In SI units, equation 8.8 becomes

(8.9)

Values of and for different positions relative to the location of the wave
crest are shown in Figure 8.12 and Table 8.1. This table also lists the average values of
and together with the heights to the centroid of the two components. The wave
forces and moments applied to each increment of height of pile projecting above the scoured
sea bed up to wave crest level, and on any underwater bracing or jacket members, are
integrated to obtain the total horizontal force on the pile or group of piles and also the
overturning moment about the point of fixity below the sea bed.
For use with equations 8.8 and 8.9 Newmark(8.7)recommends a value for CDof 0.5 to 0.6
for cylindrical members and 1.5 to 2.0 for the inertia coefficient CM. For rectangular, Hand
Isections CDcan be taken as up to 2.0. Theoretically CDis related to the Reynolds number
Reas discussed in the following section. Newmark also recommends that shielding effects
produced by closely spaced piles or bracing members should be disregarded when calculat-
ing wave forces.
BS6349 (Part 1) draws attention to the effect of impact forces (wave slam) on horizontal
members exposed to the crests of advancing waves.
Barnacle growth on piles and bracings should be taken into account by allowing an appro-
priate increase in diameter. It has been reported(8.10)that marine growths more than 200 mm
in thickness have occurred around steel piles of the North Sea gas production platforms after
about eight years of exposure. The growths extend down to sea bed where the water depths
were about 25 m. If drag forces due to marine growths are excessive, provision can be made
for the members to be cleaned periodically by divers.

8.1.4 Current forces on piles

The velocities and directions of currents (or tidal streams) affecting the structure are
obtained by on-site measurements which should include the determination of the variation
in current velocity between the water surface and the sea bed. A curve is plotted relating the
velocity to the depth and the current drag force is calculated for each increment of height of
the pile above the sea bed. Any scour below the sea bed should be provided for.
Current forces are calculated from the equation:

FD 0.5CDV^2 An (8.10)

The components of the above equation are defined in BS6349 as

FD steady drag force (kN)

CD dimensionless time-averaged drag force coefficients

 water density (tonne/m^3 )

V incident current velocity (m/sec)

An Area normal to flow (m^2 )

1/g· (du/dt)

(u/c)^2

u/c)^2 1/g· (du/dt)(

f 7.8CDhuc

2

 8 CMD·^1 g·du

dt^

kN/m^2

Piling for marine structures 411