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.