Wind overturning effect of a semi-submersible has been calculated on the basis of model test results. Measurements were made at four different displacements each at different angles of heel, totaling 13 test conditions.
The model test results are compared with results obtained from application of the ABS-rules for calculation of intact stability requirements.
Our investigation reveals that for the semi submersible in question the wind overturning moment at large angles of heel will, in general, be considerably smaller than predicted by the rules. This implies that the vessel will have a higher permitted KG and hence be able to carry a larger deck load than obtained by empirical calculation methods.
Model testing in towing tank and wind tunnel is an important tool in the process of design and construction of offshore structures and vessels. The extreme environmental conditions, e.g. from wind current and waves that the vessel will meet during its future lifetime can be simulated in model scale. On the basis of the data and observations obtained from the model tests predictions can be made of the full scale vessel's behaviour, seaworthiness etc.
The main purpose of the model tests that provide the basis for the present paper was to acquire wind force and moment data for subsequent analysis of a semi-submersible's behaviour in wind, waves and current.
The semi-submersible has a rectangular deck supported by three columns in each side above two ship shaped hulls.
The vessel is an Emergency and Support Vessel (E.S.V.), which should be able to operate at different draughts under severe weather conditions in the North Sea. In order to get as good basis as possible for calculation of the wind forces and moments, wind tunnel tests were carried out with models of the semi-submersible. Four different loading conditions and angles of heel in the range up to 25 degrees were studied. Influence of wind and direction was also included in the test programme.
Both the above-water part and the under-water part of the model were tested in order to obtain coefficient that could be applied both for calculation of wind forces and current forces. Furthermore the tests with the under-water part provide the correct centre of reaction which should be used in calculation of the total heeling arm.
For each of the four loading conditions of the semi-submersible the heeling lever for a l00 knots wind has been calculated at various heeling angles, both on basis of the wind tunnel test results and on basis of the empirical method prescribed in the ABS-rules. The intact stability requirements have been calculated and the gaining regarding permissible height of centre of gravity has been outlined.
It is a common method in codes of practice for calculation of wind effects to prescribe drag coefficients in non-dimensional form for different geometrical shapes. The wind force at a given wind speed is then proportional to the product of the wind speed squared, the shape coefficients and the projected area.