The Club would like to draw Members’ attention to an unfortunate incident where the bow thruster failure of the anchor handler vessel resulted in it landing heavily on the rig and causing damage.
The Incident
The vessel in question was one of two anchor handlers (the ‘vessel’) shifting a semisubmersible rig. During the operation, a strong wind was blowing on the port bow of the rig and a current of over 1 knot setting onto that side. The vessel was ordered to retrieve the port aft anchor which was run out at an angle of about 60° from the rig’s centre line. The anchor was lifted clear of the bottom and hauled up the stern roller. The vessel then manoeuvred slowly astern towards the rig while the anchor wire was being recovered by the windlass on the rig. As the vessel approached the rig, the rig Master asked the vessel to swing to a position on the beam of the rig to allow a better clearance to the other port anchor wire. This however brought the wind onto the beam of the vessel. To maintain this position, the bow thruster was put to full power. However, the bow thruster started to overheat and eventually tripped. The loss of the bow thruster made it difficult for the vessel to maintain position whilst the attached anchor pennant wire made it difficult to manoeuvre safely. Attempts were made to pass the anchor pennant wire from the vessel back to the rig, however the shark jaws on the vessel malfunctioned resulting in the inability to release the pennant wire. The vessel, pinned by the anchor pennant wire at the stern, was swept down onto the rig by wind and tide. She landed heavily on the port side ranging up and down causing severe damage.
Observations
- This incident shows how a number of factors can often combine to create a serious incident.
- Although this was the first time the shark jaws had failed, overheating was a recurrent problem with the bow thruster which had never been properly repaired.
- In this case, failure of either the bow thruster or the shark jaws alone would not have resulted in the incident, but when the effects of the two were combined with the vessel being in close proximity to the rig, the incident became inevitable.
Recommendations
- A Planned Maintenance System (PMS) should be in place to address the regular maintenance requirement.
- Critical equipment to be identified and it should be emphasized that these equipment are working to optimum capacity at all times.
- Any recurring machinery faults to be carefully investigated. Necessary maintenance to be carried and machinery to be ensured working in order prior undertaking critical tasks.
- The effect of environmental forces must be envisaged to ensure the vessel’s capability to hold the position or perform the manoeuvre.
- Anchor handling preparation checklist to be complied with prior undertaking operation.
Financial Cost
The rig was out of action for 10½ days. The claim for damage and loss of use exceeded the tug’s limitation fund under the 1976 Convention. The claim was settled using the limitation, but nevertheless the total cost exceeded US$450,000. The tug itself suffered heavy damage as a result of the collision.