The separation of the total motions of a tanker moored in head seas intowave frequency and low frequency components with the purpose of developingsimplified simulation computations is discussed. The relationship between thesemotion components is clarified using results of long duration model tests inirregular head seas. Results of time domain simulations using a simple modelare used to illustrate some of the problems facing the designer of a mooringsystem.


Rational design of mooring systems for permanently moored floatingproduction and/or storage systems requires accurate and statistically reliabledata on the motions of the vessel and the forces in the mooring system.

Tanker-based FPS systems moored in high seas carry out wave frequencymotions and superimposed, low frequency horizontal motions induced by mean andslowly varying environmental forces such as the second order wave drift forcesor wind gusts. Low frequency motions are generally resonant due to the factthat the excitation frequencies coincide with the natural frequency resultingfrom the mass of the vessel and the stiffness of the mooring system. Suchmotion components can be large partly due to the relatively low systemdamping.

The combined high and low frequency motions of the vessel combined with thehydrodynamic forces acting on the components of the system excite forces in themooring system. The design of the mooring system and its components requiresinsight in the frequency content, distribution and extremes of the mooringsystem loads. Data on the system loads can be obtained directly from modeltests carried out with a scale model of the vessel and the mooring system inmodel basins in which all relevant environmental effects can be generated onscale. This generally requires model basins in which it is possible to generatesimultaneously current, irregular waves and wind. Model tests of this kind aregenerally carried out in a later stage of the design process and nowadays areoften carried out to confirm previous design decisions made on the basis ofexisting data for similar systems or based on computations.

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