Structural Modeling and Response Analysis in the New ISO Standard for the Site Specific Assessment of Mobile Jack-Up Units
Authors: Tan Pao-Lin , Mobbs Brad , Perry Michael John , Stiff John James , Stock Douglas J. 
Published: April 2012

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.

Introduction

The major changes affecting site assessment, introduced by Panel 3, aredescribed and include:Additional assessment load cases Fd, for extreme storm event and earthquakehave been added to 19905, and the formulation of the assessment load case forincluding action factors has also changed to be generally applicable for theoriginal assessment load cases in 5-5A and the additional assessment load casesin 19905.The Drag-Inertia method is a popular time domain method for estimating thecontribution of dynamics to the response of a jack-up. The method had, however, been subject to some criticism as it produced results which were, in somecases, less conservative than other methods. The Drag-Inertia method is easy toimplement and produces values that are stable; both qualities are desirable fora site assessment. During the development of 19905, a study was performed whichsuggested factoring the Dynamic Amplification Factors (DAFs) produced by theDrag-Inertia method as a practical way to bring the DAFs in line with the meanDAF results produced by the Winterstein method. The simple factor is a functionof period ratio, Tn/Tp.As 19905 was developed, a potential un-conservatism came to light forjack-ups operating close to resonance. In order to control this, and to ensurethe reasonably possible action combinations are assessed, additional actioncombinations have been suggested for jack-ups operating close to resonance. Additionally, there are new restrictions on the use of the Single Degree ofFreedom (SDOF) method.As computers become faster it is possible to include increasing levels ofnonlinearity into analyses. It is now possible to undertake a " one-stage" analysis of a jack-up in which the nonlinearities of the foundation, thestructure and the loading are included within a time domain assessment. Thetime histories of the responses are then used to develop the most probablemaximum extreme (MPME) values that are used to check performance. This paperdescribes the analysis, and the proposed method for developing the partial loadfactors.