Wave                          


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Waves

Wind-driven waves are a major source of environmental forces on offshore production facilities. Such waves are random in nature, vary in height and length, and may approach a platform form more than one direction simultaneously. For these reasons the intensity and distribution of the forces applied by waves are difficult to determine. Because of the complex nature of the technical factors which must be considered in developing wave-dependent criteria for the design of floating platforms, specialized knowledge in the fields of meteorology, oceanography, and hydrodynamics should be applied.

Because of the random nature of the sea surface, a seastate is usually described in terms of a few statistical wave parameters such as significant wave height, spectral peak or significant wave period, spectral shape and directionality. Other parameters of interest can be derived from these. Refer to API RP 2A for a more detailed discussion on seastate representation.

The design significant wave height should e determined based on the design recurrence interval and wave data. The wave data used to determine the design should include available measured data and storm hindcast data as well as ship observations. The wave height versus wave period relationships for the design seastate should be accurately determined from oceanographic data for the area of operation. The period can significantly affect surge and sway amplitudes and mean drift forces.

Wave force

Model tests

Model test data may be used to predict wave forces for mooring system design provided that a representative underwater model of the unit is tested. Care should be taken to assure that the character of the flow in the model test is the same as the character of the flow for the full-scale unit.

Wave frequency vessel motions

The motions of the vessel at the frequency of the waves are an important contribution to the total mooring system loads, particularly in shallow water. These wave frequency motions can be obtained from regular or random wave model test data, or computer analysis using either time or frequency domain techniques. The frequency domain technique, which is most frequently used, involves first determining the response amplitude operator (RAO) as a function of frequency over the full range of wave frequencies. A response spectrum can be obtained by integrating the RAO with a wave spectrum. The response spectrum is then integrated and the square root is taken to determine the rms response. Finally the significant and maximum responses can be obtained by using appropriate distribution factors.

Wave frequency motions have six degrees of freedom: surge, sway, heave, pitch, roll, and yaw. They are normally considered to be independent of mooring stiffness.

Mean wave drift force

The mean wave drift force is induced by the steady component of the second order wave forces. The determination of mean drift force requires advanced motions analysis computer programs or model tests. Design curves for estimating mean wave drift forces for semi-submersibles are provided in API RP-2P

Low frequency vessel motions

Low frequency motions are induced by the low frequency component of the second order wave forces. These forces follow a quadratic law where, in the case of a regular wave, the force is proportional to the square of the wave height. However, in irregular waves, the low frequency motions due to the second order forces will not increase as rapidly with wave height as the forces themselves since the resistance to the vessel motions due to viscous drag and wave periods also increase with wave height. This means that the forces in the mooring system do not increase as fast as the forces on the vessel.

Proportions of previous 3 force components

The drift forces, in general, are quite small compared to the first order forces. Because of this, the drift motions do not play a significant role in the motions in the vertical plane (i.e., roll, pitch, and heave motions), where large hydrostatic restoring forces are present. However, in the horizontal plane (i.e., surge, sway, and yaw motions), where the only restoring forces present are due to mooring or dynamic positioning systems and production risers, the motions produced by the drift forces can be substantial. This is particularly true at frequencies near the natural frequency of the mooring. Therefore in general, only low frequency surge, sway and yaw motions are included in a mooring analysis.

Low frequency motion of a moored vessel is narrow banded since it is dominated by the resonant response at the natural frequency of the moored vessel. The motion amplitude is dependent on the stiffness of the mooring system and the system damping, therefore a good estimate of damping is critical in computing low frequency motions. Methods for predicting the low frequency motions are still in a state of development. There is a substantial degree of uncertainty in the estimation.

       

                                                                     

 

 

 

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Last modified: 10/16/06