Challenges of Ice Model Tests on Moored DP Assisted Structures

Abstract

Abstract The combination of decreasing ice volume and thickness in Arctic regions results in extended activities of the oil and gas companies in these areas. Actually installed offshore structures are mostly fixed or moored ones. Problematic for moored structures especially with a turret position leading to an indifferent encounter angle is the heading of the vessel regarding the ice drift direction. That is why the idea was raised to combine moored offshore structures (such as FPSO's) with a dynamic heading control system. By such a system the heading can be adjusted to the optimal encounter angle of either waves or ice. Thereby the resistances especially due to changes of the ice drift direction shall be significantly reduced. The advantage of this system in comparison to a completely DP controlled vessel is, that the ice loads are mainly taken by the passive mooring system which does not need any fuel and is able to handle with high resistances while only the heading is controlled by the active thrusters. In case thrusters are installed at the front of the vessel which faces the ice, they may help to protect the turret, mooring lines and risers by their wash from ice contact. Additionally the vessel may operate independently in case of an emergency release of the turret due to critical ice loads on the mooring system. To investigate the benefits and challenges of moored and heading controlled vessel a new test set-up for ice model tests was developed at HSVA. The test set-up combines the already existing knowledge for moored structures with the recently developed DP system for model tests in ice. The DP system is a result of the recent research project DYPIC (www.dypic.eu) and is operating in a new designed mode which only controls the heading of the vessel. During testing the mooring system is simulated by lines leading from the turret to an underwater framework. The mooring characteristic (stiffness) is simulated by an individual combination of springs submerged and fixed on the framework. Above the water surface the model is tracked by an optical tracking system which is connected to the modified DP system. The optical tracking system and the submerged framework are fixed to the carriage. The ice drift is simulated by the movement of the towing carriage. The paper will describe the test set-up possibilities for the new test method as well as the experiences with the first ice model tests. Especially the challenges according to the modified dynamic heading algorithm shall be discussed. Based on analysis of the ice model test results the future work on the algorithm with regard to full scale vessels will be pointed out.