Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 214

Therefore, the objectives of an investigation should always be kept in mind
and the planning effort and selection of equipment and techniques should be
aimed at achieving the objectives at the lowest cost. If there is leeway in sched-
uling of the work, the work can be undertaken in suitable weather; if there is
no leeway, larger equipment may be needed to cope with more severe condi-
tions. The drilling and sampling techniques must be adapted to the marine
environment in order to produce the desired results within a reasonable time.
Once the desired field investigation has been completed, further investiga-
tionisneededinthelaboratorytoevaluateandtoobtainsoilpropertiesand
to apply the field and laboratory results to the design problem.
It is appropriate to mention at this point that a fairly wide range of results
can be obtained, particularly in the field, depending upon several factors. It is
essential that the factors influencing the results be evaluated in interpreting
the data and in applying the results to the design.
Offshore soil investigation provides useful information in connection with
platform installation as well as design. An accurate water-depth determination,
made in connection with obtaining a sample exactly at the seabed, gives the
engineering office the required information, and high-quality samples and
data near the seabed provide very useful information relative to jacket support
before piles are driven, jacket leg penetration below the seabed and soil-pile
interaction under lateral load.
The observations made during drilling and sampling should be recorded,
because they can also provide some indication of potential problems in pile
installation. The scope of work and execution of an investigation should encom-
pass all possible elements important to design and construction.
4.2.1 Performing an Offshore Investigation
An investigation at sea must begin by identifying the location, which is ordinarily
done by a survey boat. After the correct location has been established, a buoy
may be dropped to mark the location for drilling. The survey vessel also assists
in running the anchors for the drilling vessel. In most cases, while drilling and
sampling are in progress, the survey vessel remains in the area to serve as an
emergency stand.
Most offshore investigations done at the present time utilize self-propelled
vessels, usually of the oilfield-supply class. Such vessels have adequate deck
space and can easily be outfitted for drilling. A typical vessel length is in the
range of 40 to 70 m. The vessel is equipped with winches, cables and anchors
for four-point mooring. Anchor lines usually approach about eight times water
depth.
Anchors may weigh as much as, or more than, 2.5 tons, to give reasonable
assurance of maintaining station in adverse conditions of wind, wave, current
and mud-line soils. As much as possible, the bow of the vessel is oriented