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.
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
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.
Hossain, Muhammad Shazzad (U. of Western Australia) | Randolph, Mark Felton (U. of Western Australia) | Safinus, Stefanus (U. of Western Australia) | Cassidy, Mark Jason (U. of Western Australia) | Krisdani, Henry (Keppel Offshore & Marine) | Purwana, Okky Ahmad (Keppel Offshore Technology Development) | Quah, Matthew
Jack-up geotechnical hazards such as unpredicted leg penetration, rapid legpenetration and punch-through continue to occur at an increasing rate despiteefforts by the jack-up industry to minimize these risks. Improvements ininstallation guidelines and site specific assessment are essential to achievesafe jack-up rig installation. Assessment of spudcan penetration is one of thekey aspects required in a jack-up site specific assessment. An accurate spudcanpenetration prediction underpins reliable site specific assessment. In thispaper, current practice for spudcan penetration prediction is reviewed. Designapproaches to obtain a spudcan penetration curve from field penetrometer dataare proposed. The design approaches to predict spudcan penetration are thenincorporated into an integrated jack-up installation system. The systemcalculates the spudcan penetration curve based on penetrometer data and thenclosely monitors progression of spudcan installation. The aim is to assistjack-up operators in making decision on what measures should be taken during anoffshore installation to prevent or mitigate a potential geotechnicalhazard.
Jack-up Installation and Associated Potential Geohazards
Most of the world's offshore drilling in water depths up to 150 m is performedfrom mobile jack-up rigs (see Figure 1). A unique feature of jack-up rigs istheir self-installing capability. This differentiates the design of its spudcanfoundations from most conventional offshore and onshore foundations, as theirperformance has to be reevaluated each time the jack-up installs at a new site.Prior to commencing jack-up operations, spudcans are routinely proof tested bystatic vertical preloading, with each of the (usually) three spudcans installedsequentially or simultaneously. Geotechnically, this increases the size of thetheoretical yield envelope in combined vertical, moment and horizontal loadspace, and thus ensures each spudcan has sufficient reserve capacity in anextreme storm event (SNAME, 2008). For three-legged jackups, preloading isaccomplished by pumping seawater into holding tanks within the hull. Thiscauses the spudcan foundations to penetrate into the seabed until the load onthe spudcan is equilibrated by the resistance of the underlying soil. Thepreload is then dumped and the hull is elevated to provide an adequate air-gapduring subsequent operation.
Several potential geohazards arise due to this method of installation. Hazarddescriptions and statistics collated from reported case histories are providedin McClelland et al. (1981), Sharples et al. (1989), CLAROM (1993), Jack et al.(2001, 2007), Kvitrud et al. (2001), Hunt & Marsh (2004) and MSL (2004).Rapid leg penetration and spudcan ‘punch-through' were identified as the mostcommon geohazards that lead to serious consequences. These incidents mostlyoccur during installing and preloading jack-up rigs in stratified deposits,where a surface or an interbedded strong layer overlays a weaker layer (Figure2). Punch-through may lead to structural damage or failure of the rigs and evenrisk safety of the personnel on board (Aust, 1997; Maung & Ahmad, 2000;Brennan et al., 2006; Kostelnik et al., 2007; Chan et al., 2008). Due to theseinstabilities, the jack-up may also collide with the adjacent fixed platform itis drilling for. Punch-through is defined as a sudden leg penetration due to adrop in bearing capacity (see Figure 2), and where the legs cannot be jackedfast enough to maintain the hull at a horizontal level. The extent to whichrapid leg penetration may be controlled by the rig, avoiding punch-through, hasimproved with advances in jack-up technology and geotechnical knowledge.
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.