Hydrodynamic loads during the deployment of ROVs

Offshore operators understandably seek to operate remotely operated vehicles (ROVs) for as long as possible and in the widest range of sea conditions. Accurate predictions of the hydrodynamic loads are important at the design stage as well as in operation, particularly during the launch and recovery phases when snatching of the tether may occur. There is some speculation that calculation methods currently advocated in guidelines lead to an over-estimation of the hydrodynamic forces and consequently to unduly restrictive operability constraints. The present paper has measured wave forces on a 1/8 scale model of a widely used ‘workclass’ ROV, as well as on a solid box of similar envelope dimensions, and compared these against Morison's equation using coefficients derived from three methods. It is concluded that simple linear theory using total (substantive) derivatives, together with a Morison coefficient Cm≈1.5, can provide good estimates of the loading even in waves of quite high steepness, perhaps for height-to-wavelength ratios up to 0.08; i.e., in practice, up to wave breaking.