Ship to Ship mooring incident

Outline:

Specialist operations require extra diligence.

What the reporter told us:

A ship to ship transfer was taking place between a 106,000DWT tanker (discharging) and a 40,000DWT tanker (loading), the transfer was completed at 10:36 and cargo hose disconnection was completed at 10:42.

Shortly afterwards, at 11:00, the person in overall advisory control (POAC) informed both vessels to prepare to commence the unmooring operation due to rapidly deteriorating weather conditions. The departure checklist was completed at 11:12, by which time the actual weather conditions were wind NE 25kts, gusting 30kts, with a NE swell of 3m (both elements exceeded the agreed weather criteria for the STS operation).

The agreed unmooring plan called for the larger vessel’s fore and aft wires to be cast-off first. This was to be followed by the simultaneous release of all the smaller tanker’s head and stern lines.

At 11:25 the smaller tanker’s two (2) aft spring lines parted. At this time, the larger ship requested the smaller tanker to run off its three (3) headlines and two (2) forward back springs as his crew (large tanker) were unable to release them.

At 11:30 the unmooring operation was completed. Subsequently, the mooring ropes released into the water were returned to the smaller tanker by a service vessel.

Additional Information: The deterioration in the weather had been forecast but not until later in the day. There were no tugs available at the site of the STS transfer.

CHIRP comment:

Ship to Ship (STS) transfers are specialist operations fraught with potential hazards with parting mooring lines being high on the list of possible dangers.

  • Two dissimilar vessels will each have their respective pitch, roll, heave, surge, yaw, and sway movement periods, potentially in opposition to each other at any given moment. This can put tremendous snatch loading on the mooring lines.

Image courtesy of Witherby Publishing

 

  • This differing movement makes balancing the load on the mooring lines more difficult than conventional mooring operations.
  • For this reason, ship to ship transfers should be carried out only under favourable weather conditions with constant monitoring required to ensure that the agreed weather parameters are not exceeded – especially the sea and swell conditions.
  • Weather forecasts are more important to a vessel engaged in an STS operation than they are at sea, due to the proximity of obstructions and hazards.
  • The authority to cease STS operations rests with both ships involved, either one can stop the operation on the grounds of safety.
  • STS operations will normally have their own mooring requirements but if not, or if there is any dispute, then the OCIMF Mooring Guide (MEG4) should be considered the definitive mooring guide.
  • All crew members of vessels involved in STS operations should be fully conversant with all aspects of the agreed standard operating procedures including any special arrangements for quick release of mooring lines if this becomes necessary. Regular emergency preparedness exercises for unmooring should be practised.
  • It is important that all mooring lines in each of the three (3) groups, breast lines, spring lines, and head / stern lines are of the same size, construction, breaking strength and length to ensure equal tension on all lines. Dissimilar characteristics within a group can lead to rapid parting of mooring lines.

Image courtesy of Witherby Publishing

 

 

Report Ends………………………….

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