Soil Penetration

Soil Penetration is an option for Small Body Hydrodynamic Data.

The soil penetration models apply for both skirted and piled structures.
The soil penetration model is related to a small-volume body, which means that all rotations are excluded for the model.
There are three conditions which are considered in the modeling of soil resistance to seabed penetration, these are:

1. The structure causes no pressure build-up such that the soil resistance against vertical movement is from vertical friction forces and resistance on the skirt/pile tip area.
2. The structure has cells that are enclosed, with small ventilation channels. These channels allow water flow in and out of the cells. Suction may be applied after a predefined time. The net pressure force inside the cells adds to other accelerating forces.
3. As for 1) but the body is allowed to slide horizontally with a friction force in the horizontal direction, depending on the penetrating depth.

For skirted structures, soil resistance to penetration into the seabed is dependent on:

  • The degree of cyclic motion of the structure.

  • How much of the total skirt area is directly in contact with the sediments.

  • Whether the cells within the skirts are completely enclosed or not in soil around the structure’s perimeter.

For model 1 and 2 the penetrating body is arrested in horizontal direction at landing.
This is accomplished by setting the horizontal velocities and the accelerating forces in horizontal direction, equal to zero.
This implies a simplification, no sliding along the sea bed is included.

The body motion will be finally stopped when either:

  • The soil friction equals the net downward force from the structure, or

  • The body reaches its depth of full penetration

The soil penetration models should be combined with depth dependent hydrodynamic coefficients to account for any proximity effects from the sea bed on added mass and damping properties.

The modeling of soil reaction forces includes:

  • Soil buoyancy force (if the soil can be regarded as a fluid)

  • Soil friction and bearing force

For structures with closed compartment also:

  • Pressure forces in undisturbed soil, including suction.

  • Pressure buildup in a fractured compartment caused by hydrodynamic forces related to the underside of the closed compartments. These forces may give additional fractures.

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