The Experimental and Theoretical Study of a Tension Leg Platform in Deep Water

ABSTRACT The offshore industry is going into deep water exceeding 3,000 ft. The tension leg platform Js available for exploratory drilling, supporting production equipment, and work over operations in such deep water. The moored stable platform with a buoyancy exceeding the weight has tensioned vertical anchor cables. The tension leg generally consists of wire ropes with elasticity that cannot be ignored in deep water. The elongation of the anchor cables results in roll, pitch, and heave motions of the structure. These three motions are small and, therefore, have no significant effect on the actual field operations; however, in extremely deep water the vertical motions are not negligible. In order to evaluate the effect of elasticity of the anchor lines in deep water, we made a 1/50 scale model of a tension leg platform that has four vertical cylinders and four anchor lines. Hydrodynamic forces acting on the cylinders were calculated by considering the radiation and wave exciting forces vertically and the drag and inertia forces horizontally. We recognized through the theoretical study and model experiments that more elasticity in the tension cables would result in less tension variation of the cables in motion response by waves, and investigated the nonlinearity of motion response to regular waves under steady state forces such as wind, current and wave drift. INTRODUCTION Floating vessels, such as a drill ship and a semisubmersible drilling rig, experience more heave, pitch, and roll motions as sea conditions become rougher and water depths greater, resulting in costly downtime. However, the tension leg platform, which is held firmly in place by vertical anchor cables, results in less motion response to heave, pitch, and roll even in deep water. This is because the cable tension prevents vertical motions, and acts to lessen horizontal motions. The tension leg-platform concept is adaptable to several specialized types of offshore oil structures such as a production platform and a workover unit for servicing subsea wells. Similarly, the amplitude of the tension Variations between the maximum end the minimum cable tension by a regular wave could change, depending upon the magnitude of the steady state forces. Fig. 7 shows the tension variations of Cables 1, 2, 3, and 4 in Case B, in which the external forces ere horizontal force components of a wave with a period and height of 1.5 seconds and 10 cm, respectively, end steady-state forces. The natural periods of heave, pitch, and surge of the 1/50 size model me shown in Table 1. Cases A, B, C, and D indicate the different property characteristics of the cables as shown in Fig. 5. (There are no legs in Case D). Nevertheless natural periods of surging in Case: A, B, and C are not very different; that is, these periods. not significantly dependent upon the elasticity of the tension cables.