Abstract For a self-elevated jack up unit, localized failure and collapse may take place during installation phase under severe sea state conditions。 In order to minimize the loss, an innovative method is proposed and incorporated in finite element analysis to predict the structural behaviours before construction。 With a full model of a jack-up unit being built, numerical simulations could be carried out under different load and boundary conditions。 A lot of information such as displacement and member force etc。 during jacking operations could be obtained from simulation results and these valuable data would be much helpful for re-design of the structure and could be used as guidance for site installation as well。83693

Keywords: Jack-up unit; Finite element method; Numerical simulation; Control algorithm; Seabed fixity; Rack phase difference

1。Introduction

As a very useful and movable tool for drilling oper- ations of oil and gas industry in shallow water (i。e。 less than 100 m deep), jack-up units have been used widely for several decades。 In general, a jack-up structure consists of a hull and three K-lattice legs resting on spudcan footings。 Each leg has three chords。 When the structure is towed to site and preloaded to the desirable penetration, the hull is then jacked up。

According to the design code [1], attention should be paid to the four particular design conditions such as transit, installation, elevated states and retrieval during conceptual design of a jack-up unit。 A lot of effort has been put on the transit and the elevated states in past years。 Non-linear dynamic behaviours under various wave and current loading conditions were studied by Spidsøe et al。 [2] on an integrated leg-hull system。 With a

* Corresponding author。 Fax: +65-6419-1280。

E-mail address: tanxm@ihpc。a-star。edu。sg (X。M。 Tan)。

two-dimensional non-linear finite element model being built, dynamic responses of an offshore jack-up unit due to environmental loads have been investigated with parametric study by Williams et al。 [3,4]。 Non-linear analysis for elevated jack-up units has been carried out by Cassidy et al。 [5] with constrained new wave meth- odology。 Karunakaran et al。 [6] and Springett et al。 [7] have done a lot of analysis on the full-scale measure- ments obtained from the instrumented jack-up plat- forms。 Graaf et al。 [8] examined structural failure or overturning of jack-up units by physical uncertainty in the extreme environmental loads。 Furthermore, charac- teristics of a jack-up platform operating at two different locations in the North Sea and the effects from uncer- tainty in load and boundary conditions were analyzed by Leira et al。 [9]。 As a result, a lot of important findings have been obtained from these research works。 How- ever, the important installation phase––jacking opera- tions––has been touched rarely by  researchers。

The hull is jacked up by motors through pinions ro- tating along racks on the chords。 The pinions are con- nected to the jack-cases, which are fixed tightly on the

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hull。 Normally, there are four pinions supported by each chord。 The pinions are motivated by motors through gearboxes。 Rack phase difference (RPD), defined as the displacement difference between the averaged position of the pinions on each chord of one leg and that of the lowest pinion group of the same leg, is used to judge the inclination of the leg towards the hull。 If RPDs of one or more legs are too high, adjustment should be performed。 In order to protect the structure from damage, guides, i。e。 upper guides, lower guides and wear plates, are installed for each chord。 Usually, the clearances between guides/wear plates and chords are very small。 As a typical problem, guide-to-leg contact and friction may  occur,  giving  some  clamping  moments  and even