Safe Shutdown for Tankers

In a world where autonomous ships are no longer the stuff of science fiction and unmanned ships might soon transit the oceans safely with minimal direct human interaction, this article explores whether it is now time for more autonomous cargo systems to address the oft-blamed “human error” from accident reports?

Emergency Shutdowns on Cargo Systems

Emergency Shutdown (ESD) systems have long been a requirement of the IMO IGC Code for the carriage of liquefied gases in bulk.

An ESD system acts as a link between the ship and the terminal. It can be automatically activated either by pre-defined conditions, such as high tank levels or high pressure or manually activated by an emergency button. If it is activated, by either party or an abnormal condition, then a sequence of events is triggered designed to return the cargo system to a static condition so that any remedial action can be taken in a timely and safe manner. This should include the structured closing of valves both on board and ashore and the tripping of pumps and compressors.

This safety system works well on gas carriers but would it work on a conventional tanker? Is there even a need for it?

We must ask ourselves: how many times do we read about the overfilling of cargo tanks because cargo tank ullages were not properly monitored? Or when a ship desperately tries to contact the terminal to stop pumping?

Industry issues recommendations

A recent paper co-published by the Oil Companies Marine Forum (OCIMF) and the Chemical Distribution Institute (CDI) discusses the subject of emergency shutdown systems for oil and chemical tankers.

The primary recommendation is that linked ESD systems should be used for oil and chemical transfers, including ship-to-ship (STS) transfers.

Amongst the core recommendations made is the requirement for shut down of cargo transfers in abnormal conditions, such as high tank level, high or low tank pressures, fire or gas detection or excessive movement in relation to cargo manifold.

A key feature of any ESD system is the ability to stop the cargo operation in a safe manner. No party, be it ship or shore, should have to shut a valve against a full flow of incoming liquid. A system that links the ship and shore can allow either party to activate a pre-defined and controlled shutdown procedure. This process protects the system against unacceptable pressure surges (which may have the potential to make the situation worse) and brings the transfer operation to a static condition safely.

Whilst the current ESD system on liquefied gas carriers is well established, one of the biggest flaws is the lack of standardisation. The type of umbilical connection is dependent on the terminal and a gas carrier may have to carry and be familiar with a number of different connections to be accepted into a port.

OCIMF and CDI propose a standard 5-pin twist connector that should offer a degree of standardisation. This should allow any tanker to berth at any terminal and be linked with the ability to transmit a signal from ship to terminal or vice versa.

If these recommendations become widely adopted then hopefully we will see an end to overfilling of cargo tanks. This in turn should reduce the risk of oil spills from tankers during cargo operations.

A copy of the paper published by OCIMF and CDI can be read here.

Author: Rod Maclennan