A Philosophy for the Integrity Assessment of Jackup Units
- S.F. Leijten (Shell Intl. Petroleum Mij. B.V.) | Michael Efthymiou (Shell Intl. Petroleum Mij. B.V.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- June 1991
- Document Type
- Journal Paper
- 125 - 130
- 1991. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 4.3.4 Scale, 4.5 Offshore Facilities and Subsea Systems, 4.1.2 Separation and Treating, 1.6 Drilling Operations
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Summary. Survey results show that current methods to assess the structural integrity of jackups require revision. This paper presents improvement guidelines and proposes a common industry practice. It also stresses the need for site-specific location assessments and describes Shell's efforts to find a workable solution.
Self-elevating mobile units, commonly called jackups, comprise a variety of shapes and fashions. Structures range from the liftboats and workboats used in the Gulf of Mexico, swamp areas, and shallow waters, to the more familiar exploration units used worldwide and to the production and accommodation platforms used in several areas (see Fig. 1). The degree of mobility varies from the typical one move per week to one move every few years. The reliability of mobile units, particularly jackups, has received less attention than that of fixed platforms because of the diversity of their applications, the limited duration of their assignments, and their less overall economic importance. This started to change around 1980, when the use of jackups for year-round drilling and in deeper waters, particularly in the North Sea, increased. Greater use necessitated particularly in the North Sea, increased. Greater use necessitated a thorough review of existing practices, which identified a number of shortcomings. The discussion in this paper is limited to elevated jackups used for exploration or development drilling or as production units. Field transit and siting operations are beyond the scope of this paper. Accident statistics are a reasonably objective yardstick by which to measure performance and to identify potential sources of concern, if they are used in the proper context. While interpretation of absolute figures is usually difficult, comparison with other relevant statistics is appropriate to judge overall performance. On the basis of such comparisons, it is possible to set a qualitative target for overall safety/reliability performance. To translate such a qualitative target into a practical verification procedure, it is necessary to define clear evaluation methods and procedure, it is necessary to define clear evaluation methods and acceptance criteria. Again, comparison of such methods and criteria with those of other relevant applications can identify potential shortcomings that may require correction and/or further development of achieve the safety/reliability target. Because of their similar duties and behavior, fixed platforms are comparable with elevated jackups. The vast experience gained in the past 20 years from offshore fixed platforms contributes significantly to a philosophy for the integrity assessment of jackups. This paper uses that experience where appropriate.
The quality of a product or service is often expressed in terms of nonconformance to specific requirements. Similarly, the performance of jackups may be expressed in terms of their failure rate. Table 1 compares exposure and accident statistics from the Worldwide Offshore Accident Databank (WOAD) for elevated jackups and fixed platforms. The exposure statistics illustrate the vast experience platforms. The exposure statistics illustrate the vast experience with fixed platforms compared with that of jackups, with a relative exposure ratio of approximately 15. The figures quoted refer to jackups in use and exclude units that are (temporarily) laid up. "Accidents" in WOAD are those events or conditions that have caused damage to supporting structures or equipment and environment and those causing death or injury to individuals. Typical occupational accidents, involving single persons without potential to cause material damage or serious consequences to other potential to cause material damage or serious consequences to other persons aboard, are not included. persons aboard, are not included. The incidence of the most severe accidents (those that resulted in total loss of the unit) suggests two things. First, the frequency rate of accidents resulting in a total loss for jackups is considerably higher than that for fixed platforms. Second, the frequency rate for fixed platforms improves significantly with time (from 7.3 to 1.1), but such a trend is not observed for jackups (from 162 to 97). To evaluate the structural performance of elevated jackups, it is necessary to focus on a subset of the total losses, namely those caused by environmental overloading or foundation failure, and to exclude losses from blowout, fire, explosion, collision, or transportation of the unit. This subset is summarized in Table 1. The total-loss frequency rates of fixed platforms and jackups owing to environmental loading or foundation failure show trends similar to those observed for the accident frequency rates discussed earlier. We can conclude that the structural reliability of fixed platforms is greater than that of elevated jackups and that the disparity may be increasing (compare 1970-79 with 1980-87). The lower structural reliability of jackups than of fixed platforms and the lack of visible improvement over the years are reasons for concern, particularly when units are moved to deeper waters and harsher particularly when units are moved to deeper waters and harsher environments and when platforms are worked over, both of which increase the risk for these installations. The question arises whether the jackup performance is adequate and, if not, what the target should be.
Elevated-Jackup Performance. Target
To set a jackup performance or reliability target, it is useful to compare jackups with fixed platforms in terms of application and consequences of failure. Consider the following three uses for jackups (see Fig. 2): production, development drilling over fixed platform, and exploration drilling. There is no doubt that jackups used for production perform essentially the same duties as a fixed production production perform essentially the same duties as a fixed production platform and thus require the same reliability performance. When platform and thus require the same reliability performance. When a jackup unit is used for development drilling over a fixed platform, the different reliability requirements of the two units increases the risk for the installation with the higher individual reliability. Thus, the reliability of the jackup in such an application should match that of the fixed platform. A direct reliability comparison of a jackup and a fixed platform used for exploration drilling is less evident. The common thread with the previous scenarios is the personnel on board. The concern for the safety of personnel governs the selection of performance or reliability criteria, regardless of whether the jackup unit operates at one or many locations during 1 year or whether a jackup or fixed platform is used. The risk for personnel per unit of time should be equal. If we accept this personnel per unit of time should be equal. If we accept this principle, two questions arise: (1) is it correct to consider the principle, two questions arise: (1) is it correct to consider the structural reliability separate from the overall reliability and (2) are fixed platforms overdesigned (as suggested by some)? Of course, it is the overall reliability that matters when overall risk to personnel and facilities is evaluated. For instance, if overall reliability is governed by blowouts, improving structural reliability would make little sense. Performance data of elevated jackups (from WOAD) suggest that about 50% of total losses are related to structural failures (including foundation failures). Thus, improving structural reliability is worthwhile because it greatly affects over-all reliability. Efforts to reduce risk from other hazards should, of course, also continue. It is difficult to judge from performance data whether fixed platforms are overdesigned. platforms are overdesigned. SPEDE
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