The Benchmarking of the New ISO for the Site-Specific Assessment of Mobile Jack-Up Units

摘要

The paper presents the logic behind, and the results of, a series of comparison analyses between a number of jack-up site-specific assessments using both the new ISO 19905-1 on the site-specific assessment of mobile jack-up units [Ref. 1] and the Society of Naval Architects and Marine Engineers Technical & Research Bulletin 5-5A [Ref. 2]. The study shows that the differences in the results of analyses between those undertaken using 5-5A and ISO 19905-1 are generally small, although one area of potentially larger difference, which was intentionally excluded from the comparison, (the normally lower kinematics reduction factor in 19905) could lead to lower wave loads in 19905 calculations.rnISO 19905 is largely based on 5-5A, however, the ISO development incorporated some major changes, including:rn1. Enhancements to calculation methodsrn2. Significant changes in the geotechnical calculations, including the method of calculating acceptable bearing capacity, incorporation of deep penetration cases, distinction between backfill and backflow, etc.rn3. Changes to enhance compatibility with other ISO documents in the 19900 series, particularly ISO 19902 addressing Fixed Offshore Structures [Ref. 3] (e.g. the structural code checks for tubular members)rn4. Introduction of additional structural member checksrn5. Re-structuring of 5-5A into a more logical ISO formatrn6. Introduction of "benign" changes required for ISO compatibility (e.g. introduction of apparent and intrinsic wave periods), although some proved to be less benign than othersrn7. Additional loadcases for jack-ups operating close to resonancernGiven so many changes, there was concern that the ISO may not be useable (e.g. there would be gaps, inconsistencies, and serious transposition errors), and even if an analysis could be undertaken using it, the analysis results would be significantly different from a similar assessment using 5-5A.rnTo ensure the compatibility and acceptability of ISO assessments results, a two part benchmarking process was undertaken:rnPart 1 Contract a single consultant to ensure the complete ISO could be used to produce an answer. In effect, could an analysts start an analysis using the document and arrive at a solution - regardless of whether the solution was correct. This work resulted in some important changes.rnPart 2 Contract four consultants to assess four jack-up designs, and compare the results to a comparable series of assessments based on SNAME Bulletin 5-5A. In most cases each jack-up was assessed by two consultants with the analyses aligned at specific points. Alignment allowed differences in ISO interpretation to be identified, and ensured that results did not diverge as they progressed. A detailed sample calculation "Go-By" was also produced.rnThe results highlighted differences between 5-5A and ISO 199005, and also some areas of interpretation difference. There were a number of difficulties by consultants aligning at the required alignment points. Fortunately the differences between consultants were generally not that great that this caused problems, but it has made comparisons between the results considerably more complex. A number of corrections were made to the ISO prior to its final acceptance, and a considerablernnumber of typographical errors were discovered. The studies brought out a number of areas where it was acceptable to use arnmore favorable interpretation from ISO, and where new calculation methods had not been fully incorporated into 5-5A. Itrnalso showed that some of the required changes in the ISO were less benign than had been originally thought (e.g. the effectsrnof intrinsic and apparent wave period). While the consultants often found it difficult to follow the new ISO, a few E-mailrnexchanges normally sufficed in correcting problems, and when comparable analysis routes were taken through 5-5A and ISO,rnsimilar results emerged. Given the large number of changes included in the ISO, this was a successful outcome.rnOne final thought on the use of the ISO is that any consultant that has used 5-5A in the past will likely be able to use thernISO without having to take on significant new technology, but it should not be assumed that the transition from 5-5A to ISOrnwill be easy. There are a large number of small differences, and if the analyst is not extremely careful, these can be lost in arn"scan" reading of the ISO. The ISO needs to be read carefully and used in its entirety. In almost all cases the differencesrnbetween 5-5A are intended, and sometimes they can be very subtle.