In a previous paper, Ref. 1, the authors presented typical results of wind tunnel tests obtained with a number of models of semi-submersibles for determination of wind overturning forces and moments.
The present paper deals with wind tunnel tests for the determination of the forces and moments on the underwater part of semi-submersibles or ships exposed to a flow field, such as a uniform current.
The test method described was initially meant as an adequate way of obtaining the position of the centre of the reacting hydrodynamic forces that has to be used in the calculation of wind overturning moments in connection with the stability analysis of semi-submersible platforms. However, the method has turned out to be a relatively fast and inexpensive way to obtain a good estimate of the forces and moments that a uniform current will introduce.
Typical test results are presented in no dimensional form for semi-submersibles and ships. The validity of the test method has been investigated by comparison between results of towing tank tests and wind tunnel tests with models of the same vessels.
The test results reveal that the centre of the hydrodynamic forces, defined as the total moment divided by the horizontal force, is in some cases below the keel.
It is important to have as good a knowledge as possible to the extreme effects of the environmental conditions, i.e. wind, waves and current, that a vessel will have to encounter during its life-time.
Of primary concern in that connection is of course the safety of the vessel and its personnel. The vessel has to be designed and built to survive at all possible modes of operation. In order to be able to plan and predict safe operations as accurately as possible, extensive studies and investigations have to be made already at the design stage. Such studies and investigations often imply carefully planned model tests in model tanks and in wind tunnels.
During the process of model testing, the feasibility of the design is checked, and the influence of possible changes can be tested before the final decisions are made.
On the basis of the data and observations obtained from the model tests, predictions of the full scale behavior can be made by the use of more or less complex computer simulations.
The main purpose of the wind tunnel tests that form the basis for the present paper was to acquire data that could be used for the calculation of forces and moments from wind and current in connection with studies concerning:
Wind overturning moments
Dynamic positioning
Anchoring forces
Maneuvering simulations.
The data presented have been obtained from wind tunnel tests with the underwater models of four different semi-submersibles and two conventional single hull ships. For the semi-submersibles, both conditions of even keel and various conditions of pure heel and combined heel/trim are treated. The two ship models were both tested on even keel. Most of the model tests were performed for clients. For the reason of confidentiality, only scarce information is given about the main particulars of the vessels.
