Permanent anchors::Anchor


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Permanent anchors These are used where the vessel is permanently or semi-permanently sited, for example in the case of lightvessels or channel marker buoys. The anchor needs to hold the vessel in all weathers, including the most severe storm, but needs to be lifted only occasionally, at most – for example, only if the vessel is to be towed into port for maintenance. An alternative to using an anchor under these circumstances, especially if the anchor need never be lifted at all, may be to use a pile driven into the seabed.

Permanent anchors come in a wide range of types and have no standard form. A slab of rock with an iron staple in it to attach a chain to would serve the purpose, as would any dense object of appropriate weight (for instance, an engine block). Modern moorings may be anchored by sand screws, which look and act very much like oversized screws drilled into the seabed, or by barbed metal beams pounded in (or even driven in with explosives) like pilings, or by a variety of other non-mass means of getting a grip on the bottom. One method of building a mooring is to use three or more conventional anchors laid out with short lengths of chain attached to a swivel, so no matter which direction the vessel moves, one or more anchors will be aligned to resist the force.

Mushroom anchor

Mushroom anchor on the lightship Portsmouth in Virginia.

The mushroom anchor is suitable where the seabed is composed of silt or fine sand. It was invented by Robert Stevenson, for use by an 82-ton converted fishing boat, Pharos, which was used as a lightvessel between 1807 and 1810 near to Bell Rock whilst the lighthouse was being constructed. It was equipped with a 1.5-ton example.

It is shaped like an inverted mushroom, the head becoming buried in the silt. A counterweight is often provided at the other end of the shank to lay it down before it becomes buried.

A mushroom anchor will normally sink in the silt to the point where it has displaced its own weight in bottom material, thus greatly increasing its holding power. These anchors are only suitable for a silt or mud bottom, since they rely upon suction and cohesion of the bottom material, which rocky or coarse sand bottoms lack. The holding power of this anchor is at best about twice its weight until it becomes buried, when it can be as much as ten times its weight.<ref>Moorings. INAMAR.</ref> They are available in sizes from about 10 lb up to several tons.

Deadweight anchor

This is an anchor which relies solely on being a heavy weight. It is usually just a large block of concrete or stone at the end of the chain. Its holding power is defined by its weight underwater (i.e. taking its buoyancy into account) regardless of the type of seabed, although suction can increase this if it becomes buried. Consequently, deadweight anchors are used where mushroom anchors are unsuitable, for example in rock, gravel or coarse sand. An advantage of a deadweight anchor over a mushroom is that if it does become dragged, then it continues to provide its original holding force. The disadvantage of using deadweight anchors in conditions where a mushroom anchor could be used is that it needs to be around ten times the weight of the equivalent mushroom anchor.

Screw anchor

Screw anchors can be used to anchor permanent moorings, floating docks, fish farms, etc. These anchors must be screwed into the seabed with the use of a tool, so require access to the bottom, either at low tide or by use of a diver. Hence they can be difficult to install in deep water without special equipment.

Weight for weight, screw anchors have a higher holding than other permanent designs, and so can be cheap and relatively easily installed, although may not be ideal in extremely soft mud.

High-holding-power anchors

There is a need in the oil-and-gas industry to resist large anchoring forces when laying pipelines and for drilling vessels. These anchors are installed and removed using a support tug and pennant/pendant wire. Some examples are the Stevin range supplied by Vrijhof Ankers. Large plate anchors such as the Stevmanta are used for permanent moorings.

Anchor sections
Intro  Overview   Evolution of the anchor    Small boat anchors    Permanent anchors    Anchoring gear   Anchor warps   Anchoring techniques   In heraldry  See also  References  Bibliography  Further reading  External links  

Permanent anchors
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