Blowing and pigging of pipelines at terminals poses inherent risks for the terminal and a chemical tanker. Frequent damages to tanks have occurred. If there are doubts about the shore operation or signs of problems ashore the OOW must immediately request clarification, reports Chemical Tanker Guide.
Cargo monitoring equipment maintenance
The pre-cargo meeting shall include full information regarding the final stage of cargo operations – any blowing or pigging. Standard procedures must be followed, including throttling the manifold valve and venting the tank. Line clearing shall be considered as critical operation and subject of risk assessment.
Vessel should confirm during the pre-operation meeting with the terminal if the pigging / blowing operation would be towards the ship’s tank. This should be avoided if the ship’s tanks are nearly full.
Maintenance of cargo monitoring equipment, such as pressure sensors, must be done at the earliest opportunity after any defects are found. Ship’s manifold must not be left unattended during blowing or pigging operations and manifold valves must be closed during idle periods.
Onboard a chemical tanker an incident occurred during loading operations at a US Gulf port.
The port slop tank (PST) had been nominated for loading of Crude Degummed Soybean Oil. The cargo equipment was checked and inspected as per company requirements prior to arrival, during which the PST remote pressure transducer was found inoperative.
Bulk loading into the port side tank was completed at 0615 on the day of the incident with the ship lines and shore hose blown shortly after.
At 1830 the Terminal asked the vessel to re-open the PST for further blowing and commenced blowing the shore line into PST. The Terminal advised the vessel to keep the manifold open. At 2315 the Duty Officer heard a heavy striking noise on deck and noticed air blowing out of the PST pressure valve under high pressure. Cargo operations were stopped immediately. On investigation it was found that cargo was migrating from PST into COT 6 port. All adjacent tanks were inspected and cargo was also found in the WBT 6 port.
What Went Wrong (Critical Factors)
Although the terminal advised they were blowing the lines, actually the terminal was pigging the line. The terminal incorrectly considered blowing and pigging as similar operations.
Communication and information from the terminal to the ship interface was inadequate
The ship’s PST manifold valve was kept opened from 1830 till the time of the incident. This was requested by the terminal. The terminal did not provide an estimated time of completion for the clearance of the shore pipeline system, thus the ship’s crew did not know when to anticipate completion. The manifold valve was left unattended during mentioned period.
Whilst the pressure transmitter in the PST was inoperative, the pressure in the tank was monitored on the local tank gauge. A spare pressure transmitter was available on board but it had not been fitted.
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Source: Chemical Tanker Guide