Criteria for good anchor design

Anchor parameters can be scaled from geometrically proportional anchors using the scale rules in table A.

There are several attributes of an anchor which are crucial in assuring its effective performance:

•The anchor must offer a high holding capacity; a result of the fluke area and shank design in combination with penetration and soil type.

•The design of the anchor should be such that the anchor is capable of being used successfully in practically all soil conditions encountered over the world, ranging from very soft clay to sand, corals and calcarenites.

•The fluke/shank angle of the anchor should be easily adjustable, allowing the anchor to be quickly

26deployed in different soil conditions.

•The design must be so conceived and produced that the high loads common in practice can be resisted and that the anchor can be easily handled, installed, retrieved and stored.

•The penetration of an anchor depends upon its shape and design. Obstructing parts on the anchor should be avoided as much as possible.

•The stability of an anchor encourages its penetration and, consequently, its holding capacity. Efficient stabilisers are an integral part of a good anchor design.

•The shank must permit passage of the soil.

•The surface area of an anchor fluke is limited by the required structural strength of the anchor.

•The anchor design must have optimal mechanical strength to fulfil requirements and stipulations of the classification societies.

•The anchor should be designed to ensure an optimum between structural strength of the anchor and holding capacity.

•The anchor should be streamlined for low penetration resistance.

Scale influence

table A

Soil strength is generally expressed in terms of the shear strength parameters of the soil. The soil type is classified mainly by grain size distribution.

In general, the soil types encountered in anchor design are sand and clay (Grain diameter from 0.1 µm to 2 mm). However, mooring locations consisting of soils with grain sizes above 2 mm, such as gravel, cobbles, boulders, rock and such, also occur. Clay type soils are generally characterised by the undrained shear strength, the submerged unit weight, the water content and the plasticity parameters. The consistency of clays is related to the undrained shear strength. However, American (ASTM) and British (BS) standards do not use identical values. The undrained shear strength values Su can be derived in the laboratory from unconfined unconsolidated tests (UU) (table B).

On site the values can be estimated from the results of the Standard Penetration Test (SPT) or Cone Penetrometer Test (CPT). An approximate relation between shear strength and the test values are shown in table C.

Undrained Shear Strength (kPa)

table C

The mechanical resistance of sandy soils is predominantly characterised by the submerged unit weight and the angle of internal friction, ϕ. These parameters are established in the laboratory. An approximate correlation between the angle ϕ and the relative density of fine to medium sand is give in table D.

The undrained shear strength of clayey soil can also be estimated based on manual tests.

•In soft clay the thumb will easily penetrate several inches, indicating an undrained shear strength smaller than 25 kPa.

•In firm (medium) clay the thumb will penetrate several inches with moderate effort, indicating an undrained shear strength between 25 kPa and 50 kPa.

•Stiff clay will be easily indented with the thumb but penetration will require great effort, indicating an undrained shear strength between 50 kPa and 100 kPa.

•Very stiff clay is easily indented with the thumbnail, indicating an undrained shear strength between 100 kPa and 200 kPa.

•Hard clay is indented with difficulty with the thumbnail, indicating an undrained shear strength larger than 200 kPa.

The rock strength can generally be described by its compressive strength (table E).

A classification system for soil based on the carbonate content and grain size of the soil (Clark and Walker), is shown on page 48 of this chapter.