Natural resource derivatives like petroleum-based products are crucial for the world economy. These invaluable products include crude oil, or in other words, naturally extracted liquid compounds from which various kinds of petroleum products are derived or the derivatives themselves, like refined oils or gases. Specially designed oil tankers and LNG carriers transport these crucial products across the globe.
However, appropriate care must be taken to avoid accidents as these products are highly flammable, which means they tend to catch fire with the slightest trigger. It essentially means that with the interference of any causal factors near its vicinity, it can burst into raving flames in no time.
Speaking of these factors, a spark or fire is the main reason for these disasters. Remember seeing those large ‘No smoking’ boards in hazardous places like petrol pumps or gas stations?
While smoking is also prohibited onboard ships, especially in the enclosed spaces within the hull, there can still be other means for catching fire. They include accidental gaseous leakages, overheating of any machinery or systems, electrical sparks or short circuits, and so on.
However, while these aspects are considered for quality control, it can still be said that to err is human. So, it cannot be asserted with 100% certainty that everything will be perfect and ideal throughout a vessel’s service life, and there can never be emergencies or disasters.
But from the point of view of risks and hazards, we can always strive for two things: even in the doomed instance of something unwanted, we can still put in the best efforts to reduce the severity or impact of the aftermath or upshot.
Moreover, give in our sincere efforts to at least reduce the chances or the probability of something unwarranted happening. Once again, by the laws of nature, no likelihood of occurrence or non-occurrence, in this case, can be zero but yes, even trying our best to keep the value of this probability as low as possible is itself a positive trait. Hence, for the prevention of fire, this needs to be kept in mind.
All classification rules and regulations and statutory guidelines for all vessels have a special designation for fire hazards. You may have read about the Fire Safety Protection Plan for prevention or at least mitigation of fire and the Escape Plan for safe evacuation and escape of crew or passengers in a fire emergency. Now, for tankers, it is evident that the concern for fire prevention and mitigation is very high, given the cargo they carry.
While crude oil carriers or tankers have a significantly high degree of safety considerations, tankers carrying refined products like natural gases or petroleum derivates have an extremely high degree of safety rules as they are even much more flammable than crude or unprocessed products.
While we shall discuss some of these safety measures in our future publications, for now, let us discuss an essential feature of oil tankers, the mast risers.
What is a mast riser?
Now, looking at the nomenclature, please do not get confused! This has nothing to do with mast, which is also very much a marine term, and alludes to those vertical erections over the main deck, which have been those supports for sails in older ships to various tower-like installations for telecommunication or other purposes in modern vessels.
A mast riser is essentially a device or, rather, a system to relieve the pressure accumulated inside the cargo tanks of oil tankers. Now by pressure, we mean what? Most liquid substances, when entrapped or enclosed within a bound space, until and unless filling the entirety of the given space, emits a vapour that occupies the remaining volume within that space.
Now, while their chemistry may differ from that of soft drinks, even petroleum products tend to emanate vapours around them when enclosed. These keep on accumulating over time if not allowed to escape. The pressure builds as these vapours accumulate without being allowed to escape. This is the case in tankers holding petroleum products in enclosed tanks or holds.
Now, these crude oil vapours are equally inflammable as their original liquid form. Moreover, by the chemistry of inflammable substances, the combustibility is directly proportional to the pressure. Henceforth, as the pressure surmounts within the closed tank spaces, the risks associated with inflammability develop manifold.
The vapours not only become highly pressurised but also put pressure on the fluid medium it co-occupies. Now, as liquids are incompressible, it is once again a problem. Henceforth, there should be means to prevent this unwanted pressure from building in large amounts and allow the accumulated vapours to escape from time to time. This is precisely the role of a mast riser.
Design and Construction of a Mast Riser
The mast riser is nothing but a valve and comes under the category of cargo tank venting systems. In appearance, it resembles a pipe-like installation that originates above the tanks or spaces and projects above the main deck level.
It is a ventilation or venting component. The exposed part or the portion of the riser that projects above the main deck plate is 6 metres for all practical purposes.
The inlet is located inside the cargo hold, while the outlet is situated outside. Within the cargo tank, there may be various piping systems that are sometimes combined into a single common outlet above the main deck. These pipes serve as venting systems to release accumulated vapors from the tanks or holds. An isolation valve connects these venting lines and the mast riser.
Additional valves, such as bypass valves, pressure valves, and vacuum valves, may be installed in conjunction with the mast riser. Below the mast riser is the VOCON valve, which stands for Vapour Pressure Release Control Valve. This valve controls the closing pressure of the mast riser system based on operational requirements and settings made remotely.
The mast riser is activated only when the internal pressure of the tank or hold reaches a high level. It is essential for releasing vapors and maintaining safe pressure levels during loading and unloading operations. The mast riser is equipped with a flame arrestor at the outlet for added safety measures.
Apart from fire prevention, the mast riser and other venting systems are crucial for expelling vapors before loading liquid cargo to ensure a smooth and efficient process. Another similar system, the Vapour Emission Control System (VEPC), redirects gases to the shore via pipelines during loading and unloading operations.
Most liquid cargo vessels have multiple mast risers customized based on tank size, cargo properties, and regulatory requirements. These risers are typically automated for easy operation and require regular cleaning and maintenance to prevent blockages. The watertight design of mast riser outlets ensures protection against water ingress during adverse weather conditions.
In conclusion, the proper functioning of mast risers and venting systems is vital for the safe and efficient transportation of liquid cargo on vessels.