Civil and Building Engineering.

The initial step in an offshore investigation is to refer to a detailed map of the topography of the sea bed. Such a survey will be available from the owner of the site. In addition to the location of the structure to be constructed, the topography of the sea bed in the general area should be studied. There may
be evidence of the existence of stream beds that have been refilled with soft material. A further step would be to review geophysical tests that have been performed at the site. In the North Sea, it is customary to use high-power sparker profiling with a spacing of 200 to 1000 m (Andresen et al., 1978).

An investigation of the character of the soils at an offshore site usually requires a special vessel, such as that shown in Figure 4.14. The daily cost for the drilling vessel is substantial, so double crews are necessary to allow the subsurface investigation to continue around the clock. A weather window is necessary to allow the vessel to stay on site for the few to several days required to complete the work on site.

Over the several decades that offshore borings have been made, procedures have been developed to allow the work to proceed with dispatch. Each rig includes a mechanism, as shown in Figure 4.14, to compensate for the heave of the ship. The ship goes to the drill site and is anchored in multiple directions; a casing is then placed on the ocean floor, and soil exploration proceeds.

Drilling is accomplished through the drill string, and drilling fluid may be used to maintain the excavation if necessary. The common procedure is to lower a sampling tool with a wire line; the wire line can raise a weight, and
the tool is driven into position. The end of the drill string and the sampling tool are shown on the left-hand side of Figure 4.15 (Richards and Zuidberg, 1983). The sample is retrieved by lifting the wire line, and the borehole is advanced by drilling to the next position for sampling.

An alternative method of sampling is shown on the right-hand side of Figure 4.15. The wire line lowers a hydraulic piston and sampling tube. The tool can be activated from the drill ship, and the thin-walled sampler is pushed into place. A number of other tools may be lowered; however, the hammer sampler is the most common because of the extra work in latching more complex tools to the bottom of the drill string.
Emrich (1971) reported on a series of studies aimed at comparing the results of the properties of clays obtained by various techniques in borings done to a depth of 91 m at an offshore site near the Mississippi River. Samples were taken using a hammer sampler with a diameter of 57 mm, with a hammer sampler with a diameter of 76 mm, with an open-push sampler with a diameter of 76 mm, and with a fixed-piston sampler with a diameter of 76 mm. Unconfined compression tests were performed on samples from each of the three methods of sampling. In addition, field vane tests were performed, and miniature vane tests were performed at the ends of some of the samples within the sampling tubes.

The data were analyzed by plotting the results on the same graph, with the results from the fixed-piston sampler being taken as the correct value. Compared to results from the fixed-piston sampler, the following results were obtained: 57-mm hammer sampler, 64%; 76-mm hammer sampler, 71%; open-push sampler, 95%. The results from the vane tests were scattered but were generally higher than those from the fixed-piston sampler. Sampling disturbance can explain the lower values of shear strength for some of the
methods described above. To reduce the effect of sampling disturbance, some investigations subjected the specimens from the hammer sampler in a triaxial apparatus to a confining pressure equal to the overburden pressure. This procedure could be unwise if the soil at the offshore site is underconsolidated, as are many recent offshore deposits. Many offshore investigations are performed at a site where the loads of
the piles have been computed and the borings are to be carried deep enough so that information is available on the required penetration of the piles, employing the guidelines of the appropriate governing authority, such as the American Petroleum Institute (API, 1987). Because offshore platforms are frequently designed to sustain horizontal loads during a severe storm, some piles will be subjected to large tensile loads. The exploration must alert the construction managers if soils at the site may be impossible to penetrate, even
with a very-heavy-impact hammer, so that proper procedures can be in place to allow construction to proceed without undue delay. Factors involving soil properties that are critical to the design of piles under
axial loading are numerous. The reader is referred to Chapters 10, 11, 12, 13, and 14 for further reading.