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Wind Wind is a significant design factor. The wind conditions used in a design should be appropriately determined from collected wind data and should be consistent with other environmental parameters assumed to occur simultaneously. Two methods are generally used to assess effects of wind for design: 1) Wind forces are treated as constant and calculated on the basis of the 1-minute average velocity. 2) Fluctuating wind force is calculated on the basis of a steady component, based on the 1-hour average velocity, plus a time-varying component calculated from a suitable empirical wind gust spectrum. The choice of the treatment depends on the system parameters and goals of the analysis. Either approach may give more severe loads, depending on the system moored and the wind spectrum used. The time varying component in Method 2 is also known as low frequency wind force. Methods for predicting low frequency wind force are still in a state of development. There is a substantial degree of uncertainty in the estimation, particularly in the wind energy spectrum which is derived from measured wind data. Most of the wind data to date are measured on land. The validity of these data for offshore environments is questionable. Some wind data have been collected recently in a few offshore areas, and these data have indicated greater low frequency content than wind data measured on land. Therefore caution should be exercised in selecting the spectrum to ensure that it adequately represents wind energy at the low frequencies typically associated with natural frequencies of moored structures. If a wind energy spectrum cannot be derived with confidence, the first approach of treating wind force as constant may be more desirable. The design wind speed should refer to an elevation of 33 ft (10 meters) above still water level. Wind force Model tests Model test data may be used to predict wind loads for mooring system design provided that a representative model of the unit is tested, and that the condition of the model in the tests, i.e. draft, deck cargo arrangement, etc., closely matches the expected conditions that the unit will see in service. Care should also be taken to assure that the character of the flow in the model test is the same as the character of flow for the full scale unit. Wind force calculation 1) Constant wind force: The steady state force due to wind acting on a moored floating production unit can be determined using the following equation: Fw = CwΣ(CsChA)V2w where:
The projected area exposed to the wind should include all columns, deck members, deck houses, trusses, crane booms, derrick substructure and drilling derrick as well as that portion of the hull above the waterline. In calculating wind areas, the following procedures can be followed: · The projected area of all columns should be included. · The blocked-in projected area of several deck houses may be used instead of calculating the area of each individual unit. However, when this is done, a shape factor, Cs, of 1,10 should be used. · Isolated structures such as derricks and cranes should be calculated individually. · Open truss work commonly used for derrick mast and booms may be approximated by taking 60% of the projected block area of one face. · Areas should be calculated for the appropriate hull draft for the given operating condition. · The shape coefficients can be used. · Wind velocity increases with height above the water. In order to account for this change, the height coefficient, Ch, is included. · The following equation may be used to adjust the wind velocities of various average time intervals: Vt = α Vhr where:
2) Low frequency wind force Wind force can be treated as constant or a combination of a steady component and a time varying component. The time varying component is also known as low frequency wind force. Similar to low frequency second order wave forces, low frequency wind force also induces low frequency resonant surge, sway, and yaw motions. Low frequency wind forces are normally computed from an empirical wind energy spectrum. Low frequency wind and wave forces are normally combined to yield low frequency vessel motions due to both effects.
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