Key Points To Be Considered for Safe Pigging Operation On Chemical Tanker


A chemical tanker is scheduled to load various grades of cargo. Some of the cargoes have a very high melting points and the cargoes are therefore likely to solidify at ambient temperatures. Once the tanker is safely moored to the berth, the chief officer completes the ship-shore safety checklist with the loading master during the pre-cargo operation meeting. They also discussed and focussed on the hazards to be aware of during the loading operation. However, issues such as the notice period required by the ship prior to line clearing, the time required for a pig to travel along the line, the pressures and venting capacity of the ship’s reception tank and the communication routines during the entire operation were not discussed and agreed during the meeting.

The terminal advises the vessel that the shore line to the vessel will be cleared by means of pigging using nitrogen gas after each cargo has been loaded. Once the loading operation is complete, the ullage spaces must be purged down to less than two per cent oxygen.

The vessel commences loading operations. As some of these cargoes have a high melting point, the crew on board are very careful to ensure that all lines, bends and connections are fully cleared throughout the loading operations. According to the ship-shore safety checklist, the terminal will notify the vessel before commencement of the pigging operation.

With a few hours left before completion of the loading operation, the duty officer notices a sudden increase in pressure in a number of the cargo tanks. He immediately contacts the terminal to reduce the loading rate and to ask if the pigging operation has commenced. In spite of repeated calls, the duty officer receives no response from the terminal and after several failed attempts at contacting the terminal, the duty officer asks the deck crew at the manifold to advice if they have seen someone from the terminal on the jetty. Whilst trying to communicate with shore the duty officer hears a loud bang. The deck crew reports an explosion in one of the cargo tanks due to too high a pressure.

One of the main factors in the above incident was the fact that the crew and the shore personnel had underestimated the importance of clear communication lines, resulting in key personnel not having a common understanding of the operation itself.

How to improve by lessons learnt

Based on the above case and the keywords below, you should now perform an onboard risk assessment of the incident and the factors which led to it. Bear in mind the company’s SMS procedures and onboard practice when answering the questions listed below.

  • What are the hazards associated with pipeline clearing operations? Have these hazards been communicated to the involved ship’s personnel prior to the cargo operation?
  • What are the main contents of the ship-shore safety checklist? Which topics should be included in the pre-cargo operation meeting?
  • What should be done if there are any discrepancies between what was agreed before loading and the actual loading operations?
  • What are the “pressure” and “vacuum” settings on your vessel’s PV valves?
  • What are the alarm settings for high and low pressure for the individual tanks?
  • What is the maximum loading rate when loading in only one tank?
  • What are the dangers associated with “pigging” operations?
  • What systems do you have in place to stop loading if there is no response from the terminal in an emergency? Are emergency shutdown procedures discussed during ship-shore safety meeting?

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Source: GARD


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