Figure 5.1 Typical gas carrier loading arm
Figure 5.2 Loading arm operating envelope
Connecting systems between hard arms and ships are of two types:
• Bolted flanges — these are basic bolted flange connections.
• Quick connect/disconnect coupling (QCDC) — QCDCs are used to speed up the connection and disconnection operation. The coupling is under full manual control but most often has hydraulic operation of the clamping/unclamping jaws (see Figure 5.3). During cargo pumping, the ship/shore joint is maintained by a positive mechanical lock, independent of the hydraulic power supply. In certain applications, the QCDC can benefit the safety of personnel. Such areas can include off-shore ports where ship movement is a problem.
Emergency release system
An important development in hard arm design has been the introduction of emergency disconnect arrangements for use in the event that the limits of the design operating envelope are approached or if some other emergency occurs. This special connection is fitted in addition to the normal means of connection (bolted flange or a QCDC as described above).
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-
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