Depending on the type of gas ( LPG, LNG or CNG) there are a variety of shipping methods available.
1-Pipeline transport
2-Gas tank truke
Two and three layers tankers with high safety coefficient for LPG and CNG transportation
3-gas carrier
A gas carrier (or gas tanker) is a ship designed to transport LPG, LNG or liquefied chemical gases in bulk.
Types:
Fully pressurized gas carrier
The seaborne transport of liquefied gases began in 1934 when a major international company put two combined oil/LPG tankers into operation. The ships, basically oil tankers, had been converted by fitting small, riveted, pressure vessels for the carriage of LPG into cargo tank spaces. This enabled transport over long distances of substantial volumes of an oil refinery by-product that had distinct advantages as a domestic and commercial fuel. LPG is not only odourless and non-toxic, it also has a high calorific value and a low sulphur content, making it very clean and efficient when being burnt.
Today, most fully pressurised oceangoing LPG carriers are fitted with two or three horizontal, cylindrical or spherical cargo tanks and have typical capacities between 3,500 and 7,500 m3 (120,000 and 260,000 cu ft). However, in recent years a number of larger-capacity fully pressurised ships have been built, most notably a series of 10,800 m3 (380,000 cu ft) ships, built in Japan between 2003 and 2013. Fully pressurised ships are still being built in numbers and represent a cost-effective, simple way of moving LPG to and from smaller gas terminals.
Semi-pressurised ships
These ships carried gases in a semi-pressurized/semi-refrigerated state however due to further development semi-pressurised/fully refrigerated gas carriers had become the shipowners’ choice by providing high flexibility in cargo handling. These carriers, incorporating tanks either cylindrical, spherical or bi-lobe in shape, are able to load or discharge gas cargoes at both refrigerated and pressurised storage facilities.
Ethylene and gas/chemical carriers
Ethylene carriers are the most sophisticated of the gas tankers and have the ability to carry not only most other liquefied gas cargoes but also ethylene at its atmospheric boiling point of −104 °C (−155 °F). These ships feature cylindrical, insulated, stainless steel cargo tanks able to accommodate cargoes up to a maximum specific gravity of 1.8 at temperatures ranging from a minimum of −104 °C to a maximum of +80 °C (176 °F) and at a maximum tank pressure of 4 bar.
Fully refrigerated ships
They are built to carry liquefied gases at low temperature and atmospheric pressure between terminals equipped with fully refrigerated storage tanks. However, discharge through a booster pump and cargo heater makes it possible to discharge to pressurized tanks too. The first purpose-built, fully refrigerated LPG carrier was constructed by a Japanese shipyard, to a United States design, in 1962. Prismatic tanks enabled the ship’s cargo carrying capacity to be maximised, thus making fully refrigerated ships highly suitable for carrying large volumes of cargo such as LPG, ammonia and vinyl chloride over long distances. Today, fully refrigerated ships range in capacity from 20,000 to 100,000 m3 (710,000 to 3,530,000 cu ft). LPG carriers in the 50,000–80,000 m3 (1,800,000–2,800,000 cu ft) size range are often referred to as VLGCs (Very Large Gas Carriers). Although LNG carriers are often larger in terms of cubic capacity, this term is normally only applied to fully refrigerated LPG carriers.
The main type of cargo containment system utilised on board modern fully refrigerated ships are independent tanks with rigid foam insulation. The insulation used is quite commonly polyurethane foam. Older ships can have independent tanks with loosely filled perlite insulation. In the past, there have been a few fully refrigerated ships built with semi-membrane or integral tanks and internal insulation tanks, but these systems have only maintained minimal interest. The large majority of such ships currently in service have been constructed by shipbuilders in Japan and Korea.
Liquefied natural gas (LNG carrier)
The majority of LNG carriers are between 125,000 and 135,000 m3 (4,400,000 and 4,800,000 cu ft) in capacity. In the modern fleet of LNG carriers, there is an interesting exception concerning ship size. This is the introduction of several smaller ships of between 18,000 and 19,000 m3 (640,000 and 670,000 cu ft) having been built in 1994 and later to service the needs of importers of smaller volumes.
-Cargo containment systems
A cargo containment system is the total arrangement for containing cargo including, where fitted:
-A primary barrier (the cargo tank),
– Secondary barrier (if fitted),
– Associated thermal insulation,
-Any intervening spaces, and
-Adjacent structure, if necessary, for the support of these elements.
For cargoes carried at temperatures between −10 and −55 °C (14 and −67 °F) the ship’s hull may act as the secondary barrier and in such cases it may be a boundary of the hold space.
The basic cargo tank types utilised on board gas carriers are in accordance with the list below:
-Independent Type
-Independent Type ‘A’
Type A Independent Tanks are prismatic and supported on insulation-bearing blocks typically consisting of wooden chocks and located by anti-roll chocks located at the top of the tank inside the void space and anti-flotation chocks located inside the void space usually just above the double bottom tanks. The tanks are normally divided by a centreline liquid-tight bulkhead; by this feature, together with the chamfered upper part of the tank, the free liquid surface effect is reduced and thus the virtual rise of the Centre of gravity and the stability is increased. When these cargo tanks are designed to carry LPG (at −50 °C), the tank is constructed of fine-grained low-carbon manganese steel or even stainless steel as seen in the Maersk J class Ships. The hold space (void space) in this design is normally filled with dry inert gas or Nitrogen but may be ventilated with air during a ballast or gas free passage. The Conch design has been developed for carriage of LNG (at-163oC). The material for these cargo tanks has to be either 9% nickel steel or aluminium. The maximum allowable relief vapour setting (MARVS) is < 0. 7 bar.
-Independent Type ‘B’
Type B Independent Tanks are generally spherical and welded to a vertical cylindrical skirt, which is the lone connection to the ship’s main hull. The hold space (void space) in this design is normally filled with dry inert gas or Nitrogen but may be ventilated with air during a ballast or gas free passage. A protective steel dome covers the primary barrier above deck level, and insulation encloses the outside of the primary barrier surface. This containment system has been used for carriage of LNG. The material of construction is either 9% nickel steel or aluminium.[9] The maximum Allowable relief vapour setting (MARVS) is < 0. 7 bar.
-Independent Type ‘C’
Type C Independent Tanks are deck pressure vessels or cylindrical pressure tanks mounted horizontally on two or more cradle-shaped foundations. The tanks may be fitted on, below or partly below deck and be both longitudinally and transversely located. Lobe-type tanks are commonly used at the forward end of the ship, to improve the poor utilization of the hull volume. This containment system is used for LPG, Ethylene and small scale LNG carriers. The material, if used for the construction of tanks designed to carry ethylene, is 5% nickel steel. The maximum Allowable relief vapour setting (MARVS) is > 0. 7 bar.
– Membrane
Some other types such as:
-Internal insulation Type ‘1’
-Internal insulation Type ‘2’
-Integral
have been fully designed and approved but have not been commercially used yet.
