Offshore Structure Loads and Strength - Offshore Structures: Design, Construction and Maintenance - page 47

Actual stress = moment/section modulus = 8.9 (10 6 )/371 = 24,000 kN/m 2 ●

Unsupported length = 6.22 m ●

Allowable stress (ksi) = 12,000 C b /(member length × depth/area of flange) = ●

1.4)] = 9.35 ksi = 64,466 kN/m 2

12,000 × 1/[622 × 26/(9 × > actual stress

2.4.2 Wind Loads

V 2 Wind loads/one stair channel = C d × ½ × ρ × × ½ stair width ●

C d (drag coefficient) = 2 ●

(air intensity) = 1.3 kg/m 3

ρ

●

V (wind velocity) = 29 m/sec

●

29 2 Wind loads/one stair channel = 2 × ½ × 1.3 × × 0.6 = 655.98 N/m = ●

0.66 kN/m 2

Moment = (wind load/m) × inclined length × projected length/8 =

●

0.66 × 6.22 × 4.4/8 = 0.23 m/kN

Actual stress (gravity + wind) = moment/section modulus = (8.9 + 0.23)
(10 6 )/371 = 24,609 kN/m 2

●

< allowable stress × 1.33

Note that wind loads on a channel in the minor axis direction are neglected
because horizontal bracing is provided.

2.5 OFFSHORE LOADS
Offshore loads that most affect platforms are waves, tide and current. For this
discussion, some terminology related to sea water levels has to be introduced:

High-high water level (HHWL) ●

Mean-high water level (MHWL)

●

Mean water level (MWL), equal to mean sea level (MSL) and still water
level (SWL)

●

Low-low water level (LLWL), equal to the chart datum (CD) ●

Mean-low water level (MLWL) ●

The mean-low water level is less than the mean-high water level by 304 mm (1 ft).
Tides are the rise and fall of sea levels caused by the combined effects of the
gravitational forces exerted by the moon and the sun and the rotation of the
Earth. Most places in the ocean usually experience two high tides and two
low tides each day (semidiurnal tide), but some locations experience only
one high and one low tide each day (diurnal tide). The times and amplitude
of the tides at the coast are influenced by the alignment of the sun and
moon, by the pattern of tides in the deep ocean, as shown in Figure 2.3 ,and
by the shape of the coastline and near-shore bathymetry.
Most coastal areas experience two high and two low tides per day. The
gravitational effect of the moon on the surface of the Earth is the same when it
is at its zenith as when it is at its nadir. The moon orbits the Earth in the same