Oil Tanker

Overview

An oil tanker carries crude oil or refined products as liquid bulk, loaded through deck manifolds into a block of cargo tanks and discharged by the ship's own pumps. The largest common type, the Very Large Crude Carrier, is around 330 m long and 300,000 deadweight tonnes, holding roughly two million barrels of crude. Three features set a modern tanker apart from any other cargo ship: a Cargo Tank block built into a double hull, a Cargo Handling System of large pumps that can empty the ship in a day, and an Inert Gas Plant that keeps the tank atmosphere too oxygen-poor to burn. The hull is assembled from around 70 prefabricated Hull Blocks and driven by a single slow-speed Two-Stroke Main Engine.

Double hull and cargo tanks

After the Exxon Valdez grounding, MARPOL banned new single-hull tankers. Every modern tanker wraps its cargo in a double hull: the Cargo Tanks are bounded by an inner shell, and the space between inner and outer shell is given over to segregated ballast in Web Framed wing and double-bottom tanks. A grounding or low-energy collision tears the outer shell and floods ballast space without breaching the oil boundary. Each Hull Block is built around this arrangement, and the cargo-boundary Hull Welds are fully inspected because they are the primary barrier against spillage.

The cargo block is subdivided by oiltight Tank Bulkheads into around fifteen tanks. Subdivision limits the free-surface effect of liquid sloshing, which would otherwise wreck the ship's stability, and contains a single-tank leak. Every tank is internally protected by a Tank Coating System against the corrosive mix of crude, water bottoms and inert-gas acids, and fitted with Crude Oil Washing Machine crude-oil-washing nozzles, Cargo Level Gauge gauging, a high-velocity P/V Valve vent and suction Suction Bellmouths feeding the pumps.

Cargo handling

Discharge is the demanding operation: shore facilities provide pressure for loading, but the ship must lift its entire cargo over the manifold for discharge. The Cargo Handling System uses four large Cargo Pumps, traditionally steam-turbine driven from the ship's boilers, sited low in a dedicated pump room and moving several thousand cubic metres per hour each. As tanks empty and the main pumps lose suction, Stripping Pumps clear the last residue. Crude is routed through Cargo Deck Lines to the midship Cargo Manifold where shore loading arms couple on. The pump room is a hazardous, gas-prone space kept ventilated by explosion-protected Pump-Room Fans. The whole sequence is run remotely from the Cargo Control Room.

Crude oil washing is part of the discharge cycle: Crude Oil Washing Machine nozzles spray pressurised crude against the tank surfaces to wash off the heavy clingage that would otherwise be left behind, recovering cargo and cutting the slop that has to be disposed of. It replaced the old practice of seawater washing that discharged oily water overboard.

Inert gas and fire safety

A tank that is part full of crude has an explosive vapour space above the liquid, and an empty tank after discharge is worse. The Inert Gas Plant solves this by cooling and scrubbing boiler flue gas in a seawater IG Scrubber and pushing it into the tanks with IG Blowers, holding the oxygen content below 5% so no mixture can ignite. An Oxygen Analyser continuously confirms the limit, and a Deck Water Seal water barrier stops hydrocarbon gas flowing back along the IG main toward the machinery spaces. The tanks are kept inerted from loading through the ballast voyage until gas-freeing for tank entry.

Propulsion and shafting

A VLCC is driven by a single six- or seven-cylinder slow-speed two-stroke Two-Stroke Main Engine of around 80-90 cm bore, delivering 25-30 MW directly to the Propeller Shaft. Each Cylinder Assembly runs an oil-cooled Piston through a Crosshead inside a Cylinder Liner, with the Exhaust Valve and fuel valves in the Cylinder Cover. Power passes through the Shaft Line and its Stern Tube to a fixed-pitch Propeller of nearly 10 m, steered by a Rudder and Steering Gear. Service speed is 15-16 knots: like a bulk carrier, the tanker trades speed for fuel economy on long crude routes.

Ballast, accommodation and systems

Because cargo space can never hold ballast, the Ballast System uses the segregated double-hull tanks, filled by Ballast Pumps through a treated discharge via a Ballast Water Treatment Unit. The crew of around 25 lives in the aft Deckhouse, in Crew Cabins served by an HVAC Plant that holds the accommodation at slight overpressure against cargo vapour, with the Bridge Console above and a stern Lifeboat Station. Electrical power comes from three Auxiliary Gensets while the Boilers raise the steam that drives the cargo pumps and heats the cargo. The Piping Systems is the densest on any cargo ship: cargo, ballast, crude-wash, inert-gas, fuel and fire mains together run to many thousands of spools, flanges and valves.