Optimal Short-Range Routing of Vessels in a Seaway

An investigation of the optimal short-range routing of a vessel in a stationary random seaway is presented. The calculations are performed not only in head seas but also in oblique waves. The evaluation of the added drag is performed by computing the time average wave force acting on the vessel in the longitudinal direction. Subsequently, the added drag is superimposed on the steady drag experienced by the ship as it advances in calm water. In this manner, the fastest path between the origin point A and the destination point B can be evaluated, taking into account operational constraints. To obtain the fastest path between two points, the underlying structure and properties of the maximum mean attainable speed are analyzed. This detailed analysis allows us to demonstrate the fastest path for the problem without any operational constraints to be a straight line. Subsequently, the solution is reevaluated for scenarios where the original optimal path violates at least one of the operability criteria considered. For that case, a fastest path is found to be a path consisting of one waypoint, that is, a two line segment path. In addition to providing a closed-form fastest-path solution for the case of no operational constraints, a bound is provided for travel time error for more general speed functions in the case where a straight line path is followed.

Prediction of Slamming Occurrence on Catamaran Cross Structures

In this paper, we study the problem of wave slamming on the cross structures of both fast ferry type catamarans and ocean going racing sailing catamarans. The emphasis is given to the prediction of the statistical distributions of slamming occurrence and slamming pressure magnitudes in a random seaway. A partly non-linear high-speed strip theory sea-keeping program is used to calculate the vessel motions and the relative motions between any part of the hull and the sea surface, including slamming impact velocity. Impact velocities are classified in 5 groups, and slamming pressures calculated for each group. To calculate vessel motions of heeled sailing catamarans a strip method for an asymmetric multi-hull is developed; the theory and initial results are presented. An investigation into the effect of sail forces on motions and slamming occurrence is also performed. The sail forces are found to be an important factor in predicting motions of sailing catamarans. The procedure proposed in this paper gives the necessary information to estimate the maximum slamming pressures the vessel is likely to encounter and equally importantly the expected frequency of lighter slams, which is useful for fatigue calculations.

MOB Platform Nonlinear Dynamics in a Realistic (Random) Seaway

In this work, our previously developed approach is extended to include parametric excitation. This approach makes use of a closed-form analytic solution, which is exact up to the first order of randomness and takes into account the unperturbed (no forcing or damping) global dynamics. The result of this is that very large amplitude nonlinear vessel motion in a random seaway can be analyzed with similar techniques used to analyze nonlinear vessel motions in a regular (periodic) seaway. The practical result is that dynamic capsizing studies can be undertaken considering the true randomness of the design seaway. The capsize risk associated with operation in a given sea spectrum can be evaluated during the design stage or when an operating area change is being considered. Moreover, this technique can also be used to guide physical model tests or computer simulation studies to focus on critical vessel and environmental conditions which may result in dangerously large motion amplitudes.