We use the following formula for pipe racks, based on equating a pipe group to a single equivalent pipe. Unfortunately I'm not sure of the original source/reference.
Fw = Cd*p*(dmax + dave*a)
where:
'Fw' is the unit wind load [kN/m]
'Cd' is the drag coefficient for the largest pipe in the group
'p' is the wind pressure [kPa]
'dmax' is the largest diamter in the pipe group [m]
'dave' is the average diamter of the pipe group [m]
'a' is a factor for the number of pipes in the group
1 pipe, a = 0; 2, 0.70; 3, 1.19; 4, 1.53; 5, 1.77; 6, 1.94; 7, 2.06; 8, 2.14; 9, 2.20; 10, 2.24; >10, 2.30