The Incident:
An Oil Tanker (VLCC – Very Large Crude Carrier) was discharging its cargo. Ballasting was planned in the early hours of the day in order to correct the trim/List. The engine room got flooded to a level of two metres above tank top (to just below the engine room floor plating). Had the flooding incident lasted a few minutes longer, serious machinery damage could have resulted. The incident was reported to occur during ballasting operations, when the configuration of pumping from and to ballast tanks was altered. During this process the cover plate of the sea chest strainer was blown off as the wing nuts (clamps) slipped. This resulted in seawater ingress into the engine room.
Engine room personnel instructed the cargo officer (Deck officer on watch during the cargo operations) in charge of ballasting to stop the ballasting operations (without mentioning reasons).
As per company procedure, the overboard suction valve and overboard discharge valve were opened to ‘de-energize’ the pumping system. Since a direct connect between open water and engine room got created the situation went worse. Sea water started flooding the engine room spaces more forcibly than before. This caught the engine room personnels immediate attention and immediately the cargo officer was instructed to close overboard valves. Thus the ingress of water was reduced and other contingency measures were followed. Emergency Bilge Suction valve was put in use to reduce the water level in the engine room bilges. Terminal and port controls were alarmed appropriately and the incident was logged in Engine room Logbook.
Root cause:
- Malfunctioning of one of the actuators of the valves in the ballast pipelines caused the valve to close in 1.5 – 2 seconds (normally 15 – 20 seconds)
- This created a pressure shock in the ballast system
- Subsequently, this pressure shock caused the cover plate of the sea strainer to blow off
- The actuator of the valve which was closing in 1.5 – 2 seconds was replaced by vessel crew in early 2014 without verification of the opening/closing time of the valve
- Communication between engine room personnel and bridge personnel was not sufficiently detailed
- This resulted in incorrect actions being taken which caused a worsening of the situation
- Ballasting procedure of manufacturer was not followed. The recommended procedure described that pump(s) should be stopped before operating valves in the ballast lines. This was not followed
- Introduced vessel-specific procedures for ballasting and added this instruction to the official document list of vessel
- Following the hierarchy of control, the company recognised it could not eliminate nor substitute the ballast process on board. Therefore, it focused on the following engineering controls by: − Improving visual presentation for valve operations on the bridge/Cargo Control Room (CCR) to emphasize valve position and open lines in the ballast system introduction of interlock system to eliminate simultaneous operations of valve(s) and pump(s)
- The Engine Department should try out the emergency bilge suction valves and record the same in the engine room logbook. Most cases, the Engine staff do not test the proper functioning of this valve and carry out a table top drill for the same
- Sea chest and other sea water line filters need proper attention when tightening them after filter cleaning. The clamps has to be tightened even and proper in ‘opposite first’ fashion. It is always essential to put the sea water line filters in use and test them against leaks after purging out air effectively. If you choose to keep the sea-chest filters isolated after cleaning as a standby, ensure that the sea chest cover nuts are tight and there are no leaks after change over
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Are we missing some thing here !! The report did not mention which part of the strainer cover caused the lid to blow off. Was it because of the clamp slipped off or because some of the clamp bolt sheared off. In normal case these 6 clamps are designed to stand at least 1.5 times of the highest working pressure. If the clamp bolt sheared off under the strain, it could be due to the corroded bolt or / and un-even tightening of the clamp butter fly nuts. If we can examine the sheared cross section of the bolt, we can understand if there is any crevice corrosion propagated from thread root. If some bolts were over tightened, load will be taken by only these bolts, instead of all 6 bolts and it gave up first, leaving the remaining bolts to take up all load and lead to ultimate blow up of the lid.
Dear Mr. Ali,
Thank you for your comments. Though the wing nuts slipped off, the report emphasis was on the lack of communication between the deck and engine staffs whereby there was an abnormal pressure surge in the ballast line – which ultimately caused the strainer cover wing nuts to slip and thus flooding. The vessel or investigation did not reveal anything significant on the clamps/wing nuts shearing-off. The main root cause was pressure surge in ballast line and sea suction as well as overboard valves were kept open which caused sea water to gush in.