Experimental and Numerical Studies of Moored Ships in Level Ice

Abstract

Moored ships are believed to be feasible for marine operations in ice-covered waters. A variety of ice features, such as broken ice, level ice, ridged ice and icebergs, will pose potential challenges with respect to design and operations. This thesis presents studies on the actions of level ice on moored ships and the resulting vessel response. Focus was put on both analysing physical measurements and deriving suitable numerical models. The thesis is thus articulated around three parts: analysis of ice failure modes, vessel response and mooring forces from model tests of a concept called the Arctic Tandem O_oading Terminal;analysis of ice forces and mooring forces from model tests of a moored simplified hull;development of methodology for numerical modelling of moored ships in level ice with constant drift direction. A concept for o_oading of hydrocarbons in ice-infested waters was tested in the Large Ice Model Basin at the Hamburg Ship Model Basin (HSVA). The Arctic Tandem offloading Terminal consists of two vessels; a turret moored offloading icebreaker, and a shuttle tanker moored in tandem at the stern of the offloading icebreaker. Studies of the coupled response of both vessels to ice actions and the resulting tandem mooring forces show that the concept was stable in yaw when the ice drift direction changed slowly in level ice. The response of the offloading icebreaker moored alone in level ice with variable drift direction and the corresponding ice failure modes were investigated with both slow and sudden changes of ice drift direction. The study underlines how ice actions and the ice failure modes depend on the relative angle between the ice drift direction and the vessel's heading, the hull shape and the vessel's response. The test campaign showed that with the present concept, the magnitude of the mooring forces in severe ice drift events can be comparable to those experienced in ridges. Actions from level ice with variable drift direction should therefore be considered as a possible design criterion for moored ships in certain areas.

Model tests of a moored simplified hull in level ice with constant drift direction have been performed at HSVA. The hull was instrumented to measure the local ice actions on the bow, as well as the mooring forces and the surge response. The test setup enabled studies of the dynamic properties of both ice and mooring forces by independently varying the ice drift speed and the stiffness of the mooring system. Average mooring forces and local ice forces on the bow increased with the ice drift speed, except for the softest mooring system, and were highest for the soft mooring system and lowest for the stiff one at all speeds. A semi-empirical method for modelling local ice forces on the bow was developed based on observations from the model tests. Ice actions are split into actions in the vicinity of the waterline, caused by breaking of intact ice and rotation of broken ice oes, and actions below the waterline, caused by ice-hull friction. The method is probabilistic and includes speed dependence.

Numerical modelling of the surge response of moored ships interacting with intact level ice with constant drift direction was performed with two different approaches

for the ice actions: the ice was modelled as an elastic beam on an elastic foundation and applied on a simplified 2D hull design;the above-mentioned semi-empirical local ice force formulation was implemented and applied on a model of the offloading icebreaker. Ice forces depended on the penetration of the ship into the ice and enabled feedback effects of the surge response on the ice actions. Such effects were mainly present at low ice drift speeds and often induced large mooring forces. The local ice force formulation induced mooring forces comparable to those measured in model tests of the same hull. The author believes that the method is suited for studies of the dynamic response of moored ships in level ice with constant ice drift direction and can be extended to level ice with variable ice drift direction.