Liquefied Petroleum Gas Ships

Ships carrying LPG are categorised by their cargo containment system.

Fully Pressurised Tanks

The capacity of fully pressurised ships is usually less than 2000 m³b or propane, butane or anhydrous ammonia carried in two to six uninsulated horizontal cylindrical pressure vessels arranged below or partly below deck. These independent tanks of Type C are normally designed for working pressures up to 17.5 kg/cm² which corresponds to the vapour pressure at 45°C, the maximum ambient temperature the vessel is likely to operate in.
The ship has a double hull extending over the bottom and bilge area, the secondary barrier being provided by low temperature (notch ductile) steel at the inner bottom, sloping bilge tank, part side shell, and sloping bottom of topside tank. Transverse bulkheads may be single or double plate (cofferdam) type between cargo holds. Insulation can be either on the tank or the secondary barrier for this type of ship.

Semi-Pressurised (Or Semi-Refrigerated) Tanks

The capacity of semi-pressurized ships ranges up to about 5000m³ the cargoes carried being similar to fully pressurized ships. The independent Type C tanks are generally constructed of ordinary grades of steel suitable for a temperature of -5°C and are designed for a maximum pressure of about 8kg/cm². The outer surface of the tank is insulated and refrigeration or reliquefaction plant cools the cargo and maintains the working pressure. Cargo tanks are often horizontal cylinders mounted on two saddle supports and many designs (see Figure 26) incorporate bio-lobe tanks to better utilize the under deck space and improve payload.

Fully-Refrigerated Tanks

The capacity of fully-refrigerated ships ranges from 10 000m³ to 100 000m³ the smaller ships in the range being multi-product carriers whilst the larger vessels tend to be single product carriers on a permanent route, Tanks fall almost exclusively into the prismatic, independent Type A category with tops sloped to reduce free surface and bottom corners sloped to suit the bilge structure. In most cases they are subdivided along the centreline by a liquid-tight bulkhead, which extends to the underside of the dome projecting through the deck, which is used for access and piping connections, etc. The tanks sit on insulated bearing chocks, attached to brackets in the following areas, double bottom tank tops, top sloping sides against the saddle ballast tanks and the flat sides, there is a clearance between the chocks and tank to allow thermal movement, the clearance must not be excessive, otherwise in heavy weather, the tank will bounce and generate fractures in the surrounding structures, the tank surfaces are accessible for inspection, the chocks prevent the tank floating off if the hold is flooded. Tanks are constructed of notch ductile low temperature steel for the minimum operating temperature of -43°C the boiling point of propane.
The ship has a double hull extending over the bottom and bilge area, the secondary barrier being provided by notch ductile low temperature steel at the inner bottom, sloping bilge tank, part side shell and sloping bottom of topside tank. Transverse bulkheads may be single or double plate (cofferdam) type between cargo holds. Insulation can be either on the tank or the secondary barrier (Void Space) for this type of ship.
When the liquid gas is inflammable, the void space will be purged through with dry inert gas to an oxygen level of <1%, and slightly pressurised. This is a safety factor in the event of a collision, also reduces corrosion in the surrounding steel work.

International Insulation Tanks

Which are non-self-supporting and consist of thermal insulation materials, the inner surface of which is exposed to the cargo supported by the adjacent inner hull or an independent tank. There are two types:

‘Type 1’, where the insulation or combination of insulation and one or more liners act only as primarily barrier. The inner hull or independent tank forms the secondary barrier.

‘Type 2’, where the insulation combination of insulation and one or more liners act as both the primary and secondary barrier and are clearly distinguishable as such.

Independent Tanks

These are self-supporting and independent of the hull. There are three types:
‘Type A’, which are designed primarily using standard traditional methods of ship-structural analysis. LPG at or near atmospheric pressure of LNG may be carried in such tanks .
‘Type B’, which are designed using more sophisticated analytical tools and methods to determine stress levels, fatigue life and crack propagation characteristics. The overall design concept of these tanks is based on the so called ‘crack detection before failure principle’ which permits their use with a reduced secondary barrier (see Figure 25). LNG is normally carried in such tanks.
‘Type C’, which are designed as pressure vessels, the dominant design criteria being the vapour pressure. Normally used for LPG and occasionally ethylene.

Integral Tanks

Those tanks which form a structural part of the ships hull and are influenced in the same manner and by the same loads which stress the adjacent hull structure. These are used for the carriage of LPG at or near atmospheric conditions, butane for example, where no provision for thermal expansion and contraction of the tank is necessary.

Liquefied Natural Gas (LNG)

LNG is natural gas from which most of the impurities such as sulphur and carbon dioxide have been removed. It is cooled to or near its boiling point of –165°C at or near atmospheric pressure and is transported in this form as predominantly liquid methane. Methane has a critical temperature of –82.5°C, i.e. the pressure and temperature above which liquification cannot occur, so that methane can only be liquefied by pressure at very low temperatures.

Liquefied Gas Carriers

A large number of ships are in service which are designed to carry gasses in liquid form in bulk. Most of these ships are designed to carry liquefied petroleum gas (LPG), whilst a much smaller number of ships are designed to carry liquefied natural gas (LNG).