General trends concerning the wind overturning forces and moments on semisubmersibles are drawn from the results of wind tunnel tests carried out with a number of different semisubmersibles, each in variousloading conditions.
In (1) it was demonstrated thaL for beam wind conditions large discrepances exists between the results from empirical calculation methods as outlined in rules and regulations and results obtained from wind tunnel tests. It was assumed that lift forces were the main reason for this lack of correlation.
The present paper deals primarily with combined heel/pitch for the most cirtical wind direction close to a diagonal direction. Some typical results obtained from wind tunnel tests are presented and the importance of the lift components is demonstrated.
IE has now become a normal practice for many designers and owners of semisubmersibles to carry out wind tunnel tests in order to obtain as exact knowledge as possible to thawind forces and moments that have to be encountered.
It is realized that the empirical calculation methods outlined in rules and regulations cannot possibly cover every conceivable layout without leading to overpessimistic .windforces for many designs. Too conservative estimates of windforces will result in a number of consequences all of which will be on the detrimental side of the cost benefit of the semisubmersible. Tne windforces are important for decisions concerning:
Anchoring systems design. Dynamic positioning strategy. Intact and damaged stability calculations. Deck load carrying capacity In the various codes of practice for calculation of wind effects it is a common method to prescribe drag coefficients in non-dimensional form for different geometrical shapes. The wind force at a given wind velocity is then proportional to the wind speed squared, the shape coefficient and the projected area. The wind moment is the product of the wind force and the arm between the centre ofaction of the wind force and the centre of reaction. This method does not include lift forces. It could be characterized as a two-dimensional way of treating the problem.
Semisubmersibles are, however, three-dimensional bodies typically with front and side areas of almost equal size, and with a large extension in the wind direction. The wind-flow over the body will therefore create a pressure distribution resulting in both a drag and a lift force. Figure 1 illustrates how this flow pattern might look, and where pressure and suction are likely to occur.
The only way of obtaining exact knowledge of the sizes of the lift and drag forces and the resulting moments is by carrying out wind tunnel testswith a carefully scaled model of the semisubmersible. During the last 2-3 years a number of wind tunnel tests have been performed with various designs for different clients at out laboratory. For this presentation some typical results and common trends concerning the wind effects have been extracted from the wind tunnel test results.