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Reliability of jack-ups under extreme storm conditions
Marine Structures 14 (2001) 523}536
Technical note
Reliability of jack-ups under extreme storm conditions A.C. Morandi *, I.A.A. Smith, G.S. Virk Global Maritime, 11767 Katy Freeway, Suite 660, Houston, TX 77079, USA Global Maritime, 12 Craven Street, London WC2N 5PB, UK Global Marine Drilling Company, 777 North Eldridge, Houston TX 77079, USA
Abstract The present paper is concerned with a technical review on the reliability of jack-ups. It covers recent developments on the application of reliability theory to o!shore structural and foundation systems and their potential impact on the standards for assessment of jack-ups and o!shore structures in general. The e!ects of risk-reducing features inherent in jack-ups that di!erentiate their risk assessment from other structures such as jackets are also investigated. In the light of the above, past comparisons of jack-up and jacket reliability are reviewed and tentative suggestions are made for improving reliability assessments and calibration procedures to be adopted in the development of the ISO standard for jack-ups. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Jack-up; Reliability; Safety; Risk; Standards; Criteria; ISO
1. Introduction Mobile drilling units in general and jack-ups in particular play a vital role in the o!shore industry. Historically, the structural safety of these units has been considered acceptable. Since standards and procedures for their assessment were known to vary, a recommended practice for site assessment of jack-ups was developed by a Joint Industry Project and published by the Society of Naval Architects and Marine Engineers (SNAME) in their T&R Bulletin 5-5A in 1994 [1]. The stated objectives of this document were [2]: to promote consistency of assessment practice, to provide
* Corresponding author. Tel.: #1-281-558-3690; fax: #1-281-558-0541. E-mail address: [email protected] (A.C. Morandi). 0951-8339/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 1 - 8 3 3 9 ( 0 0 ) 0 0 0 5 7 - 5
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guidance in view of a trend for using jack-ups in more dynamically sensitive locations (deep water and harsh environment) and to transfer technology developed for "xed structures in those areas which could be benexcial to jack-up assessment. Bulletin 5-5A [1] was expected to promote more consistent reliability levels thus, minimising the potential for isolated cases where safety could be of concern. However, industry experience has indicated that this document tends to impact a large portion of existing rigs. Although experience indicates that jack-ups have been safely operated over the past 40 years, the acceptance criteria given in Bulletin 5-5A [1] result in unfavourable assessments in cases where the track record suggests otherwise. This may lead to expensive remedial measures with a potential impact on marginally economic "elds. The debate has gained renewed importance as ISO TC67/SC7/WG7 is now developing an international standard for mobile drilling units including jack-ups. Notwithstanding the present concern regarding the applicability of the Bulletin's recommended practice, this document is being proposed as a basis for the ISO standard. Furthermore, ISO procedures will require a certain degree of consistency between the format of the jack-up standard and that of the standard for "xed structures now reaching its "nal stages. Care must be taken to keep requirements for the design of new "xed structures from being imposed carte blanche on the assessment of existing jack-ups. The objectives of this paper are: E In concise terms, give an appreciation of the recent application of reliability theory in the development of o!shore standards. The di!erences between the evolution of standards for "xed structures (API RP 2A [3] in particular) and the development of the Bulletin 5-5A [1] criteria are highlighted. E Investigate the impact of risk-reducing features inherent to jack-ups that di!erentiate their safety assessment from that of "xed structures. In particular, clarify potentially misleading interpretations of jacket and jack-up comparative reliability. A few preliminary suggestions are also given on a way forward in developing the ISO standards for jack-ups.
2. Risk, safety and reliability Safety assessment in many industries (o!shore, shipping, etc.) is evolving towards a framework of devolved responsibility, which concentrates on management and organisation for safety and encourages individual companies to establish targets for safety performance [4}6]. This evolution is driven by a general acceptance of the inadequacy of past prescriptive safety regimes due to a number of limitations [5], such as lack of clarity in goals and objectives, slow response to technological developments, imbalance in safety margins and an often reactive approach to disasters and their media coverage. Safety is increasingly seen as a quality that re#ects a state of acceptable risk concerning human lives, hardware, software and the environment [5,6]. The main challenges are then to determine acceptable risk levels and to
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translate them into standards that can be followed by practising engineers. The target safety levels adopted should, in principle, re#ect a number of di!erent aspects [7]: E Failure cause and mode. E Di!erent phases in the platform life-cycle (temporary/long term). E Reliability analysis (RA) methodology and, in particular, the speci"c uncertainties modelled and the basis for the modelling adopted. E Potential consequences of failure * does failure of a component leads to failure of the system? What are the consequences of failure in terms of fatalities, pollution and economic loss? E The expense and e!ort necessary to reduce the risk of failure. In particular, reducing risk in the design stage tends to be much less expensive than doing so for an existing structure. Some of the di!erent approaches used in determining target safety levels can be summarised as follows: E Economic criteria. This approach attempts to quantify the risks and costs associated with repair or replacement in case of failure, delay in operations, pollution, fatalities, injuries, etc. It has the advantage of permitting rational decisions to be made in view of quanti"ed information. However, it may be sensitive to the accuracy in the risk and cost estimates. In addition it should, in principle, account for all potential modes of failure, a task of questionable feasibility. Finally, assigning a cost to the loss of human life or of large-scale damage to the environment tends to be a very controversial task. E Accepted risk levels. Here the decision is based on a parameter describing the accident rate so far tolerated by society for a particular activity. For example, the fatality rate (FAR) for a given application can be estimated from accident experiences and a percentage allocated for structural failure (e.g. 10%), which will then form the basis for the target structural reliability. A &consequence factor' can be also introduced. The major shortcoming is the sensitivity of the results to the available statistics of accidents. E Reliability implicit in existing standards. This approach is based on using design cases meeting existing standards as a basis for calibrating the new standard under development. The method is clearly simplistic and will seldom re#ect the di!erent aspects a!ecting safety described above. However, it tends to be robust in relation to the modelling assumptions if the same reliability analysis procedure is used in establishing the reliability of existing designs and in calibrating the acceptable criteria in the new standard. Such acceptance criteria would then re#ect the same average reliability as existing practice but with a reduced variability. The main shortcoming of the method is the sensitivity to the choice of the design cases. Existing designs will tend to re#ect varying degrees of optimisation and designer preferences that may change with time. If care is not taken these aspects may lead to the average reliability implied by the new standard to deviate substantially from that implied by existing practice.
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In addition, the dominant causes of failure tend not to be related to the structure but to errors arising from human, procedural or organisational errors. A comprehensive discussion on this subject is given by Bea [6]. The potential approaches to manage the risk of human error are: E Reduce the incidence and severity of these errors. E Reduce the e!ects by implementing structural redundancy and damage tolerance in design. E Increase detection and remediation by means of inspection, maintenance and repair. Some of the above aspects are outside the scope of a structural design/assessment standard, but the key point here is that utilising larger safety factors is not an e!ective or e$cient way to achieve reliability in structures. Resources are best focused at clear and concise design/assessment procedures that will promote safety and in implementing systems with a high tolerance to human error.
3. Jack-up reliability 3.1. General Reliability analysis (RA) is the basic tool normally used to quantify safety margins when calibrating design/assessment standards. Detailed descriptions of the method, its applications and limitations in various industries can be found in the literature [6}13]. In simple terms it permits safety to be assessed by means of statistical models of capacity vs. demand. Such models should incorporate both Type I (natural) uncertainties and Type II (modelling) uncertainties. The fundamental concept is inherently simple but its implementation is not. Reliability theory is ideally suited for systems (however complex) designed &from the bottom}up' with parts that have already been extensively tested * aeroplanes and computers can be mentioned in this context. However, o!shore structures are increasingly designed &from the top}down' to save time and estimating their reliability is a complex task. Jackets designed to API RP 2A [3] (from the 9th Ed. onwards) and jack-ups assessed to historical practice are complex systems with a successful track record. Their reliability is essentially 100% but minus a certain value (or probability of failure p ), where tends to be small and di$cult to quantify either from databases D of accidents or from "rst principles of engineering. A Physics Nobel Laureate and one of the brightest and sharpest minds of this century, Richard Feynman gives an excellent insight into the issues discussed above in his investigation [14] into the causes of the disaster involving the Space Shuttle Challenger * a system designed &from the top}down'. But, despite such limitations and when clearly understood in a comparative framework, RA is a powerful tool in achieving the twin goals of quantifying and standardising safety.
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Design/assessment standards usually implement safety via explicit and implicit reserves. Explicit reserves include conservative combinations of functional requirements (e.g. variable load in most critical position), use of a large return period for the environmental load combination and explicit safety factors. In addition, designers will often include extra margins as their prerogative. Implicit reserves include conservatism in interpreting site-speci"c data (metocean, soils, etc.) and in a large number of assumptions used in analysis (wave, wind and current in-line, lower bounds to member failure test data, etc.). Both explicit and implicit reserves need to be accounted for if more realistic reliability levels are to be estimated. 3.2. Explicit safety reserves Table 1 illustrates the explicit safety factors contained in various standards: API RP 2A * North Sea Variant [15], Bulletin 5-5A - North Sea Annex - Draft 4 [16], Bulletin 5-5A [1], Bulletin 5-5A - 3rd Draft [2] (representative of historical jack-up assessment criteria). The information in Table 1 is given in terms of WSD-type safety factors. In some cases the comparison is approximate and opened to interpretation as some of the practices considered follow an LRFD format. The values given should be seen as an illustration only and not as a recommendation for design/assessment criteria. It is informative to re-examine the development process of API RP 2A [3] (Fig. 1) and that of Bulletin 5-5A [1]. Both used the simplest approach to calibration * based on the average reliability of existing structures. However, API RP 2A [3] is converging towards criteria that recognise the consequences of failure and that a di!erent cost}bene"t balance applies to existing structures in comparison with new designs. In addition, care has been taken so that the load and resistance factor (LRFD) version re#ects the average reliability of the working stress (WSD) version that originated it. Bulletin 5-5A [1], on the other hand, seems to have led to jack-up assessments that can deviate sharply from historical WSD practice. In fact, the explicit safety margins
Table 1 Comparison of explicit safety factors in various practices Practice
Storm RP (Manned)
Air gap (m)
Leg length (m)/Leg inclination
Overturning (SF)
Brace buckling (SF)
Preload (SF)
API - (NS)
100y (JP)
*
*
&1.30}1.47
*
Bull. 5-5A NS Annex Bull. 5-5A RP } Draft 3
100y (JP)
1.5 (with Subsid.) 1.5 at 100y 0.0 at 10,000y 1.5 1.5
1.5/Yes
1.32
&1.30}1.47
&1.25
1.5/Yes 1.5/No
1.32 1.10
&1.30}1.47 1.00
&1.25 1.00
50y (IE) 50y (IE)
IE * Independent extremes; JP * joint probability; SF * Safety factor.
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Fig. 1. Key points in the evolution of API RP 2A [3].
within Bulletin 5-5A [1] show a remarkable convergence towards those of the North Sea calibration of API RP 2A [15]. The key point is that the more stringent criteria given in Bulletin 5-5A did not arise from a rational analysis of risk levels on the basis of either accident rates or "rst engineering principles but from its calibration process. There is also a concern that the complexity in the analysis procedures contained in Bulletin 5-5A [1] does not contribute towards reducing the likelihood of human errors in their interpretation. 3.3. Implicit safety reserves * load recipe and P} ewects Historical practice tended to adopt a &smooth' value of the drag coe$cient C for B the complete leg, but with no reduction in kinematics or no current blockage. The Bulletin 5-5A [1], on the other hand, recommends a recipe with default C values for B &rough' and &smooth' parts of the leg which tend to increase loading while current blockage and a wave kinematics factor (implemented as a wave height reduction factor) tend to reduce it. The comparison here is not simple and is opened to interpretation * in the past not all assessors accounted for an increase in the diameter of the tubulars to account for marine growth. However, the industry perception is that the Bulletin's recipe is more conservative than historical practice in many cases. This is being aggravated in the draft NS Annex [16] with the use of a smaller reduction in wave kinematics, although it is claimed that this is balanced by the use of 100-year loading in conjunction with joint probabilities and 1-h wind loading. The other aspect is inclusion of P} e!ects. While in historical WSD practice these e!ects would be added at 100% of the environmental loading, in the Bulletin 5-5A [1] LRFD format they are introduced at 125% of the environmental loading. The scienti"c reasoning behind the modelling adopted within Bulletin 5-5A [1] is not being questioned. However, looking on the overall balance the conclusion here is again that Bulletin 5-5A [1] departs from historical practice and leans on the conservative side.
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4. Reliability of jack-ups vs. jackets A number of implicit safety reserves not dealt with in the previous sections will also a!ect reliability. The HSE study into the reliability of jackets and jack-ups [17,18] attempted to deal with explicit and implicit safety margins in a comprehensive manner. Another important aspect is that the structures were resized as close to the code limits (API RP 2A-LRFD [3] for the jacket and Bulletin 5-5A [1] for the jack-up) as possible. Preliminary results of such study, with simpli"ed foundation models and excluding foundation failure, were presented at the last Jack-up Conference [17]. The "nal results including detailed foundation modelling and foundation failure were issued last year [18]. In the meantime a number of aspects have been highlighted that potentially impact on the conclusions of that study. These will be dealt with in this section. 4.1. Environmental loading * irregularity, spreading and regional variation Real waves tend to be three-dimensional and exhibit a signi"cant degree of spreading and irregularity. In simple terms, a wave kinematics factor is usually adopted to reduce the #uid velocity (and consequently the hydrodynamic forces) so that forces obtained from a regular wave analysis are approximately similar to those obtained from more accurate irregular wave analyses. In the HSE study [17,18] the sample jacket was re-sized to be at the limit of API RP 2A-LRFD [3] using the environmental load recipe recommended in that standard, with a wave kinematics factor of 1.0 (no reduction in #uid velocity). However wave kinematics factors of 0.95 may be used in jacket design and values as low as 0.91 have been recently proposed [19]. The sample jack-up was re-sized to the limit of Bulletin 5-5A Rev. 0 [1] using the environmental load recipe recommended therein, including a wave kinematics factor via the wave height reduction factor of 1.60/1.86. Both structures were then subject to reliability analysis using a common environmental load recipe. The consensus at the time was that such a common load recipe should be similar to that of API RP 2A-LRFD [3] and no di!erentiation should be made between the two structures at this stage. However, it has been recently brought to the industry's attention that a larger reduction in kinematics would be justi"ed for jack-ups due to the typically large distance between legs (e.g. 50}60 m) [20]. Another important aspect of real waves is their random nature. Load factors in LRFD are usually justi"ed by the uncertainty associated with the randomness of the wave environment. The HSE study [17,18] assumed a load factor of 1.35 in re-sizing the jacket and a load factor of 1.25 in re-sizing the jack-up. However, a load factor of 1.25 has been recently proposed in North Sea calibration studies for "xed structures [15]. The following is an estimate of the impact of the above factors in the estimated reliability [18]: E Reference case * system structural reliability, basic HSE study: (p ) /(p ) "10.7 (The *order of magnitude+ diwerence). D U D
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E Case 1 * same as reference case, except that jacket design load factor is reduced from 1.35 to 1.25: (p ) /(p ) "3.2. D U D E Case 2 * same as Case 1, except with a net 5% change in kinematics: (p ) /(p ) "0.80. D U D E Case 3 * same as Case 1, except with a net 9% change in kinematics: (p ) /(p ) "0.24, D U D where p is the structural system failure probability. D Cases 2 and 3 apply if the jacket is re-sized for a smaller wave kinematics factor (0.95 and 0.91, respectively), in addition to the smaller load factor of 1.25 considered in Case 1, and the same load recipe is then applied to both platforms in reliability analysis. If, in addition, a di!erence was introduced in the reliability analysis to account for the larger leg spacing between jack-up legs, the comparison would be even more favourable to the jack-up than Case 3 indicates. The cases considered above exclude "xity. This would also make the comparison more favourable to the jack-up. The above numbers highlight the sensitivity of reliability estimates to key global parameters and the need for proper interpretation and quali"cation of these results. It is also highlighted that the "nal conclusions of the HSE study [18] did not indicate a substantial di!erence in reliability between jackets and jack-ups. 4.2. Loss of air gap and deck inundation Loss of air gap and deck inundation have a large impact in reliability due to the following factors: E Large increase in hydrodynamic loading. E Large increase in the uncertainty associated with hydrodynamic loading. E Potential increase in dynamic sensitivity. The "rst two aspects have been extensively covered in recent work presented in the Air Gap Workshop [20}22] promoted by HSE/E&P Forum. The third aspect has not been covered in detail, but cases have been highlighted [23] where global jacket response to loading on the substructure showed limited dynamic ampli"cation, but the response to wave-in-deck was dynamically sensitive (increase in linear DAF from &1.05 to &1.50). In the HSE study, deck inundation e!ects were not accounted for in the reliability analyses. The consensus was that the uncertainties in the calculation methods would dominate the results and that similar air gap requirements were used at the time for both platforms. However, recent studies for "xed structures [22] showed the e!ect of their inclusion to be more important than the associated uncertainties. In addition, di!erent air gap requirements are being proposed for "xed and jack-up structures. A 10,000-year air gap is being recommended in the Bulletin 5-5A NS Annex [16] for
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Table 2 Air gap exceedance * "xed structures (to HSE Guidance Notes) [21] Location/% Northern NS 75% 30% Central NS 65% 10% Southern NS 30% 20%
Return period (years)
(10,000 (1000 (10,000 (1000 (10,000 (1000
Fig. 2. Air gap requirements * typical North Sea situation [22].
jack-ups. However, many existing jackets could not meet 10,000-year criteria as easily as jack-ups could. Subsidence only aggravates the problem. ISO 13819-2 ("xed structures) stopped short of recommending 10,000-year air gap and states simply that *where possible, deck height should be chosen such that the frequency of wave impact on the deck is compatible with the target failure rate of the substructure+ [22]. Table 2 illustrates the estimated percentage of "xed structures in the North Sea for which the return periods associated with loss of air gap are inferior to 1000 years and/or 10,000 years. These values are based on the crest height predicted by the HSE guidance notes and the situation tends to improve in many cases with the use of more sophisticated statistics. Fig. 2 illustrates return periods associated with exceedance of typical air gap requirements for the North Sea (50-years crest#1.5 m clearance) [22]. Typical maximum wave heights for di!erent return periods, normalised by the 50-years signi"cant wave height are shown in the vertical axis. Fig. 2 also illustrates the impact of di!erent calculation methods (referred as SE1 * generalised Pareto distribution/extreme value analysis of surface elevation, SE2 * generalised Pareto
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distribution/extreme value analysis of signi"cant wave height and still water level [22]) and di!erent locations (NNS * Northern North Sea, CNS * Central North Sea). Even with the use of the most favourable calculation method, meeting 10,000year criteria is a challenging task for "xed structures in the Northern North Sea. It is also important to recognise that both jacket and jack-up realised high levels of structural reliability and to place the values obtained in the context of all the other potential risks. Fig. 2 illustrates that the return periods implied by the structural system reliability levels obtained in the HSE study (excluding deck inundation e!ects) [17,18] would exceed 100,000 years and fall outside the graph. In this case, extra expenditure on meeting increased structural requirements would have minimal impact in increasing overall reliability. Existing jack-ups can adjust to higher air gap requirements (such as the proposed 10,000-year values in the NS annex) much more easily than existing jackets. This factor alone should change any perception that jack-ups are less reliable than jackets. 4.3. Human and organisational factors The key importance of human and organisational factors was addressed in Section 1 of this paper. A number of factors inherent in jack-up design and operation provide a safety reserve against these factors: E Most units tend to spend a smaller proportion of their lifetime under limiting conditions leading to smaller degradation of member/joint integrity. E More e!ective procedures are in place to control dead and live loads. E The consequences of subsidence (such as loss of air gap) can be dealt with by jacking the hull up. E Jack-ups have a more favourable inspection regime (dry-docking) and the critical components are often not underwater, improving the inspectability and repairability. E The HSE Study [17,18] showed the jack-up to be more redundant and that jack-up system failure was driven by chord failure while jacket system failure was driven by failure of less robust components such as braces. Quantifying the impact of the above aspects would be a very challenging task and these were not covered in detail in the HSE study [17,18]. However these features limit the risks associated with human error, a major component in overall risk levels, and need to be kept in mind when qualifying the results obtained from reliability analysis. 4.4. Foundation reliability The HSE study initially considered simpli"ed foundation models for both the jacket and the jack-up. At this stage the jack-up was assumed as pinned at the spudcans. Structural reliability was then evaluated at a component as well as at a system level. The results obtained were reviewed and quali"ed in the previous sections of this paper.
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In the "nal stage of the HSE study, detailed models were implemented for the jacket and the jack-up foundations [18]. The jacket detailed foundation model consisted of p}y, t}z and q}z curves. The jack-up detailed foundation model included "xity by means of a hardening plasticity model. Based on such detailed models, foundation failure occurred before structural component failure for both the jacket and the jack-up. As a consequence, foundation system reliability was low in comparison with structural system reliability [18]. At the present time, it is unclear if the low foundation reliability represents a real safety issue or is purely a consequence of the conservative modelling currently adopted in the assessment of foundation capacity. With that in mind, it is relevant to investigate the structural component reliability results obtained by extrapolating the sti!ness predicted by these more detailed models prior to foundation failure. On this basis the detailed modelling of jacket foundation sti!ness would not substantially modify the jacket structural component reliability. On the other hand, the detailed modelling of jack-up foundation sti!ness with the inclusion of "xity by means of a hardening plasticity model would have a large favourable e!ect and reduce the jack-up structural component failure probability by a factor of 11.7. In summary, the inclusion of "xity is an additional factor that tends to make the comparison more favourable to the jack-up. However, the industry has yet to "nd a consensus on how much "xity and capacity can be adopted at the high load levels typical of extreme storms. In addition, much clearer agreement by the foundation community is needed in relation to the following factors: E Boring/sampling/testing procedures/strength characterisation. E Loading rate and cyclic e!ects/non-linear FE modelling. E Ageing of piled structures. These aspects are addressed in the literature for "xed structures [9] but not in detail for jack-ups. A particularly important observation is that maximum platform response to storm loading is limited to only a small number of cycles of high amplitude, (Fig. 3), and the use of reduction factors on soil strength based on high-cycling, high-amplitude tests needs to be further investigated. In view of the behaviour of irregular storm seas it is also felt that improved procedures are needed in pushover analysis to capture foundation system failure more accurately. An ambitious program to instrument several jack-up classes is under way by members of the IADC (International Association of Drilling Contractors) in order to determine "xity under extreme storm conditions. The program would stretch for over a period of three years beginning at 1999. It will cover jack-up operations in the US Gulf of Mexico and in the North Sea and possibly also in West Africa and Canada.
5. Conclusions and way forward This paper has highlighted some of the challenges in quantifying the reliability of complex systems such as "xed structures and jack-ups and the inherent variability
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Fig. 3. Typical response to storm loading (West Epsilon Jack-up, H "9.3 m) [24].
associated with the results obtained, either from databases of accidents or from "rst engineering principles. It follows that extreme care must be taken in comparing reliability levels obtained by di!erent methodologies. For example, the reliability levels estimated during the calibration of Bulletin 5-5A [1] should not be compared with those obtained during the calibration of API RP 2A-LRFD [3]. Foundation reliability is another area where further research is needed in order to obtain more consistent reliability estimates. However, if properly interpreted and quali"ed the results of RA provide useful insights into the safety inherent in the design and location assessment of these systems. Recent results demonstrating the imbalance between requirements for air gap and those for structural strength in "xed structures are an example of the positive application of RA. The results of the detailed RA performed, on the basis of "rst engineering principles, as part of the HSE study on &Reliability of Jackets vs. Jack-ups', clearly dismiss any suggestions that jack-ups are less reliable than jackets, even under the assumption of both structures operating in the same location for their entire lifetime. In fact, the qualitative considerations in this paper point towards the jack-up being more reliable than the jacket. This conclusion is in agreement with the successful track record of jack-up structures over the past 40 years. Turning to the issue of developing design/assessment standards, it is believed the process should encompass all key stakeholders in the industry and society in general and meet the challenge of the &triple bottom line': economical feasibility, human/ environmental safety and social responsibility. Under-regulation has a cost in terms of risks to human lives and environment, but over-regulation has also a cost in harming business and employment prospects.
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Rule development and prescriptive legislation are not a satisfactory way of improving safety. This should be achieved by dealing with the main causes of accidents, which are often related to human, procedural and organisational factors. Increasing safety factors is not an e$cient or e!ective way of achieving the goal of increased safety. Structural redundancy and robustness are the way forward here. It is also important to recognise the di!erences in the cost}bene"t balance between new designs and existing structures. At present, newly developed LRFD standards tend to be used mostly when requested by operators. There is widespread concern in the industry that these standards have yet to deliver an improved product as the increased expenditure associated with their implementation seems not to be o!set by a reduction in overall costs and risks. The fault is obviously not with the method itself but with its implementation. API RP 2A [3] has been considerably developed and enhanced over recent years, accommodating consequence-based criteria as well as re-quali"cation criteria. It is perceived by the major stakeholders in industry and public as a solid basis for the development of an ISO standard. On the other hand, SNAME Bulletin 5-5A [1] is yet to complete the positive evolution experienced by API RP 2A [3]. Jack-up assessments based on Bulletin 5-5A [1] in many cases depart signi"cantly from historical practice. Such departures do not seem to be a consequence of a detailed risk assessment of these units and may be purely a consequence of the calibration process adopted. Various avenues can be explored in order to improve Bulletin 5-5A [1] to re#ect state-of-the-art technology in the development of o!shore standards. Revisiting its calibration seems a priority task. It is also felt the traditional North Sea approach of linking failure of a structural component directly to fatality or injury and then setting annual target reliability levels for manned platforms is not ideal. A more modern approach is one in which di!erent balances are sought in dealing with the risks a!ecting the platform structure and those a!ecting human lives. The structural safety is dictated by economics. On the other hand, human safety is the overriding concern at any time and is treated separately by limiting a fatality risk index to a level as low as reasonably practicable. Acknowledgements The authors convey their appreciation to the Jack-up Committee of the International Association of Drilling Contractors (IADC) for their "nancial support and for the technical discussions during the course of this work.
References [1] Society of Naval Architects and Marine Engineers. Site speci"c assessment of mobile jack-up units. SNAME T&R Bulletin 5-5A, 1st ed., Rev. 0}1994, Rev. 1}1997. [2] International Association of Drilling Contractors. Third Draft of RP and items for possible further study. IADC Memorandum, 07/02/1990.
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[3] American Petroleum Institute. Recommended practice for planning, designing and constructing "xed o!shore platforms. API RP 2A, WSD 20th ed., 1993; WSD Section 17, 1996; Draft WSD 21th ed., 1999; LRFD 1st ed., 1993. [4] UK O!shore Operators Association. A framework for risk related decision support. UKOOA Industry Guidelines, Issue 1, May 1999. [5] Vassalos D. Shaping ship safety: the face of the future. Marine Tech SNAME 1999;36(2):61}76. [6] Bea R. Human and organisational factors in the safety of o!shore structures. In: Guedes Soares C, editor. Risk and reliability in marine Technology. Rotterdam: Balkema, 1998. [7] Moan T. Target levels for structural reliability and risk analysis of o!shore structures. In: Guedes Soares C, editor. Risk and reliability in marine technology. Rotterdam: Balkema, 1998. [8] Thoft-Christensen P, Baker MJ. Structural reliability theory and its applications. Berlin: Springer, 1982. [9] Bea R, Valle O. Key issues associated with development of reassessment and requali"cation criteria for platforms in the Bay of Campeche, Mexico. In: Proceedings of the International Workshop on Platform Requali"cation, 17th OMAE, Lisbon, 1998. [10] Bea R, Lara L, Heredia E, Valle O, Valdes V. Reliability criteria for design and requali"cation of pipelines and risers in the Bay of Campeche, Mexico. In: Proceedings of the 18th OMAE, Newfoundland, Canada, 1999. [11] Frieze PA. Reliability analysis in theory and practice. In: Smith CD, Dow RS, editors. Advances in marine structures-2, London: Elsevier Applied Science, 1991. [12] Mansour AE, Faulkner D. On applying the statistical approach to extreme sea loads and ship hull strength. Trans Roy Inst Naval Architects (RINA) 1973;115:277}314. [13] Palmer A. The limits of reliability theory and the reliability of limit state theory applied to pipelines. Paper OTC 8218, OTC'96, Houston, 1996. [14] Feynman RP. &What do you care what other people think?'. W W Norton, 1988. [15] Turner RC. Partial safety factor calibration for North Sea adaptation of API RP 2A-LRFD. In: Proceedings of the Institution of Civil Engineers, Water Maritime & Energy, 101, June 1993. [16] SNAME OC-7 Panel, North Sea Annex. 4th Draft, 1999. [17] Morandi AC, Frieze PA, Birkinshaw M, Smith D, Dixon A. Jack-up and jacket platforms: a comparison of system strength and reliability. In: Boswell LF, D'Mello C, Supple WJ, editors. Proceedings of the Sixth International Conference on The Jack-up Platform. Hove: ATS Press, 1997. [18] Health and Safety Executive. Reliability of "xed and jack-up structures * "nal report, July 1998. O!shore Technology Report. HMSO, in preparation. [19] Efthymiou M, Tromans P. Derivation of reliability based load factors. Draft ISO Standard. Commentary C3, March 1996. [20] Tromans P. Walk-In, Air Gap Workshop, UK Health & Safety Executive and E&P Forum, London, 14-15/06/1999. [21] Smith D. Introduction to Air Gap Workshop, UK Health & Safety Executive and E&P Forum, London, 14-15/06/1999. [22] Smith D. Birkinshaw M, Bolt HM. Airgap determination * meeting the challenge. In: Air Gap Workshop, UK Health & Safety Executive and E&P Forum, London, 14-15/06/1999. [23] Schmucker DG, Cornell CA. Dynamic behaviour of semi-ductile jackets under extreme wave and wave-in-deck forces. In: Proceeding Behaviour of O!shore Structures, 1994. [24] Morandi AC, Karunakaran D, Dixon AT, Bvrheim M. Comparison of full-scale measurements and time-domain irregular sea analysis for a large deepwater jack-up. Paper 8828, OTC'98, Houston, 1998.
Technical note
Reliability of jack-ups under extreme storm conditions A.C. Morandi *, I.A.A. Smith, G.S. Virk Global Maritime, 11767 Katy Freeway, Suite 660, Houston, TX 77079, USA Global Maritime, 12 Craven Street, London WC2N 5PB, UK Global Marine Drilling Company, 777 North Eldridge, Houston TX 77079, USA
Abstract The present paper is concerned with a technical review on the reliability of jack-ups. It covers recent developments on the application of reliability theory to o!shore structural and foundation systems and their potential impact on the standards for assessment of jack-ups and o!shore structures in general. The e!ects of risk-reducing features inherent in jack-ups that di!erentiate their risk assessment from other structures such as jackets are also investigated. In the light of the above, past comparisons of jack-up and jacket reliability are reviewed and tentative suggestions are made for improving reliability assessments and calibration procedures to be adopted in the development of the ISO standard for jack-ups. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Jack-up; Reliability; Safety; Risk; Standards; Criteria; ISO
1. Introduction Mobile drilling units in general and jack-ups in particular play a vital role in the o!shore industry. Historically, the structural safety of these units has been considered acceptable. Since standards and procedures for their assessment were known to vary, a recommended practice for site assessment of jack-ups was developed by a Joint Industry Project and published by the Society of Naval Architects and Marine Engineers (SNAME) in their T&R Bulletin 5-5A in 1994 [1]. The stated objectives of this document were [2]: to promote consistency of assessment practice, to provide
* Corresponding author. Tel.: #1-281-558-3690; fax: #1-281-558-0541. E-mail address: [email protected] (A.C. Morandi). 0951-8339/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 1 - 8 3 3 9 ( 0 0 ) 0 0 0 5 7 - 5
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guidance in view of a trend for using jack-ups in more dynamically sensitive locations (deep water and harsh environment) and to transfer technology developed for "xed structures in those areas which could be benexcial to jack-up assessment. Bulletin 5-5A [1] was expected to promote more consistent reliability levels thus, minimising the potential for isolated cases where safety could be of concern. However, industry experience has indicated that this document tends to impact a large portion of existing rigs. Although experience indicates that jack-ups have been safely operated over the past 40 years, the acceptance criteria given in Bulletin 5-5A [1] result in unfavourable assessments in cases where the track record suggests otherwise. This may lead to expensive remedial measures with a potential impact on marginally economic "elds. The debate has gained renewed importance as ISO TC67/SC7/WG7 is now developing an international standard for mobile drilling units including jack-ups. Notwithstanding the present concern regarding the applicability of the Bulletin's recommended practice, this document is being proposed as a basis for the ISO standard. Furthermore, ISO procedures will require a certain degree of consistency between the format of the jack-up standard and that of the standard for "xed structures now reaching its "nal stages. Care must be taken to keep requirements for the design of new "xed structures from being imposed carte blanche on the assessment of existing jack-ups. The objectives of this paper are: E In concise terms, give an appreciation of the recent application of reliability theory in the development of o!shore standards. The di!erences between the evolution of standards for "xed structures (API RP 2A [3] in particular) and the development of the Bulletin 5-5A [1] criteria are highlighted. E Investigate the impact of risk-reducing features inherent to jack-ups that di!erentiate their safety assessment from that of "xed structures. In particular, clarify potentially misleading interpretations of jacket and jack-up comparative reliability. A few preliminary suggestions are also given on a way forward in developing the ISO standards for jack-ups.
2. Risk, safety and reliability Safety assessment in many industries (o!shore, shipping, etc.) is evolving towards a framework of devolved responsibility, which concentrates on management and organisation for safety and encourages individual companies to establish targets for safety performance [4}6]. This evolution is driven by a general acceptance of the inadequacy of past prescriptive safety regimes due to a number of limitations [5], such as lack of clarity in goals and objectives, slow response to technological developments, imbalance in safety margins and an often reactive approach to disasters and their media coverage. Safety is increasingly seen as a quality that re#ects a state of acceptable risk concerning human lives, hardware, software and the environment [5,6]. The main challenges are then to determine acceptable risk levels and to
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translate them into standards that can be followed by practising engineers. The target safety levels adopted should, in principle, re#ect a number of di!erent aspects [7]: E Failure cause and mode. E Di!erent phases in the platform life-cycle (temporary/long term). E Reliability analysis (RA) methodology and, in particular, the speci"c uncertainties modelled and the basis for the modelling adopted. E Potential consequences of failure * does failure of a component leads to failure of the system? What are the consequences of failure in terms of fatalities, pollution and economic loss? E The expense and e!ort necessary to reduce the risk of failure. In particular, reducing risk in the design stage tends to be much less expensive than doing so for an existing structure. Some of the di!erent approaches used in determining target safety levels can be summarised as follows: E Economic criteria. This approach attempts to quantify the risks and costs associated with repair or replacement in case of failure, delay in operations, pollution, fatalities, injuries, etc. It has the advantage of permitting rational decisions to be made in view of quanti"ed information. However, it may be sensitive to the accuracy in the risk and cost estimates. In addition it should, in principle, account for all potential modes of failure, a task of questionable feasibility. Finally, assigning a cost to the loss of human life or of large-scale damage to the environment tends to be a very controversial task. E Accepted risk levels. Here the decision is based on a parameter describing the accident rate so far tolerated by society for a particular activity. For example, the fatality rate (FAR) for a given application can be estimated from accident experiences and a percentage allocated for structural failure (e.g. 10%), which will then form the basis for the target structural reliability. A &consequence factor' can be also introduced. The major shortcoming is the sensitivity of the results to the available statistics of accidents. E Reliability implicit in existing standards. This approach is based on using design cases meeting existing standards as a basis for calibrating the new standard under development. The method is clearly simplistic and will seldom re#ect the di!erent aspects a!ecting safety described above. However, it tends to be robust in relation to the modelling assumptions if the same reliability analysis procedure is used in establishing the reliability of existing designs and in calibrating the acceptable criteria in the new standard. Such acceptance criteria would then re#ect the same average reliability as existing practice but with a reduced variability. The main shortcoming of the method is the sensitivity to the choice of the design cases. Existing designs will tend to re#ect varying degrees of optimisation and designer preferences that may change with time. If care is not taken these aspects may lead to the average reliability implied by the new standard to deviate substantially from that implied by existing practice.
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In addition, the dominant causes of failure tend not to be related to the structure but to errors arising from human, procedural or organisational errors. A comprehensive discussion on this subject is given by Bea [6]. The potential approaches to manage the risk of human error are: E Reduce the incidence and severity of these errors. E Reduce the e!ects by implementing structural redundancy and damage tolerance in design. E Increase detection and remediation by means of inspection, maintenance and repair. Some of the above aspects are outside the scope of a structural design/assessment standard, but the key point here is that utilising larger safety factors is not an e!ective or e$cient way to achieve reliability in structures. Resources are best focused at clear and concise design/assessment procedures that will promote safety and in implementing systems with a high tolerance to human error.
3. Jack-up reliability 3.1. General Reliability analysis (RA) is the basic tool normally used to quantify safety margins when calibrating design/assessment standards. Detailed descriptions of the method, its applications and limitations in various industries can be found in the literature [6}13]. In simple terms it permits safety to be assessed by means of statistical models of capacity vs. demand. Such models should incorporate both Type I (natural) uncertainties and Type II (modelling) uncertainties. The fundamental concept is inherently simple but its implementation is not. Reliability theory is ideally suited for systems (however complex) designed &from the bottom}up' with parts that have already been extensively tested * aeroplanes and computers can be mentioned in this context. However, o!shore structures are increasingly designed &from the top}down' to save time and estimating their reliability is a complex task. Jackets designed to API RP 2A [3] (from the 9th Ed. onwards) and jack-ups assessed to historical practice are complex systems with a successful track record. Their reliability is essentially 100% but minus a certain value (or probability of failure p ), where tends to be small and di$cult to quantify either from databases D of accidents or from "rst principles of engineering. A Physics Nobel Laureate and one of the brightest and sharpest minds of this century, Richard Feynman gives an excellent insight into the issues discussed above in his investigation [14] into the causes of the disaster involving the Space Shuttle Challenger * a system designed &from the top}down'. But, despite such limitations and when clearly understood in a comparative framework, RA is a powerful tool in achieving the twin goals of quantifying and standardising safety.
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Design/assessment standards usually implement safety via explicit and implicit reserves. Explicit reserves include conservative combinations of functional requirements (e.g. variable load in most critical position), use of a large return period for the environmental load combination and explicit safety factors. In addition, designers will often include extra margins as their prerogative. Implicit reserves include conservatism in interpreting site-speci"c data (metocean, soils, etc.) and in a large number of assumptions used in analysis (wave, wind and current in-line, lower bounds to member failure test data, etc.). Both explicit and implicit reserves need to be accounted for if more realistic reliability levels are to be estimated. 3.2. Explicit safety reserves Table 1 illustrates the explicit safety factors contained in various standards: API RP 2A * North Sea Variant [15], Bulletin 5-5A - North Sea Annex - Draft 4 [16], Bulletin 5-5A [1], Bulletin 5-5A - 3rd Draft [2] (representative of historical jack-up assessment criteria). The information in Table 1 is given in terms of WSD-type safety factors. In some cases the comparison is approximate and opened to interpretation as some of the practices considered follow an LRFD format. The values given should be seen as an illustration only and not as a recommendation for design/assessment criteria. It is informative to re-examine the development process of API RP 2A [3] (Fig. 1) and that of Bulletin 5-5A [1]. Both used the simplest approach to calibration * based on the average reliability of existing structures. However, API RP 2A [3] is converging towards criteria that recognise the consequences of failure and that a di!erent cost}bene"t balance applies to existing structures in comparison with new designs. In addition, care has been taken so that the load and resistance factor (LRFD) version re#ects the average reliability of the working stress (WSD) version that originated it. Bulletin 5-5A [1], on the other hand, seems to have led to jack-up assessments that can deviate sharply from historical WSD practice. In fact, the explicit safety margins
Table 1 Comparison of explicit safety factors in various practices Practice
Storm RP (Manned)
Air gap (m)
Leg length (m)/Leg inclination
Overturning (SF)
Brace buckling (SF)
Preload (SF)
API - (NS)
100y (JP)
*
*
&1.30}1.47
*
Bull. 5-5A NS Annex Bull. 5-5A RP } Draft 3
100y (JP)
1.5 (with Subsid.) 1.5 at 100y 0.0 at 10,000y 1.5 1.5
1.5/Yes
1.32
&1.30}1.47
&1.25
1.5/Yes 1.5/No
1.32 1.10
&1.30}1.47 1.00
&1.25 1.00
50y (IE) 50y (IE)
IE * Independent extremes; JP * joint probability; SF * Safety factor.
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Fig. 1. Key points in the evolution of API RP 2A [3].
within Bulletin 5-5A [1] show a remarkable convergence towards those of the North Sea calibration of API RP 2A [15]. The key point is that the more stringent criteria given in Bulletin 5-5A did not arise from a rational analysis of risk levels on the basis of either accident rates or "rst engineering principles but from its calibration process. There is also a concern that the complexity in the analysis procedures contained in Bulletin 5-5A [1] does not contribute towards reducing the likelihood of human errors in their interpretation. 3.3. Implicit safety reserves * load recipe and P} ewects Historical practice tended to adopt a &smooth' value of the drag coe$cient C for B the complete leg, but with no reduction in kinematics or no current blockage. The Bulletin 5-5A [1], on the other hand, recommends a recipe with default C values for B &rough' and &smooth' parts of the leg which tend to increase loading while current blockage and a wave kinematics factor (implemented as a wave height reduction factor) tend to reduce it. The comparison here is not simple and is opened to interpretation * in the past not all assessors accounted for an increase in the diameter of the tubulars to account for marine growth. However, the industry perception is that the Bulletin's recipe is more conservative than historical practice in many cases. This is being aggravated in the draft NS Annex [16] with the use of a smaller reduction in wave kinematics, although it is claimed that this is balanced by the use of 100-year loading in conjunction with joint probabilities and 1-h wind loading. The other aspect is inclusion of P} e!ects. While in historical WSD practice these e!ects would be added at 100% of the environmental loading, in the Bulletin 5-5A [1] LRFD format they are introduced at 125% of the environmental loading. The scienti"c reasoning behind the modelling adopted within Bulletin 5-5A [1] is not being questioned. However, looking on the overall balance the conclusion here is again that Bulletin 5-5A [1] departs from historical practice and leans on the conservative side.
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4. Reliability of jack-ups vs. jackets A number of implicit safety reserves not dealt with in the previous sections will also a!ect reliability. The HSE study into the reliability of jackets and jack-ups [17,18] attempted to deal with explicit and implicit safety margins in a comprehensive manner. Another important aspect is that the structures were resized as close to the code limits (API RP 2A-LRFD [3] for the jacket and Bulletin 5-5A [1] for the jack-up) as possible. Preliminary results of such study, with simpli"ed foundation models and excluding foundation failure, were presented at the last Jack-up Conference [17]. The "nal results including detailed foundation modelling and foundation failure were issued last year [18]. In the meantime a number of aspects have been highlighted that potentially impact on the conclusions of that study. These will be dealt with in this section. 4.1. Environmental loading * irregularity, spreading and regional variation Real waves tend to be three-dimensional and exhibit a signi"cant degree of spreading and irregularity. In simple terms, a wave kinematics factor is usually adopted to reduce the #uid velocity (and consequently the hydrodynamic forces) so that forces obtained from a regular wave analysis are approximately similar to those obtained from more accurate irregular wave analyses. In the HSE study [17,18] the sample jacket was re-sized to be at the limit of API RP 2A-LRFD [3] using the environmental load recipe recommended in that standard, with a wave kinematics factor of 1.0 (no reduction in #uid velocity). However wave kinematics factors of 0.95 may be used in jacket design and values as low as 0.91 have been recently proposed [19]. The sample jack-up was re-sized to the limit of Bulletin 5-5A Rev. 0 [1] using the environmental load recipe recommended therein, including a wave kinematics factor via the wave height reduction factor of 1.60/1.86. Both structures were then subject to reliability analysis using a common environmental load recipe. The consensus at the time was that such a common load recipe should be similar to that of API RP 2A-LRFD [3] and no di!erentiation should be made between the two structures at this stage. However, it has been recently brought to the industry's attention that a larger reduction in kinematics would be justi"ed for jack-ups due to the typically large distance between legs (e.g. 50}60 m) [20]. Another important aspect of real waves is their random nature. Load factors in LRFD are usually justi"ed by the uncertainty associated with the randomness of the wave environment. The HSE study [17,18] assumed a load factor of 1.35 in re-sizing the jacket and a load factor of 1.25 in re-sizing the jack-up. However, a load factor of 1.25 has been recently proposed in North Sea calibration studies for "xed structures [15]. The following is an estimate of the impact of the above factors in the estimated reliability [18]: E Reference case * system structural reliability, basic HSE study: (p ) /(p ) "10.7 (The *order of magnitude+ diwerence). D U D
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E Case 1 * same as reference case, except that jacket design load factor is reduced from 1.35 to 1.25: (p ) /(p ) "3.2. D U D E Case 2 * same as Case 1, except with a net 5% change in kinematics: (p ) /(p ) "0.80. D U D E Case 3 * same as Case 1, except with a net 9% change in kinematics: (p ) /(p ) "0.24, D U D where p is the structural system failure probability. D Cases 2 and 3 apply if the jacket is re-sized for a smaller wave kinematics factor (0.95 and 0.91, respectively), in addition to the smaller load factor of 1.25 considered in Case 1, and the same load recipe is then applied to both platforms in reliability analysis. If, in addition, a di!erence was introduced in the reliability analysis to account for the larger leg spacing between jack-up legs, the comparison would be even more favourable to the jack-up than Case 3 indicates. The cases considered above exclude "xity. This would also make the comparison more favourable to the jack-up. The above numbers highlight the sensitivity of reliability estimates to key global parameters and the need for proper interpretation and quali"cation of these results. It is also highlighted that the "nal conclusions of the HSE study [18] did not indicate a substantial di!erence in reliability between jackets and jack-ups. 4.2. Loss of air gap and deck inundation Loss of air gap and deck inundation have a large impact in reliability due to the following factors: E Large increase in hydrodynamic loading. E Large increase in the uncertainty associated with hydrodynamic loading. E Potential increase in dynamic sensitivity. The "rst two aspects have been extensively covered in recent work presented in the Air Gap Workshop [20}22] promoted by HSE/E&P Forum. The third aspect has not been covered in detail, but cases have been highlighted [23] where global jacket response to loading on the substructure showed limited dynamic ampli"cation, but the response to wave-in-deck was dynamically sensitive (increase in linear DAF from &1.05 to &1.50). In the HSE study, deck inundation e!ects were not accounted for in the reliability analyses. The consensus was that the uncertainties in the calculation methods would dominate the results and that similar air gap requirements were used at the time for both platforms. However, recent studies for "xed structures [22] showed the e!ect of their inclusion to be more important than the associated uncertainties. In addition, di!erent air gap requirements are being proposed for "xed and jack-up structures. A 10,000-year air gap is being recommended in the Bulletin 5-5A NS Annex [16] for
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Table 2 Air gap exceedance * "xed structures (to HSE Guidance Notes) [21] Location/% Northern NS 75% 30% Central NS 65% 10% Southern NS 30% 20%
Return period (years)
(10,000 (1000 (10,000 (1000 (10,000 (1000
Fig. 2. Air gap requirements * typical North Sea situation [22].
jack-ups. However, many existing jackets could not meet 10,000-year criteria as easily as jack-ups could. Subsidence only aggravates the problem. ISO 13819-2 ("xed structures) stopped short of recommending 10,000-year air gap and states simply that *where possible, deck height should be chosen such that the frequency of wave impact on the deck is compatible with the target failure rate of the substructure+ [22]. Table 2 illustrates the estimated percentage of "xed structures in the North Sea for which the return periods associated with loss of air gap are inferior to 1000 years and/or 10,000 years. These values are based on the crest height predicted by the HSE guidance notes and the situation tends to improve in many cases with the use of more sophisticated statistics. Fig. 2 illustrates return periods associated with exceedance of typical air gap requirements for the North Sea (50-years crest#1.5 m clearance) [22]. Typical maximum wave heights for di!erent return periods, normalised by the 50-years signi"cant wave height are shown in the vertical axis. Fig. 2 also illustrates the impact of di!erent calculation methods (referred as SE1 * generalised Pareto distribution/extreme value analysis of surface elevation, SE2 * generalised Pareto
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distribution/extreme value analysis of signi"cant wave height and still water level [22]) and di!erent locations (NNS * Northern North Sea, CNS * Central North Sea). Even with the use of the most favourable calculation method, meeting 10,000year criteria is a challenging task for "xed structures in the Northern North Sea. It is also important to recognise that both jacket and jack-up realised high levels of structural reliability and to place the values obtained in the context of all the other potential risks. Fig. 2 illustrates that the return periods implied by the structural system reliability levels obtained in the HSE study (excluding deck inundation e!ects) [17,18] would exceed 100,000 years and fall outside the graph. In this case, extra expenditure on meeting increased structural requirements would have minimal impact in increasing overall reliability. Existing jack-ups can adjust to higher air gap requirements (such as the proposed 10,000-year values in the NS annex) much more easily than existing jackets. This factor alone should change any perception that jack-ups are less reliable than jackets. 4.3. Human and organisational factors The key importance of human and organisational factors was addressed in Section 1 of this paper. A number of factors inherent in jack-up design and operation provide a safety reserve against these factors: E Most units tend to spend a smaller proportion of their lifetime under limiting conditions leading to smaller degradation of member/joint integrity. E More e!ective procedures are in place to control dead and live loads. E The consequences of subsidence (such as loss of air gap) can be dealt with by jacking the hull up. E Jack-ups have a more favourable inspection regime (dry-docking) and the critical components are often not underwater, improving the inspectability and repairability. E The HSE Study [17,18] showed the jack-up to be more redundant and that jack-up system failure was driven by chord failure while jacket system failure was driven by failure of less robust components such as braces. Quantifying the impact of the above aspects would be a very challenging task and these were not covered in detail in the HSE study [17,18]. However these features limit the risks associated with human error, a major component in overall risk levels, and need to be kept in mind when qualifying the results obtained from reliability analysis. 4.4. Foundation reliability The HSE study initially considered simpli"ed foundation models for both the jacket and the jack-up. At this stage the jack-up was assumed as pinned at the spudcans. Structural reliability was then evaluated at a component as well as at a system level. The results obtained were reviewed and quali"ed in the previous sections of this paper.
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In the "nal stage of the HSE study, detailed models were implemented for the jacket and the jack-up foundations [18]. The jacket detailed foundation model consisted of p}y, t}z and q}z curves. The jack-up detailed foundation model included "xity by means of a hardening plasticity model. Based on such detailed models, foundation failure occurred before structural component failure for both the jacket and the jack-up. As a consequence, foundation system reliability was low in comparison with structural system reliability [18]. At the present time, it is unclear if the low foundation reliability represents a real safety issue or is purely a consequence of the conservative modelling currently adopted in the assessment of foundation capacity. With that in mind, it is relevant to investigate the structural component reliability results obtained by extrapolating the sti!ness predicted by these more detailed models prior to foundation failure. On this basis the detailed modelling of jacket foundation sti!ness would not substantially modify the jacket structural component reliability. On the other hand, the detailed modelling of jack-up foundation sti!ness with the inclusion of "xity by means of a hardening plasticity model would have a large favourable e!ect and reduce the jack-up structural component failure probability by a factor of 11.7. In summary, the inclusion of "xity is an additional factor that tends to make the comparison more favourable to the jack-up. However, the industry has yet to "nd a consensus on how much "xity and capacity can be adopted at the high load levels typical of extreme storms. In addition, much clearer agreement by the foundation community is needed in relation to the following factors: E Boring/sampling/testing procedures/strength characterisation. E Loading rate and cyclic e!ects/non-linear FE modelling. E Ageing of piled structures. These aspects are addressed in the literature for "xed structures [9] but not in detail for jack-ups. A particularly important observation is that maximum platform response to storm loading is limited to only a small number of cycles of high amplitude, (Fig. 3), and the use of reduction factors on soil strength based on high-cycling, high-amplitude tests needs to be further investigated. In view of the behaviour of irregular storm seas it is also felt that improved procedures are needed in pushover analysis to capture foundation system failure more accurately. An ambitious program to instrument several jack-up classes is under way by members of the IADC (International Association of Drilling Contractors) in order to determine "xity under extreme storm conditions. The program would stretch for over a period of three years beginning at 1999. It will cover jack-up operations in the US Gulf of Mexico and in the North Sea and possibly also in West Africa and Canada.
5. Conclusions and way forward This paper has highlighted some of the challenges in quantifying the reliability of complex systems such as "xed structures and jack-ups and the inherent variability
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Fig. 3. Typical response to storm loading (West Epsilon Jack-up, H "9.3 m) [24].
associated with the results obtained, either from databases of accidents or from "rst engineering principles. It follows that extreme care must be taken in comparing reliability levels obtained by di!erent methodologies. For example, the reliability levels estimated during the calibration of Bulletin 5-5A [1] should not be compared with those obtained during the calibration of API RP 2A-LRFD [3]. Foundation reliability is another area where further research is needed in order to obtain more consistent reliability estimates. However, if properly interpreted and quali"ed the results of RA provide useful insights into the safety inherent in the design and location assessment of these systems. Recent results demonstrating the imbalance between requirements for air gap and those for structural strength in "xed structures are an example of the positive application of RA. The results of the detailed RA performed, on the basis of "rst engineering principles, as part of the HSE study on &Reliability of Jackets vs. Jack-ups', clearly dismiss any suggestions that jack-ups are less reliable than jackets, even under the assumption of both structures operating in the same location for their entire lifetime. In fact, the qualitative considerations in this paper point towards the jack-up being more reliable than the jacket. This conclusion is in agreement with the successful track record of jack-up structures over the past 40 years. Turning to the issue of developing design/assessment standards, it is believed the process should encompass all key stakeholders in the industry and society in general and meet the challenge of the &triple bottom line': economical feasibility, human/ environmental safety and social responsibility. Under-regulation has a cost in terms of risks to human lives and environment, but over-regulation has also a cost in harming business and employment prospects.
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Rule development and prescriptive legislation are not a satisfactory way of improving safety. This should be achieved by dealing with the main causes of accidents, which are often related to human, procedural and organisational factors. Increasing safety factors is not an e$cient or e!ective way of achieving the goal of increased safety. Structural redundancy and robustness are the way forward here. It is also important to recognise the di!erences in the cost}bene"t balance between new designs and existing structures. At present, newly developed LRFD standards tend to be used mostly when requested by operators. There is widespread concern in the industry that these standards have yet to deliver an improved product as the increased expenditure associated with their implementation seems not to be o!set by a reduction in overall costs and risks. The fault is obviously not with the method itself but with its implementation. API RP 2A [3] has been considerably developed and enhanced over recent years, accommodating consequence-based criteria as well as re-quali"cation criteria. It is perceived by the major stakeholders in industry and public as a solid basis for the development of an ISO standard. On the other hand, SNAME Bulletin 5-5A [1] is yet to complete the positive evolution experienced by API RP 2A [3]. Jack-up assessments based on Bulletin 5-5A [1] in many cases depart signi"cantly from historical practice. Such departures do not seem to be a consequence of a detailed risk assessment of these units and may be purely a consequence of the calibration process adopted. Various avenues can be explored in order to improve Bulletin 5-5A [1] to re#ect state-of-the-art technology in the development of o!shore standards. Revisiting its calibration seems a priority task. It is also felt the traditional North Sea approach of linking failure of a structural component directly to fatality or injury and then setting annual target reliability levels for manned platforms is not ideal. A more modern approach is one in which di!erent balances are sought in dealing with the risks a!ecting the platform structure and those a!ecting human lives. The structural safety is dictated by economics. On the other hand, human safety is the overriding concern at any time and is treated separately by limiting a fatality risk index to a level as low as reasonably practicable. Acknowledgements The authors convey their appreciation to the Jack-up Committee of the International Association of Drilling Contractors (IADC) for their "nancial support and for the technical discussions during the course of this work.
References [1] Society of Naval Architects and Marine Engineers. Site speci"c assessment of mobile jack-up units. SNAME T&R Bulletin 5-5A, 1st ed., Rev. 0}1994, Rev. 1}1997. [2] International Association of Drilling Contractors. Third Draft of RP and items for possible further study. IADC Memorandum, 07/02/1990.
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