Civil Engineering Reference
In-Depth Information
Probably the best way to determine water depth is to use a sounding weight
on the small wire line used for sampling and to use a wire-line counter to mea-
sure the length of line. The weight must be adequate to minimize current effects
but must be designed so that it defines the seabed and does not sink into a very
soft bottom. The water depth can be confirmed on the first sampling attempt.
World-wide predictions of seasonal weather conditions and sea states are
available from several sources. Short-term forecasts are also available in most
offshore areas. Predicted tide and current tables can be obtained for almost
any area. All of this information is useful in design planning and jacket
installation.
If the platform is located in an area where significant tide changes occur, it is
necessary to make frequent observations to define the tide cycle and thereby
control the sampling depth.
A plot of time versus water depth can be used to reduce water depth to any
desired datum. The time, date and measured water depth should be recorded at
the start of a boring. Tide variations can sometimes be recorded by a suitable
fathometer.
In areas of very soft, underconsolidated soils, it is necessary to exercise very
careful control of drilling fluid weight to counteract the tendency of the material
to squeeze into the drilled hole and up into the drill pipe. Failure to do so can
result in very severe sample disturbance. Furthermore, in such areas, the prob-
lem of handling disturbance on recovered samples is also quite severe.
Once a boring has penetrated a granular soil formation, it is essential that
drilling mud having suitable viscosity and gel properties be used to stabilize
the drilled hole and to prevent caving; commercial saltwater gel is excellent
for this purpose. Particularly in glacial deposits, coarse granular material such
as cobbles or boulders may be encountered and may make drilling extremely
difficult. The presence of rock formations within the depth of investigation
requires that special tools and procedures be used.
In most cases, gas may be present within formations penetrated by soil bor-
ings and flows of water may also be encountered. The normal procedure of using
a blowout preventer on cased holes is not easily applied to the wire-line method.
Sometimes a large mobile rig may be used to drill at a location where a plat-
form may be installed later, and it may be desirable to make a soil boring from
the mobile rig. If space is available, the boring can be made without interference
with the normal rig activity by placing a soil-boring rig on board. The drilling
and sampling procedures are identical to those used from a floating vessel. The
large rigs, either jack-ups or floaters, provide a relatively stable base for soil-
boring operations.
Sometime the soil boring is performed using a large oilfield rig. If the soil-
boring rig cannot be accommodated, it is possible to use the large drilling rig
and its crews, working under technical supervision, to make the soil boring.
The large mobile rig and its support equipment may cost $25,000 to $50,000
per day; therefore, the soil boring may be quite expensive. This approach should
Search WWH ::
Custom Search