Electrical Power and Instrumentation
Electrical instruments used in hazardous areas should be of flameproof or intrinsically safe design (see 4.8). The use of wandering electric power leads should be disallowed.
Aluminium
Portable aluminium alloy equipment, such as ladders and "Zip-Up" scaffolding should not be used in hazardous areas; as a smear of aluminium on rusty steel can cause an incendive spark if subsequently struck. Thus the potential for ignition can remain after the equipment has been removed from the area. Alloys containing more than 6% magnesium are potentially more dangerous than those with a lower magnesium content.
Mobile Telephones
The use of mobile telephones and pagers on deck or in terminals should be strictly limited to those certified as intrinsically safe.
Insulation Flanges and Ship/Shore Bonding Cables
Electrical arcs may occur when connecting or disconnecting cargo connections between ship and shore if the hose or hard arm provides an electrical path between ship and jetty structures. Electrical current will flow through this path due to differences in potential between the ship and the jetty. Such differences may be increased by imbalance between the cathodic protection applied to each structure. Although the resultant potential difference between ship and jetty is small, the electrolytic cell is large. Accordingly, and given that the electrical resistances in the cargo connection is small, a heavy current of many amperes may flow through the cargo connection. This current, on being interrupted, can produce arcs of incendive energy at the manifold.
The original intention of the ship/shore bonding cable was to provide an alternative path for this current but, in practical terms, such cables have been shown to be ineffective. Since it is ineffective and presents a hazard by virtue of the current it carries, the use of the bonding cable is no longer recommended. The introduction of an electrical discontinuity in the cargo hose or hard arm by means of aninsulating
flange or a length of electrically discontinuous hose, as appropriate, is effective in eliminating the current and any sparking.
For reasons of accessibility, insulating flanges are usually located at the lower end of the outer arm of the hard arm (see also 5.1.4).
Although the potential dangers of using a ship/shore bonding cable are widely recognised, attention is drawn to the fact that some national and local regulations may still require a bonding cable to be connected. If a bonding cable is insisted upon, it should first be visually inspected to make sure it is mechanically sound. The connection point on board ship for the cable should be well clear of the manifold area. There should always be a switch on the jetty, in series with the bonding cable, and of a type suitable for use in a hazardous area. It is important to ensure that the switch is always in the off position before connecting or disconnecting the cable. Only when the cable is properly attached and in good contact with the ship should the switch be closed. The cable should be attached before the cargo hoses or hard arms are connected and removed only after they have been disconnected.
|
|
|
Where national authorities require a ship/shore bonding cable to be used it is still recommended that insulation flanges be fitted.
Chapter 3
Principles of Gas Carrier Design
This chapter provides an overview of the written standards covering gas carrier construction. It starts with an outline of the various Gas Codes agreed by the International Maritime Organisation. Later sections discuss the essential elements of design such as cargo containment systems and Ship Types.
In reading this chapter it is important to realise that apart from written standards there are some aspects of gas carrier construction which are covered by the additional requirements of experienced shipowners.
3.1 DESIGN STANDARDS AND SHIP TYPES
3.1.1 The Gas Carrier Codes
The overall layout of a gas carrier is similar to that of the conventional oil tanker from which it evolved. The cargo containment system and its incorporation into the hull is, however, very different due to the need to carry cargo under pressurised, or refrigerated conditions; or under a combination of pressure and refrigeration.
Gas carriers designed for pressurised cargoes can usually be identified by cylindrical or spherical tanks which may project through the deck. Similarly the LNG carrier with spherical tanks protruding above the main deck can be easily recognised by its distinctive profile and much larger size. Gas carriers designed to carry their cargo at atmospheric pressure in prismatic tanks are not easily distinguishable from oil tankers except by their freeboard which is significantly greater. This greater buoyancy results from cargoes of a much lower density than most oils and the requirement to have totally segregated tanks for ballast.
|
|
|
To examine the design of these ships in greater detail, readers should consult the Gas Codes and the rules of the major ship classification societies which give guidance on the requirements of the Gas Codes.
The Gas Codes, developed by IMO, apply to all gas carriers regardless of size. There are three Gas Codes and these are described below.
Дата добавления: 2018-02-28; просмотров: 383; Мы поможем в написании вашей работы! |
Мы поможем в написании ваших работ!