07 Aug 13 - 14:46
Japan P&I Club Loss Prevention Bulletin No.25
According to the investigation by the Japan Marine Accident Tribunal in 2004, 40% of ships anchoring under typhoon conditions experienced their anchors dragging. Generally speaking, dragging the anchor happens in rough weather and weighing the anchor may take some time in such circumstances.
Even if the anchor can be successfully raised in rough weather conditions, further time may be required to restore the ship to full manoeuverable condition. The usual weighing operation, therefore, may be abruptly transformed into an emergency procedure requiring experienced skills and cool judgment.
This refers to how and why dragging the anchor occurs and how to deal with a dragging situation.
Accidents Involving Ships at Anchor
Accidents involving ships at anchor usually occur when the anchor drags and the vessel drifts without holding power, leading to collisions and/or groundings or strandings. The following considerations should be taken into account:-
① It can take some time to realise the anchor is dragging, despite the ship drifting. A vigilant bridge watch is, therefore, essential.
② It takes some time to weigh the anchor and restore the ship to full manoeuverable condition, even though the ship may be drifting for that period. Contingency plans must be in place to ensure rapid response times.
③ During the period beginning with the detection of dragging to the time full control is achieved over the ship'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, no serious accident should occur just because a ship is dragging its anchor, provided there is enough space around it for manoeuverability and enough time available to restore it to a fully controlled condition.
The considerations outlined above become of even greater importance in the case of a crowded anchorage where there may be insufficient space between vessels to deal timely with emergencies such as dragging anchor and drifting out of control. The master of an arriving vessel should satisfy himself first that the anchorage is safe in all respects before committing himself to anchoring.Masters and deck officers are advised to familiarise themselves with the following concepts in order to prevent as far 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 time and space required to regain control of the vessel if the anchor drags.
The reason why an anchor drags
A ship's anchor drags due to the impact of external forces on it which exceeds the holding power of the anchor and cable.
Masters and deck officers should be aware of how various parameters, such as the scope of cable in relation to the depth of water and the effects of wind, wave and tidal forces on the vessel, can in turn exert excessive forces on the anchor and cable system leading to break-out of the anchor from the ground and dragging. In the above connection, there still remain some empirical or "rule of thumb" methods of assessing the scope of anchor cable required under various circumstances of water depth and expected weather conditions, for example:
Empirical or Rule of Thumb Methods for Assessing the Minimum Required Length of Anchor Chain
Where:
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
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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 Bulleting Preventing an Anchor from Dragging
Source: Japan P&I Club
