Jun 132011
 

Petro-chem tanker AlgoCanada

How does ventilating a cargo tank blow-up a ship’s bowthruster compartment? According to Canada’s Transportation Safety Board recently released investigation of the explosion aboard AlgoCanada on 24 July 2009 it went like this:

The apparent wind across the vessel’s bow creatFFull reportull reported an area of lower pressure in the forecastle. This, coupled with the open PV valves, induced a flow of gasoline vapours from the cargo tank into the forecastle through the drying unit and the modified non-return valve. Once the heavier-than-air vapours passed through the tank-drying unit, they settled downward and into the bow thruster compartment.

This compartment contained a heater unit that was automatically controlled by a thermostat, along with electrical lighting fixtures and a ventilation fan, none of which were explosion-proof. Smoke and burn patterns on the electric heater indicated that it was likely the ignition source.

 

Migration of flammable vapours from cargo tank to bow thruster compartment 1. PV valve 2. Double non-return valve 3. Forecastle door 4. Air-drying unit 5. Bow thruster compartment 6. Trunk

Says the report:

 

“Valve Modifications

It was the practice on the AlgoCanada to use the tank-drying system to vent and gas-free the cargo tanks. However, the air flow of the tank-drying unit was found to be inadequate for this purpose. Modifications to the double non-return valve were subsequently performed, which included the removal of 1 of the 2 valve plates. This improved air flow and shortened ventilation times.

Neither the valve’s manufacturer nor the classification society was consulted regarding the valve’s modification, which had effectively rendered it a single, non-return valve. Given that the vapours were subsequently able to migrate into the forecastle and bow thruster compartment, the remaining single valve plate was not an effective seal.

Use of Inappropriate Ventilating Equipment

When ventilating cargo tanks without first inerting, it is essential that a sufficient volume of air be supplied to ensure the atmosphere in the tank passes through the flammable range as rapidly as possible. The AlgoNova‘s P&A Manual stated that such volumes were to be moved by portable water-driven fans through tank-cleaning hatches. It was practice on board, however, to use the fixed tank-drying system, even though this had not been approved or designed for ventilation. Combined with the modification to the double non-return valve, this practice allowed migration of explosive vapours into the bow thruster compartment.

Use of Inert Gas

An effective method for reducing the risk of explosion in a cargo tank is to reduce the amount of oxygen to a level that does not support combustion. This can be done by replacing existing air with inert gas, which is a recommended best practice for all product tankers, regardless of tonnage. For example, the International Safety Guide for Oil Tankers and Terminals (ISGOTT) states that, “During a ballast passage, cargo tanks other than those required to be gas free should remain in the inert condition and under positive pressure to prevent ingress of air.”

In this occurrence, when the AlgoCanada left Sydney, Nova Scotia, in ballast, the cargo tanks’ atmosphere was in an “over-rich” condition. Although the vesselwas equipped with an IG system, this was not used. Moreover, the crew was not familiar with the ship’s IG system, nor were there procedures to maintain cargo tanks in an inerted condition.

It is accepted practice to purge the tanks of hydrocarbon vapour using IG prior to venting cargo tanks with air. In this manner, the transition from the inerted condition to gas free is done without passing through the flammable range bounded by the upper and lower explosive limits. 15 Had the cargo tanks been inerted during discharge and subsequently purged before venting, there would have been insufficient explosive vapour available to migrate to the bow thruster compartment and the risk of an explosion occurring greatly reduced.

Further, as demonstrated by this occurrence, vessels that do not inert cargo tanks or follow accepted procedures for purging and tank venting are at increased risk of fire and explosion, particularly during critical ventilation operations.

Safety Management Systems

According to the International Management Code for the Safe Operation of Ships and For Pollution Prevention (ISM Code), a critical objective of a vessel’s SMS is for a company to “assess all identified risks to ships, personnel and the environment and establish appropriate safeguards.” 16 This is achieved through plans, instructions and written procedures for key shipboard special and critical operations to ensure safe operation of ships and protection of the environment. Typically, many of these plans and instructions are found in onboard documents used to detail a SMS.

The ISM Code also addresses the company’s responsibilities for resources and personnel, making reference to qualifications, competence, manning, familiarization and, in particular, the training of officers and crew.

Shipboard Procedures

At the time of this occurrence, Algoma’s SMS did not ensure safeguards in the form of procedures and practices relating to ventilation and the requirement for the use of IG—2 common, critical safety tanker operations. Furthermore, the system did not refer to or incorporate the P&A Manual’s guidance regarding the appropriate equipment to use for tank ventilation. This meant the crew relied on informal verbal instructions and their own previous experience when venting tanks, both of which proved insufficient.

As a consequence, without formal procedures and training to mitigate the risks associated with tanker operations, the effectiveness of the vessel’s SMS was reduced, placing the vessel, crew and environment at greater risk”.

Findings as to Causes and Contributing Factors

  1. The inappropriate practice of using the tank-drying equipment for cargo tank ventilation allowed the migration of explosive vapours into the bow thruster compartment.
  2. The modification of the double non-return valve reduced its effectiveness and contributed to the migration of explosive vapours into the forecastle and bow thruster compartment.
  3. It is likely that an electric heater in the bow thruster compartment provided the ignition source for the explosion.
  4. Without formal procedures and training to mitigate the risks associated with tanker operations, the effectiveness of the vessel’s safety management system (SMS) was reduced.

Full report

 

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