The symmetric dynamic behaviour of two types of Very Large Floating Structure (VLFS) is investigated. The structures are of pontoon (or mat like) and semi-submersible type and have the same beam, length and displacement. The responses for these stationary and free-floating structures in regular head waves are investigated using the three-dimensional hydroelasticity theory, applicable to structures with arbitrary shape. The “dry analysis” is carried out by discretising the structures using beam and shell finite elements, as appropriate. The solution of the fluid-structure interaction problem is achieved through a pulsating source distribution whereby the mean wetted surface of either structure is discretised using four-cornered panels. The symmetric dynamic characteristics of both structures are compared, both in vacuo (e.g. natural frequencies and mode shapes) and in water (e.g. generalised added mass and hydrodynamic damping). Predicted responses such as vertical deflections and direct stresses, in regular head waves, are also discussed and compared.

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