The Terminal-Equipment and Instrumentation

Terminal equipment is more difficult to describe than ship's equipment due to the lack of international design standards. However, this chapter describes some of the essential aspects, including the primary piece of cargo transfer equipment — the jetty based marine loading arm or hard arm. Furthermore, the fitting of Emergency shut­down systems and Emergency Release Couplings is recommended in the text. This recommendation is in addition to a suitable means of closing down ship and shore operations in unison — most often best achieved by an interlinked system.

The chapter also describes various types of shore storage tanks and provides some advice on jetty fire-fighting equipment.

5.1 CARGO TRANSFER SYSTEMS

Cargo transfer systems vary widely between the different liquefied gas trades. For example in the LNG trade there are relatively few terminals — numbering close to fifty. At LNG loading terminals some condensate cargoes may also be handled. Here also gas processing plant can be found and large scale liquefaction equipment is fitted. LNG receiving terminals are usually connected to public utilities such as gas distribution companies or electrical generating companies but sometimes large gas customers such as steelworks are included. In addition to storage tanks, LNG receiving ports will also be outfitted with large scale vaporising equipment.

In contrast, when dealing with the LPGs and chemical gases there are many hundreds of terminals worldwide. Loading terminals can range from oil refineries to chemical or fertilizer plants and for discharge purposes the variety of ports are too numerous to cover.

Design standards vary considerably from country to country and from company to company and this variation can also be apparent at the jetty. Here, it will be found that cargo transfer is usually accomplished by the use of loading arms (hard arms) or hoses. In recent years, there has been a steady increase in the use of hard arms for the transfer of LPGs, however, since the beginning of the LNG trade, hard arms have been used universally.

5.1.1 Hoses

All hoses should be purchased for the purpose intended and specified against a suitable national standard (for example, the rubber hoses used for cargo transfer often

comply with British Standard BS 4089 — Hoses and Hose Assemblies for LPG and composite hoses are often manufactured to British Standard 5842). Accordingly, at the point of purchase, important specifications to be relayed to manufacturers are the minimum and maximum operating temperatures and pressures. The compatibility of the hose material with the cargo must also be addressed.

During operations, the proper handling of hoses is particularly important and hoses of all types should be correctly supported in a hose cradle. This will help to ensure that manufacturers' recommendations on minimum bending radius are met. Care should also be exercised when rigging or moving hoses to ensure that they are not damaged or laid against sharp edges which could weaken the hose.

Hoses should be inspected frequently (before each transfer operation) and tested at specified intervals (generally, not exceeding six months). Where appropriate, pressure testing should be supplemented with measurements for elongation under pressure and for electrical continuity. It is good practice for terminals to provide hoses with break-away couplings (see 10.5.2).

There are three types of cargo hose suitable for liquefied gases. These can be of composite, rubber or stainless steel construction.

For LNG, they can be of composite construction or of corrugated stainless steel, but, in general the composite type is preferred. For LPG, hoses may be of similar construction to those used for LNG but, hoses of synthetic rubber manufacture may also be used.

Hoses of composite design have been used for LNG transfer, but this has only ever been necessary in emergency situations. Normally, all LNG cargo transfers are carried out through hard arms with the ship alongside a jetty. These hoses are normally manufactured from special polypropylene films and fabrics supported by inner and outer wire helixes.

Further information on the subject of cargo hoses is to be found in Reference 2.12 and 2.38).

For hoses carried on board ship, the Gas Codes should be consulted, where additional specifications will be found.

5.1.2 Hard arms (loading arms)

A typical marine loading arm used for the transfer of liquefied gas is shown in Figure 5.1. The arm is fitted with swivel joints to provide the required movement between the ship and the shore connections. A counter-balance is provided to reduce the deadweight of the arm on the ship's manifold connection and to reduce the power required to manoeuvre the arms into position.

The range or operating envelope of the hard arm is determined by the tidal variation and changes of ship's freeboard whilst loading or discharging. In addition, an allowance is provided in case the ship ranges fore and aft along the jetty or drifts away from the berth. Figure 5.2 shows a typical operating envelope.

Maximum allowable angular limits in the arms are typically 150° at the apex angle; 15° on the arm tuckback from vertical; 10° for arm elevation above the horizontal; and about 45° for slew or luff.

For further information on hard arm specifications Reference 2.22 and 2.43 are recommended.

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