On the Low-Frequency Surge Motions of Vessels Moored in High Seas

Moored floating structures for drilling, production and/or storage purposes are being installed in ever increasing water depths and in areas where the environmental conditions are also more severe. As a result of the increased water depth, the elasticity of the mooring system increases.With the increase in the elasticity of the mooring, the low-frequency horizontal motions induced by low-frequency second-order wave drift forces also become larger. These low-frequency motion components completely dominate the horizontal motions and as a consequency also the mooring forces. Therefore it is necessary to investigate the low-frequency motions for those situations where greater water depths and more extreme sea conditions occur.In this paper, results are given of an investigation into the low-frequency surge motions of a large storage tanker moored in a linear system in deep water subject to high head seas. Results of model tests are compared with results of computations. For the computations the term wave damping" has been introduced. It will be shown that the wave damping significantly influences the low-frequency surge motions. A good agreement between the measured and computed low-frequency surge motions has been found."