Criteria for anchor holding capacity

The holding capacity of an anchor is governed by the following parameters:

• The fluke area, which is limited by the strength of the anchor design.

•The penetration of the anchor. The penetration of the anchor is governed by the soil type (deep penetration in very soft clay and shallow penetration in sand), the anchor type (design), the type of mooring line that is used (chain or wire rope) and the applied load.

An increase in fluke area or an increase in the penetration depth of the anchor results in a higher holding capacity.

In the following paragraphs, the influences on the anchor penetration are further clarified.

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Streamlining of the anchor

A streamlined anchor is very important for optimal penetration in the soil. As can be seen in fig. 2-01 and fig. 2-02, an anchor which has protruding parts will encounter much more soil resistance and consequently will not penetrate as deep as a more streamlined anchor with the same fluke area.

Shank shape

A square shank, which is common for most older type single shank anchors, will cause penetration resistance due to the fact that the soil can not pass easily past the shank. A clod of soil will form underneath the shank, effectively increasing the resistance of the soil (fig. 2-03). Bevelling the shank allows deeper penetration.When the single shank is replaced by a twin shank construction (for instance Stevpris, FFTS), usually two thin parallel steel plates, the soil can more easily pass through and past the shank, and consequently the twin shank anchor can penetrate deeper (fig. 2-04).

Mooring line

An anchor connected to a wire rope mooring line will penetrate deeper than the same anchor connected to a chain mooring line (fig. 2-05 and fig. 2-06). This is caused by the higher lateral resistance (penetration resistance) along the chain mooring line. This effect is noticeable in all soil conditions, but especially in very soft clay where very deep penetration can be obtained. The holding capacity of a chain mooring line, due to friction in and on the seabed, is larger than the holding capacity of a wire rope mooring line.

When an anchor reaches its ultimate holding capac ity, i.e. it will not resist any higher loads, at shallow penetration a wedge shaped piece of soil (in front and above the anchor) will fail. The holding capacity of the anchor can then be described as a combination of the following parameters (fig. 2-07 and fig. 2-08):

•The weight of the anchor (A).

•The weight of the soil in the failure wedge (B).

•The friction of the soil in the failure wedge along fracture lines (C).

•Friction between fluke surface and soil (fluke area) (D).

•The bearing capacity of shank and mooring line (E).

•The friction of the mooring line in and on the soil (E).