Failure of AHC cylinder causing oil leak to deck

The incident occurred when an offshore vessel was engaged in the installation of a jumper line in 690 m of water. 

The vessel was stood off at a safe distance from a drill centre; the crane was stationary at the time and waiting in AHC mode. A bang from the crane was heard from inside the crane cab and the crane automatically switched to normal mode (AHC off).

A leak was spotted from the crane's right hand side. As soon as it was observed that there was an oil leak from the bottom of the AHC cylinder, the crane was stopped, and the alarm raised to contain the oil. The client’s jumper was lost as unrecoverable.

Our Member noted that had the incident occurred exactly over the drill centre, major damage could have been caused to subsea assets.

The bottom of the AHC cylinder before the work started on a temporary repair to seal it

Scoring seen inside the cylinder

• The leak was spotted promptly, and the alarm raised.
  
  
• No oil was spilled to sea, all of it (approx. 150 litres) was contained on board.
  
  
• Engineers were able to remove the remaining bolts from the bottom plate on the AHC cylinder, create a temporary seal and secure the plate with new bolts, which enabled the crane to be recovered to deck.

Our members’ investigation noted the following:

• Poor design of AHC cylinder – Lack of pressure sensor on vacuum side drain line.
  
  
• Lack of maintenance – there was no planned overhaul of the cylinder in the manufacturer’s manual or specification.
  
  
• Lessons had not been learned from a similar incident the year before involving a high pressure HP seal on a cylinder on a sister vessel.
  
  
• The root cause of the AHC failure on the low-pressure side of the accumulator was contamination of the hydraulic system, either from the system itself or from particles that had entered the system when it was first connected.

During normal active heave operation, the piston travels up and down inside the cylinder and this movement controls the movement of the hook wire in relation to the vessel’s roll. This ensures that subsea, the hook remains in the same place as the vessel is rolling.

The low pressure seals became worn. This allowed oil to leak past the piston into the vacuum side. At the time of the incident the seal had failed so badly that the leak oil choked the drain lines and oil built up in the vacuum space behind the piston. The vessel then rolled; the piston began to travel compensating the hook wire. As the piston moved down the cylinder, it hit the wall of built-up oil, this transferred the force through the incompressible oil onto the small surface area of the cylinder end cover, shearing the bolts and blowing the oil out onto deck. The cylinder is not designed to take any pressure on this side of the piston.  

Investigators believe that at some point, contaminated oil has passed through the cylinder causing damage to the seal and scoring the cylinder. The source of this contamination could have been during an overhaul of other components and has entered the oil and found its way to the active heave cylinder.

It was not known at what point in the cylinder’s lifetime this damage was sustained because the cylinder had never been inspected inside. In its lifetime the piston and seals have travelled many thousands of kilometres inside the cylinder as the active heave will move every time the ship is rolling during crane use.