Wind heeling moments for MODUs.
Question: Can I use GHS to calculate wind heeling moments for a Mobile Drilling Unit so that it agrees with the standard methods for MODUs? Answer: GHS uses a wind speed gradient which takes into account the lower speed of wind close to the water. It is calibrated such that the speed is equal to the nominal speed at 10 meters above the water. See the WIND command in the GHS User's Manual for details. The standard method for MODUs applies height coefficients to wind areas. Following is a comparison between the GHS and MODU height factors for wind speed:
Pressures in kg/m2 at 50.00 knots (25.74 m/sec) height MODU_Ch GHS_c(h)^2 pMODU pGHS diff% 10.00 1.00 1.00 41.27 42.34 2.61 46.00 1.30 1.39 53.65 58.85 9.71 137.00 1.60 1.71 66.03 72.32 9.54 244.00 1.80 1.89 74.28 80.01 7.71
The pGHS column is what you get from GHS when you use the WIND command with /CD: 1.0. It is up to 10% conservative relative to the MODU table. (The 2.61% difference is due to the different air densities: MODU uses 1.222 and GHS uses 1.254 kg/m3.) You can reduce the difference by using a CD factor (on the wind command) of about 0.95 rather than 1.0. As an alternative to the wind speed, GHS allows the wind pressure to be given directly as a function of height above water. Following is an example of how this can be done conveniently: `Sets wind pressure profile using Res. 749 height coefficients: MACRO MODUWIND MACRO T SET P = %%92 TIMES {C} TIMES {WS} TIMES {WS} DIV 1000 SET T = "{T} {P} @ %%91" // VARIABLE (STRING) T VARIABLE (REAL) P,C,WS UNIT MT SET C = 0.5 TIMES 1.222 DIV 9.807 SET WS = %1 TIMES 0.5147 `Converts knots to m/sec SET T = "" .T 0.0 1.00 .T 15.3 1.10 .T 30.5 1.20 .T 46.0 1.30 .T 61.0 1.37 .T 76.0 1.43 .T 91.5 1.48 WIND (PRESS) {T} / .MODUWIND 50 `For a 50-knot wind.

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