On August 15, 2018 an explosion rocked the VLCC Desh Vaibhav. The vessel had just off-loaded 270,000 mt of crude from the Persian Gulf and her tanks were empty at the time of the explosion. Tragically, several crew members were killed in the incident.
The explosion is reported to have occurred in a forward empty tank. Now, we all know that an empty tank can me more dangerous than a full tank. A full tank usually does not have enough of a vapor space to allow a flammable atmosphere. The empty tank is filled with the vapors from its previous cargo. In this case, the previous cargo was crude oil whose vapors are extremely flammable.
Many experts at the time stated that due to the tanks previous cargo the explosion was not unusual. One would think that this was a logical conclusion. Flammable vapors can explode if they are confined.
So, why do I say that it should it be unusual to hear that this empty crude oil tank exploded? And, why is important that land-base firefighters care about this at all?
First, lets look at the history of the marine transport of crude oil. Many years ago, crude oil tankers were merely single hulled vessels with very few safety features. There were also not many restrictions on the venting of the tanks after unloading. In fact, many times after unloading, the tank covers were just left open to allow the tanks to vent by themselves. These openings were supposed to be covered with a flame screen but that was often either not done or the flame screens were not well maintained, and many contained holes.
If the vapors escaping the venting tanks reached a source of ignition and those vapors were within the flammable range, the vapors would ignite and burn back to the open tank causing a devastating explosion.
One such incident occurred on Dec. 17, 1976 in Los Angles harbor aboard the SS Sansinena, a tanker ship which had just unloaded her cargo of 20 million gallons of crude oil. The cargo tanks were left open to vent the tanks. This venting was legal at the time if there was a sufficient breeze. On this night however, the breeze was very slight and the heavy crude oil vapors wafted along the deck until they found an ignition source. The resulting explosion killed 9 people, broke the ship in two, and blew the deck off the ship and sent it 200 feed inland.
That incident changes some of the rules and even prompted some changes to tanker design. The SS Sansinena had a mid-ships bridge with an officer accommodation area over the product tank. After this explosion no accommodation blocks could be located above the product tanks. Another change will be mentioned below.
So, that’s for a tanker at dock. The VLCC Desh Vaibhav was underway when the explosion occurred. Let’s go back in history again to look at this issue. Many years ago, numerous crude oil tankers were experiencing explosions mid-ocean while returning home after discharging their cargo. Investigators were looking for any hot work being performed during the explosion which might have led to the ignition of vapors.
In many cases the crews were adamant that there was no hot work being carried out at the time of the explosion. However, the consensus of the investigators and the ship owners at the time was that the crew members were lying about the hot work because the only other operation being carried out at the time of the explosion was the washing of the tanks.
At that time tank cleaning was being carried out with water, under high pressure, ejected from nozzles either permanently mounted in each tank or from portable nozzles which would be brought out and mounted into deck openings. Again, these nozzles were only using water. That could not cause an ignition……Or could it?
After many of these explosions, which damaged or destroyed the vessels and killed many crew members, the owners wanted a review of the water washing procedures. Engineers were able to duplicate the tank cleaning procedure for study and found that when the water left the end of the nozzle and was traveling to the sides of the tank the stream built up a static electric charge. Just as the water stream was about to touch the side of the tank the electric charge would jump, in the form of a spark, from the stream to the tanks wall. The atmosphere inside of the tank with the crude oil residue was highly flammable and confined in the tank. The explosion was so violent that it often blew holes in the vessels decks and hull plating and in many cases caused the total loss of the ship.
We have just found another issue that need regulations. It was determined that tankers with flammable cargos would need to be fitted with a “gas inerting system”. In 1974 the US Coast Guard formulated regulations requiring Inert Gas Systems (IGS) on all crude oil tankers over 100,000 tons that were built AFTER 1974. This rule did not affect existing ships and only applied to ships operating in US waters. After the SS Sansinena disaster the rule was extended to include all oil tankers over 20,000 tons and operating in US waters. The International reaction took much longer to provide regulatory measures to counter this problem.
In 1982, the International Maritime Organization (IMO) rules required IGS on all new oil tankers above 20,000 deadweight built after May 1982. The IMO rule was amended in May 1985 to include existing tankers.
What exactly is a Gas Inerting System? In a cargo tank containing flammable vapors there will be a vapor area above the cargo which can be flammable if there is or was a flammable cardo in that tank. If we inject another gas, which is inert, and will not support combustion, into that vapor space in enough quantities, there will no longer be a flammable atmosphere.
The International Convention for Safety of Life at Sea (SOLAS) 1974 requires the Inert Gas System be capable of delivering inert gas with an oxygen content in the inert gas main of not more than 5% of volume. By maintaining a positive pressure in the cargo tanks at all times, with an atmosphere not having an oxygen content greater than 8% by volume, the tank atmosphere is rendered non-flammable. For an added safety margin, the figure of 5% is used.
And where do we find this inert gas? One of the most common inert gasses in Nitrogen. Many ships with IGS systems will have nitrogen generators aboard. However, this can be expensive.
But someone in the marine community looked up that his vessels exhaust stack and said, “that’s an inert gas being ejected out into the atmosphere. Why don’t we use that to inert the tanks and save money! And they did.
Most IGS systems aboard tankers today are supplied by the vessels own exhaust gas. Of course, the gasses must be cooled and cleaned before you would inject it into a cargo tank. That is why these vessels also have exhaust gas scrubbers. The cleaned exhaust gas is inserted into the vapor area of the cargo tank until the oxygen content of the vapor area is below 5%. At that percentage there can be no combustion.
When the ship is off-loading, the inert gas is continued to fill the empty cargo space and maintain the inert atmosphere. When the tanks are filled the surplus inert gas is vented out of risers on the deck or the vapors are returned to the loading facility.
Some of these vessels will have both the exhaust gas IGS and a Nitrogen generator which would be used as a back-up should the ships engine shut down or be unable to keep the O2 level below 5%
Safety alarms are set to go off if the oxygen level increases and the loading/un-loading operation would be shut down until the condition is corrected.
So, let us get back to my initial question from the start of this article. Why is it unusual that the VLCC Desh Vaibhav exploded? That vessel should have had a required IGS system which, if operating properly, should have made an explosion inside of a cargo tank impossible.
We will have to wait for a full investigation to find the answer but I’m willing to bet that something failed aboard which prevented the IGS system from doing its job.
This is important to land-based Firefighters because the first question I, as a marine firefighter responding to a fire aboard a tanker either loading or unloading would be, “Is the IGS system functioning properly? What is the O2 level in the tanks?”
If the IGS system is not operating properly then both firefighters and mariners should be aware that the International SOLAS regulations for Inert Gas Systems requires that the system can be augmented from outside sources via required fixed piping for that purpose.
And if you’re a mariner, those questions should be even more important. You will be on that vessel while its underway and you will not be able to just evacuate to the dock as I would!
Until next time. Stay safe out there.
For more information, go to www.marinefirefighting.com