Structural Acceptance Criteria in the New ISO for the Site-Specific Assessment of Mobile Jack-Up Units
Authors: Frieze Paul Alexander , Lewis David Roger , Stiff John James 
Published: April 2012

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.

Introduction

T&R Bulletin 5-5A was first published in 1994, and up until that point hadnot been extensively used in practice - it was a brand new document that hadbeen developed over the preceding six years by the Jack-Up Joint Industry Study(JU-JIS). In 1997, when there was a move to take 5-5A and develop it into anISO jack-up site-specific standard, advantage was taken of the opportunity toimprove the layout of the document, to update it to incorporate changes in thebase documents that had been used in its original development (e.g. AISC LRFD[ref. 4]), and to incorporate lessons learnt from the first few years of itsuse. It was also obvious that this ISO standard would have to be compatiblewith the other standards in the 19900 series (of which ISO 19905-1 is part), and particularly the Fixed offshore structures standard, ISO 19902.

Early discussion took place as to the overall structure of the AcceptanceCriteria that should be included in the standard. The intent of 5-5A was thatit was a Load and Resistance Factor Design (LRFD) based assessment procedure, but it became clear early on in the use of 5-5A that it could be misinterpretedto have the load factors applied as ‘load effect’ factors, after the responseanalysis. For a linear system there is no difference, but for a non-linearjack-up system, the differences can be significant. The ISO was required to bein a Partial Factor format but there was no requirement that it be an LRFDstandard. Hence, early on, there were discussions concerning the merits ofconverting the load factors to factors on the input parameters, such asmetocean factors. This was relatively quickly dismissed; however, there was anobvious need to clear up any possible misunderstanding that the assessmentshould follow a LRFD based approach. The discussions about the type of codechecks are beyond the scope of this paper, but it is important to set the stageso that there is an understanding of the amount of work needed to convert 5 5Ato the ISO. It is also important to realize that, whereas 5-5A had all theAcceptance Criteria in a single section, the new ISO would have the generalacceptance criteria in one clause, and the structural member checks in aseparate clause.