Anchoring Systems

Marine Outfittings Prof. Dr. Yousri Welaya

Anchoring Systems

An anchor system provides a simple reliable means of safely holding a ship in a

relatively fixed position in shallow water without the use of the ships main or

secondary power source. The anchor system may also be used to assist in

stopping or controlling the ship in an emergency to avoid grounding or collision.

The anchor system consists of an anchor, made of cast steel with forged steel

fittings, shackled to a chain which is engaged by a chain sprocket driven by a

windlass; this may be powered electrically, hydraulically, or by steam. When the

anchor is retrieved it is normally stowed in an anchor pocket or against a bolster

surrounding the hawse pipe with the chain stowed in a chain locker.

Two anchors are usually deployed when the swing circle of a ship using a single

anchor is too large for the available anchorage. The radius can be reduced by

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using more than one anchor. The anchors are placed well apart so that a line

drawn between the two anchors is perpendicular to the current.

Anchor Characteristics

The capability of an anchor to dig into the bottom when dragged depends on the

shape of its flukes, the angle between the flukes and the shank, and the fluke

tripping arrangement. Anchor efficiency is measured as follows:

To obtain maximum holding power the load is applied through the chain so that

the anchor shank remains parallel to the bottom. This condition is obtained by

using an anchor chain of sufficient length and weight for a given depth to assure

that the chain describes a catenary parallel to the bottom at the anchor shackle

when the pull of the chain is equal to the maximum holding power of the anchor.

The essential characteristics of an ideal anchor can be summarized as

follows:

Self Orientation

The anchor should engage the sea bed rapidly and orientate into its normal burial

attitude, irrespective of drop attitude or nature of sea bed (e.g. sand or mud).

Short Scope Capability

The anchor should be capable of engaging, orientating, and burying at long or

short scope. If the anchor can operate with a high cable angle at the sea bed,

less cable is needed or, alternatively, anchoring is possible at greater water

depth for a given length of cable.

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Deep Burial

Since holding power is a direct function of the depth of burial of the fluke of an

anchor below the sea bed surface, the anchor should penetrate as deeply as

possible to achieve the highest holding power. Penetration resistance should

also be low.

Low Breakout Force

It should have a breakout force which is as low as possible compared to its prior

holding pull. This is between 50% and 100% for conventional high holding power

anchors.

No Moving Parts

Jamming of pivoting fluke anchors may result in failure to bury and develop high

holding power.

No Stock

The stock is an easily damaged part of many high holding power anchors which

may lead to fouling of anchoring and pennant lines. It offers high resistance to

sea bed penetration and may produce handling and stowing difficulties. If the

stock becomes damaged, it causes loss of stability at the sea bed surface

resulting in loss of holding power.

Types of Anchor

Ships bower anchors are usually one of two general types; stock or stockless

anchors. In each of these categories there are a large number of variations.

1. The stockless Anchor

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When a variety of sea bottoms are expected, to be encountered, ships turn to the stockless anchor for reliability and versatility. It has a relatively short shank. The flukes are allowed to swing about 45o to either side of the shank to permit anchor to dig into sea bottom. The stockless anchor has good holding power to weight ratio and is the easiest type of anchors to stow with its shank in hawse pipe. The disadvantage is its tendency to disengage flukes by gradually turning over. The weight ranges from 100 kg to 25 tonne.

2. The stock Anchors

These are characterized by a transverse bar, or stock which orients the flukes

in the proper position to dig in one fluke when the anchor is dragged along the

bottom.

Danforth Anchor

The stock is incorporated in the anchor head. This allows stowage of a stock

type anchor with its shank in a hawse pipe. The fluke motion is limited to

about 30o each side of the shank axis. Its weight ranges from 130 kg to 14

tonne.

The Snug Stowing Anchor

It has stocklike projections made integral with the crown which give stability

against rotation when dragged, and it develops holding power comparable to

the stockless anchor. The small crown nests into the hawse pipe, which has

no shell bolster, allowing the flukes to lie snugly against the shell.

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Flipper Delta Anchor

Major advantages of this anchor:

Proven superior high holding power capacity - Excellent efficiency (weight / holding power) - Open construction for smooth and good penetration in different kinds of soil - No rotation, which means no decrease of holding capacity and no dragging of the anchor - Easy dismantling for transport purposes

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JIS Type Stock Anchor

This anchor is used for relatively small ships

3. The Bruce Anchor

This anchor is of one-piece construction and is self-orientating at the sea bed

surface irrespective of drop attitude. It penetrates the sea bed while

orientating and has complete roll stability. The Bruce anchor has higher

holding power and higher tolerable cable angles than other anchors. Also the

breakout force is as low as 20 - 25% of the prior holding pull.

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4. The Mushroom Anchor

It is named for its shape and is used on light-ships, canal barges, and for

anchoring permanent mooring buoys.

5. The Stern Anchor

A stern anchor is used to keep a ship from swinging with the current in

restricted areas. It may be housed in a hawse pipe so located that the anchor

will clear the rudder and propeller when dropped.

Anchor Chains

The bower anchor chains required by classification society rules may be stud-link

chains of normal, high or extra high-strength steel. Anchor chains are

manufactured in 15-fathom shots (27.4 m in length). The chain has an enlarged

link at each end of the shot to accommodate the joining shackles used to link up

the chain. The first shot is sometimes made 30 fathoms long to allow anchoring

in shallow water without passing a joining shackle over the wildcat.

The wildcat is a special type of cog-like windlass drum whose faces are formed

to fit the links of the anchor chain. The rotating wildcat causes the chain to be

slacked off when lowering the anchor, or hauled in when raising it.

Chain connecting links are designed for easy assembly and disassembly. The

one shown below is made by Baldt and consists of a c-link and two closing caps.

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The caps are mated to the c-link and after assembly are secured by a stainless

steel tapered pin and backed by a lead plug.

Some of the anchor chain fittings are shown below:

Anchor 'D' endshackle

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Pear shackle type Baldt

Shortlink chaincables

Studlink chaincables & fittings

Swivel

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Determination of Anchor and Chain Sizes

Ships are normally equipped with a minimum of two bower anchors and anchor

chains. The chain is usually much heavier than would be required to withstand

the tensile load alone. The chain hangs in a catenary between the hawse pipe

and the bottom, and must be selected so that the lower end of the catenary will

be horizontal when the tension at the anchor shackle is equal to the maximum

holding power of the anchor.

Chain and anchor sizes may be determined by calculation or classification

society rules. The general design condition is:

Chain Proof Load > Anchor Holding Power > Applied Maximum Load

To calculate the size of the anchors and chains required, the wind load and

current load on the ship must first be determined for the most severe conditions

under which the ship will be expected to remain at anchor. The anchor system is

also subjected to dynamic loads as the ship surges, sways and yaws and is

forced by wave action.

H = Z . FR

where H = total resistance corrected for dynamic forces FR = total wind and current load Z = factor to approximate the dynamic effect = 1.25 - 1.50 for small ships with fine lines = 1.50 1.70 for large more fully shaped ships = 1.75 2.00 for ships with more blunt shapes

Based on the selected type of anchor H/W (Holding Power / Weight ratio) could

be determined from the following table:

Type of Anchor H/W Mushroom 2.5 Stockless 5.0 Danforth 16

Required Anchor Weight (in air) W = H H/W

Chain size and scope are determined to suit the anchor size and depth in which

the ship will anchor using the catenary equation:

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where s = scope of chain, fathoms y = water depth + height of hawse pipe above water w = chain weight (in air) per unit length

The sizes of anchors and chains also may be determined by reference to tables

provided by the classification societies. The sizes are tabulated according to an

equipment number determined from characteristic dimensions of the ship by

use of formulae given in the rules.

Anchor Handling Arrangements

The anchors are housed against the forward side shell, sometimes in specially

recessed pockets. The anchor is shackled to the anchor chain (cable). The chain

passes through the shell via the hawse pipe on to the forecastle deck. It travels

over the chain stopper and on to the windlass wildcat (cable lifter) drum. From

the wildcat it drops vertically down through the "Spurling pipe" to the Chain

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Locker under the deck. The bitter end of the chain is connected to bitts fixed in

the chain locker bulkhead using the bitter pin, which should be able to be

released from outside the locker to "slip" the Anchor. This would occur if the

Windlass brake has slipped in a storm for example and you have reached "the

bitter end".

The most common arrangement utilizes a stockless anchor and a horizontal

windlass wherein the wildcats are mounted on horizontal shaft as shown in

figure.

http://en.wikipedia.org/wiki/File:Ship_anchor_windlass_diagram.gif

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The normal windlass arrangement utilizes one prime mover to drive two

declutchable wildcats and also two warping ends (gypsy heads). The warping

ends are not declutchable and rotate continuously when the windlass is in use.

When mooring light line speeds of 0.75 1.0 m/sec are required.

The windlass should be able to heave a certain weight of chain (cable). This full

load is generally between 4 6 times the weight of one anchor and the speed of

haul at full load is usually between 0.12 0.2 m/sec (LR requirement is 0.15

m/sec).

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For very wide ships such as large tankers, and for ships with large bulbs, it is

necessary to split the windlass and set each wildcat at an angle to the centerline

in order to obtain proper leads to the hawse pipe.

Large passenger ships and naval ships generally are equipped with a vertical

windlass where each wildcat is mounted on a vertical shaft.

This is driven through spur and worm gears by electric motors located on the

deck below, thus lowering the centre of gravity. Capstan heads, driven by the

windlass machinery, usually are fitted on each side outboard of the wildcats for

handling the forward mooring lines. Vertical windlasses have the advantages of

greater wrap of the chains when dropping the anchor, and the machinery is more

easily maintained at sea. It also allows a flexible angle of pull (which means rope

or chain can be run out to different fairleads).

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Chain Stopper

A chain stopper made of cast steel is usually placed between the wildcat and the

hawse pipe in line with the run of the anchor chain. It is used to hold the anchor

chain in place while the ship is riding at anchor or the anchor is fully housed. The

stopper is not designed to stop the moving chain, but only hold it in place. In the

one shown in the following figure a hinged bar is designed to fall between two

vertical links and hold the chain in place.

Devils Claw

The devil's claw is a device that is used as a chain stopper to grab and hold an anchor chain. It consists of a turnbuckle, usually attached at the base of the anchor windlass, and a metal hook with two curved fingers that grab one link of a chain.

After hoisting the anchor and setting the windlass brake, the claw is placed on a chain link and the turnbuckle is tightened to take up the tension on the chain. If more than one stopper is used, the turnbuckles can be adjusted to evenly distribute the load.

A devil's claw cannot be released while it is under tension. To release it, the tension must first be taken up by the windlass brake. Then the turnbuckle can be loosened and removed.

http://en.wikipedia.org/wiki/Turnbuckle

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Types of Anchor Handling Machinery

1. Horizontal Shaft Windlass

The steam driven spur-geared windlass with a two cylinder engine is the type

used on many tankers. The speed is controlled by the engine throttle. The engine

may be installed on the deck below. This protects engine location and provide

access at all times for overhaul.

A similar windlass powered by an electric motor is the most common type fitted

on merchant ships. Speed variation is achieved by an electric controller which

gives multiple speeds with DC and full and quarter speeds with AC. The

exception of using this type is where there is danger of spark- initiated explosions

or fires because of the nature of the cargo carried.

2. Vertical Shaft Windlass

This is driven through spur and worm gears by electric motors located on the

deck below. Vessels equipped with AC may use electro-hydraulic machinery to

drive the windlass at varying speeds. The power is supplied by a constant speed

electric motor driving a variable stroke hydraulic pump which is piped to a

hydraulic motor which drives the windlass through shafting and gearing. The

speed and direction of rotation are regulated by varying the stroke and reversing

the discharge of the pump end.

http://en.wikipedia.org/wiki/File:Securing_devils_claw.gif

Anchoring Systems

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