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%
ú10.00     1.00     1.00      41.27    42.34     2.61
ú46.00     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} /STEPWISE
  /
  .MODUWIND 50 `For a 50-knot wind.