Iceberg shape characterization for damage assessment of accidental impacts with ships and offshore structures

The northern areas are becoming more and more attractive because off the large oil and gas reservoir that is there. In this area there is a harsh environment and the possibility to collide with an iceberg increases as the number of vessels in the area increases. The project is a master thesis that was trying to find appropriate shapes of the contact area of an iceberg in relation with the ship scantlings that can be used for design against iceberg impacts. This thesis investigated an accident with a return period of 10 000 years, which correspond to an ALS (accidental limit state) type of event. This type of event allows for some plastic deformation in the structure as long as it does not danger human life, have significant pollution or financial consequences. By dividing the calculation in two, first by calculation of the energy in the iceberg relative to the ship and then impose the energy in a collision with the ship. To simulate the collision between the ship and the iceberg there were used non-linear finite element method by using the computer code LS-DYNA 971. The ship structure and iceberg were modeled in PATRAN with shell elements for the ship and solid elements for the iceberg. The icebergs were modeled as a cone with a radius in front where the angle of the cone and radius of the front were varied. The ice material was modeled with a material model that was developed by an earlier Phd. student at NTNU. This corresponds to a shared energy design of the ship according the NORSOK codes three categories. The three categories are ductile design, shared energy design and strength design. The shipside scantlings was also varied with a ship without ice strengthening, a 1C ice strengthened ship and a 1A ice strengthened ship. The ship structure was limited to only a section of the shipside modeled with an elasto-plastic material model. In the analysis the iceberg was pushed into the shipside with a constant speed and the forces, deformations and pressures were measured.The result was showing a tendency of that an iceberg with the largest radius in front equal to half of the web frame spacing and a cone angle higher than about 50 degrees was the most onerous for the shipside, but were not tested any icebergs with larger radius