201123 Improper use of tank venting system

An aframax tanker of 115,000 tons dwt was loading crude oil with 3 x 16Ó arms connections. The hourly loading rate was about 10,000 cu m/hr and six cargo tanks were receiving the cargo simultaneously: two tanks per connection. Each cargo tank was fitted with a P/V high velocity valve of a through capacity of 3,000 cu m/hr at the opening set point of 1400 mmwg. The secondary venting system consisted of a P/V breaker and a mast riser.

As per terminal regulations, the vessel was required to use the Vapour Emission Control System (VECS), the vapour being sent back to shore. The high pressure alarm on the VECS line was set to 1260 mmwg (90 per cent of the P/V set opening pressure).

Before starting the loading operations, the master held a meeting with all the personnel involved in the cargo transfer operations. Among other instructions, he emphasised that at no time during the cargo transfer should vapour be released to the atmosphere. Accordingly, close monitoring of the pressure in the IG/vapour return line was required.

About an hour after loading commenced, the pressure on the VECS line was observed to be 650 mmwg, but a pumpman on deck noticed that the P/V valve of No 5 starboard cargo tank was open on the pressure side and was releasing vapour to the atmosphere. He immediately took a rope, climbed on the top of the P/V valve support and secured the P/V in the closed position and reported via radio to the chief mate. The master, who was in his cabin at that time, listened in on the radio conversation and without delay, rushed to the No 5 cargo tank to inspect the P/V valve for probable malfunctioning.

The P/V valve was still leaking even though it was lashed. The master immediately removed the lashing and the P/V valve opened fully on the pressure side. Investigation revealed that the IG isolating valve of that particular tank was locked in the closed position and obviously being isolated from the vapour return system, the tank was being pressurised, causing the P/V valve to lift.

Root cause/contributory factors
The day before arrival at the loading terminal, as planned, the chief mate ordered the pumpman to ensure all the IG isolating valves of the tanks were freed and operating properly and to lock them in the fully open position. Due to oversight, the pumpman had left the IG isolating valve of No 5 starboard tank locked in the closed position.
The chief mate failed to do a final check and the wrong status of the tank's IG isolating valve went unnoticed.
Immediate corrective actions

The master ordered the terminal to stop cargo transfer;
The master together with chief mate and pumpman reconfirmed that all the tanks were lined up properly and that all the IG isolating valves were locked in the correct open position;
While the loading was suspended, the crew was briefed about the potentially catastrophic consequences that can result from improper line up of the cargo transfer systems;
After cargo transfer was resumed, the master, the chief mate and crewmembers closely monitored all the P/V valves and the pressure in the vapour return line and the loading was completed without further incident;
After the vessel's departure from the load port, a special safety meeting was held during which the master explained to the crew the forces that tank boundaries can be subject to due to over-pressurisation.

Preventive actions
The incident has been communicated to the fleet;
The DPA has supplemented this notification with simplified data for easy reference highlighting the hazards arising from incorrect line-up of the venting/VECS system;
The incident will be discussed in future company seminars;
Improved training programmes to be delivered to crew;
Management to consider retro-fitting of pressure sensors in cargo tanks on all vessels in the fleet.