Wave-Loading Experiments on Elements of Jackup Legs at Large Scale

Abstract

ABSTRACT Results of wave loading experiments on chord sections and isolated bays of two typical jack-up leg designs are discussed. The tests were carried out at a large scale (approx. 1:2 for chords and approx. 1:4 for bays) in regular and random waves with large wave heights (up to 2.4 metres). This large scale is necessary to obtain flow conditions which are representative for full scale offshore conditions. INTRODUCTION Wave induced forces are a major contribution to the overall loading on a jack-up platform. They play an important role in the design of a jack-up platform or in the analysis of existing jack-ups, when their fitness for use at a specific site is considered. While a large amount of experimental data has been gathered on smooth and rough circular cylinders in planar oscillatory flow and in waves, very little information is available fromtests with jack-up legs in such conditions. Itis generally assumed in the jack-up industry that the wave loading on jack-up platforms canbe based on experimental data obtained in stationary flow in wind tunnels. In view of the application of such platforms in deeper water and more severe environmental conditions, this situation is not satisfactory. Therefore, a joint industry project was carried out to improve knowledge and understanding of the wave loading on jack-up legs. The basic objectives of this project were:to determine wave forces on typical jack-up legs under representative wave conditions and with the smallest possible scale effects andto achieve results for engineering applications which have a more general bearing than on the tested legs only. This paper describes some results of wave loading experiments on elements (chords and isolated bays) of jack-up legs, carried out within the scope of this joint industry project. In view of the second objective, smaller elements (chords) were tested beside larger elements (bays) to enable determination of interaction coefficients. Bracings were not tested separately in this programme, because a large amount of literature is already available on circular members in stationary and oscillating flow. For the extrapolation of the laboratory data to full scale offshore conditions, it is essential to obtain these data in post-criticalflow conditions. To achieve the required Reynolds- numbers, the tests were therefore carriedout at a large scale (approx. 1:2 for the chords and approx. 1:4 for the bays) in regular and irregular waves with large wave heights (maximum wave height in irregular wave trains up to 2.4 metres). This paper concentrates on the execution and the results of the wave loading experiments. In future publications, attention will be focused on the composition of elementary results to bay and leg results and the comparison of the composed results with measured results to check on interactions.