M2258

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15th August 2024

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Good teamwork averts a serious incident

Initial Report

Two pilots boarded an LNG carrier before making an approach along a fairway to an offshore LNG terminal. The weather was rough, with a long swell. Four tugs were made fast before proceeding at approximately 5kts.

As the chief pilot prepared to make a planned alteration of course, both engines of the tug attached to the centre lead forward failed within minutes of each other, and the tug was overtaken and pulled hard against the LNG carrier’s bow by the towing line.

Because of the risk of damaging or capsizing the tug, the chief pilot could not conduct the planned turn while the stalled tug was still attached, but through a combination of good bridge teamwork and skilful control of the remaining three tugs, the LNG vessel was slowly manoeuvred alongside without further incident.

Throughout the incident, the co-pilot liaised with the stalled tug for regular updates and with two standby tugs in the vicinity, directing one to assist the disabled tug in detaching its towing line and pulling it to safety. At the same time, the second tug assisted in manoeuvring the LNG vessel. The pilots also provided regular updates to the port authority.

A combination of the sea state and the tug’s ballast arrangement was found to have caused the sea chest to become starved of cooling water for the generators, which were automatically shut down to preserve the equipment.

CHIRP Comment

CHIRP is aware of 3 similar tug events published in 2024, and readers may be aware of the case in 2019 in which a large passenger ship lost propulsion and nearly foundered because the ship’s motion in rough weather caused the oil pressure to drop, shutting down the engines.

In Bow Tug Operations, a manual by Henk Hensen, he writes, “Bow tug operations of a ship having headway are very risky.” The International Harbour Masters Association recommends a 6-knot speed limit for such operations.

Tugs sometimes undertake bow-to-bow (reverse) towing because it keeps their propellors further away from the pressure fields around the towed vessel’s bow. This method also ensures that if the tug propulsion fails (as in this incident), it is pulled alongside the towed vessel by its bow, which reduces the risk of capsizing.

However, when reverse-towing (as in this incident), there is a risk that the tug becomes trimmed by the bow, lifting the stern (and sea-chest water intake) out of the water and starving the sea chest of cooling water to the generators. This risk is lessened by towing more slowly, adjusting the vessel’s trim, and ensuring that the sea chest vents are open so that any air trapped inside can be expelled to allow it to re-fill.

The automatic shut-down system protects the equipment from overheating and being damaged or even catching fire. However, many systems do not have a manual override for use in emergencies. Tug vessel operators are advised to investigate how their equipment would react in a similar scenario, whether it would alert the operators before shutting down, and then develop emergency procedures accordingly. A checklist that includes the operating area (open water, sheltered water), draft and trim, ballast arrangement, and type of towing for the job will ensure that the risks of a blackout are mitigated.

This incident was successfully mitigated without injury or damage because of the close integration of the pilots, effective bridge team management, and close coordination between the vessels and port authority. Everyone understood their role and responsibilities, and information exchanges were clear and effective. The speed of response of all parties demonstrated their readiness to respond to an unplanned incident.

Key Issues relating to this report

Situational Awareness — Tug crews should be alert to the changing dynamics operating on the tug hull, especially in open waters. A simple checklist for the type of towing arrangement would ensure an adequate draft for all tow stages, and that equipment is correctly configured.

Teamwork — The pilots and the bridge team worked excellently to manage the situation, highlighting the effect of adequate resources and training.

Alerting – From a technical perspective, there appeared to be insufficient warning concerning the generators shutting down. Temperatures would have risen quickly once the cooling water could not reach the generators.

Design – Given the nature of towing operations and the increasing use of ASD tugs in narrow channels, towing from the bow has considerable benefits concerning performance and girting safety. The change in trim that results if ballast is not correctly applied needs to be factored into design considerations.