With the development of the offshore oil and gas industry, mobile jack-updrilling platforms are increasingly required to operate in deeper waters andharsher environments. The improvement of the jack-up site-specific assessmentpractice is vital for safely meeting this demand. In soft clayey seabeds, thespudcan foundations of the jack-up platform penetrate deeply into the soil, andcomplete or partial backflow occurs. In recent years, a number of studies haveinvestigated the performance of spudcan foundations in soft clay to improve therelevant recommendations in the industry guideline published by SNAME. In thispaper, a brief review of recent research in this area is first provided. Then,a force-resultant model that is suitable for performing integratedsoil-structure analysis is proposed. An example of the application of thismodel is finally provided, and important comparisons with the SNAME model aredrawn.
Although they were originally designed and built for shallow waters, mobilejack-up platforms are now more broadly used for offshore drilling activities.Due to the development of the offshore oil and gas industry, the demand forjack-ups to operate in deeper waters and harsher environments has increased,requiring improved site-specific assessment practices for both economical andsafety considerations. In many offshore areas, such as the Gulf of Mexico, theseabed consists of soft clayey soil, which often features an increasingundrained shear strength profile with a small intercept at the seabed surface.Jack-up installation in such soil conditions often results in significantfoundation embedment; embedments up to several spudcan diameters are common.The SNAME T&PB 5-5A guidelines are often used by the industry to performsite-specific assessments for the suitability of jack-up platform (SNAME,2008). The recommendations in the SNAME guidelines, however, do not reflect themechanisms of a deeply embedded spudcan in soft clay, but they are derived fromthe observed behavior of shallowly embedded foundations. Conservatism in theSNAME guidelines exists and in practice this can result in an unfavorablesite-specific assessment for a jack-up platform. This paper will brieflyhighlight some recent studies on the behavior of spudcans in soft clay andcompare these to the SNAME guidelines. Then, a plasticity force-resultantfooting model appropriate for spudcans in soft clay is proposed. Examplejack-up analyses with the proposed new model are provided, highlighting thedifferences in prediction with this new model.
The reinstallation of jack-up units near a pre-existing footprint is one of thechallenges currently faced by the jack-up industry. Footprints have an unevensoil surface and heterogeneous strength characteristics. As a consequence, thespudcan re-installation is resisted by an eccentric and/or inclined reactionfrom the soil. This results in spudcan and leg tilting that is in turn resistedby the development of a bending moment in the leg, which may compromise thestructural integrity of the jack-up unit.
The paper presents an overview of the research performed at UWA on footprintspudcan interactions, highlighting (i) the relative contributions of thefootprint geometry and soil heterogeneity to the development of bending momentsin the spudcan leg and (ii) the necessity of modelling the spudcan and legfixities correctly to assess the forces generated in the spudcan and leg duringreinstallation accurately.
Purwana, Okky Ahmad (Keppel Offshore & Marine) | Perry, Michael John (Keppel Offshore & Marine) | Quah, Matthew (Keppel Offshore Technology Development) | Cassidy, Mark Jason (U. of Western Australia)
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.