Anchorage Choice Implicated in Grounding
A temporary anchorage may appear suitable when the stay will be short and the weather forecast is good. But what if the wait for the intended berth is extended and the weather deteriorates? How can anchorages that pose unique risks be identified?
This is the question addressed in the latest Nautical Institute news, based on report M03L148 from the Transportation Safety Board of Canada.
The vessel, loaded to a draught of 13.5 metres, was upbound in a river estuary. Before arrival, the master was advised the intended berth was not yet free and to anchor at a short term, ad-hoc anchorage downriver from the port. The vessel was anchored using seven shackles of chain in position A (see diagram), which gave a swing circle as indicated by C. The weather forecast for the next few days was relatively good; as per the agent’s arrangement, the captain released the pilot.
The main engine, windlass and other equipment were kept on emergency standby and the position of the vessel was monitored frequently. The next day, the pilot of another vessel in transit reported to VTS that the anchored vessel was a hazard to shipping due to its proximity (B in the diagram) to the upstream end of the main fairway (E). He also indicated that the vessel had probably dragged its anchor although it was still within its swing circle. Concerned about the safety of the navigation, safety of the vessel and given that there was no pilot on board, he suggested that the vessel be moved.
This suggestion triggered several communications between VTS, the harbour master, and the vessel’s agent. Assuming that the vessel would be docked within the next 24 hours and with no adverse short-term weather forecast, a decision was made not to move the vessel. However, the berth did not free up within 24 hours. Two days later the vessel was still at the same anchorage when the weather began to deteriorate, with winds increasing first to 20, then 30 and more knots.
As the winds increased, and with an ebb tide running northeast at 2.5 knots the vessel began to drag anchor. The master tried to maneuver the vessel but it continued to be driven by the wind and current toward the shore; during the attempts to maneuver the anchor was weighed. About an hour after the initial dragging the vessel’s crew reported they had been pushed aground (D on diagram).
After re-floating, the damage survey showed the vessel had sustained damage to its bottom plating in way of the forepeak and ballast tanks 1, 2, and 3. Additionally, three blades of the propeller were deformed and two were fractured. Also, the port bilge keel in way of ballast tank 3 had buckled over a length of about one meter.
Some of the findings and lessons learned from the official report were:
* Considering the large size and deep draught of the vessel, the low keel clearance, the strength of the currents and winds and its limited room for maneuver, the vessel was not anchored safely in an appropriate anchorage.
* A risk-based approach was not used by various agencies to identify anchorages that pose unique risks. This in turn permitted vessels to be assigned anchorages that may not be suitable.
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* Water depth to draught ratio (Wd/D) is an important factor to consider when anchoring in strong currents, as smaller ratios will cause greater forces to act on the hull (and thus may cause the vessel to drag anchor). For example, a large full-bodied vessel in a five knot current will experience nearly three times as much hydrodynamic force on its hull with a Wd/D ratio of 1.2 than had the ratio been 3.
Captain Paul Drouin, the editor of the Nautical Institute's Mariners' Alerting and Reporting Scheme (MARS) reports notes: When your Wd/D ratio equals 1 you are most assuredly aground.