This report describes the analysis procedures and the results of a series of model tests performed at the Offshore Technology Research Center (OTRC) in order to measure the current forces and moment on FPSO’s. Both towing tests and current tests with a fixed model are performed.
The towing tests show very high inertial oscillations, disturbing the measurements. Some are caused by imperfections of the basin rails and can be excluded by choosing a suitable time window, but a lot of high frequency oscillations cannot be excluded. This leads to a very high standard deviation of the measurements. The towing tests are performed at three drafts, six angles of attack and seven speeds. Towing tests at the highest speeds show reasonable agreement with earlier publications on this subject, but the low speed measurements are very scattered and inconsistent.
Current tests are performed at one draft, six angles of attack and three speeds. There are far less high frequency oscillations in the measured signal of the current tests and therefore the standard deviation is much better. The measurements during the current tests agree very well with earlier publications and the large scattering and differences at low speed measurements do not occur.
There are strong reasons to believe that the low speed current test measurements are much more reliable than the low speed towing test measurements. With the used test settings and current speeds, the tow tests and current tests are not exchangeable. Therefore the conclusions in this report are mainly based on the current tests and only qualitative conclusions are based on the towing tests.
Identifying the magnitude of different components in the forces is difficult. One conclusion follows from this however; the ITTC-1957 formula for viscous friction drag does not give reliable results at the low Reynolds numbers of those tests.
Average values for the current force and moment coefficients based on the OTRC current tests are given in the concluding chapter. These values agree reasonable with earlier publications on some points, but there are some differences. The major differences are:
- The longitudinal current force can be much stronger than earlier predictions indicate, but there is a large spreading.
- Bilge keels increase the lateral current force coefficient with about 50 percent.
- The asymmetry of the bow and stern of the ship generates a lift force acting on the ship when it is perpendicular to the current, resulting in a large longitudinal current force.