Analysis of accidental iceberg impacts with large passenger vessels and FPSOs

Abstract

Nowadays, on account of the rapid increase in the global warming, the extent and thickness of sea ice in the Arctic region is diminishing at a very fast pace. It has even been forecasted that the Arctic region will be ice free in the very near future. Owing to this fact, the Arctic waters are increasingly becoming attractive to different classes of society because of its immense reservoirs of oil and gas, short ship routes in the NE and NW region and attractive tourist places. These activities face the major hurdles from harsh environmental conditions like ice loads, insufficient infrastructure in the Arctic regions and the threats from impact of large ice features.

The ships or offshore structures need to be sufficiently strengthened to resist the extreme ice impacts. However, very few data or models do exist that can quantify the extreme ice actions, and in addition the reliability of those data is open to question. For instance, the ice going vessels or offshore structures are designed based on pressure-area curves, but most of the P-A curves are based on Ultimate Limit State (ULS) design whereas the P-A curves for Accidental Limit State (ALS) are very rare. The work carried out in this thesis aims to study the response of structures subjected to those accidental ice impacts.

The thesis work can be regarded as a continuation of work carried out by Ekaterina Kim in department of Marine technology, NTNU. In addition, considerable improvements and progress have been achieved in quantifying the accidental ice loads in terms of a novel numerical model. Both FEM and coupled FEM-SPH techniques have been efficiently applied in ice modelling and validated against existing Pressure-Area curves. In addition, computationally demanding spatial envelope curves have been plotted using which the existence and spatial variation of high pressure zones most commonly known as HPZs are effectively studied. Moreover, further step has been taken in applying the FEM-SPH ice modelling in large scale impact simulations. In order to resemble accidental ice impacts, the ice has been modelled significantly harder and successfully applied in collision simulations. The pressures corresponding to the hardest ice surpassed the existing analytical curves by a huge margin.

For large scale simulations, FPSO and a large passenger vessel have been chosen. Certain structural scantlings are different between these two structures. The impacts using the modelled hard ice produced extensive deformation on these structures. Furthermore, the ice shape effect in accidental collisions is studied and it turned out that the tabular bergy pit produced massive deformations in structure and it can rightly be considered as the best shape for the analysis concerned with accidental ice impacts. Direct ice impacts seemed to produce more deformation on the structure when compared with oblique ice impacts. The impacts simulated using decoupled approach provides a conservative estimate of force levels and subsequent deformations.

Structures with lesser thickness deformed more during the ice impacts, however lower force levels are recorded. Similar trend has been observed in the simulations performed using lower steel grades. From the coupled collision simulations, it has been noted that the kinetic energy of ice plays a very important role in determining the maximum possible deformation experienced by the structure.

Finally, some simplified analytical formulas have been developed for estimating the crushing of ice and deformation of structures subjected to direct and oblique ice impacts. The developed formulas are successfully applied and validated using the collision data obtained from numerical simulations. In addition coupled FEM-DEM ice modelling in LS DYNA has been proposed and compared with that of FEM and FEM-SPH ice modelling.