Offshore Structure Loads and Strength - Offshore Structures: Design, Construction and Maintenance - page 45

Wind loads on downstream components can be reduced due to shielding by
upstream components. The extreme quasistatic global action caused by wind
should be calculated as the vector sum of the total above wind loads on all objects.
When wind loads are important for structural design, wind pressures and result-
ing local loads should be determined from wind tunnel tests on a representative
model or from a computational model representing the structure and considering
the range and variation of wind velocities. Computational models should be vali-
dated against wind tunnel tests or full-scale measurements of similar structures.
The code equation, which is the same for the different codes, can be used to
calculate the wind force on the structure:

V 10 = wind speed at height 10 m
10 = reference height, m
z=desired elevation, m

Table 2.15 lists some design wind pressures for a 100-year storm with a
sustained wind velocity of 125 mph.
For the wind load on the topside, there are usually a series of beams or trusses
that serve as a shield from the wind. Therefore, according to Table 2.16 (from
API), there is a reduction (shielding) factor for the wind load. For example,

TABLE 2.15 Design Wind Pressures at 125 mph for a 100-Year Storm

Pressure, kN/m 2 (Ib/ft 2 ) Structure Member

Flat surfaces, such as wide flange beams, gusset plates,
sides of building, etc.

2.9 (60)

Cylindrical structural members 2.3 (48)

Cylindrical deck equipment ( L =4 D ) 1.4 (30)

Tanks standing on end ( H ≤ D ) 1.16 (25)

Note, L is the member length, D is the member diameter and H is the cylindrical tank height

TABLE 2.16 Shielding Factors

Component Shielding Factor

Second in a series of trusses 0.75

Third or more in a series of trusses 0.50

Second in a series of beams 0.50

Third or more in a series of beams 0.00

Second in a series of tanks 1.00