A laboratory investigation has been made of the effect of. surface roughness on the force experienced by vertical surface piercing cylinders in waves. It was found that the wave force was increased significantly and a simple analysis using the Morison equation indicated that the drag coefficient had been increased by around 60%. It is suggested that these results are relevant to full scale structures.
Offshore structures suffer a prolific accretion of marine growths on their submerged members, especially on those just below the sea surface where the velocity and acceleration of the water particles is greatest. This has two consequences as far as the forces experienced by the members are concerned: Firstly, the gross roughness increases the diameter, D, of the member thereby increasing the wave force, F, according to the Morison equation:
(Mathematical equation available in full paper)
Secondly, the roughness may alter the flow field around the member by changing the behaviour of the fluid. boundary layer. This. will result in changes of the drag and inertia coefficients of the Horison equation (Cd, Cm respectively). The drag coefficient of circular cylinders in steady uniform lows is known to be dependent on the surface roughness (see eg 1,2) and Sarpkaya (3) has shown that both coefficients, Cm and Cd, are affected by a change in the surface condition in plane oscillatory flows. There is, however no known existing information regarding the effect of surface roughness on the force experienced by cylinders in waves, as the results published of previous laboratory studies have always related to smooth cylinders.
In the present study an attempt was made to simulate rigid marine growths such as barnacles and limpets by attaching garnet paper to the surface of a cylinder; the forces it experienced in waves were then compared with those of a smooth cylinder. The effect of flexible growths such as seaweed and anemonies is also of interest but was not included in this study.
The experiments were conducted in the NMI No 2 Towing Tank, Teddington, UK, which was 2.26 m deep and 6.1m wide.
As one of the aims of the study was to obtain, if possible, results which would be applicable to full scale structures in the wave loading regime where the drag component dominate, it was necessary to achieve high Reynolds numbers and high Keulegan- Carpenter numbers simultaneously. These numbers both increase with wave height and therefore the waves were generated by towing an inclined flap, immersed to a depth of 5cms, at velocities up to 4m/s (Hogben (4)), to enable greater wave heights to be produced than would otherwise have been possible in any other NMI facility at that time. The flap was brought to rest part of the way down the tank leaving the : associated wave-train to continue to the experimental working section.
