Japan P&I Club Loss Prevention Bulletin No.25
According to the investigation by the Japan Marine AccidentTribunal in 2004, 40% of ships anchoring under typhoonconditions experienced their anchors dragging. Generally speaking, dragging the anchor happens in roughweather and weighing the anchor may take some time in suchcircumstances.
Even if the anchor can be successfully raisedin rough weather conditions, further time may be required torestore the ship to full manoeuverable condition. The usualweighing operation, therefore, may be abruptly transformedinto an emergency procedure requiring experienced skills andcool judgment.
This refers to how and why dragging the anchor occurs andhow to deal with a dragging situation.
Accidents Involving Ships at Anchor
Accidents involving ships at anchor usually occur when theanchor drags and the vessel drifts without holding power,leading to collisions and/or groundings or strandings. Thefollowing considerations should be taken into account:-
① It can take some time to realise the anchor isdragging, despite the ship drifting. A vigilant bridgewatch is, therefore, essential.
② It takes some time to weigh the anchor and restorethe ship to full manoeuverable condition, even thoughthe ship may be drifting for that period. Contingencyplans must be in place to ensure rapid responsetimes.
③ During the period beginning with the detection ofdragging to the time full control is achieved over theship’s manoeuverability, the vessel may run dangerously close aboard, or into another ship or structure,or into shoal water.
Unless heavy weather causes the vessel to capsize, noserious accident should occur just because a ship is draggingits anchor, provided there is enough space around it formanoeuverability and enough time available to restore it to afully controlled condition.
The considerations outlined above become of even greaterimportance in the case of a crowded anchorage where theremay be insufficient space between vessels to deal timelywith emergencies such as dragging anchor and drifting outof control. The master of an arriving vessel should satisfyhimself first that the anchorage is safe in all respects beforecommitting himself to anchoring.Masters and deck officers are advised to familiarisethemselves with the following concepts in order to prevent asfar as possible a ship from dragging its anchor:-
① How and why dragging the anchor occurs.
② Difficulties with vessel manoeuverability while dragging an anchor.
③ The assessment of what constitutes a safe anchorage, including contingency plans involving the timeand space required to regain control of the vessel ifthe anchor drags.
The reason why an anchor drags
A ship’s anchor drags due to the impact of external forceson it which exceeds the holding power of the anchor andcable.
Masters and deck officers should be aware of how variousparameters, such as the scope of cable in relation to the depthof water and the effects of wind, wave and tidal forces onthe vessel, can in turn exert excessive forces on the anchorand cable system leading to break-out of the anchor fromthe ground and dragging. In the above connection, therestill remain some empirical or “rule of thumb” methods ofassessing the scope of anchor cable required under variouscircumstances of water depth and expected weather conditions, for example:
Empirical or Rule of Thumb Methods for Assessing the Minimum Required Length of Anchor Chain
d: Water depth(m)
L: Minimum Required Length of Anchor Chain(m)
・Japanese Publication Theory of Ship Operation
Fine weather: L=3d+90m
Rough weather: L=4d+145m
・United Kingdom Publication Theory of Ship Operation
L=39 ×√ d
Traditional Means of Detecting a Dragging Anchor
① Checking the ship’s position, to confirm whether it is placed outside of a turning circle.
(The most currently reliable way of checking whether the anchor is dragging is to carefully monitor the vessel’s position by visual and electronic means to confirm whether it remains within a swinging circle defined by the scope of anchor cable and the distance from the forecastle to the bridge. If it deviates from the circle, the ship is likely to be dragging its anchor. Reliance should not be placed on a single method of fixing the ship’s position. Cross-check whenever possible with an alternative means.)
② The bow cannot stand against the wind.
③ The ship’s side against the wind hasn’t changed.
④ Checking to see there is no slacking of chains just before a ship’s side against the wind turns.
⑤ Checking whether there are extraordinary vibrations through the anchor chains.
⑥ Checking the course recorder in case it does not indicate a “figure-of-eight” motion locus.
The above methods remain well-tried but, of course, only confirm that the anchor is dragging. They do not predict when dragging is likely to commence.
According to one current study, an analysis of anchor dragging has shown that there are two associated phenomena, or stages, to the process which indicate that dragging may be about to occur before it is detected by the more usual methods outlined above.
Learn more information on the Japan P&I Club BulletingPreventing an Anchor from Dragging
Source: Japan P&I Club
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