Chemical Tanker Explosions

I am currently serving on a Type 2/3 chemical/product tanker built in 1980. The vessel is currently on passage to the Far East with a full cargo of chemicals. I have over 25 years command experience, mainly on chemical tankers. These have ranged from quite sophisticated stainless steel ships in the 3, 000 to 10, 000 dwt range to the larger ships with mainly coated tanks.

One of the biggest jobs on a chemical tanker is tank cleaning. This is sometimes an arduous and prolonged task that can go on around the clock. There is always subtle pressure to get the ship cleaned and ready for the next cargo as quickly as possible, generally without any delay between ports. This can put a lot of pressure on the Chief Officer and deck crew. Short cuts are often taken. I have seldom seen a cargo tank gas freed before washing commences. It is normal practice to start washing directly. A newer chemical tanker, built to the IBC Code, is required to follow strict gas freeing procedures if built after 1994. There is no such requirement for a ship built prior to 1994 or built under the old BCH Code (prior to 1986), to follow such rules. In general, direct washing is not considered to be a problem due to the small size of tanks involved. The tanks are generally well coated or of stainless steel construction. They normally gas free very rapidly during the washing process. Venting at the end is more for drying than for gas freeing. I am convinced that the majority of explosions that occur at sea are due to tank cleaning without gas freeing first in non-inerted cargo tanks. It is quite likely that the initial source of ignition is due to either faulty electrical items within the hazardous area or an item such as a tank cleaning machine being dropped into a tank which is not gas free. I do not think that other sources of ignition (electrostatic generation from impinging water jets) are a likely source of the problem. Chemical tankers have much smaller tanks than crude oil tankers.

I also feel that there is a lack of training and awareness of potential problems. My present employer is more involved in the product and crude oil trade. Our chemical tankers are relatively new to the Company. We have very good crews who regularly return to the same ships but we still lack that full chemical tanker awareness that you would find with one of the major chemical tanker operators. There you will find that chemical knowledge has been passed down through the years and the crew have often never sailed on any other type of vessel. Sadly, I hear that this wealth of experience might be disappearing as the major players in the industry are seeking to replace these crews with ever cheaper crews who cannot possibly have the same experience or knowledge. As far as I know, most of the incidents I have heard about refer to ships owned by smaller companies but which were often on Time Charter to the bigger Owners in the market at the time of the incidents.

Inerting tanks would save a lot of problems. The more modern chemical tankers have nitrogen plants capable of supplying large amounts of N2 but I am not sure if this is sufficient or of high enough quality to inert all tanks for all cargoes carried that have a low flash point. Many chemical tankers, like my present command, are chemical/product tankers. They carry IGG (inert gas generator) plants which burn gas oi l instead of using flue gas. This gives a much better quality inert gas but it is not suitable for most chemicals carried. Even at peak performance there is still the risk of slight soot discolouration in cargo tanks which is very unacceptable in most chemical trades. For this reason there is a great reluctance among chemical tanker men to use the IG. When carrying CPP cargoes (clean petroleum products) the IGG system should be used. There might be a reluctance to do this because of lack of practice with the system, for example on a chemical tanker carrying products for first time in several years. Fortunately, my present vessel carries products fairly often so most of the crew are well versed in the operational procedures for the plant. Like any shipboard equipment, it is much better to use it as much as possible rather than leaving it idle. It can save a lot of problems in the long run.

Another interesting problem facing chemical tanker owners is the problem of venting under controlled conditions. This means that all air and gas displaced during loading is to be vented out through PV (pressure/vacuum) valves. The normal setting for these on a chemical tanker is 2000 mmWG (or 2.74 psi). As the PV valve only opens at this pressure it is clear that the actual tank pressure during loading will be above that. Although supposedly designed to withstand higher pressures than 2000mmWG, we are experiencing inter tank cracking occurring. At the same time the vacuum setting is -350 mmWG (about 0.5 psi). This vessel's tank system is designed to -700 mmWG. During discharging under such controlled conditions, we can generate a large vacuum, especially in some tanks where the maximum venting capacity is well below the designed discharge rate of the pumps! A lot of these problems are only more recently coming to light because we are now being forced more and more to follow the rules. Structural failures on chemical tankers due to this reason are not uncommon. In the past, chemical tankers used open venting methods nearly all the time, despite the fact that for many cargoes, controlled venting was required. I recently had a very senior surveyor, (actually the District Manager in a large country), for a major Classification Society, say to me that PV valves were only safety valves and not for normal venting during loading! What this all means is that chemical crews face the choice of either following the rules and risking damage due to over pressurization or vacuum, or surreptitiously flouting the rules by using open venting as well to relieve the pressure or vacuum. By trying to do this without being seen to do it, there is far more chance of an accident occurring as safeguards like flame screens are not necessarily in place. Owners are actively looking at legitimate ways to solve this problem but it is not easy or cheap to do. A major part of this problem could be solved by using shore vapour recovery more. Regrettably, this is not always available. Any berths that have it do not always have it in operation. In most cases, the berth can only cope with loading one grade at a time with their vapour recovery system, this means extra time for the vessel on the berth. Berth time is very critical in the tanker industry, less than honest berth Owners will do their best to bend the rules to avoid having to pay demurrage.