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Collaborating Authors
Results
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.
Development of an Integrated Jack-up Installation System
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 | Purwana, Okky Ahmad (Keppel Offshore & Marine) | Quah, Matthew (Keppel Offshore Technology Development)
Abstract 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 (Figure 2). 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.
- Asia (1.00)
- Oceania > Australia (0.94)
- Europe (0.68)
- North America > United States > Texas (0.28)
- North America > Cuba > Gulf of Mexico (0.93)
- Asia > Indonesia > Northwest Java Sea > Sunda Basin (0.93)
- Asia > Taiwan > Taipei Basin (0.91)