Piping Arrangement - Conventional Oil Tanker Basics

12/14/2016

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Piping Arrangement The pipes leading from the cargo tanks to the pumps are termed bottom lines, from the pump-room up to deck are called risers. The lines on deck are termed deck lines. The lines which lead from the deck to the tanks are called drop lines. Besides these, there are Crude Oil washing lines on deck (COW lines). The COW main line usually branches o from the main discharge line in the pump-room. It further branches out to the various tanks on deck. There is also a small diameter line (Marpol line) which is used to discharge the last part of the cargo from the ship. In the cargo tanks, the pipes terminate in a bellmouth. A tank may have two bellmouths – one main and one smaller stripper bellmouth. Alternatively, one bellmouth may serve the purpose of main as well as stripping discharge. The piping system has evolved over the years to cater to varying cargo requirements. In a product tanker which is designed to carry many grades, we see that there are many more pipes so that many grades can be catered to. In a crude oil tanker, the piping is straightforward and simple. There are three basic types of pipeline systems: 1. Direct Line system 2. Ring main system 3. Free ow system. Each system has their uses and is designed to full a need in a particular type of vessel.

Direct Line system https://www.cultofsea.com/tanker/piping-arrangement-conventional-oil-tanker-basics/

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It consists of lines running longitudinally in the centre tanks and branching out to bellmouths in the centre and wing tanks. The system is uncomplicated and found on some crude carriers.

The advantages are that: 1. it is easy to operate and less training of personnel is required 2. as there are fewer valves, it takes less time to set up the valve system before commencing a cargo operation 3. contamination is unlikely, as it is easy to isolate each section. The disadvantages are that: 1. the layout is not as versatile 2. very rigid system which makes it dicult to plan

Ring-main systems It is also called the circular system. This type of piping system provides for the handling of several dierent types of oil. A particular tank can be pumped out either by a direct suction line or through another line by use of a cross-over. The system is very versatile.

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Free-ow system In this system, the oil ows freely into the aft most tanks when the interconnecting gate valves are opened. Main suction bellmouths in a full free ow tanker will only be provided in the aft tanks. However, each tank is generally provided with a small stripping line. This system has the distinct advantage of having lesser and less complicated piping system in the tanks and is suitable for large tankers which usually do not carry many grades of oil. Obviously, the exibility of operations is comparatively less as compared to other piping systems. Some ships are also designed as part free ow i.e. free ow system only between certain tanks, which is a hybrid or cross between a full free ow system and a ring main system.

Valves: The valves used in tankers for cargo can be either buttery valves, gate valves or sometimes rarely even globe valves. At pump discharges, a non-return ap valve is usually tted. A detailed description of the merits and demerits of each type of valve will be outside the scope of this course. The valves may be hydraulically, pneumatically or manually operated. Manual valves can have an extended spindle to enable operation of a tank valve from the deck. It is important to note that the valve closure period is deliberately slow to avoid surge pressures. In the hydraulic and pneumatic system, the timing is normally pre-set to a safe limit and rarely requires to be adjusted. However, in manually operated valves the closing time is regulated by the operator. Dangerous surge pressure can build up which can rupture a pipeline or a part of it even in a remote location. The higher the uid pressure in the pipeline, the greater will be the risk of rupture and pollution. The most critical period is the last 25% https://www.cultofsea.com/tanker/piping-arrangement-conventional-oil-tanker-basics/

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when closing the valve or the rst 25% when opening it. One can actually hear the liquid squeezing through the valve. The rate of closure during this period should be especially slow and controlled.

Buttery Valve

Globe Valve

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FILED UNDER: TANKERS TAGGED WITH:CRUDE OIL, DIRECT LINE, FREEFLOW, OIL TANKER, PIPING, RING MAIN, VALVES

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Brooke McAvoy I appreciate all of the diagrams you provided. This makes it much easier to understand and comprehend Guest

the purpose of the dierent systems. It is interesting to me that having a slow valve closure period helps to avoid surge pressures. I would have never thought of this solution, it’s pretty cool. Thank you for the detailed and interesting article!

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