Dragging Anchor

Report Text:

Whilst at anchor with 5 shackles in the water, it was observed that the anchor was dragging. NW wind 30 knots, wave height 2 metres, depth of water 14 metres, ground was mud based material.
The consequence if no timely had been taken could be,

(a) collision with another vessel in the vicinity
(b) impact the safe navigation of another vessel in the river, narrow channel or fairway
(c) catch an underwater cable or pipeline and damage.
Corrective Action: Main Engine was already on ‘Standby’ with 5 minutes’ notice. Immediately heave up the anchor and dropped again at nearby position with 7 shackles in water. Carefully checked the anchor position and continued to monitor the anchor position.
Lessons Learned: Before anchoring, carefully check anchorage depths, bottom type, vessels at anchor in vicinity, current, wind speed and direction. Often with this type of vessel, dredging at anchor is a risk if the wind speed is more 30-35 knots. Before anchoring this type of vessel, the anchorage position should be near the edge of the anchorage and clear of any vessel on the predicted way of the driftage.
We then tried an exercise with the bridge team and the predicted flow scenario whilst dredging the anchor and eventually fouling a pipeline. We were unable to find in our manuals any information. We did find a few words in “The Mariner’s Handbook” but believe to avoid any delay in a similar case we need a Marine Information Notice with detailed guidance.
The reporter also highlighted as useful reference material in the Nautical Institute’s MARS Report Number 201045 and Australian Transport Safety Bureau Report no. 260-MO-2008-012. The report relates to Submarine gas pipeline damaged by an anchor. Quote: The Mariner’s Handbook notes that if it is suspected that a ship has fouled a gas pipeline with its gear or anchors, excessive weight should not be placed on the gear as it could damage the pipeline and the ship ‘could face an immediate hazard by loss of buoyancy due to gas aerated water or fire/explosion’. Given the high risk and because many pipelines were laid before accurate GPS receivers became commonplace, it would be prudent to be cautious rather than completely rely on the accuracy of their charted locations. In essence, the only appropriate course of action if a ship has, or is suspected to have, snagged its anchor on a gas pipeline is to avoid placing weight on the anchor cable and to slip the cable as soon as possible. Had this been done in this case, the gas pipeline probably would not have ruptured. Unquote

CHIRP Comment

Seaways June 2014 Article “Safe Anchorages – A Mariner’s Perspective” provides commentary on the need for consideration of the depth of water, the holding ground, nearby obstructions and to make best use of communications and weather forecasting services.

The importance of keeping a close lookout and constantly monitoring the position of the vessel is emphasised. The use of appropriate position lines on significant land based features to provide additional assistance in monitoring the position is recommended.

Modern Radar and Electronic chart displays will often have features to create guard rings which can also provide for additional position monitoring. Also it is important to remind Bridge teams of the danger created by removing too many layers on ECDIS charts and thereby losing information on underwater obstructions. Refer to The Nautical Institute “Navigator” magazine Issue number 5 for guidance.