Numerical analysis of steel tubular member response to ship bow impacts

Abstract

Offshore platforms and ship traffic can be close in proximity in some areas. This has become more likely as the maritime traffic as well as the number of offshore platforms has increased over the past decades. Although incidents involving collisions between these two structures have a low chance of occurrence, the effects of a ship impact, when occurs, can result in hazardous scenarios regarding the integrity of the platform. Current risk analysis of jacket installations is very limited when it comes to jacket legs being head-on impacted by vessels. As a result bow forces are usually estimated by assuming rigid offshore structures although it is commonly agreed that this simplification may not lead to accurate predictions of the contact force between ship and platform as the structural deformation of the platform also absorbs a significant amount of impact energy. On the other hand, deformations in the platform members are commonly predicted under the assumption that the ship hulls are strong enough to be treated as rigid. The present paper describes a detailed finite element model developed for impact analysis of merchant vessel bows against tubular members, representative of offshore jacket legs.The model comprises a general supply vessel in the range of 2000 ton-5000 ton displacement and a vertical steel pipe representative of a jacket leg. Nonlinear inelastic responses of both the ship and tubular structures are considered. The numerical results are checked against tests by other authors to verify the accuracy of the model. The verified model is used to perform parametric simulations. Different geometrical parameters such as member length, wall thickness and diameter of the tubular members, as well as the boundary conditions, axial preloading and dynamic aspects such as the impact velocity and the strain rate effects are considered in the analyses to examine the performance of the platform under vessel impact. Based on intensive numerical results, discussions are made with respect to the accuracy of the current code of practice in offshore platform design to resist possible vessel impact.