LPG AND LNG GAS CARRIER

Liquefied Petroleum Gas (LPG)

#  LPG is the name originally given by the oil industry to a mixture of petroleum hydrocarbons principally propane and butane and mixtures of the two. 

# LPG is used as a clean fuel for domestic and industrial purposes.These gases may be converted to the liquid form and transported in one of

three conditions.

(1) Solely under pressure at ambient temperature. 

(2) Fully refrigerated at their boiling point (−30 °C to −48 °C). 

(3) Semi-refrigerated at reduced temperature and elevated pressure.

# A number of other gases with similar physical properties such as ammo-

nia, propylene and ethylene are commonly shipped on LPG carriers. These

gases are liquefied and transported in the same conditions .

#  Except ethylene which boils at a much lower temperature (−104 °C) and which is therefore carried in the fully refrigerated or semi-refrigerated condition.

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. 

LIQUID PETROLEUM GAS CARRIER

Ships carrying LPG are categorized by their cargo containment-:

FULLY PRESSURIZED TANKS 

> The capacity of fully pressurized ships is usually less than 2000 m3

 of propane, butane .

> 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

> The tanks can be constructed from ordinary grades of steel, are mounted in cradle-shaped foundations, and if below deck are fitted with domes protruding through the deck to which are fitted all connections. 

> Wash bulkheads are fitted in very long tanks. The shape of the tanks generally prevents good utilization of the underdeck space.

SEMI- PRESSURIZED ( OR SEMI- REFRIGERATED ) TANKS

> The capacity of semi-pressurized ships ranges up to about 5000 m3

 the cargoes carried being similar to fully-pressurized ships. 

> The independent Type C tanks  are designed for a maximum pressure of about 8 kg/cm2

> The outer surface of the tank is insulated and refrigeration or

reliquefication plant cools the cargo and maintains the working pressure.

> Cargo tanks are often horizontal cylinders mounted on two saddle supports and many designs incorporate bio-lobe tanks to better

utilize the underdeck space and improve payload.

FULLY-REFRIGERATED TANKS 

>  The capacity of fully-refrigerated ships ranges from 10 000 m3

 to 100 000 m3 .

>  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. 

>  Transverse bulkheads may be single or double plate (cofferdam) type between cargo holds. 

>  The ship has a double hull extending over the bottom and bilge area.

LIQUEFIED NATURAL GAS CARRIER

INDEPENDENT TYPE A TANKS 

>  Early LNG ships were fitted with self-supporting tanks of

aluminium alloy having centreline bulkheads. 

> The balsa wood insulation system was attached to the inner hull (secondary barrier) and each insulated hold contained three tanks. 

> Later vessels built with tanks of this category have adopted a prismatic tank design.

INDEPENDENT TYPE B TANKS 

>  An independent Type B tank containment systemTanks con-

sist of either an aluminium alloy or 9 per cent nickel steel sphere welded to a vertical cylindrical skirt of the same material which is its only connection to the hull.

>  The sphere expands and contracts freely all movements being compensated for in the top half of the skirt.

> The outer surface of the sphere and part of the skirt is covered with a polyurethane foam insulation.

>  The system is fitted with a partial secondary barrier consisting of a drip tray under the tank and splash shields at the sides.

> In accordance with its Type B notation, each tank is provided with sensors which will detect leakage and allow timely repairs before any crack reaches critical proportions. 

MEMBRANE TANKS 

> Two common membrane tank designs are those developed and associated with the French companies Gaz Transport and Technigaz. 

 BY GAZ TRANSPORT -:

* The Gaz Transport system uses a 36 per cent nickel-iron alloy

called ‘Invar’ for both the primary and secondary barriers.

* Invar has a very low coefficient of thermal expansion which makes any corrugations in the tank structure unnecessary. 

* The Invar sheet membrane used is only 0.5 to 0.7 mm thick which makes for a very light structure. Insulation consists of plywood boxes filled with perlite. 

BY TECHNIGAZ  -:

* The Technigaz system utilizes a stainless steel membrane system where

tanks are constructed of corrugated sheet in such a way that each sheet is

free to contract and expand independently of the adjacent sheet. 

* This forms the inner primary barrier and a balsa insulation and secondary barrier similar to that fitted to the Independent Type A tanks.

SEMI-MEMBRANE TYPE B TANKS

* The rectangular tank consists of plane unstiffened walls with moderately sloped roof and rounded edges and corners which are not supported so that expansion and contraction is accommodated. 

* The tank is of 15 to 25 mm thick aluminium alloy supported on a layer of PVC insulation and the partial secondary barrier is made of plywood 25 mm thick integral with a PVC foam insulation.