Deep sea motion under higher sea states

Abstract

Surface waves, induced due to different sea states, greatly affect the dynamics and control of the vehicles operating undersea or on the sea surface. In certain missions involving longer operation time, vehicle may encounter disturbances that are induced by higher sea states, forcing to guide the vehicle to safer depths. In the moderate sea states, taken up to sea state 3, these disturbances are accounted for the motion closer to the sea surface and considered negligible as vehicle moves down to few meters. In case of a rough and higher sea states, disturbances may be experienced even down to hundred meters. This paper attempts to provide a three-dimensional generalization of disturbances in deep sea operating vehicles, by simulating their 6dof motion under higher sea states. A sea state model is realized in terms of inertial forces and moments that vehicle would experience during the motion. An analytical formalism is derived to estimate the induced forces and moments integrated over a given vehicle arbitrarily oriented in the motion. Three limiting cases are considered in this work: (1) the deep water waves: 0.5 < ho/λ <∞ (2) the intermediate depth waves: 0.05 < ho/λ < 0.5 and (3) the shallow water waves: 0 < ho/λ < 0.05. For illustration, derived forces and moments are applied to a well known autonomous underwater vehicle (AUV) known as REMUS (Remote Environmental Monitoring Unit) taken as reference vehicle for the analysis. Slender shape of REMUS closely approximates the known vehicles like submarine, remotely operated vehicle (ROV) and unmanned ocean vehicle (UOV) to which these results are applicable. Numerical results show that in case of deep water wave at lower sea state, the disturbance no longer remains significant after certain depth. On the other hand in shallow and deep water wave case at higher sea states, the disturbance is found significant, affecting the dynamics of the underwater vehicles, down to larger depths of operation. Deep water wave case is further taken up for detailed study of vehicle motion in three dimensions.