According to Britannia Club, ships anchor for a variety of operational reasons however anchoring can lead to the loss of control over the ship and ultimately to the loss of anchors or chains, grounding, collision or damage to property.
In particular, dragging anchor is defined as the loss of the holding power of the anchor system. If this loss is sudden, it may require an immediate reaction to retain control over the ship. Once the anchor starts dragging, the change in the ship’s position may occur quite rapidly, especially in adverse weather and current.
It is important to note “dragging anchor” refers to unintended loss of holding power, however “dredging anchor”, is intentional and can be used for manoeuvring purposes.
Causes of dragging anchor
The reasons for dragging anchor typically extends beyond the anchor itself. In the publication “Anchoring Guidelines: A Risk-Based Approach” [1], INTERTANKO summarises the root causes of anchoring incidents as “mainly related to poor seamanship, lack of planning and mishandling of equipment”.
A frequent immediate cause for dragging anchor is rough weather, which results in exceeding the operational limits of the anchoring equipment. In heavy weather, it may be necessary to avoid anchoring altogether by proceeding to open sea. When anchoring under extreme conditions, for example in typhoon or area of high winds/swell, always consider that other ships in the anchorage are also likely to experience dragging anchor.
In case of weather deteriorating whilst in the anchorage, delaying the decision to depart from the anchorage for too long may result in a difficulty in recovering the anchor. Weighing the dragging anchor may require additional time. Furthermore, in heavy wave/swell, the risk of anchor loss at the time of heaving up from the bottom increases significantly – this is because the ship’s vertical motion may result in rapid overloading of the anchor chain.
Dragging anchor might be manageable if the amount of space available to the ship allows for it. However, the ship is likely to be in proximity of other ships or navigational hazards and the tolerance for uncontrolled movement may be very low. As the speed of dragging anchor in strong wind may quickly reach about 4 knots, the amount of time available to regain control of the situation is typically very limited.
Dragging anchor is more likely to occur when the ship is light (in ballast condition) and has increased windage. Therefore, increasing the ship’s draught and/or reducing the trim by the stern are effective precautions against dragging anchor. It may also be necessary to restrict other activities which increase windage, such as opening hatch covers or extending gantry cranes. There have been cases where opening large hatch covers in strong wind became the ultimate trigger for dragging the anchor.
Safety Checklist
ANCHORING
- Establish the depth of water, under keel clearance, nature of bottom, any subsea hazards, which anchor to use and how much cable to pay out based on the traffic conditions and the available sea room
- Select the position of the anchorage and plan the approach with the anchor party, taking into account the expected duration of stay at the anchorage
- Ensure the ship’s swinging circle has been determined and plotted on the chart and ECDIS from the nearest land, ship and charted obstructions
- Mark out no-go-areas on nautical charts so that such areas are readily apparent to navigating officers in an emergency
- Monitor the prevailing and forecasted weather, tidal stream and flow changes
- Keep the anchor party and main engine on standby or on short notice
DRAGGING ANCHOR
- Monitor the ship’s position and movement of other ships in the vicinity, as well as the weather and sea conditions by keeping sharp lookout and using all available means such radars or terrestrial objects
- Confirm whether the ship is dragging anchor by carefully monitoring the ships past position trail, observing any zigzag pattern from the initial let-go anchor position, and ensuring the ship stays within the swinging circle
- Monitor the movements of adjacent ships and the scope of their anchor cable, and any decrease in their closest point of approach (CPA), bearing in mind that all ships do not have the same scope of swinging circle and do not swing uniformly
- Check the anchor cable leading regularly, examining for any slacking or vibration on the chain cables
- Keep the anchor not in use on standby, ready for immediate dropping when required
- Call the master whenever there are any changes in circumstances or when there is doubt of dragging anchor
- Execute the contingency plans as per Company’s Safety management System (SMS) to ensure rapid response times
- Get the anchor party to prepare the windlass and contact the engine room to get main engine for manoeuvring
- Inform ships in the vicinity and harbour authority of possibility of ship dragging anchor
- Request for pilot, if applicable, to re-anchor the ship and/or request for tugboat service to hold the ship in position
