Tether analyses of offshore triceratops under ice force due to continuous crushing

Abstract

Oil exploration and production in ultra-deep ice-infested waters demands an innovative structural form to withstand the ice loads. Offshore triceratops is one such new-generation innovative, compliant platforms, which show good adaptability in extreme sea state conditions even under the combined action of wind, waves, and current. As tethers are the crucial components in compliant platforms like a triceratops, it is of paramount importance to study the effect of dynamic ice loads on tether tension variation. The main aim of this study is to carry out the tether tension analysis under the ice force action in different ice sea states. The ice force time history is developed from the continuous crushing ice spectral model, and numerical studies are carried out using Ansys Aqwa by applying the ice force on buoyant legs. The study reveals that the tether tension variation is less than 10% even under extreme ice sea state. The parametric studies are also carried out to assess the effect of ice velocity, ice crushing strength, and ice thickness on tether tension variation. It is found that the maximum stress developed in the wires of tethers is very less compared to the yield stress of the material, ensuring no tether failure. However, a large number of stress cycles may lead to fatigue failure of tethers. Hence, fatigue analysis is also carried out to assess the fatigue damage and service life of tethers. The results show that fatigue damage increases from normal to extreme ice sea state. The results are also compared with open-water load case. Studies carried out under lock-in ice condition shows the increase in the fatigue damage and reduction in the service life of the tethers.