This paper describes laboratory based experiments to determine drag and inertia coefficients for cylinders covered with marine fouling. Real, rather than imitation fouling has been used in all cases. Various types of hard and soft fouling and long flapping seaweeds have been investigated. The experimental approach, which is new, is also described. The results indicate that marine fouling may cause a greater increase in fluid loading than is generally assumed.
Marine fouling can increase the fluid loading on a jacket structure by more than 100% yet there has been very little experimental work carried out to examine this problem. Some full scale experiments have been undertaken offshore [1, 2] but these are expensive and time-consuming as bad weather is usually needed to obtain the fluid velocities and Reynolds number range of interest. In the laboratory some tests have been done both with real [3, 41 and artificial fouling [5, 6]. Blumberg [3] attached oyster shells artificially to a cylinder which was towed beneath the water surface in a test tank. Unfortunately the cylinder was not sufficiently immersed to prevent a surface wave disturbance being generated which significantly affected the results. Miller [4] tested cylinders with barnacle cover in a wind tunnel. However, all the attendant soft fouling which grows on barnacles was removed. Tests with artificial marine fouling have been carried out in the U.S.A. [6] and also on behalf of Lloyd's Register of Shipping. How well artificial marine fouling represents actual marine fouling is again open to conjecture. In the case of hard fouling such an approximation is probably good, but for soft fouling and long flapping seaweed Father poor. The limited amount of other research that has been undertaken is well summarized by Sarpkaya and Isaacson [7].
Because of the paucity of appropriate data, results from artificially roughened cylinders are usually adopted in design work. Fouling is considered to increase the diameter of the tubular member by a certain amount, 50-100 mm being commonly used allowances, and to increase the roughness giving a drag coefficient (CD) about 70% higher than that for a smooth cylinder [7]. This latter figure, being largely based on the results of experiments with sand covered cylinders, is of necessity a rough and ready "ball-park" value. In the North Sea, marine growth is monitored and when it exceeds the design allowance it is scraped off at considerable expense.
To enable better decisions to be made both at the design and also at the maintenance stage a more accurate picture of how marine fouling affects fluid loading is needed. This paper describes experiments conducted with real fouling, both hard and soft, in the laboratory.
