Comparison of ISO 19905-1 Framework and a Plasticity-based Spudcan Model for Jackup Foundation Assessments
Authors: Purwana Okky Ahmad , Perry Michael John , Quah Matthew , Cassidy Mark Jason 
Published: April 2012

The requirements and guidance for site-specific assessment of independent legjackup units in ISO 19905-1 provides a new framework for modellingsoil-structure interactions and performing foundation stability checks. Thisnew guideline has evolved from SNAME T&RB 5-5A recommended practice whichhas been extensively used in practice and is deemed to be robust and consistentfor routine assessment purposes. While the SNAME and (by association the) ISOguidelines have gained acceptance in the industry, advanced plasticity modelsthat describe the entire load-displacement behaviour of the spudcan footinghave also developed and become an established feature in academic publications. However, the plasticity model is still rarely used in practice due to theperceived complexity involved in their implementation and integration with thejackup structural model. This paper compares the use of a plasticity model withthe ISO 19905-1 framework for jackup foundation assessments and concentrates onbehavior of a sandy seabed. Discussion on the resulting foundation responsesand their implications to the foundation acceptance are demonstrated. Thispaper is an extension of the previous study performed by the authors using theSNAME method for jackup analysis in clay and sand.

Introduction

ISO 19905-1 [Ref. 1] is a new guideline for assessing the stability of jackups, including their foundation performance, and will be in force from 2012. Thisnew guideline has evolved from SNAME T&RB 5-5A recommended practice [Ref.2], which is routinely used for site-specific assessment of jackups, with somerevisions made for consistency and more guidance added for clarity. In SNAMEand ISO, a framework to incorporate soil-structure interaction effects, whichin most circumstances benefit jackup stability, has been provided in sufficientdetails for practical applications. Simplifications made in the foundationanalysis method, particularly for accommodating non-linear foundation fixityenable the framework to be integrated with structural models for globalresponse analysis in a relatively straightforward manner. That no empiricalparameters have to be defined by the analysts has also allowed consistentanalysis results.

Assessment using ISO and SNAME is often carried out using a secant modelapproach in a Step 2b procedure. In situations where the foundation does notmeet the acceptance criteria with the Step 2b check, more advanced foundationanalyses such as full non-linear load-displacement models or finite elementmethods are allowed. Recent developments for implementing the load-displacementbehaviour of spudcans have concentrated on establishing numerical models with aplasticity framework. Incorporation of these models into a structural analysiscode has enabled assessments of the ultimate foundation capacity of a jackupunder extreme environmental loading. Despite its robustness, implementation inroutine jackup assessments is still very limited due to the perceivedcomplexity involved. As an alternative to more sophisticated analysis, if thefoundation capacity envelope is exceeded it is common practice to " virtually" expand the bearing capacity envelope at the expense of foundation settlement. This " displacement check" results in additional foundation penetrations andcorresponding additional loads to be accommodated by the structure. While thishas been practiced by many analysts when implementing SNAME, this approach isformally adopted and referred to as Step 3a check in ISO.