Pile Design and Construction Practice, Fifth edition

418 Piling for marine structures
In recent years many of the offshore oil and gas fields were becoming or have become
exhausted requiring the platforms and base structures to be dismantled and removed to
restrict future pollution of the marine environment. The search for sources of oil and gas has
been extended to deep-water areas where the piled jacket-type structure are not economically
feasible because of the limitations of available construction equipment to operate in
deep water and the associated sea conditions.
In the 1990s and continuing the present day a new outlet for the offshore industry has
arisen in the installation of wind turbines. Offshore wind farms are required, from consideration
of visual intrusion to be located at least 5 km from the shore line, but it has been
possible to find sea areas having water depths sufficiently shallow to permit the construction
of piled foundations for the turbines. The design and construction of wind farms present severe
problems for the engineer which have been reviewed by Bonnett (8.16) and by Ffrench et al. (8.17)
They give examples of wind turbines with rotor diameters up to 90 m, weighing with the
associated machinery some 250 tonne mounted at a height of 70 m above sea level. At
peak wind force conditions the dynamic forces generated by the turbines can act concurrently
with peak wave action on the supporting structure to cause cyclic overturning
moments on the foundations. A dominant design problem is in providing sufficient stiffness
in the combined machinery and foundation system so that its natural frequency exceeds that
of the excitation forces.
It has been possible, with the present generation of wind turbines, to erect them on a
single large diameter pile (monopile) foundation. Tubular steel piles 5.4 m in diameter have
been driven in water depths up to 20 m using equipment of the type shown in Figure 3.7.
Penetration depths of piles are determined from considerations of resistance of the soil to
dynamically applied horizontal and vertical forces taking into account the possibility of
sea-bed scour increasing the overturning moments. The risks of degradation of the soil
around the shaft and beneath the base of the piles need to be assessed. The present limitations
on the size of piles which can be handled and driven by available equipment may require
the use of three piles driven through a sea-bed template carrying a tripod substructure for
the foundations of the next generation of heavier turbines. Ffrench et al. (8.17) describe rotor
diameters of 126 m for turbines of 4.5 to 5 MW capacity. Bonnett (8.16) refers to the unsuitability
of codes of practice for the design of building structures to deal with the problems
involved with wind turbines. He refers to a code used for structures erected in France (8.18) .
Certifying authorities for oil and gas production usually demand a specific safety factor for
a 100-year wave combined with the corresponding wind force and maximum current velocity,
referred to as the design environmental conditions. The maximum forces due to operations on
the platform such as drilling are combined with specified wind and sea conditions, and are
known as the operating environmental conditions. The American Petroleum Institute (8.19)
requires the safety factors on the ultimate bearing capacity of piled foundations not to be
less than the minima given in Table 8.3.
The reader is referred to design and construction recommendations in the current
publications of the American Petroleum Institute (8.19,8.20) and the UK Department of Energy (6.3) .
Construction methods have been described in detail by Gerwick (8.21) .
8.3 Pile installations for marine structures
Where marine structures are connected to the shore, as in the case of a jetty head with a
trestle approach, the piles may be driven either as an ‘end-on’ operation with the piling