Figure 5.3 Quick connect/disconnect coupling

The emergency release system (ERS) forms a second stage emergency shut-down system in addition to the first stage emergency shut-down system described in 6.8. Its purpose is to provide a means to quickly uncouple the hard arms with minimal spillage in an emergency. The system incorporates instrumentation to monitor the ship's position; it also includes alarm and control systems. The physical disconnection is achieved by means of a Powered Emergency Release Coupler (PERC) installed in each hard arm, as shown in Figure 5.1.

A typical PERC is illustrated in Figure 5.4 and consists of a clamped flange interposed between two ball or butterfly valves. Two hydraulic actuators are mounted on the upper part of the coupler: one for the clamp ring; the other operating both valves via special linkage. In emergency operation, the two valves close first: this is followed by the release of the clamped coupling. On release, the lower part of the PERC and its attendant valve remain attached to the ship's manifold whilst the arm, with the upper part of the PERC and its valve is free to rise clear of the ship. Mechanical and hydraulic interlocks prevent the coupling from releasing before the valves are closed. The space between the two valves is kept as small as possible to minimise liquid spillage.

The ERS acts in conjunction with the shore ESD system and firstly, will initiate shut­down of the transfer operation via the ship/shore link, before acting on the PERCs. The valves adjacent to the PERC close rapidly and this is typically achieved within five seconds. In the cargo loading situation such rapid closure could give rise to excessive pressure surges. This must be catered for in some way and it is common to find shore-

IN EMERGENCY BALL VALVES CLOSE AND COUPLING DISCONNECTS

Figure 5.4 Powered emergency release coupling (PERC)

based surge control facilities fitted for this purpose. As an alternative, increased pipeline scantlings may be considered or special operating procedures may be followed. (For further information see also 10.5.2).

5.1.3 Vapour return

The provision of a vapour return facility between ship and shore at both loading and discharging terminals depends on a number of factors such as, economics, cargo transfer rates, distance of jetty from storage tanks, product pressures and cargo temperatures. In the LNG trade, vapour returns are always fitted. In the LPG trade they are becoming more common but are usually connected to the ship for safety reasons and might only be operated if high shipboard pressures become difficult to contain.

Where a vapour return facility is available, the vapours generated during loading may be transferred to the shore by a ship's compressor or vapour blower. Alternatively, a terminal vapour blower or compressor may be used. In the latter case, loading rates are independent of the ship's vapour return capacity, although they may be limited by the shore reliquefaction plant capacity.

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