Work on structural integrity for semi submersibles exposed to bergy bits - Integrated analysis of ice structure impacts

Abstract

Bergy bits and growlers (i.e., glacial ice features with a waterline diameter < 15 m) travelling with waves are identified as risk to the integrity of offshore structure and ships operating in the high North. It is generally more difficult to detect and monitor these small ice features and apply concurrent ice management operations. Therefore, it is important to study the corresponding damage assessment of a potential impact with such glacial ice features. Traditionally, while carrying out a structural damage assessment, the ice feature is often treated as a rigid body, whereas structural deformation and fractures are simulated with, e.g., NonLinear Finite Element Method (NLFEM). However, this method is too conservative and neglects that the ice feature crushes and dissipates part of the impact energy (often obtained with the so-called external mechanics). On the other hand, it is possible to treat both the ice and the structure deformable, and a fully coupled simulation can be carried out with NLFEM. This method can capture the physical essence that the impact energy dissipation is shared by both ice crushing and structural deformation, thus leading to a less conservative structural damage assessment. However, this method is often too computationally expensive and can only take care of the so-called 'internal mechanics' part. In this paper, we propose an integrated approach. This approach is mainly based on the innovative Simulator for Arctic Marine Structures (SAMS) to calculate the impact energy following the principle of 'external mechanics'. Afterwards, the simulated results are integrated with NLFEM simulations to re-construct the shared impact energy dissipation between both the ice feature and structure. A case study regarding a glacial ice feature (diameter = 15 m) impacting a semi-submersible is carried out in this paper. This integrated simulation approach (i.e., SAMS + NLFEM) is proved to be significantly more effective than the fully coupled analysis; more versatile and less conservative than the limiting scenario analysis when dealing with the impact between ice features and man-made structures.