Hossain, Muhammad Shazzad (The University of Western Australia) | Ngo, Vinh Triet (The University of Western Australia) | Kim, Youngho (The University of Western Australia) | Cassidy, Mark Jason (The University of Western Australia)
This paper reports a measure for easing spudcan-footprint interaction issues, with the efficiency of perforation drilling tested through model tests carried out at 1g. The soil conditions tested simulate a moderate seabed strength profile close to the mudline, with undrained shear strength of 25 kPa. The most critical reinstallation location of 1D (D = spudcan diameter) was investigated. An existing footprint depth of 0.66D was considered, with the evidence from a half-spudcan test. A ‘shower head’ apparatus was employed to perform perforation drilling operation on an optimized pattern. Three spudcan tests were conducted on un-perforated footprint, perforated footprint and level ground. The spudcan was rigidly connected to a leg instrumented by three sets of bending strain gauges and one set of axial strain gauges to record the vertical load applied and the bending moment generated during testing. Free lateral displacement of the leg was allowed during the installation. The removal of soil (through perforation) inside the spudcan perimeter, with an area of 9% perforated, reduced the induced maximum moment and horizontal sliding distance by 39% and 23~52% respectively. This was supported by the observed failure mechanisms: catastrophic sliding failure turned to a near vertical penetration.
‘Mobile’ Jack-Up Rig and Spudcan-Footprint Interaction Issues
Most offshore drilling in shallow to moderate water depths (< 150 m) is performed from self-elevating jack-up rigs due to their proven flexibility, mobility and cost-effectiveness (CLAROM, 1993; Randolph et al., 2005). Today’s jack-ups typically consist of a buoyant triangular platform supported by three independent truss legs, each attached to a large 10 to 20 m diameter spudcan. After the completion of the task, the legs are retracted from the seabed, leaving depressions (referred to as a crater or ‘footprint’) at the site, as shown in Figure 1. Jack-ups often return to sites where previous operations have left footprint in the seabed. This is, for examples, to drill additional wells or service existing wells; installing structures such as jackets, wind turbines (Killalea 2002; Osborne and Paisley 2002; InSafeJIP 2010).
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.