Crane motion reference unit (MRU) malfunctions after overheating

A vessel was engaged in inspection of a floating production storage and offloading unit (FPSO) mooring chain in around 80 metres of water, with the AHC crane being utilised to lift and hold sections of the mooring chain off the sea bed for cleaning and inspection by ROVs.

During one lift while operating in the AHC mode, the crane unexpectedly disengaged from AHC mode and the wire slowly paid out approximately three to four metres, lowering the chain and then coming to a complete stop all without operator intervention.

There was no damage to the crane, mooring chain or the ROV. All activities were stopped, the chain lowered and the seabed and the crane de-rigged. Once the condition of the crane was established and a risk assessment had been conducted, the rigging was recovered to deck and the crane taken out of service.

The chief engineer and electrical technical officer of the vessel investigated the incident in co-operation with the crane manufacturer, who was able to remotely interrogate the crane’s computer diagnostic system. The following details were recorded:

• Crane type – active heave compensated offshore knuckle – jib
  
  
• Crane safe working load (SWL) – 70 tonnes (T)
  
  
• Lift weight – 44T at time of incident
  
  
• MRU location – crane pedestal – mounted in lower section
  
  
• Winds – 7-10 knots
  
  
• Sea state – 0.5-1.2 metres (calm conditions)
  
  
• Ambient temperature – 32 °C
  
  
• Solar radiation – high.

With regard to the causes of the incident, the following points were noted:

• The MRU returned a signal error that resulted in the AHC mode being deactivated by the high-high-position deviation alarm. This was confirmed by the crane log print out.
  
  
• Activation of the AHC high-high-position deviation alarm activated a stop condition for the AHC mode and therefore the crane control system reacted accordingly by shutting down the AHC mode.
  
  
• The MRU produced spurious signals due to overheating:
  
  
  • the calm conditions, high ambient temperature and solar radiation resulted in reduced cooling capability of the crane hydraulic power unit (HPU) cooling circuit.
    
    
    • As a result of this and the AHC being in use, there was a very high ambient temperature within the upper and lower crane pedestal. This high temperature caused the internal temperature within the MRU junction box to increase over time, which in turn increased the MRU operating temperature, and caused it to supply a spurious signal
      
      
  • the air-conditioning (AC) unit in the crane pedestal tripped out resulting in reduced cooling to the pedestal-mounted MRU junction box.