GOM is aging and this is clearly indicated among all configuration categories, especially within the CGOM region where nearly 40% of the well protectors and over a third of the fixed platforms are over 30 years old.

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Table 2 Average annual number of structures installed and removed in the Gulf of Mexico via water depth and planning area (1996–2001) Water depth range (ft)

Installed

Removed

WGOM

CGOM

GOM

WGOM

CGOM

GOM

0–10 11–20 21–30 31–40 41–50 51–75 76–100 101–125 126–150 151–175 176–200 Subtotal

ð0; 0Þ ð0; 0Þ ð0:2; 0:4Þ ð0:8; 0:4Þ ð2:8; 1:9Þ ð7:6; 1:9Þ ð2:4; 1:7Þ ð1; 0:7Þ ð1:4; 1:1Þ ð0:8; 0:8Þ ð1; 1:2Þ ð18; 3:8Þ

ð2:8; 2:5Þ ð6; 2:9Þ ð11:2; 5:2Þ ð13:4; 4:0Þ ð5:8; 1:3Þ ð18; 6:4Þ ð11; 4:3Þ ð8:2; 4:1Þ ð6:6; 2:5Þ ð5:2; 3:3Þ ð5; 2:6Þ ð93; 12:6Þ

ð2:8; 2:3Þ ð6; 2:9Þ ð11:4; 5:4Þ ð14:4; 3:6Þ ð8:6; 2:2Þ ð25:6; 6:0Þ ð13:4; 4:8Þ ð9:2; 4:8Þ ð8; 2:9Þ ð6; 3:1Þ ð6; 3:8Þ ð111; 13:2Þ

ð0:2; 0:4Þ ð0; 0Þ ð0; 0Þ ð0:6; 1:3Þ ð2:6; 1:8Þ ð7:8; 6:5Þ ð4:6; 4:4Þ ð2; 2Þ ð0:8; 1:3Þ ð1:2; 0:8Þ ð0:8; 0:8Þ ð20:6; 18:6Þ

ð2:6; 2:7Þ ð16:2; 8:6Þ ð11:8; 6:6Þ ð10; 5:2Þ ð14:6; 7:7Þ ð19:8; 6:4Þ ð8; 2:2Þ ð6:8; 7:4Þ ð4:2; 1:5Þ ð2:8; 0:8Þ ð3:8; 1:5Þ ð100:6; 17:8Þ

ð2:8; 2:8Þ ð16:2; 8:6Þ ð11:8; 6:6Þ ð10:6; 6:2Þ ð17:2; 8:5Þ ð27:6; 10:9Þ ð12:6; 3:1Þ ð8:8; 9:2Þ ð5; 2:1Þ ð4; 0Þ ð4:6; 1:8Þ ð121:2; 21:3Þ

201–656 657–2624 2624þ Subtotal

ð3:6; 1:5Þ ð1:2; 0:4Þ ð0:2; 0:4Þ ð5; 1:6Þ

ð16:2; 4:9Þ ð1:4; 1:5Þ ð1:2; 0:8Þ ð18:8; 5:2Þ

ð19:8; 6:3Þ ð2:6; 1:9Þ ð1:4; 1:1Þ ð23:8; 6:7Þ

ð1:4; 1:1Þ ð0:4; 0:5Þ ð0; 0Þ ð1:8; 1:2Þ

ð5:4; 2:6Þ ð0; 0Þ ð0; 0Þ ð5:4; 2:6Þ

ð6:8; 3:3Þ ð0:4; 0:5Þ ð0; 0Þ ð7:2; 3:3Þ

Total

ð23; 4:1Þ

ð111:8; 13:7Þ

ð134:8; 14:8Þ

ð22:4; 8:7Þ

ð106:0; 17:9Þ

ð128:4; 21:6Þ

The number of active structures in gulf waters is shown in Tables 5. There are nearly 1200 caissons in the GOM which comprise one-third of the total number of structures. Nearly all caissons are located within the 0–200 ft water depth range. The purpose of most caissons and well protectors is simply to protect wells from damage. Well-protector jackets are multi-piled structures that do not support drilling or production equipment. There are roughly 400 well-protectors structures in gulf waters, or about 12% of the total number of structures. The oil and gas from caissons and well-protector jackets are typically transported through flow lines to a production platform, the most common structure in the GOM. There are at present over 2300 fixed platforms in the GOM. In Table 6 the average age of structures removed from the GOM is shown along with the standard deviation of the mean. Structures in the WGOM tend to be removed, on average, at an earlier time than their counterparts in the CGOM which could be due to smaller field size, faster production rates, or other geologic-based conditions; e.g., most fields in the WGOM are gas fields which exhibit a quick depletion rate. Caissons in the CGOM are removed after about 16 years of service while caissons in the WGOM have a significantly shorter lifespan of 7 years. Observe that the standard deviation values in all cases are greater than 50% of the value of the mean, and so it is clear that there is wide variability in structure removal ages within categories and across water depth and configuration type. Also, with the exception of well protectors in the WGOM, there is not a significant difference in the average age of removal across water depth categories.

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Table 3 Average annual number of structures installed and removed in the Gulf of Mexico via water depth and planning area (1991–2001) Water depth range (ft)

Installed

Removed

WGOM

CGOM

GOM

WGOM

CGOM

GOM

0–10 11–20 21–30 31–40 41–50 51–75 76–100 101–125 126–150 151–175 176–200 Subtotal

ð0:1; 0:3Þ ð0; 0Þ ð0:1; 0:3Þ ð0:6; 0:5Þ ð1:8; 1:8Þ ð8; 3:9Þ ð3:9; 3:2Þ ð1:4; 1:0Þ ð1:3; 1:1Þ ð1:6; 1:6Þ ð0:8; 0:9Þ ð19:6; 5:9Þ

ð2:4; 2:3Þ ð7:2; 3:2Þ ð10:9; 4:3Þ ð10:3; 4:8Þ ð7:9; 3:5Þ ð19:3; 7:1Þ ð12:3; 4:3Þ ð8:8; 3:8Þ ð6; 2:6Þ ð5:1; 3:3Þ ð4:8; 2:5Þ ð95; 13:3Þ

ð2:5; 2:2Þ ð7:2; 3:2Þ ð11; 4:4Þ ð10:9; 4:9Þ ð9:7; 3:2Þ ð27:3; 5:8Þ ð16:2; 6:4Þ ð10:2; 4:5Þ ð7:3; 3:1Þ ð6:7; 4:2Þ ð5:6; 3:2Þ ð114:6; 14:2Þ

ð0:1; 0:3Þ ð0; 0Þ ð0; 0Þ ð0:3; 0:9Þ ð1:9; 1:4Þ ð6:7; 5:3Þ ð3:6; 3:3Þ ð1:6; 1:6Þ ð1:4; 1:8Þ ð1:5; 1:8Þ ð1:1; 1:3Þ ð18:2; 7:3Þ

ð2:2; 2:2Þ ð17; 9:2Þ ð14:6; 8:5Þ ð10:5; 3:7Þ ð13; 7:2Þ ð20:1; 9:6Þ ð8:6; 3:8Þ ð5:5; 6:1Þ ð4:8; 4:7Þ ð2:7; 2:5Þ ð3:6; 1:7Þ ð102:6; 20:1Þ

ð2:3; 2:3Þ ð17; 9:2Þ ð14:6; 8:5Þ ð10:8; 4:3Þ ð14:9; 7:7Þ ð26:8; 12:8Þ ð12:2; 4:7Þ ð7:1; 7:5Þ ð6:2; 5:2Þ ð4:2; 2:8Þ ð4:7; 2:3Þ ð120:8; 22:8Þ

201–656 657–2624 2624þ Subtotal

ð3:5; 1:9Þ ð0:9; 0:6Þ ð0:2; 0:4Þ ð4:6; 2:0Þ

ð15:2; 5:8Þ ð1:2; 1:2Þ ð0:6; 0:8Þ ð17; 6:0Þ

ð18:7; 7:4Þ ð2:1; 1:6Þ ð0:8; 1:0Þ ð21:6; 7:6Þ

ð1:6; 1:3Þ ð0:2; 0:4Þ ð0; 0Þ ð1:8; 1:4Þ

ð5; 2:1Þ ð0; 0Þ ð0; 0Þ ð5; 2:1Þ

ð6:6; 2:7Þ ð0:2; 0:4Þ ð0; 0Þ ð6:8; 2:7Þ

Total

ð24:2; 6:2Þ

ð112; 14:6Þ

ð136:3; 16:1Þ

ð20; 7:4Þ

ð107:6; 20:2Þ

ð127:6; 23:0Þ

The data entries are denoted by the coordinate pair ðm; sÞ; where m represents the mean and s the standard deviation of the structure data per water depth and planning area category Gi;j :

Age Category

40+ 31-40

Fixed

21-30

Well Protectors Caissons

11-20 1-10 0

50 100 Number of Structures

150

Fig. 5. Age distribution of active structures in 0–200 ft in the Western GOM (1947–2001).

3.4. Structure removals by configuration type and method of removal The application of explosive techniques varies widely throughout gulf waters as shown in Table 7. In Table 7 the number of structures removed (R ¼ RðGi;j Þ) and the number removed by explosive techniques (RE ¼ RE ðGi;j Þ) are shown as a function of

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472

Age Category

40+ 31-40

Fixed

21-30

Well Protectors Caissons

11-20 1-10 0

100

200

300

400

Number of Structures Fig. 6. Age distribution of active structures in 0–200 ft in the Central GOM (1947–2001).

Table 4 The age distribution of active structures in shallow water (0–200 ft) by configuration type and planning area (2001) Age distribution (year) Caissons

Well protectors

W (%) C (%) G (%) W (%) 40þ 31–40 21–30 11–20 1–10

0 o1 o1 33 63

8 13 16 30 33

7 12 15 30 33

0 11 13 40 36

Fixed

C (%) G (%) W (%) C (%) G (%) 20 19 22 21 17

18 19 21 23 19

o1 4 23 47 25

11 23 23 21 23

9 20 23 25 23

W, C, G denote WGOM, CGOM, and GOM.

Table 5 The number of active structures by configuration type and planning area in the Gulf of Mexico (2001) Water depth range (ft.) Caissons

Well protectors

Fixed

Total

WGOM CGOM WGOM

CGOM

WGOM CGOM WGOM CGOM

0–200 201–656

79

1073

47 8

355 20

252 100

1617 378

366 108

3058 398

Total

79

1073

55

375

352

1995

474

3456

water depth and planning area beginning from the year 1986. Although multiple techniques may be used to sever conductors and piling, severing is usually categorized as either explosive or nonexplosive. If explosives are used in any amount and at any stage of the decommissioning project, then the method is considered explosive. Beginning in 1986 companies planning to remove offshore structures with explosives were required to obtain a permit from the MMS, and hence only data from this period of time onward is available. The data set represents

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Table 6 Average age of structures upon removal by water depth, configuration type, and planning area (1947– 2001) Water depth range (ft)

0–200 201–656

Caissons

Well protectors

Fixed

WGOM

CGOM

WGOM

CGOM

WGOM

CGOM

(7.1, 4.7)

(15.9, 10)

(16.3, 11.3) (7.3, 3.8)

(17.4, 10.2) (17, 10.9)

(9.3, 5.3) (10.4, 6.2)

(17.1, 10.8) (12.6, 7.9)

The data entries are denoted by the coordinate pair ðm; sÞ; where m represents the mean and s the standard deviation of the structure data per water depth and planning area category Gi;j : Table 7 Number of structures removed (R), structures removed by explosive technique (RE ), and the percentage of explosive removals (pE Þ as a function of water depth and planning area (1986–2001) WGOM R

CGOM RE

Water depth range ðftÞ 0–10 1 11–20 21–30 1 31–40 4 41–50 31 51–75 78 76–100 41 101–125 19 126–150 20 151–175 17 176–200 16 201–656 23 657–2624 2 2624þ

pE (%)

R

GOM RE

pE (%)

20 210 208 150 155 238 109 81 60 34 44 64

11 71 145 88 107 130 61 45 49 23 26 49

55 34 70 59 69 55 56 56 82 68 59 77

R

RE

pE (%)

21 210 209 159 186 316 150 100 80 51 60 87 2

12 71 146 91 129 164 81 57 64 33 39 66 1

54 34 70 59 69 52 54 57 80 65 65 76 50

1

100

1 3 22 34 20 12 15 10 13 17 1

100 75 71 44 71 63 75 59 81 74 50

Water depth range ðmÞ 0–60 228 131 61–200 23 17 200þ 2 1

57 74 50

1309 64

756 49

58 77

1537 87 2

887 66 1

58 76 50

Total

59

1373

805

59

1626

954

59

253

149

For the convenience of the reader the data is classified in terms of both a feet and meter categorization.

about 80% of the total structure removals, which for the purposes of analysis, is considered representative of GOM conditions. The percentage of structures removed using explosive techniques is calculated as pE ðGi;j Þ ¼

RE ðGi;j Þ : RðGi;j Þ

474

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The data in Table 7 is presented in terms of both feet and meter water depth ranges for the convenience of the reader. The percentage values depicted need to be interpreted carefully, however, since the values are dependent7 upon the selection of the water depth categories employed. An additional problem in interpreting the value of pE is that the percentage calculation may be based on only a handful of data, and under such circumstances, one cannot assign much confidence to the values as being ‘‘representative’’ of conditions in the region. This is particularly a problem throughout the shallow water (0–40 ft) and deep water (657–2624 ft) categories of the WGOM where only a few structures have thus far been removed. With these exceptions noted, however, there does not appear to be a significant difference between the application of explosive techniques over the WGOM and CGOM planning area. This is quite reasonable since there is no reason why explosive techniques would be different across planning area unless the structure types, age,8 or year of removal are dramatically different. The data in Table 7 supports the assertion that there is not a strong planning area dependence on pE ; and so we can aggregate over planning area and consider the application of explosive removals throughout the GOM as representative of either the WGOM or CGOM planning area. The description of explosive removals across the GOM as a function of configuration type is depicted in Table 8. It is apparent from Table 8 that the choice of removal method depends to some extent on the configuration type of the structure, but there are no observable trends within the 0–200 ft category for any of the configuration types. It is also difficult to explain the variability that does exists, and most probably, the variation of pE with water depth is due to ‘‘noise’’ that cannot be detected. Recall that structure removal decisions are usually based on a few factors that are mostly unobservable: cost, safety, risk of failure, and technical feasibility, combined with a wide variety of structure-, site-, and company-specific criteria. The variation that exists in pE ðWi ; Tk Þ across water depth and configuration types leads us to conclude that the best indicators of pE are aggregate measures across broad water depth categories that do not differentiate between planning area. Using the categorization shown at the bottom of Table 8, observe that caissons are the most commonly removed structure using nonexplosive methods, and well protectors and fixed platforms, if removed using nonexplosive techniques, are more commonly performed in shallow waters. Caissons have an equal chance of being removed with either explosive or nonexplosive methods, and well protectors and fixed structures—more complex than caissons—realize a greater chance of an 7 The reader is encouraged to aggregate the category data along different water depth categorizations and re-compute pE to convince oneself of the sensitivity of the measure to the categorization level employed. Some researchers try to define categories that have roughly the same amount of data within each set for consistency, but this is not always possible or realistic. 8 In fact, as mentioned previously (recall Table 6), there is a difference in the average age of structures upon removal across planning area, and to the extent that a younger structure is more likely to be removed with nonexplosive technology, we would suspect WGOM structures to have a slightly lower probability of being removed with explosives as shown in the aggregated categories (pE ðP1 Þ and pE ðP2 Þ) at the bottom of Table 7.

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Table 8 Number of structures removed ðRÞ; structures removed by explosive technique ðRE Þ; and the percentage of explosive removals ðpE Þ as a function of water depth and configuration type for the Gulf of Mexico (1986– 2001) Caissons R Water depth range 0–10 14 11–20 170 21–30 137 31–40 89 41–50 99 51–75 141 76–100 51 101–125 25 126–150 9 151–175 8 176–200 201–656 657–2624 2624þ

749

Fixed

R

pE (%)

R

All

RE

pE (%)

ðftÞ 5 55 98 49 71 63 19 6 6 5

36 32 71 55 72 44 37 24 67 63

1 11 23 12 28 48 25 14 12 8 11 6

1 4 12 6 22 32 12 7 10 7 6 5

100 36 52 50 79 67 48 50 83 88 55 83

6 29 49 53 59 127 74 61 59 37 41 81

6 12 36 36 36 69 50 44 48 22 28 61

100 41 73 68 61 54 68 72 81 59 68 75

51

193 6

119 5

62 83

595 81

387 61

51

199

124

62

676

448

Water depth range ðmÞ 0–60 749 381 61–200 200þ Total

Well protectors

381

RE

RE

pE (%)

R

RE

pE (%)

21 210 209 154 186 316 150 100 80 51 60 87 2

12 71 146 91 129 164 81 57 64 33 39 66 1

57 34 70 59 69 52 54 57 80 65 65 76 50

65 75

1537 87 2

887 66 1

58 76 50

66

1626

954

59

For the convenience of the reader the data is classified in terms of both a feet and meter categorization.

explosive removal. As the water depth increases the chance of using explosives also increase across all configuration types as expected. The percentage values depicted for explosive removals for well protectors in the 61–200 m water depth range is slightly suspect, however, since it is based on only six data points. Thus far, no caissons, well protectors, or fixed structures have been removed in water depth greater than 200 m water, and the two semisubmersibles that have been removed in this water depth range are included in Table 8 for completeness. 3.5. Structure removals by year and configuration type The number of structures removed by configuration type by year is shown in Table 9 across all water depths in the Gulf of Mexico. There are no noticeable trends in the removal rates across time except caissons and fixed structures typically compete for the greatest number of removals in any given year with well protectors a distant third (Fig. 7). The percentage values pE can be considered a stochastic process as illustrated in the time series in Fig. 8, but it is preferable to ‘‘average out’’ the time

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Table 9 Number of structures removed ðRÞ; structures removed using explosive technique ðRE Þ and the percentage of explosive removals ðpE Þ in the Gulf of Mexico according to configuration type (1986–2001) Year

Caissons R

RE

Well protectors pE (%)

R

RE

Fixed pE (%)

R

Total RE

pE (%)

R

RE

pE (%)

1986 1987 1988 1989 1990

10 46 46 53

0 5 34 26

10 11 74 49

1 2 9 7 9

0 0 2 6 5

0 0 22 86 56

1 7 36 34 36

0 0 19 30 29

0 0 53 88 81

2 19 91 87 98

0 0 26 70 60

0 0 29 80 61

1991 1992 1993 1994 1995

54 44 77 42 59

26 19 49 22 40

48 43 64 52 68

16 13 30 16 9

11 9 12 14 7

69 69 40 88 78

44 40 61 66 49

36 33 41 51 34

82 83 67 77 69

114 97 168 124 117

73 61 102 87 81

64 63 61 70 69

1996 1997 1998 1999 2000 2001

48 92 35 72 49 22

13 54 14 35 37 7

27 59 40 49 76 32

15 14 11 17 19 11

8 11 8 9 13 9

53 79 73 53 68 82

56 71 29 45 66 35

29 38 13 32 42 21

52 54 45 71 64 60

119 177 75 134 134 68

50 63 35 76 92 37

42 58 47 57 69 54

Total

749

381

51

199

124

62

676

448

66

1624

953

59

variability by aggregating the RE ð Þ and Rð Þ values and calculating P RE ðTk ; tÞ pE ðTk Þ ¼ Pt t RðTk ; tÞ as shown in the last row of Table 9. The variability of pE across time for a given configuration class can be explained to some extent through the age of the structure and the water depth as demonstrated next. 3.6. Structure removals by age, water depth, and configuration type Structures that have been removed from the GOM according to planning area and age upon removal is depicted in Table 10. All structure types are aggregated within the same category and it is clear that a significant variation exists across planning areas. To wit, more than 90% of all WGOM structures are removed within 20 years of their installation—indicating small reserves, quick production, poor geologic prospects, or a combination of all these factors (recall Table 6). For the most part, structures in the GOM are removed when they reach the end of their economic life. A few structures are removed because of structural damage (collision with barge, hurricane event, etc.), with fewer still removed because of fatigue. Structures are removed near the time when they are no longer economic, and this is not (normally)

477

200 150

Fixed Well Protectors

100

Caissons

50

00 20

98 19

19

96

94 19

92 19

90 19

19

19

88

0 86

Number of Structures

M.J. Kaiser et al. / Ocean & Coastal Management 45 (2002) 459–483

Year Fig. 7. Number of structures removed by structure type in the Gulf of Mexico (1986–2001).

Probability

100 80 Caissons

60

Well Protectors 40

Fixed

20

00 20

98 19

96 19

94 19

92 19

90 19

88 19

19

86

0

Year Fig. 8. Percentage of structures removed by explosive technique in the Gulf of Mexico (1986–2001).

constrained by the design life of the structure. At the opposite end of the spectrum is the relatively long life of many CGOM fields: 15% of all CGOM structures for instance were at least 30 years old upon removal. In Table 11 the number of structures removed using explosives is depicted along with the percentage of explosive removals categorized according to age. Examine the percentage of explosive removals shown on the right side of Table 11. As the age of a structure increases so does the probability explosive methods will be employed. It is interesting to note that when the data is aggregated according to age upon removal, WGOM structures have a greater likelihood of an explosive removal relative to CGOM structures (compare to Table 7 and footnote 8). To examine the features of water depth and structure age upon removal method structure data was aggregated and then classified as shown in Tables 12 and 13. Table 12 depicts the number of structures removed as a function of water depth and age upon removal, and it is clear that the majority of structures removed from both water depth categories are within 20 years of their installation date. The data in Table 13 are more interesting, however, since the general trends observed earlier hold here with the same caveats: the percentage of structures removed using explosive

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Table 10 Number of structures removed ðRÞ and percentage of structures removed ðpÞ grouped according to age upon removal and planning area (1986–2001) Age

p ð%Þ

Number of structures (RÞ WGOM

CGOM

GOM

WGOM

CGOM

GOM

0–10 11–20 21–30 30þ

159 72 13 9

479 405 282 207

638 477 295 216

63 28 5 4

35 29 21 15

39 29 18 13

Total

253

1373

1626

100

100

100

p ¼ R=RT ; where RT denotes the total number of structures per planning area.

Table 11 Number of structures removed using explosives ðRE Þ and the percentage of all structures removed using explosives (pE ) grouped according to age upon removal and planning area (1986–2001) Age

Number of structures (RE Þ WGOM

pE ð%Þ

CGOM

GOM

WGOM

CGOM

GOM

0–10 11–20 21–30 30þ

82 50 9 8

229 253 172 151

311 303 181 159

52 69 69 89

48 62 61 73

49 64 61 74

Total

149

805

954

59

59

59

pE ¼ RE =R; where the R values are obtained from Table 10.

methods increase as a function of age upon removal for the 0–60 m category and is dominated by the application of explosive removals in the 61–200 m water depth category.9 The general trends observed in Table 11 for the application of explosive techniques also apply to individual configuration type and water depth categories as shown in Tables 14 and 15. From Table 14 it is observed that across all configuration types, the use of nonexplosive methods is most common in the 0–10 year category, and as the age of the structure increases, so does the likelihood that explosive methods will be applied. In Table 15 the percentage of structures removed using explosive technique as a function of water depth, age upon removal, and configuration type is presented. Blank entries indicate that no structures of the given categorization were removed.

9

The number of structures in the 61–200 m group, however, especially for the 21–30 and 30þ age categories, are too small to draw meaningful conclusions.

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Table 12 Number of structures removed ðRÞ and percentage of structures removed ðpÞ as a function of water depth and age upon removal in the Gulf of Mexico (1986–2001) Water depth range (m)

0–10

21–30

30þ

Total

441 36 0 477

283 12 0 295

213 3 0 216

1537 87 2 1626

29 41 — 29

18 14 — 18

14 3 — 13

100 100 100 100

11–20

Age upon removal 0–60 61–200 200þ Total

600 36 2 638 p ð%Þ

0–60 61–200 200þ Total

39 41 100 39

p ¼ R=RT ; where the RT denotes the total number of structures throughout the GOM.

Table 13 Number of structures removed using explosive techniques and the percentage of explosive removals (pE ) as a function of water depth and age upon removal in the Gulf of Mexico (1986–2001) Water depth range (m)

0–10

21–30

30þ

Total

276 27 0

173 8 0

156 3 0

888 66 1

63 75 —

61 67 —

73 100 —

58 76 50

11–20

Age upon removal 0–60 61–200 200þ

283 28 1 pE ð%Þ

0–60 61–200 200þ

47 78 50

pE ¼ RE =R; where the R values are obtained from Table 12.

4. Short-term historic forecast model 4.1. General methodology A short-term forecast is developed that employs historical data to infer short-term future behavior. The basic hypothesis is that structures removed in the short-term future will follow near-term historical trends. This hypothesis is expected to be reasonable over a short time horizon, say 5 years or less, but cannot be applied over a long-term period. Historic average performance serves as a baseline level of activity and is relatively easy to perform, although the hypothesis is a simplistic view with no underlying physical basis except that the near-term future will mimic the short-term

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Table 14 Number of structures removed ðRÞ; number of structures removed by explosive technique ðRE Þ; and the percentage of structures removed by explosives (pE ) categorized according to age and configuration type in the Gulf of Mexico (1986–2001) Age

Caissons

Well protectors

R

RE

pE (%)

0–10 11–20 21–30 30þ

295 204 157 93

116 115 83 67

39 56 53 72

75 52 36 36

Total

749

381

51

199

R

Fixed

RE

pE (%)

R

RE

pE (%)

40 35 21 28

53 67 58 78

266 221 102 87

154 153 77 64

58 69 75 74

124

62

676

448

66

Table 15 Percentage of structures removed using explosive techniques categorized by configuration type and water depth in the Gulf of Mexico (1986–2001) Age

Caissons 0–60 m

61–200 m

Well protectors

Fixed

0–60 m

61–200 m

0–60 m

61–200 m 76 76 67 100 75

0–10 11–20 21–30 30þ

39 56 53 72

52 67 58 77

100 67 100

55 68 77 73

Total

51

62

83

65

past. This does not mean that the forecast is inherently inaccurate, but rather, it remains difficult to justify the numbers and defend the forecast on methodological grounds. Criticism is also associated with historic trending since structure installation data is not considered directly and the methodology can only be considered ‘‘valid’’ over a short-term period. Nonetheless, short-term historic averages serve as useful indicators and describe general baseline performance. The methodology to forecast structure removals is a three-step procedure: Step 1. Forecast RðGi;j ; Tk Þn Step 2. Forecast pE ðGi;j ; Tk Þn Step 3. Compute RE ðGi;j ; Tk Þn : The two basic assumptions required concern the forecast methodology employed for RðGi;j ; Tk Þn and pE ðGi;j ; Tk Þn : A forecast quantity will be denoted by a superscript * and historic averages will be denoted through the bracket notation / S: The historic 10-year average /RðGi;j ; Tk ÞS is used to estimate RðGi;j ; Tk Þn while the historic average /pE ðGi;j ; Tk ÞS is used to proxy pE ðGi;j ; Tk Þn : A1. RðGi;j ; Tk Þn ¼ /RðGi;j ; Tk ÞS A2. pE ðGi;j ; Tk Þn ¼ /pE ðGi;j ; Tk ÞS:

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Table 16 Average annual number of structures removed in the Gulf of Mexico via water depth, configuration type and planning area (1991–2001) Water depth Caissons Well protectors Fixed Total range (m) WGOM CGOM GOM WGOM CGOM GOM WGOM CGOM GOM WGOM CGOM GOM 0–60 61–200

10.9 0

57.1 0

68 0

9.6 1

20.3 1

29.9 15.5 2 1.9

41.3 4.9

56.8 36.0 6.8 2.9

118.7 5.9

154.7 8.8

Total

10.9

57.1

68

10.6

21.3

31.9 17.4

46.2

63.6 38.9

124.6

163.5

Table 17 Expected number of structures removed using explosive techniques over a 5-year period by water depth, configuration type, and planning area (2002–2007) Water depth Caissons Well protectors Fixed Total range (m) WGOM CGOM GOM WGOM CGOM GOM WGOM CGOM GOM WGOM CGOM GOM 0–60 61–200

28 0

146 0

174 0

30 4

63 5

93 9

50 7

134 18

184 25

108 11

343 23

451 34

Total

28

146

174

34

68

112

57

152

209

119

366

485

A short-term forecast of the number of structures removed is then given by the relation RE ðGi;j ; Tk Þn ¼ /RðGi;j ; Tk ÞS/pE ðGi;j ; Tk ÞS: 4.2. Model results The average annual number of structures removed in the GOM as a function of water depth, configuration type and planning area is depicted in Table 16 based on a 10-year time horizon. The data presented in Table 8 for the removal rates using explosive techniques are employed to proxy future values. Model results are depicted in Table 17. The expected number of structures removed using explosive techniques over a 5-year period is obtained through the product of the annual number of structure removals (Table 16) by the probability of an explosive removal (Table 8), and then multiplying the final result by five to adjust to the 5-year time horizon: i.e., as reported in Table 17 for CGOM well protectors in 0–60 m; RðG1;2 ; T2 Þn ¼ J5ð20:3Þð0:62Þn ¼ 63; where the bracket J n is used to round up to the nearest integer. The number of structures in the GOM expected to be removed using explosive techniques over a 5year time horizon is summarized in Table 17.

482

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5. Conclusions A statistical description of the explosive removal of offshore structures in the Gulf of Mexico has been presented based on historical data collected by the US Minerals Management Service. The influence of factors such as water depth, planning area, configuration type, structure age, and time upon the application of explosive removals have been examined, and generally-held industry beliefs appear to be ‘‘valid’’, namely, that the application of explosive techniques appear to increase with (i) water depth, (ii) structure complexity, and (iii) age upon removal. Most of the structures that have been removed from the GOM are in the Central planning area (84%) with the remaining removals distributed throughout the Western GOM. Explosive technology was employed in 954 of the 1626 structures decommissioned to date—representing in aggregate a 59% explosive removal rate. Caissons were equally likely to be removed with either explosive or nonexplosive methods, while wellprotector jackets employed explosives 62% of the time and fixed structures were removed with explosives 66% of the time. The application of explosive methods increase with the complexity of the configuration type, water depth and age of the structure upon removal. The influence of planning area on the application of explosive removals was shown not to be a significant factor, and while the probability of an explosive removal is dependent upon time in a stochastic manner, the time dependence was integrated out of the data set for consistency. Company practice in the application of removal techniques was also not considered in the analysis but may be a factor that sheds additional light on the decommissioning process. The number of structures expected to be removed from the GOM using explosive techniques was forecast over a 5-year horizon using historic average values and classified according to water depth, planning area, and configuration type. The short-term forecast, although methodologically problematic with no physical basis for prediction, serves as a useful guide to baseline activity levels.

Acknowledgements This paper was prepared on behalf of the US Department of the Interior, Minerals Management Service, Gulf of Mexico OCS region, and has not been technically reviewed by the MMS. The opinions, findings, conclusions, or recommendations expressed in this paper are those of the authors, and do not necessarily reflect the views of the Minerals Management Service. Funding for this research was provided through the US Department of the Interior and the Coastal Marine Institute, Louisiana State University.

References [1] O’Connor P. Case studies of platform re-use in the Gulf of Mexico. International Conference on the Re-Use of Offshore Production Facilities, Netherlands, October 13–14, 1998.

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