As UK MAIB reports in its most recent Safety Digest, a vessel was discharging cargo alongside at a not always afloat but safely aground (NAABSA) tidal river berth just before low water.
The vessel’s mooring lines came under increasing tension as the tide ebbed and the vessel settled aground at the berth. Suddenly, the head rope started juddering and veered in dramatic jolts. As the ship started to slide away from the quayside the fore spring, stern spring and stern rope all snapped in quick succession. This left only the head rope keeping the vessel connected to the shore and the ship slipped 20m out into the river, coming to rest on an off-lying shoal.
The crew were able to run spare lines ashore and, as the tide started to flood, pull the ship back alongside. Similar accidents had occurred twice before at the same berth, the most recent just 12.5 hours earlier on the last ebb tide, which also involved the same vessel sliding 20m into the river. Following some rapid hydrographic survey work it became clear that the slope of the riverbed at the NAABSA berth had changed. The mud was now on an incline and was serving to draw ships away from the quayside. The resulting strain on the mooring lines was too much when the vessel settled aground. On parting, the mooring lines snapped back some 40m at their greatest extent. Dredging quickly resolved the primary cause of this accident by levelling off the riverbed adjacent to the quay.
Credit: UK MAIB
Lessons learned
- Monitor → For a NAABSA berth to be safe to use it needs to be flat and without any significant incline. A riverbed can easily change its profile following heavy rain, extended dry periods or due to significant building works either up or downstream. Regular surveys of the berth area are necessary to determine the profile and depths and whether any debris is present that might cause a hazard. The last survey in this case had been carried out 6 months before the incident but there had been heavy rainfall in the intervening period.
- Risk → Snapback of mooring lines can be dramatic and result in fatalities. It was fortunate that no one was standing near the lines when they parted. Such risks need to be managed and operations ceased immediately should the risk of snapback become apparent.
- Teamwork → This event was the third in a sequence of similar accidents at this berth. Berth owners and operators need to be sufficiently resourced to ensure the prompt and effective investigation of accidents and that appropriate action is taken to avoid a recurrence.
