Ultimate Strength and Capacity Assessment of Ice Class Vessel Operating in Ice

Abstract

In an ongoing project between DNV GL and a customer, the ultimate strength and damage on a ship exposed to ice loading are to be assessed. The vessel has the DNV GL class notation Ice(1A*), but is now considered for operation in more severe ice conditions than it initially was designed for. The purpose of the thesis is to analyze the response of a hull model subjected to different loads and to determine the residual capacities after first yield. Ice properties and numerical models will also be reviewed.

Ice as a material is complex because its properties are strongly dependent on parameters such as strain, strain rate, porosity, salinity, crystal structure and stress-state. Moreover, the load distribution exerted by ice on impacting structures is highly irregular due to brittle failure, including fracturing, crushing and spalling. These properties make ice diffcult to model. The developed numerical models all have their flaws and there are currently no "verifed" material models for ice, despite decades of research. The main issue seems to be their ability to reproduce the irregular load distributions.

Class rules use a simplied approach where it is assumed that all energy will be dissipated by the ice, implying crushing of ice. When exceeding design load level, the hull will start to deform plastically and the residual capacity will be utilized. Ice class vessels are actually expected to experience loads somewhat larger than the design loads, and some deformation is accepted. The rules do, however, only have a single design point at first yield and there are no further requirements on the residual strength of the hull.

A finite element model of a part of the bow was considered. Rectangular, uniform pressure patches of different sizes were applied at different locations of the model. Linear response analyses were executed and the capacity with respect to first yield was determined. The highest stress levels occurred in the stiffeners, and loads centered the farthest away from the bulkheads were found to be the most critical. Linear capacities ranged between 1.3 MPa and 3.7 MPa. It was also found that the rule design loads are similar to the rst yield capacities calculated.

The residual capacities were determined by doing nonlinear finite element analyses. A user defined ultimate strength criterion based on maximum allowable permanent deformations was made. Permanent lateral deformation at stiffener midspan of 0.5 % or 1.0 % of the stiffener length was set as a criterion. The idea was to allow deformations that can be regarded as acceptable, and the strain levels were found to be well below material capacity using these criteria. The hull showed to exhibit ultimate strengths in the range of 1.5 to 3.6 times the linear capacity, depending on the load distribution. Two boundary conditions were considered, and they gave equal results.

Even though the hull exhibits large residual capacities after first yield, it is questioned whether this alone can be used as leverage for allowing the vessel to operate in more severe ice conditions than it was designed for. Ice loads larger than design loads are expected in the first place, so some of this residual strength is taken into account by the class notations. Numerous damages on ice class vessels have been reported, so it is clear that the simplified method used in class notations has its weaknesses.

For future work it is recommended to do analyses with nonuniform pressure patches, which is more realistic and more conservative. Doing long-term full scale measurements is also recommended. A distribution function can be fit to the the measured loads and from this a new design load can be determined using a probabilistic approach. Using the capacities calculated in this thesis, it can be determined whether the vessel can operate in harsher ice conditions than it was initially designed for or not.