The IMO International Gas Carrier Code, became mandatory in 1986.

The requirements of this code are incorporated in the rules for ships carrying liquefied gases published by Lloyd’s and other classification societies.

The code covers damage limitations to cargo tanks and ship survival in the event of collision or grounding, ship arrangements for safety, cargo containment and handling, materials of construction, environmental controls, fire protection, use of cargo as fuel etc. Of particular interest in the context of ship construction is the section of cargo containment which defines the basic cargo container types and indicates if a secondary barrier is required, i.e. a lining outside the cargo containment which protects the ships hull structure from the embrittling effect of the low temperature should cargo leak from the primary tank structure. The cargo containment types are described next.

Keel Plating

At the centre line of the bottom structure is located the keel, which is often said to form  the backbone of the ship. This contributes substantially to the longitudinal strength and effectively distributes local loading caused when docking the ship. The commonest form of keel is that known as the ‘flat plate’ keel, and this is fitted in the majority of ocean-going and other vessels (see Figure 8 above ). A form of keel found on smaller vessels is the bar keel . The bar keel may be fitted in trawlers, tugs etc., and is also found in smaller ferries.

Where grounding is possible this type of keel is suitable with its massive scantlings, but there is always a problem of the increased draft with no additional cargo capacity. If a double bottom is fitted the keel is almost inevitably of the flat plate type, bar keels often being associated with open floors, where the plate keel may also be fitted.
Duct keels are provided in the double bottoms of some vessels. These run from the forward engine room bulkhead to the collision bulkhead and are utilised to carry the double bottom piping. The piping is then accessible when cargo is loaded, an entrance to the duct being provided at the forward end of the engine room. No duct is required aft of the engine room as the piping may be carried in the shaft tunnel.

CONTROL OF LEAKAGES in hydraulic system

Leakage. Any hydraulic system will have a certain amount of leakage. Any leakage will reduce efficiency and cause power loss. Some leakage is built in (planned), some is not. Leakage may be internal, external, or both.

a. Internal. This type of leakage (nonpositive) must be built into hydraulic components to lubricate valve spools, shafts, pistons, bearings, pumping mechanisms, and other moving parts. In some hydraulic valves and pump and motor compensator controls, leakage paths are built in to provide precise control and to avoid hunting (oscillation) of spools and pistons. Oil is not lost in internal leakage; it returns to a reservoir through return lines or specially provided drain passages.

Too much internal leakage will slow down actuators. The power loss is accompanied by the heat generated at a leakage path. In some instances, excess leakage in a valve could cause a cylinder to drift or even creep when a valve is supposedly in neutral. In the case of flow or pressure-control valves, leakage can often reduce effective control or even cause control to be lost.

Normal wear increases internal leakage, which provides larger flow paths for the leaking oil. An oil that is low in viscosity leaks more readily than a heavy oil. Therefore an oil's viscosity and viscosity index are important considerations in providing or preventing internal leakage. Internal leakage also increases with pressure, just as higher pressure causes a greater flow through an orifice. Operating above the recommended pressures adds the danger of excessive internal leakage and heat generation to other possible harmful effects.

A blown or ruptured internal seal can open a large enough leakage path to divert all of a pump's delivery. When this happens, everything except the oil flow and heat generation at a leakage point can stop.

b. External. External leakage can be hazardous, expensive, and unsightly. Faulty installation and poor maintenance are the prime causes of external leakage. Joints may leak because they were not put together properly or because shock and vibration in the lines shook them loose. Adding supports to the lines prevents this. If assembled and installed correctly, components seldom leak. However, failure to connect drain lines, excessive pressures, or contamination can cause seals to blow or be damaged, resulting in external leakage from the components.

c. Prevention. Proper installation, control of operating conditions, and proper maintenance help prevent leakage.

(1) Installation. Installing piping and tubing according to a manufacturer's recommendations will promote long life of external seals. Vibration or stresses that result from improper installation can shake loose connections and create puddles. Avoid pinching, cocking, or incorrectly installing seals when assembling the units. Use any special tools that the manufacturer recommends for installing the seals.

(2) Operating Conditions. To ensure correct seal life, you must control the operating conditions of the equipment. A shaft seal or piston-rod seal exposed to moisture, salt, dirt, or any other abrasive contaminant will have a shortened life span. Also, operators should always try to keep their loads within the recommended limits to prevent leakage caused by excessive pressures.

(3) Maintenance. Regular filter and oil changes, using a high-quality hydraulic oil, add to seal life. Using inferior oil could cause wear on a seal and interfere with desirable oil properties. Proper maintenance prevents impurity deposits and circulating ingredients that could wear on a dynamic seal