Tan, Pao-Lin (American Bureau of Shipping) | Mobbs, Brad (LeTourneau Technologies) | Perry, Michael John (Keppel Offshore & Marine) | Stiff, John James (ABS Consulting) | Stock, Douglas J. (Digital Structures)
There are two ISO documents addressing the site-specific assessment of mobileoffshore jack-up units, ISO 19905-1 and ISO 19905-2 [ref. 1 and 2]. 19905-1 isthe main ISO standard while 19905-2 is a Technical Report addressing additionalinformation relevant to the assessment of jack-ups, but not critical to theanalysis (collectively referred to as 19905 in this paper). Both of thesedocuments have been developed from the SNAME T&R Bulletin 5-5A [ref. 3](herein referred to as 5-5A). During the development of 19905 from 5-5A therewas major restructuring of certain sections, particularly in the area ofresponse assessment. In addition, there were changes in both required andacceptable methods of assessment. This paper explains the most significant ofthe changes that come under the mandate of ISO TC67/SC7/WG7 Panel 3, whichdeveloped the Clauses in 19905 on Structural Modelling and Response Analysis,and provides an indication of their impact on site assessment. Other companionpapers in this OTC session on Site-Specific Assessment of Mobile Jack-Up Unitsaddress how other parts of 19905 differ from 5-5A.
The approach chosen for the ISO standard is somewhat different from that in5-5A, and it (19905) should produce a more complete and consistent assessmentof jack-up operability. The jack-up site-assessment ISO standard will beincreasingly used throughout the world, and it is vital that users understandhow the methodologies originated, and how to interpret the document.
This keynote paper describes the place, significance and development of the ISO19905-series standards (Mobile Offshore Units) within the 19900-series ofoffshore structures standards. It provides an overview of both the new standardfor the site-specific assessment of jack-ups, ISO 19905-1 [ref. 1], and thesupporting technical report, ISO/TR 19905-2 [ref. 2]. The Technical Reportcontains background information and a detailed example "go-by" calculation, afeature previously found to be extremely useful. The paper is one of seven in aseries outlining the key technical aspects of the development of ISO 19905-1and their significance in relation to the SNAME T&RB 5-5A source document[ref. 3]. The planned ISO 19905 3, site-specific assessment of floating units,will be described, but not discussed in any detail.
In 1996, ISO Work Group 7, part of Sub-Committee 7, was formed and mandated bythe parent ISO Technical Committee, ISO/TC67, to develop the jack-up siteassessment standard using SNAME T&RB 5-5A as the basis. While thedevelopment of 5-5A, first published in 1994, was rigorous, conscientious, andbased on state of the art knowledge, the published document contained much thatwas new, controversial and perceived by some to be too conservative.Consequently the IADC and others sponsored many projects to advance the processand accuracy of site assessment. Some of this work was incorporated intorevisions of 5-5A, but after 19905-1 started to be developed, much was takendirectly into that document, bypassing 5-5A. This paper provides an overview ofthe projects undertaken and the process used to develop ISOs 19905-1 and19905-2. Other papers in the session give the detailed background to thechanges from 5-5A.
The approach chosen in ISO 19905-1, although sometimes different from that inSNAME and other relevant ISO 19900 series standards, has been developed toembrace new technology whilst providing a site assessment methodology forjack-ups that is robust, calibrated against the best-practice application of5-5A, and allowing for improved technology/computing capability.
The Jack-Up site-assessment ISO will be increasingly used throughout the worldand it is vital that potential users are confident that the standard has asound provenance.
This paper describes how the structural acceptance criteria in ISO 19905-1[ref. 1] were developed from those of SNAME T&R Bulletin 5-5A (5-5A) [ref.2]. T&R Bulletin 5-5A has all the acceptance criteria, includingstructural, geotechnical, on-bottom stability, and all the dimensional checksin a single section. This was not consistent with the structure of thedeveloping ISO, so a new clause was created for the structural criteria. Thegeotechnical checks were included with other geotechnical calculations and astand-alone clause developed that listed all of the acceptance checks that hadto be undertaken. In effect, a single clause identified what needed to bechecked, and the details of the checks were moved to the relevant clause wherethe detailed calculations were enumerated. At the same time, the structuralcriteria were reformulated, in part for consistency with the provisions ofother ISO 19900-series documents (particularly ISO 19902 Fixed steel structures[ref. 3]), but also to ensure a more complete assessment was undertaken.
The paper presents the logic behind, and the results of, a series of comparisonanalyses between a number of jack-up site-specific assessments using both thenew 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 theresults of analyses between those undertaken using 5-5A and ISO 19905-1 aregenerally small, although one area of potentially larger difference, which wasintentionally excluded from the comparison, (the normally lower kinematicsreduction factor in 19905) could lead to lower wave loads in 19905calculations.
ISO 19905 is largely based on 5-5A, however, the ISO development incorporatedsome major changes, including:
• Enhancements to calculation methods
• Significant changes in the geotechnical calculations, including the method ofcalculating acceptable bearing capacity, incorporation of deep penetrationcases, distinction between backfill and backflow, etc.
• Changes to enhance compatibility with other ISO documents in the 19900series, particularly ISO 19902 addressing Fixed Offshore Structures [Ref. 3](e.g. the structural code checks for tubular members)
• Introduction of additional structural member checks
• Re-structuring of 5-5A into a more logical ISO format
• Introduction of "benign" changes required for ISO compatibility (e.g.introduction of apparent and intrinsic wave periods), although some proved tobe less benign than others
• Additional loadcases for jack-ups operating close to resonance
Given so many changes, there was concern that the ISO may not be useable (e.g.there would be gaps, inconsistencies, and serious transposition errors), andeven if an analysis could be undertaken using it, the analysis results would besignificantly different from a similar assessment using 5-5A.
To ensure the compatibility and acceptability of ISO assessments results, a twopart benchmarking process was undertaken:
Part 1 Contract a single consultant to ensure the complete ISO could be used toproduce an answer. In effect, could an analysts start an analysis using thedocument and arrive at a solution - regardless of whether the solution wascorrect. This work resulted in some important changes.
Part 2 Contract four consultants to assess four jack-up designs, and comparethe 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 analysesaligned at specific points. Alignment allowed differences in ISO interpretationto be identified, and ensured that results did not diverge as they progressed.A detailed sample calculation "Go-By" was also produced.
During the development of ISO 19905-1 [ref. 1] from Society of Naval Architectsand Marine Engineers (SNAME) publication T&R Bulletin 5-5A [ref. 2], someof the metocean actions assessment methodologies have changed. This paperdescribes two areas of change, and one area where the traditional 5-5A methodshave remained the same, but has been calibrated against the ISO metoceandocumented methodologies contained in ISO 19901-1 [ref. 3]. ISO 19901-1 is oneof the "top level" ISO documents in the 19900 series of standards that allother standards within the series are required to follow. The three main areasaddressed within this paper are:
Wave Spreading: It is well-established that seastates are generallyshort-crested and therefore produce lower overall actions than the equivalentlong-crested regular wave seastate. Extensive work has been undertaken duringthe development of 19905 to demonstrate factors that can reasonably be used toapproximate these effects. The factors included in 19905-1 are jack-up specificas they account for leg spacing and are more appropriate than theapproximations for single piles included in 19901 1.
Vertical Wind Profile: ISO 19901-1 sets out a logarithmic relationship betweenwind speed and height/averaging period, suggesting that this is more accuratethan the traditional power law profile. The environmental action clauses of19905 permit the use of power law wind profiles. A calibration of this methodis included in the 19905-2 Technical Report [ref. 4].
Intrinsic/Apparent wave period: ISO 19901-1 defines the difference between theintrinsic and apparent wave periods, and states how they should each be usedwhen calculating the wave actions on an offshore structure. The paper describesa method for modeling a random seastate to account for the fact that theactions are calculated using the intrinsic wave periods and that the dynamicsare calculated using the periodicity of the apparent wave. A simplified SDOFdynamic amplification approach is presented based on the apparent waveperiod.
SNAME [ref. 2] determines quasi-static wave loads from an applicablehigher-order wave theory such as Stoke's 5th or an appropriate order of StreamFunction and hydrodynamic coefficients that are considered realistic e.g. adrag coefficient of 1.0 for a typically rough tubular. To account for theeffects of wave spreading in short-crested seas, SNAME recommends that "Fordeterministic/regular wave force calculations it is appropriate to apply akinematics reduction factor of 0.86 in order to obtain realistic forceestimates (see Commentary). This factor may be considered to implicitly accountfor spreading and also the conservatism of deterministic/regular wavekinematics traditionally accomplished by adjusting the hydrodynamicproperties."
SNAME further recommends that the factor be applied by means of a reduced waveheight. This recommendation was largely because the software available in theearly 1990's, when SNAME was being developed, generally did not allow for theinclusion of kinematics reduction factors. The factor recommended in SNAME wasdetermined to provide a good match to the results of prior practice e.g. withfull wave height but drag coefficients in the range 0.65 to 0.70 for a (rough)tubular; it also provided a good match to the results from random seas with arealistic rough tubular drag coefficient of 1.0 and crest height increased toapproximate that of higher order wave theory (as is recommended in both SNAMEand ISO 19905-1).