200515 Uncontrolled Venting

15 Jan 2005 MARS

Uncontrolled Venting
MARS Report 200515

The vessel concerned was loading a full cargo of propylene in a NW European port. Cargo operations were well established and the vessel had been loading at full rate for several hours. As the chief officer was satisfied with the status of the deck, he took the opportunity to take some rest in preparation to topping off cargo tanks. He handed over responsibility to the third officer at about 1130 hours. At 1200 hours the normal change of watch took place, the second officer was briefed on the state of loading, cargo machinery operation and cargo line set up. The chief officer's loading program was read through and signed by the second officer, this included a remark that condensate was being returned to number 2 cargo tank only.

In order to stagger the topping off process of the cargo tanks, the cargo tank loading valves had been adjusted to achieve the desired 'spread'. To accelerate this tank level stagger, the second officer closed the condensate return valves to number two cargo tank, not appreciating that there were no other condensate return valves open elsewhere on the system. Shortly after closing the above valves, unusual noises were heard from the cargo compressor room and, on investigation, the second officer noted that the cargo condenser of the re-liquefaction plant was full and the associated compressors were operating noisily.

The chief officer was immediately called and he tried to drain the condenser via the by-pass line of the expansion valve of the re-liquefaction plant, this had no effect. On checking the return line it quickly became apparent that there were no valves open on the tank domes, a suitable valve was opened and the condenser drained. Meanwhile; the loading master had stopped loading operations as he noted cargo vapours escaping from the vent masts. One of the three cargo compressors had stopped on its high pressure safety cut out device but the remaining machines continued on line, venting to atmosphere via one of the high-pressure safety valves. Once the nature of the problem was identified the abnormalities were rectified and the vessel resumed loading within the hour.

Root Cause

At first sight it would appear that the root cause of this incident was human error by the duty officer. However, this was only the trigger point of the incident. After due investigation, which included a rigorous testing and calibration of the equipment, it was determined that, in all probability: -

  • Closing the condensate return valve to the cargo tank caused the cargo condensers to over fill.
  • This over filling caused an abnormally high pressure in the re-liquefaction plant systems
  • One of the three compressors was stopped by its high pressure cut out safety device
  • One or more of the non return valves of the cargo compressors had failed in service
  • A high pressure safety valve had opened prematurely

The remaining on line compressors continued to operate, venting to atmosphere via the cargo riser

Whilst the necessary maintenance had been carried out by the responsible officers and in accordance with the SMS system, certain items had been omitted from the procedure, specifically the inspection and maintenance of the non-return valves. The high-pressure safety valves had been tested and certified at the most recent dry-docking (less than 8 months prior to the incident).

Lessons learnt
  • Particular care should be taken with all matters relating to safety and the maintenance of cargo equipment, reliance on the electronic planned maintenance should be approached with an element of common sense. If normal maintenance of any particular part of a system is not included in the database, the necessary action to maintain this equipment and the maintenance program should be modified accordingly, a suitable non-conformity report against the system is to be raised to ensure that corrective action is taken.
  • Testing of safety devices should be quantified (rather than just an operational check) and the results recorded for comparison. This will help determine any changes in setting and/or deterioration in the accuracy of protection devices.
  • A review of current practices relating to the testing protocols and timings should be undertaken and the company systems are to be adjusted in accordance with the findings of this review.
  • The in-house training of newly promoted second officers, and those joining the company for the first time, is to be made on the computer based simulator for LPG/LNG cargo operations.

It is the Company's requirement that a port to port hand over for junior officers who are unfamiliar with the ship type should be of such a length that they include at least one full cargo operation, (on this occasion "one full cargo operation" was not completed).

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