Technigaz membrane system

The Technigaz system, shown in Figure 3.6(a), features a primary barrier of stainless steel (1.2 millimetres in thickness) having raised corrugations, or waffles, to allow for expansion and contraction. In the original Mark I design, the insulation that supports

Figure 3.6(a) Technigaz membrane containment system — larger LNG carriers

the primary membrane consisted of laminated balsa wood panels held between two plywood layers; the face plywood formed the secondary barrier. The balsa wood panels were interconnected with specially designed joints comprising PVC foam wedges and plywood scabs and were supported on the inner hull of the ship by wooden grounds.

Figure 3.6(b) Construction of the Technigaz membrane — Mark III

In the latest design (Mark III) the balsa wood insulation is replaced by reinforced cellular foam. Within the foam there is a fibreglass cloth/aluminium laminate acting as secondary barrier. Figure 3.6(b) shows a cutaway section through the Mark III Technigaz containment system.

3.2.3 Semi-membrane tanks

The semi-membrane concept is a variation of the membrane tank system. The primary barrier is much thicker than that in the membrane system, having flat sides and large radiused corners. The tank is self-supporting when empty but not in the loaded condition. In this condition the liquid (hydrostatic) and vapour pressures acting on the primary barrier are transmitted through the insulation to the inner hull as is the case with the membrane system. The corners and edges are designed to accommodate expansion and contraction.

Although semi-membrane tanks were originally developed for the carriage of LNG no commercial-size LNG carrier has yet been built to this design. The system has however, been adopted for use in LPG ships and several Japanese-built fully refrigerated LPG carriers have been delivered to this design.

3.2.4 Integral tanks

Integral tanks form a structural part of the ship's hull and are influenced by the same loads which stress the hull structure. Integral tanks are not normally allowed for the carriage of liquefied gas if the cargo temperature is below -10°C. Certain tanks on a limited number of Japanese-built LPG carriers are of the integral type for the dedicated carriage of fully refrigerated butane.

3.2.5 Internal insulation tanks

Internally insulated cargo tanks are similar to integral tanks (see 3.2.4). They utilise insulation materials to contain the cargo. The insulation is fixed inside ship's inner hull or to an independent load-bearing surface. The non-self-supporting system obviates the need for an independent tank and permits the carriage of fully refrigerated cargoes at carriage temperatures as low as -55°C.

Internal insulation systems have been incorporated in a very limited number of fully refrigerated LPG carriers but, to date, the concept has not proved satisfactory in service.

3.3 MATERIALS OF CONSTRUCTION AND INSULATION

3.3.1 Construction materials

The choice of cargo tank materials is dictated by the minimum service temperature and, to a lesser degree, by compatibility with the cargoes carried. The most important property to consider in the selection of cargo tank materials is the low-temperature toughness. This consideration is vital as most metals and alloys (except aluminium) become brittle below a certain temperature.

Treatment of structural carbon steels can be used to achieve low-temperature characteristics and the Gas Codes specify low-temperature limits for varying grades of steel down to -55°C. Reference should be made to the Gas Codes and classification society rules for details on the various grades of steel.

According to the Gas Codes, ships carrying fully refrigerated LPG cargoes may have tanks capable of withstanding temperatures down to -55°C. Usually, the final temperature is chosen by the shipowner, depending on the cargoes expected to be carried. This is often determined by the boiling point of liquid propane at atmospheric pressure and, hence, cargo tank temperature limitations are frequently set at about

-46°C. To achieve this service temperature, steels such as fully killed, fine-grain, carbon-manganese steel, sometimes alloyed with 0.5 per cent nickel, are used.

Where a ship has been designed specifically to carry fully refrigerated ethylene (with a boiling point at atmospheric pressure of -104°C) or LNG (atmospheric boiling point

-162°C), nickel-alloyed steels, stainless steels (such as Invar) or aluminium must be used for the material of tank construction.

3.3.2 Tank insulation

Thermal insulation must be fitted to refrigerated cargo tanks for the following reasons:

· To minimise heat flow into cargo tanks, thus reducing boil-off.

· To protect the ship structure around the cargo tanks from the effects of low temperature.

Insulation materials for use on gas carriers should possess the following main characteristics:

· Low thermal conductivity

· Ability to bear loads

· Ability to withstand mechanical damage

· Light weight

· Unaffected by cargo liquid or vapour

The vapour-sealing property of the insulation system, to prevent ingress of water or water vapour, is important. Not only can ingress of moisture result in loss of insulation efficiency but progressive condensation and freezing can cause extensive damage to the insulation. Humidity conditions must, therefore, be kept as low as possible in hold spaces. One method to protect the insulation is to provide a foil skin acting as a vapour barrier to surround the system.

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