Chemical Tanker

Overview

A chemical tanker carries liquid chemicals in bulk: acids, alcohols, vegetable and animal oils, solvents, caustic soda and hundreds of other products, often several different cargoes at once. That last point shapes the whole ship. A crude tanker is one big cargo split across a few tanks; a chemical tanker is a parcel ship, with around two dozen small segregated Cargo Tanks, each in its own corrosion-resistant material with its own Deepwell Pump and its own Cargo Deck Line so two cargoes can never mix. The ship is built to the IMO IBC Code, with a double hull, Tank Cleaning System and Nitrogen Blanketing System aboard. The hull comes from around 45 prefabricated Hull Blocks, and a manoeuvre-heavy trade to many small terminals favours a Main Engine & Reduction Gear driving a controllable-pitch propeller.

Tanks and materials

The defining choice on a chemical tanker is tank material. The most flexible ships use duplex stainless Tank Walls, which resist acids and a wide range of aggressive cargoes and can be cleaned to a high standard between parcels. Cheaper ships use zinc-silicate or epoxy coatings, which handle many products but rule out others. The material of each tank fixes what it can carry, so an owner trades capital cost against the range of cargoes the ship can quote for.

Subdivision into many small tanks is what lets the ship carry a dozen different parcels for a dozen charterers on one voyage. Each Cargo Tank has its own Deepwell Pump with the hydraulic motor on deck and the impeller at the tank bottom, so nothing is shared between parcels, plus a Cargo Heating Coil, a Cargo Level Gauge, a P/V Valve and fixed Tank Cleaning Machines. Tank welds in stainless are pickled and passivated to restore the corrosion resistance the heat-affected zone loses.

Cargo handling and segregation

Segregation runs through the entire Cargo Handling System. Each tank or group has a dedicated Cargo Deck Line, driven by deepwell pumps that take hydraulic power from the Hydraulic Power Pack packs, and the lines reach the ship's side at port and starboard Cargo Manifolds with reducers for the many hose sizes terminals use. Cargo Valves give positive shut-off so a cargo cannot creep from one system into another, and Drip Trays under every manifold and valve cluster catch drips before they reach the deck or the sea. The whole multi-parcel operation is run from the Cargo Control Room, where each tank's gauging, heating and valves are watched at once.

Tank cleaning

Between cargoes a tank must be cleaned to the standard the next charterer demands, which can be far stricter than the previous one. The Tank Cleaning System supplies heated wash water through the fixed cleaning machines, raises it to temperature in a Wash-Water Heater, drives it with Cleaning Pumps, and clears the dirty water through Stripping Eductors into Slop Tanks for settling and shore disposal under MARPOL Annex II. A failed wall-wash test can mean a cargo is rejected, so tank cleaning is a core commercial skill on these ships, not an afterthought.

Inerting and heating

Many chemical cargoes react with oxygen or moisture. The Nitrogen Blanketing System generates nitrogen on board with a Nitrogen Generator fed by Air Compressors, buffers it in a Nitrogen Buffer Tank, and holds a dry inert blanket over reactive parcels, checked by an Oxygen Analyser. Other cargoes are viscous or crystallise if they cool: palm oil, phosphoric acid and many waxes have to be kept warm. The Cargo Heating System circulates thermal oil from a Thermal-Oil Heater through the tank coils, using a Heat Exchanger to keep any contaminated coil flow away from the heater. Holding each parcel in its own temperature band, sometimes a dozen different setpoints at once, is part of the daily cargo-care routine.

Propulsion and manoeuvring

Parcel tankers are smaller than crude carriers and call at many terminals, so they are built for manoeuvring rather than ocean economy. The Main Engine & Reduction Gear is usually a medium-speed four-stroke diesel through a reduction gear to a Controllable-Pitch Propeller controllable-pitch propeller, which gives full thrust control and reverse without stopping the engine. Each Cylinder Unit runs a Piston in a Cylinder Liner under a valve-carrying Cylinder Head, fed by the Fuel Injection System and kept turning by the Lubrication System. A Bow Thruster and a high-lift Rudder let the ship work tight berths with fewer tugs. The CPP swings its Propeller Blades together through a Pitch Cylinder in the hub.

Accommodation and systems

A crew of around 20, trained to the chemical-tanker standard, lives in the aft Deckhouse in Crew Cabins served by an HVAC Plant kept at slight overpressure against cargo vapour. The Bridge Console handles navigation and the stern Lifeboat Station meets SOLAS. Three Auxiliary Gensets carry the heavy electrical load that the cargo-pump hydraulics, nitrogen plant and heating impose, more than a dry-cargo ship of the same size would need. A chemical tanker is, in effect, a fleet of small independent tank systems wrapped in one hull, which is why it carries far more cargo equipment per tonne than any other merchant ship.