201215 - Release of inert gas/cargo vapour mixture at berth
A tanker was berthed at a terminal in the tropics and discharging crude oil. The port is in a notified volatile organic compounds (VOC) controlled area. Soon after discharge had commenced, the terminal requested a temporary cargo stoppage without advising the reason or expected duration (presumed to be due to lack of storage tank space ashore). During this period, due to very high ambient temperature, No 2P COT pressure relief valve opened, releasing inert gas (IG)/cargo vapour mixture to the atmosphere, in breach of the applicable Annex 6 of Marpol.
1. The vessel was carrying Maya crude, a highly volatile and sour (containing hydrogen sulphide) cargo. Cargo was loaded at a higher than usual temperature (48° C) and due to the short voyage, the cargo temperature was unchanged at the discharge port;
2. Cargo tanks were only part-full, so the inerted volume was significant;
3. Prior to berthing, the tank inert gas pressures had been reduced to 70 mm WG;
4. There was a lengthy delay between vessel’s arrival and commencement of discharge, which was temporarily suspended a few hours into the operation;
5. There were abnormal heat-wave conditions at the discharge port;
6. During the stoppage, tank pressures rose significantly probably as a result of the unsaturated ullage space containing mainly inert gas;
7. After some time, No 2P COT pressure vacuum valve (PVV) lifted at between 1200-1400 mm WG as per the design parameters of the valve;
8. The Master immediately requested permission from shore to cool the tank deck with sea water from deck monitors. This was partially successful in reducing pressure by 80 mm WG;
9. All personnel were properly briefed and trained and were wearing personal multi gas detectors. Breathing apparatus sets were distributed on the main deck; 10. As no H2S alarm was activated, it is highly probable that the released vapour was mainly inert gas.
Root cause/contributory factors
1. Lack of planning – Given the prevailing heat wave conditions, the known properties of Maya Crude, and the lack of shore tank space, the terminal should have planned the berthing better, so that immediate and continuous discharge could take place and avoid over?pressure in ship’s tanks;
2. The vessel should have both anticipated and more closely observed the rise in cargo tank pressure and should have notified the terminal immediately on the developing hazardous condition.
Lessons learnt
1. In circumstances such as those described above, terminals should plan berthing only when there is sufficient space available to receive the cargo at the tanker’s optimum discharge rate;
2. Terminals (and charterer’s agents where appropriate) should freely provide vessels with timely and complete information on anticipated operational delays, to allow contingency planning;
3. Vessels must monitor cargo tank conditions continuously, with due regard for prevailing and expected ambient conditions. Company operating procedures and C/O’s standing orders for deck watchkeepers should reflect this requirement; Concerns about cargo/vessel conditions should be communicated to the terminal promptly and updated as necessary. Countermeasures should be discussed and agreed with terminals. As far as possible, such countermeasures should not lead to breaches of regulations, and practical legitimate alternatives should be considered first, e.g. Vapour Return Line. Under the Vessel General Permit for Discharges (VGP) regulations in the USA, deck washdown in US ports is an action of last resort, to be avoided except in emergency;
4. Where pressure reduction by deck cooling is unavoidable, it should be started as soon as possible, and the necessary permissions must be obtained. If appropriate, a VGP Noncompliance Report must be submitted to the authorities, explaining the reasons. The best practices listed in VGP Section 2.2.1 should be complied with to the extent possible.
Corrective/preventative actions
A fleet circular on this incident was issued for information, discussion and compliance.