Recent contributions of geotechnical centrifuge modelling to the understanding of jack-up spudcan behaviour Christophe Gaudin, Mark Jason Cassidy n , Britta Bienen, Muhammad Shazzad Hossain Centre for Offshore Foundation Systems, University of Western Australia, Perth, WA 6009, Australia article info Available online 17 January 2011 Keywords: Geotechnical engineering Offshore foundations Centrifuge modelling Spudcan Jack-up Soil–structure interaction abstract The paper presents an overview of the recent contributions of centrifuge modelling to the understanding of soil–structure interaction and the development of design and predictive methods in the field of mobile jack-up drilling rig foundations. Both advantages and limitations of the centrifuge methods are detailed and key examples are presented. The benefits provided by centrifuge modelling to the development of analysis methods that are now being used within the jack-up industry are highlighted. To conclude, industry trends and research opportunities are discussed, as is the possible role of the geotechnical centrifuge in finding solutions to these new needs. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction In the offshore oil and gas industry, most drilling operations in water depths up to around 120 m are performed from self-elevat- ing mobile jack-up units. These offshore platforms typically have a buoyant triangular hull, three independent truss-work legs and foundations, commonly known as ‘spudcans’, that approximate large inverted cones. For jack-up installation and removal from site, a rack and pinion system are used to jack the legs up and down through the deck (Fig. 1). Roughly circular in plan, spudcans typically have a shallow conical underside (in the order of 15–301 to the horizontal), some with a sharp protruding spigot. In the larger jack-ups in use today, the spudcans can be in excess of 20 m in diameter, with shapes varying with manufacturer and rig. As an alternative, some jack-ups use a mat support that connects all of the legs together. These have applicability in very soft sediments, because of the increased bearing area of the mat. Jack-up leg lengths are in the order of 100–205 m. Jack-up rigs are self-installing. They are towed to site with their legs elevated out of the water. On location, their legs are lowered to rest on the seabed. Once the jack-up has been positioned, the spudcans are jacked until an adequate bearing capacity exists for the hull to be lifted clear of the water. The spudcan foundations are then preloaded by pumping sea-water into the ballast tanks in the hull. This ‘proof tests’ the foundations by exposing them to a larger vertical load than the spudcan’s proportion of the rig’s self-weight (usually by a factor of 1.3–2). The ballast tanks are emptied before drilling operations begin. During the preloading process, challenges faced by the geotech- nical engineer include an accurate prediction of the penetration depth and ensuring the stability of the jack-up during penetration. Instabil- ities can occur due to eccentric loading of the spudcans by a slope or an existing footprint on the seabed, or by a rapid leg penetration during a ‘punch-through’ failure. In the latter, the spudcan temporarily loses vertical capacity as it punches through a layer of stronger soil into underlaying softer conditions. After the jack-up has been installed, it typically operates at the site for as little as days or as long as a number of years. Engineers must assess the jack-up stability during this operational phase prior to rig installation, with the major issue being capacity under storm loading. During a storm, environmental wind, wave and current forces impose horizontal, moments and even torsional loads on the spudcans, as well as altering the vertical load sharing between the spudcans. Geotechnical engineers must be able to describe the behaviour of spudcan footings to these combined loads. When the jack-up is to be finally moved from the site, the spudcan footings must be removed from the ground. Deep pene- trations can make this operation difficult, with the time to pull the spudcans clear being reported to exceed one month in extreme circumstances. There is an industry need for better understanding of the extraction mechanisms and the development of a more efficient extraction procedure. Before a jack-up can operate at a given location, a site-specific assessment of its installation, operation and extraction must be performed. This on-going assessment is what differentiates the jack-up analysis from that of the conventional fixed platforms and most onshore operations. The ‘‘Guidelines for the Site Specific Assessment of Mobile Jack-Up Units’’ as published by Society of Naval Architects and Marine Engineers (SNAME) has been the accepted as an industry standard (SNAME, 1994, 2008), though Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/oceaneng Ocean Engineering 0029-8018/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.oceaneng.2010.12.001 n Corresponding author. Tel.: + 61 8 6488 3732; fax: + 61 8 6488 1044. E-mail addresses: gaudin@civil.uwa.edu.au (C. Gaudin), mark.cassidy@uwa.edu.au (M.J. Cassidy), britta.bienen@uwa.edu.au (B. Bienen), hossain@civil.uwa.edu.au (M.S. Hossain). Ocean Engineering 38 (2011) 900–914