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.
As a very useful and movable tool for drilling operations 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 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] and [4]. Non-linear analysis for elevated jack-up units has been carried out by Cassidy et al. [5] with constrained new wave methodology. Karunakaran et al. [6] and Springett et al. [7] have done a lot of analysis on the full-scale measurements obtained from the instrumented jack-up platforms. Graaf et al. [8] examined structural failure or overturning of jack-up units by physical uncertainty in the extreme environmental loads. Furthermore, characteristics of a jack-up platform operating at two different locations in the North Sea and the effects from uncertainty 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. However, the important installation phase––jacking operations––has been touched rarely by researchers.
The hull is jacked up by motors through pinions rotating along racks on the chords. The pinions are connected to the jack-cases, which are fixed tightly on the 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 resulting in some failures.
To analyze a jack-up unit during jacking operations for safety evaluation of a proposed design under the specified conditions, the proposed control elements and gap elements are used for the connections between pinion and chord and the interactions between guide/wear plate and chord, respectively. These two kinds of element are incorporated into finite element analysis with extra control techniques.
The behaviours of a jack-up system during jacking operations vary with the load and boundary conditions. Wind load is one of the factors that affect the system seriously. Sometimes, wind speed is extremely high on the site. Apart from the wind load condition, another important factor is the fixity of the footings. The seabed fixity has been measured, analyzed and discussed by Springett et al. [7], Temperton et al. [10] and Nelson et al. [11] based on the instrumentation data measured on site. Generally, the fixity of a jack-up leg would be somewhere between pinned connection and fixed connection. However, according to the research done by Hoyle et al. [12], Langen [13] and SNAME report [14], legs may even be sliding if the penetrations of the spudcans to seabed are not desirable. Therefore, the effects of wind load and leg fixity on the behaviours of a jack-up unit are investigated.
Innovative numerical analysis approach based on finite element method to predict the behaviours of self-elevated jack-up units during jacking operations has been developed. Since the results such as RPDs, pinions’ vertical reaction forces, leg reactions and member forces are achieved with respect to the whole structural system in time domain, they are more accurate and more reliable than the results from the traditional method––empirical estimation. This development is of great industrial importance that a jack-up unit can be optimized before construction. Additionally, installation can be done smoothly on site with the predicted behaviours under various sea states.
With numerical studies carried out on a practical design, some conclusions are drawn as below: (1) The jacking operations would better to be carried out under low-class sea state; (2) The spudcan of each leg should penetrate to the sea bed as deep as possible during pre-loading to create good fixity and leg sliding should be avoided if possible.
In order to accomplish good quality of a practical design with low cost, more simulations should be done on the sensitivity studies by changing the above-mentioned parameters as well as guide distances, leg stiffness and motor curves.
