UK MAIB presents in its Safety Digest an incident about an anchor handling tug supply (AHTS) vessel that after finishing a routine dive inspection, the vessel moved forward to enter the port. However, the master miscalculated the height of the tide and the water was insufficient resulting to the damage of the skeg.
The Incident
The master was required to berth alongside for a routine dive inspection, so its master contacted a nearby port it visited regularly. The master requested a pilot and advised that the AHTS vessel’s draught on arrival would be 6.7m. He also advised that an under keel clearance of at least 1m was required. The port’s VTS informed the master that a pilot would embark at about 1100 the next day. As a result, the AHTS was anchored overnight.
At approximately 1000 the following morning, the VTS confirmed the pilot would be boarding at 1100 in a position 0.5nm from the port’s entrance.
For that reason, the captain manoeuvred the vessel towards the embarkation point, but at 1055 VTS advised him that the pilot was delayed by 1 hour due to other vessels’ movements. The master stopped the AHTS in the water and used the vessel’s dynamic positioning system to hold it in position.
The pilot eventually boarded at 1206. The information exchanged between the master and the pilot included the intended route, berth, and the vessel’s draught and other characteristics. Tidal conditions were not discussed. It was now less than 1 hour to the predicted time of low water.
As soon as the master/pilot exchange was completed, the vessel proceeded towards the port entrance with the chief officer having the conn. On passing the entrance breakwater, starboard helm was used to turn the vessel into a turning basin. As the turn progressed, an unusual vibration was felt, which was quickly assessed by the bridge team to be due to debris in the water. The AHTS berthed without incident and the pilot disembarked.
Later, the divers conducting the planned underwater hull inspection saw that the vessel’s skeg was damaged. The master was uncertain how this had occurred, and reported to the VTS the vibration felt during the inbound passage. Review of the vessel’s position during entry, along with the height of tide, identified that the vessel had been on its intended track.
However, it also identified that, because the inbound passage was 1 hour later than intended, and the height of tide was 0.4m less than predicted, insufficient water had been available. The vibration felt during the starboard turn resulted from the vessel taking the ground towards its stern. The damage to the skeg (see figure) required the AHTS to be dry docked for repairs.
Lessons Learned
- Water depth can change dramatically with the tide, making some ports inaccessible at certain times. Therefore, when arriving or departing a port earlier or later than intended, confirming the height of tide and depth of water available, and rechecking the intended route, are smart moves.
- Frequently, there is very little time for masters and pilots to discuss a lot of information. However, as the height of tide and the resulting water depths available along the track and alongside are fundamental to safe navigation, this information warrants being towards the top of the master/pilot exchange checklists.
- Tides are not always as predicted, but most port authorities generally have realtime tidal information to hand. If this information is not provided as a matter of course through routine VHF broadcasts or exchanges, it is in everyone’s interest to ask for it when meeting the required under keel clearance is marginal or in doubt.
