Dynamics of JBF Arctic drilling unit moored in ice

Abstract

In light of the recent increasing interest in the oil and gas developments in the arctic region, Huisman Equipment B.V. started development of a drilling semi-submersible suited for arctic condition. Model tests were performed by the Krylov Shipbuilding Research Institute to gain insight in the ice forces acting on the structure. Based upon the data gathered during these model tests a mooring system was designed using an ice loading model that was based on the means of the loads measured and assumed an ice-load independent of vessel motions. For a more accurate design of the mooring system and a better understanding of the dynamics, a dynamic model of the ice-loading and moored vessel motions is/was needed. This thesis elaborates on a model that can model the dynamic interaction between the vessel motions and the ice-loads. Based upon this model, the mooring system is optimized for the highest possible ice conditions. During the model tests the model of the JBF Arctic was retained in a fixed position while being towed through the ice. The main interest was in the interaction with level ice at operational draft. Three parameters were varied during these tests: the ice thickness, the ice velocity and the trim angle of the vessel. To model the ice loads, the first step was to create a model for the mean part of the ice loading. A very simple ice model for the mean of the horizontal force had already been made by the Krylov Shipbuilding Research Institute. This ice model only depends on the ice thickness and the ice velocity and is only valid within a certain range. As part of this thesis the model was expanded so that it would be valid for the entire range of parameters which are required during the time domain simulations. Next the transition between bending and crushing was found to be dependent on the trim angle of the vessel. The mean loads in all other five DOF’s were added based on correlations with the horizontal force or as separate entities. Several other ice phenomenon were also added to the model, such as the initial transient interaction between the approaching ice sheet and the vessel and a crude implementation of the interaction with ridges. The second step to model the ice loads, was to add the fluctuating components of the ice loading. An extensive study was done to analyze the frequency characteristics of the data gathered. The frequency characteristics of the analyzed spectrums were random, and no correlation could be found with the variables that were varied during the tests. A flat or white noise spectrum with the correct energy density was used to model the dynamic part of the ice forces. The moored vessel motions itself were modeled with AQWA DRIFT. The AQWA model was coupled with MATLAB to incorporate the mean and dynamic ice forces. Eventually a model was delivered that can be used to model the dynamics of a moored vessel in ice. This model was also used to do a comprehensive study to find the optimal mooring configuration (material type, grade and, the anchor radius, the number of lines and their layout etc.) for the JBF arctic. The final mooring system can handle 3.1m thick ice moving at 0.5 m/s or 2.4m thick ice moving 1.5 m/s.