Rigid Seabed

The pressures calculated using the above approaches generally use the linear (Airy) theory and expect the pore pressure response to be in phase with the surface waves. This is in agreement with what has been measured for sandy bottoms. In contrast, measurements made by Hirst and Richards (1978) showed that bottom pressures can be much larger than predicted by linear theory for soft clayey bottoms. In Sections 9.6.3.1 and 9.6.S.2 the case of a rigid seabed (i.e., sandy bottom) will be considered first followed by the case of a deformable seabed (i.e., soft clayey bottom). 

Seed and Rahman (1978) developed a procedure for evaluating the magnitude and distribution of wave-induced pore pressures in ocean floor deposits. The generation of excess pore pressure in terms of the number of cycles Ml required to cause initial 

Relationship between wave characteristics and -induced shear stress. (From Seed, H.B., and M.S. Rahman, Marine Geotech., 3,123-150,1978. Reprinted with permission of Taylor Francis Group.) 

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Tension leg platforms (TLP) are used mainly in deep water where rigid platforms would be both vulnerable to bending stresses and very expensive to construct. A TLP is rather like a semi-submersible rig tethered to the sea bed by jointed legs kept in tension. Tension is maintained by pulling the floating platform down into the sea below its normal displacement level. The legs are secured to a template or anchor points installed on the seabed. 

The screw-in, auger, or helical anchor uses one or more single-turn helical plates attached to a rigid shaft to screw itself into the soil. This anchor system was developed for use on land to anchor guy wires for towers and poles. It has had limited use on the seabed to anchor pipelines and cables. Anchor penetration is dependent upon reaction force and torque from the drive unit in addition to soil strength. 

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