Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 213

Chapter 4
Geotechnical Data and
Pile Design
4.1 INTRODUCTION
To begin the design of any structure, the designer must know the type of soil on
which the structure will be built. So the first step in design of an offshore struc-
ture platform is to perform a soil investigation, but the offshore soil investiga-
tion technique is not like onshore soil investigation.
A fixed offshore platform is defined as a jacket-type structure extending
above water level and supported by relatively long piles, because this type of
structure has been an industry standard for many years. Of course, nowadays
other types of structures are being built and many different designs are being
proposed, as presented in Chapter 1 , and the soil investigation requirements
and its depth depend on the type of structure, since they could vary considerably
for different types of structures.
In a traditional fixed offshore platform, the soil investigation usually con-
sists of a single boring sampled to a penetration below the expected pile-tip
penetration depth.
4.2 INVESTIGATION PROCEDURE
The final design of a fixed offshore platform is based on the best soil field test
data that can be obtained at the exact structure location. Therefore, an accurate
survey should be implemented with the soil investigation. In general, the soil
data are usually needed well in advance of any construction at a site.
The marine vessel and drilling equipment that must be provided for conduct-
ing and supporting the soil investigation should have a mooring or positioning
system and should be capable of drilling and sampling well below the maxi-
mum expected penetration of the piles. Furthermore, offshore soil investigation
is typically more expensive than similar work onshore. This is due to the cost of
the marine vessel itself, as its fee is calculated per day.
Provision should be made for on-site determinations of pile capacity versus
depth for the pile size proposed to be used in the structure. Thus, an adequate
but not excessive depth of investigation can be ensured for the pile load
anticipated.