Uncertainty in the estimation of fluid loading due to the effects of marine growth

When assessing the reliability of tubular offshore structures a major source of uncertainty is the magnitude of the fluid loading. This is usually estimated using Morison's equation which in turn requires estimates of the water particle kinematics, the size of the tubular members and the magnitude of the empirical force coefficients. This paper addresses the last two aspects both of which are greatly effected by the presence of marine growth. The paper begins by considering the a priori prediction of the type of marine growth which will occur on a North Sea structure and the way in which various types of growth tend to spread over the structure and overlay one another. Data is presented on the mean and variance of the final thickness of the growth and the size of individual species. A review of all the known laboratory and other experiments to estimate the effects of marine growth on the fluid loading of circular cylinders has been undertaken. From these data estimates of the mean and variance of the force coefficients associated with the various types of marine growth are presented and discussed.