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.
Wong, Patrick C. (ExxonMobil Development Co.) | Templeton, Jack (Sage USA) | Purwana, Okky Ahmad (Keppel Offshore & Marine) | Hugo, Hofstede (GustoMSC) | Cassidy, Mark Jason (U. of Western Australia) | Hossain, Muhammad Shazzad (U. of Western Australia) | Martin, Chris (University of Oxford)
This paper presents the new foundation assessment provisions in ISO 19905-1.The paper discusses the improvements to spudcan penetration analysis achievedby the refinement of bearing capacity formulations and by adoption of afundamentally new approach to backflow prediction which accounts for thechanges in soil flow regime and the evolving pattern of soil deformation in thevicinity of the spudcan. It also addresses the upgrades made to the SNAME 5-5Aapproach to foundation capacity and foundation stiffness reduction in sand andclay as well as the means used to better account for the effects of deepspudcan penetrations in clay. The change to, and basis for, the use of grossfoundation capacity in some of the calculations are also discussed. Finally,the paper presents the new foundation acceptance checks framework consequent tothe change from available capacity/reaction concept in SNAME 5-5A to grosscapacity/reaction approach in ISO 19905-1.
At the time of this paper submission, it was expected that ISO 19905-1 would beisssued prior to the presentation of the paper at the 2012 Offshore TechnologyConference. Upon the issue of this standard the new foundation assessmentprovisions described in this paper will be required for jack-up site-specificassessments worldwide.
Introduction and Background
The jack-up geotechnical assessment approach in ISO 19905-1 is the result ofthe concerted effort of Panel 4 under the auspices of ISO TC67/SC7/WG7. Thedevelopment of the latest assessment approach by Panel 4, made up ofexperienced geotechnical practitioners in the industry and well-respectedresearchers in academia, spanned almost two decades. Using SNAME TR5-5ARevision 2 as the starting point, the geotechnical assessment approach wasrevised and upgraded based on findings from industry studies and academicresearch projects in understanding spudcan foundation behavior over theyears.
The prior and existing recommended practices for assessment of jack-upfoundations and their performance under storm loading conditions stemmed fromindustry studies in the 1980s and early 1990s. Those recommendations weresubstantially conservative by intention, and they were limited by the researchof the time. As a result they were reliable but, in many cases, overlyrestrictive for jack-up location approvals. Not only are the new foundationassessment requirements of ISO 19905-1 better grounded in current research,they are less restrictive and more realistic while remaining reliablyconservative. This paper provides an overview of five main geotechnical areasin the geotechnical assessment approach, namely:
• Site investigation requirements
• Spudcan penetration and foundation bearing capacity
• Spudcan foundation response under combined load during storm
• Special spudcan foundation considerations (fatigue, earthquake)
• Spudcan foundation acceptance criteria