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Collaborating Authors
Offshore Facilities and Subsea Systems
Abstract 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. Introduction Jack-up units are commonly used for oil or gas exploitation in water depths upto 120 m. They consist of a buoyant triangular hull connected to threeindependent truss-work legs with a conical shape foundation (known as aspudcan) at the base. During installation, the legs are lowered into the seabedindependently, usually one after another. The loading process includes stagesof preloading where additional load (in comparison to the in-service load) isapplied on the spudcan. Once the drilling or work over is complete, the jack-upunit is removed, leaving footprints on the seabed, which may be up to 10 m deepand 30 m wide in soft clay. The reinstallation of jack-up units nearby pre-existing footprints is one ofthe challenges currently faced by the offshore oil and gas industry (Figure 2). During the installation process, vertical load is applied directly though thecentre of the spudcan. In the case of installation near a pre-existingfootprint, where the soil surface is uneven and may exhibit heterogeneousstrength, an eccentric and/or inclined reaction from the soil may be applied tothe spudcan, which will tend to cause tilting of the spudcan and lateral motionof the leg. These motions are resisted by the development of a bending momentin the leg and at the hull-leg connection, potentially threatening thestructural integrity of the jack-up leg. Although this problem has been clearly identified, there are still noguidelines to assist operators in a safe reinstallation, aside from therecommendation to monitor leg loads via the rack phase difference duringinstallation and to create an even soil surface over footprints. There isconsequently a need to understand the parameters governing thespudcan/footprint interaction and to be able to predict forces generated in thespudcan leg during reinstallation. Reinstallation scenario and problem relevance Due to their mobility, jack-up units may be moved relatively easily from onelocation to another before coming back to an existing location to drilladditional wells or to enhance the production of existing wells. During theextraction process, each leg leaves a depression on the seabed, known as afootprint, of highly disturbed strength, and with geometry varying with thesize and shape of the penetrating spudcan and the characteristics of the soil. Figure 1a presents the result of a seafloor survey undertaken after jack-upremoval that highlights the presence of footprints. A statistical studyundertaken by Berg (2004) indicated that there were approximately 1200footprints within the Shell EP Europe at the time of the survey.
- Europe (1.00)
- North America > United States > Texas (0.28)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems (0.69)
- Data Science & Engineering Analytics (0.46)
Abstract 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. Introduction 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.
- Geology > Mineral > Silicate > Phyllosilicate (1.00)
- Geology > Geological Subdiscipline > Geomechanics (0.96)
- Reservoir Description and Dynamics > Reservoir Characterization (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Platform design (1.00)
- Well Drilling > Drilling Equipment > Offshore drilling units (0.88)
Comparison of ISO 19905-1 Framework and a Plasticity-based Spudcan Model for Jackup Foundation Assessments
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)
Abstract 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.
- Well Drilling > Drilling Equipment > Offshore drilling units (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Platform design (1.00)