Good anchoring practice
Anchoring loss prevention
Anchoring is a critical operation on vessels. Shipping companies, port authorities and P&I Clubs value the safety of anchoring, which can be affected by the wrong anchoring operations and the increased traffic of ships as well as undesirable weather conditions. Club statistics shows that the direct claims relating to anchor account for up to 8% of navigation related claims. An improper anchoring could cause damage and loss to the vessel, other vessels, property and the environment. The consequential losses of grounding and collision due to anchor dragging or loss can be significant. Claims relating to improper anchoring are as follows:
- Anchor lost or twist
- Grounding or collision due to anchor dragging
- Damage to underwater cables or pipelines
- Damage to floating objects or port facilities
- Damage to navigation aids or facilities
- Damage to the marine environment (e.g. coral reef)
- Fines etc.
Good bridge management in anchoring operations is the key to achieving safe anchoring and avoiding accidents. It includes anchoring planning, risk assessment, best anchoring practices, anchoring watch keeping, etc. Ship companies should set up procedures for these critical tasks and incorporate them into the safety management system.
Surroundings of anchoring
Along with evaluating the nature of the seabed when anchoring, these are the elements to be considered: direction and strength of wind and current, sea condition, shallow water, prohibited areas, navigational aids and facilities, underwater cables and pipelines, swinging room, other anchoring vessels in the vicinity.
To avoid accidents like anchor dragging, vessels should keep a safe distance from other vessels, navigational hazards, underwater cables and pipelines. The distance to the nearest grounding line should be no less than one nautical mile.
A safe distance between vessels depends on vessel's maneuverability that could be restored from anchor dragging. There are no definite criteria to measure it.
The ICPC (International Cable Protection Committee) bulletin point out that anchor dragging whilst at anchor or under way can cause underwater cable damage, and the cause of such damage around the world has been closely monitored since the formation of ICPC. Some newly laid oil pipelines or gas pipelines might not be marked/updated on the navigation chart and Notices To Mariners.
Emergency anchor dropping might also be necessary in case of steering failure, probable collision, maneuvering in shallow waters, etc.
Risk assessment and plan to anchor
Anchoring operation is part of a passage plan, which must be carefully planned, executed and monitored. An effective anchoring plan can prevent anchor accidents and avoid any operational failure.
A detailed risk assessment of the anchoring operation should be carried out to formulate an effective plan and to make prudent decisions when facing emergencies. If you expect wind force to increase, the possibilities of anchor dragging must be part of the risk assessment. An alternative anchorage should also be prepared if the initial selected anchoring position is unavailable.
The anchor plan should be prepared by the master considering the following elements:
- The limitation of the anchoring equipment: It is only designed to hold the vessel in good holding ground, and not to hold the vessel off fully exposed coasts in rough weather.
- The available depth and type of holding ground at this anchorage. Maximum depth of anchoring must be applied. Do not anchor in depths beyond windlass hauling capacity with allowance of efficiency reduction for old windlass.
- The minimal Under Keel Clearance: in a calm weather and smooth seas condition, the UKC should be at least 20% of maximum vessel's draft in loaded condition.
- Location of the anchorage designed for the vessel.
- Tide, direction and strength of the current in the anchorage area.
- The immediate and predicted weather, wind direction and strength, visibility, sea condition of wave, swell, etc.
- The availability of adequate sea floor.
- The safety swinging circle of the vessel: A circle with a minimum radius including length of anchor chain and the vessel's Length Over All.
- The proximity of navigational hazards. An adequate safety distance to the nearest vessels and navigational facilities.
- An alternative anchorage if the initial selected anchoring position is unavailable.
- The anchor to be used with the condition of anchor, anchor chains, windlass, brake band, chain stopper, lashing devices, etc.
The master should also determine the operation mode of the engine according to the type of anchorage, weather conditions and the distance from other vessels, shoals and navigational hazards.
Anchoring operation is based on experience in handling complex anchorage and various conditions of vessels. The following points should be considered for safe anchoring:
Determining which anchor to use depends largely on the vessel and condition of the anchors. The basic principle is that the anchor must be in good holding and heaving condition.
Sternway speed: the speed over the ground need to be minimized when the vessel dropping the anchor and the chain paying out. In general, it should be limited to about 0.5 – 1.0 knots; for VLCCs, it should be from 0.25 to 0.5 knots only. Laying the chain across the ground in an orderly manner can avoid excessive strain on the chain.
Observed GPS speed might not be reliable if the speed is less than 0.5 knot. It is very difficult to be accurate at such low speed over the ground.
Anchor chain paid out: Wrong practice in dropping the anchor may cause chain entangling accidents or loss of the anchor. Most accidents are caused by uncontrolled running-out speed of the anchor chain and poor condition of the brake when dropping the anchor.
The running-out speed should be limited to 5-6 metres/sec. and the brake force must be used to control the speed.
In shallow waters, up to the depth of 25 metres, the customary practice is to let go the anchor from the hawse pipe or one meter above water by releasing the brake.
If the water depth of the anchorage is between 25 to 50 metres, release the anchor about 5 metres above the sea bottom with the windlass, and then let go the anchor by releasing the brake.
If the water depth of the anchorage exceeds 50 metres, release the anchor and the chain with the windlass until the chain walking out to the required length.
However, if the water depth is above 80 metres, do not drop anchor as the maximum anchor depth for most vessels are designed to the rule of 82 metres (three shackles). The master should check the class limitation of the vessel to ensure the windlass heaving capacity limits do not exceeded for the anchoring depth.
Length of cable: the cable length that should be released depends on factors such as water depth, draft, windage area, strength of wind and current, and anchorage congestion.
The previous information of the cable length (3.5 to 4 times the water depth) is no longer enough to prevent vessel from dragging if anchoring is affected by wind and current.
A general guide: The cable length of should be 3 times of the water depth plus 90 metres in normal condition. It should be 6 shackles under normal circumstance for a depth of 25 metres. In rough weather condition, the cable length should be 4 times the water depth plus 150 metres. Congested anchorage is one of the exceptions. For example, in Singapore Roads, there should be 3 shackles in the water for handy size vessel, and only 4 shackles in the water are acceptable for "Panamax".
Anchor in stand-by: If there are underwater cables and pipelines on the planned route of intended anchoring point, the anchor should not be lowered into the water and the clutch of the anchor should remain engaged. This prevents the anchor accidental releasing and damaging the underwater cables and pipelines.
Anchors when ship alongside berth should be properly secured with stoppers or lashings to prevent any accidental running out.
Dragging the anchor often happens in rough weather conditions, especially in tropical areas (typhoon, hurricane, etc.). Avoid anchoring when facing current turbulence in heavy weather and the vessel should then be sailed to open sea.
It has been found that anchor dragging is experienced in 40% of instances of anchoring under typhoon conditions. In such circumstances, it requires extra time and emergency procedure to handle anchor dragging and anchor weighing.
Anchor dragging would not cause serious accidents if there is enough space in the sea for maneuvering and enough time to regain control of the vessel. But in most cases there is not sufficient space or enough time as the speed of anchor dragging under wind pressure force is approximately 3 - 4 knots.
Anchor dragging rarely happens on vessels with a deep draft when compared to vessels with a light draft. It is recommended to increase the draft of the vessel to prevent dragging. The master and officers should familiarize themselves with the condition of the vessel in advance and take necessary precautions.
Checking the anchor position frequently to detect anchor dragging at early stage is of great importance. A vigilant bridge watch is essential as it can take some time to recognise anchor dragging, especially in a crowded anchorage where there is insufficient space between vessels to deal timely with emergencies.
The master and office must keep in mind that during the period beginning of detecting dragging to regaining full control of the vessel, the vessel may run into a dangerous situation, close to other ships and facilities, or underwater cables and pipelines, or shallow water.
Timely steps should be taken once anchor dragging occurs as it can affect the safety of the vessel. Anchor dragging can be reduced to a minimum if another anchor is dropped immediately.
In the event of suspected anchor dragging, anchor dragging of vessels nearby, or when the vessel is straying out of the safety swinging circle, the officers on watch should:
- Report to the master immediately
- Inform engine room to start the main engine emergently
- Have officers standing by at the anchor station
Anchoring in heavy weather
When the wind increases to BF 7, the main engine should be prepared on standby, the bridge and engine room must be at a navigation level, and officers should stay alert.
If necessary, the vessel should leave the anchorage and proceed to open sea to avoid anchor dragging. If the master decides to take such actions for safety reasons, he or she does not need to wait for instruction from VTS or port authorities.
To prevent anchor dragging in heavy weather, below are the recommended measures:
- Reduce wind area of the vessel as much as possible
- Use ballast to increase the draft of the vessel
- Reduce trim by stern as much as possible
- Trim by head and increase it if possible. This is to move the wind center backward and the hydrodynamic center forward to reduce deviation of the vessel and improve the vessel's stability.