Table 3.1 Typical insulation materials
| MATERIAL | APPLICATION | THERMAL CONDUCTIVITY watts/metre °K |
| Balsa Wood | A load-bearing insulant | 0.05 |
| Mineral Wool | Normally supplied in slabs or rolls | 0.03 |
| Perlite | Granular silicon/aluminium oxide used as bulk in-fill for hold spaces or in modular boxes | 0.04 |
| Polystyrene | Pre-formed, sprayed or foamed | 0.036 |
| Polyurethane | Pre-formed, sprayed or foamed | 0.025 |
Table 3.1 provides information on the insulation materials normally used in gas carrier construction, together with approximate values for their thermal conductivities at 10°C.
Thermal insulation may be applied to various surfaces, depending on the design of the containment system. For Type 'B' and 'C' containment systems, insulation is applied directly to the cargo tank's outer surfaces. For Type 'A' cargo tanks insulation can be applied either directly to the cargo tank or to the inner hull (if fitted) although its application to the cargo tank is more common.
As most insulation materials are flammable, great care is required at times of construction or refit to ensure that fires are avoided.
3.4 GAS CARRIER TYPES
Gas carriers can be grouped into five different categories according to the cargo carried and the carriage condition. These are as follows:
· Fully pressurised ships
· Semi-pressurised ships
· Ethylene ships
· Fully refrigerated LPG ships
· LNG ships
The first three ship types listed are most suitable for the shipment of smaller-size cargoes of LPG and chemical gases. This is normally accomplished on short-sea and regional routes. Fully refrigerated ships are used extensively for the carriage of large-size cargoes of LPG and ammonia on the deepsea routes.
3.4.1 Fully pressurised ships
Fully pressurised ships are the simplest of all gas carriers. Their containment systems and cargo handling equipment have been established for many years. They carry their cargoes at ambient temperature. They are fitted with Type 'C' tanks (pressure vessels) fabricated in carbon steel having a typical design pressure of about 18 barg. Ships with higher design pressures are in service and a few ships can accept cargoes at pressures of up to 20 barg. No thermal insulation or reliquefaction plant is necessary for these ships and cargo can be discharged using either pumps or compressors.
|
|
|
Because of their design pressure, the cargo tanks are extremely heavy. As a result, fully pressurised ships tend to be small having cargo capacities of about 4,000 to 6,000 m3, and are primarily used to carry LPG and ammonia. Ballast is carried in double bottoms and in top wing tanks. Because these ships are fitted with Type 'C' containment systems, no secondary barrier is required and the hold space may be ventilated with air.
Figure 3.3 shows a section through a typical fully pressurised ship. These ships carry cargo at ambient conditions and, as such, cargo temperatures may be different at each end of the voyage. Allowance must be made for this and certain rules apply (see 7.5.5).
When equipped with a loading heater these ships can load from a fully refrigerated terminal .
3.4.2 Semi-pressurised ships
Semi-pressurised ships are similar to fully pressurised ships in that they have Type 'C' tanks — in this case pressure vessels designed typically for a maximum working pressure of from 5 to 7 barg. Compared to fully pressurised ships, a reduction in tank thickness is possible due to the reduced pressure but this is at the cost of refrigeration plant and tank insulation. This type of gas carrier has evolved as the optimum means of transporting a wide variety of gases such as LPG, vinyl chloride, propylene, and butadiene. They are most frequently found in the busy coastal trades around the Mediterranean and Northern Europe. Today, this type of ship is the most popular amongst operators of smaller-size gas carriers due to its cargo handling flexibility.
Semi-pressurised ships use Type 'C' tanks and, therefore, do not require a secondary barrier (cargo capacities can vary from 3,000 to 20,000 m3). The tanks are usually made from low temperature steels to provide for carriage temperatures of -48°C which temperature is suitable for most LPG and chemical gas cargoes. Alternatively, they can be made from special alloyed steels or aluminium to allow for the carriage of ethylene at -104°C (see also ethylene ships). The ship's flexible cargo handling system is designed to load from (or discharge to) both pressurised and refrigerated storage facilities. A typical ship section is shown in Figure 3.4.
|
|
|
3.4.3 Ethylene ships
Ethylene ships are often built for specific trades but will also operate carrying LPGs or Chemical Gases. They normally have capacities ranging from 1,000 to 12,000 m3. Ethylene is normally carried in its fully refrigerated condition at its atmospheric boiling point of -104°C. Normally Type 'C' pressure vessel tanks are used and no secondary barrier is required. Thermal insulation and a high-capacity reliquefaction is fitted on this type of ship.
Ballast is carried in the double bottom and wing ballast tanks.
A complete double hull is required for all cargoes carried below -55°C, whether the cargo tanks are of Type 'A', 'B' or 'C'.
3.4.4 Fully refrigerated ships
Fully refrigerated ships carry their cargoes at approximately atmospheric pressure and are designed to transport large quantities of LPG and ammonia. Four different cargo containment systems have been used for these ships. They are as follows:—
· Independent tanks with single hull but double bottom and hopper tanks
· Independent tanks with double hull
· Integral tanks (incorporating a double hull), and
· Semi-membrane tanks (incorporating a double hull)
For this class of ship the most widely used arrangement is the first listed above. (The other systems have not found general favour with ship operators). Here, the tank itself is a Type 'A' prismatic free-standing unit capable of a maximum working pressure of 0.7 barg (Figure 3.1). The tanks are constructed of low-temperature steels to permit carriage temperatures of about -48°C. Fully refrigerated ships range in size from about 20,000 to 100,000m3.
There are relatively few fully refrigerated ships between 55,000 m3 and 70,000 m3. Trading patterns in the 1990s show that ships smaller than 55,000 m3 tend to be used in general tramping routes where cargo changes are frequent. Such ships may switch into the ammonia trade from time to time and in exceptional circumstances, if properly certificated as an oil tanker, they have been known to carry petroleum products. On the other hand ships of 70,000 m3 and above tend to be on long-haul bulk trades carrying similar grades between a limited number of regular ports.
|
|
|
A typical fully refrigerated ship has up to six cargo tanks. Each tank is fitted with transverse wash plates, while a longitudinal bulkhead on the centre line is provided to reduce free surface so improving ship stability. The tanks are usually supported on wooden chocks and are keyed to the hull to allow for expansion and contraction as well as to prevent tank movement under static and dynamic loads. The tanks are also provided with anti-flotation chocks to avoid lifting in case of ballast tank leakage. Because of the low-temperature carriage conditions, thermal insulation and reliquefaction equipment must be fitted.
To improve a fully refrigerated ship's operational flexibility, cargo heaters and booster pumps are often fitted to allow discharge into pressurised storage facilities. This will normally be accomplished at reduced discharge rates.
Where Type 'A' tanks are fitted, a complete secondary barrier is required (see 3.2.1). The hold spaces must be inerted when carrying flammable cargoes. Ballast is carried in double bottoms and in top side (saddle) tanks or, when fitted, in side ballast tanks.
3.4.5 LNG ships
LNG carriers are specialised types of gas carriers built to transport large volumes of LNG at its atmospheric boiling point of about -162° C. These ships are now typically of between 125,000 and 135,000 m3 capacity and are normally dedicated to a specific project. Here they often remain for their entire contract life, which may be between 20-25 years or more. Apart from a few notable exceptions during the early years of LNG transport, the containment systems on these ships are mainly of four types:
· Gaz Transport membrane (Figures 3.5(a) and 3.5(b))
· Technigaz membrane (Figures 3.6(a) and 3.6(b))
· Kvaerner Moss spherical — independent Type 'B' (Figure 3.2(a)), and
· IHI SPB Tank — prismatic (Figure 3.2(b))
These systems have already been described in 3.2. The newest containment system is the self supporting, prismatic Type 'B' (SPB) design developed by the Japanese shipbuilder IHI and this is based on the earlier Conch system. This design incorporates an aluminium tank.
|
|
|
All LNG ships have double hulls throughout their cargo length which provide adequate space for ballast. Ships fitted with the membrane systems have a full secondary barrier and tanks of the Type 'B' design have drip-pan type protection. A characteristic common to all LNG ships is that they burn cargo boil-off as fuel.
Hold spaces around the cargo tanks are continuously inerted, except in the case of spherical Type 'B' containment where hold spaces may be filled with dry air provided that there is an adequate means for inerting such spaces in the event of cargo leakage. Continuous gas-monitoring of all hold spaces is required.
In general, reliquefaction plants have been little used on LNG ships but it should be noted that a very small number of LNG ships have been fitted with reliquefaction plant suited to cater for limited boil-off. However these were never successfully operated. Being much colder than LPG, the necessary equipment is much more costly and it is currently more economic to burn the boil-off gas in the ship's main boilers (see 7.6.2). Most LNG carriers have steam turbine propulsion plants. Two medium size ships are equipped with low speed, low injection pressure, dual fuel diesel engines. Although technology exists to introduce gas-burning diesel engines the perceived greater reliability of the steam turbine has so far prevented any serious development in this direction.
3.5 GAS CARRIER LAYOUT
Gas carriers have many features which are not found on other types of tanker. Chapter Four deals specifically with cargo handling systems and highlights some of these basic differences. Other unique features can be identified from the general arrangement of gas carriers. Some specific features are outlined below.
It is not permitted for a cargo pumproom to be placed below the upper deck, nor may cargo pipelines be run beneath deck level; therefore, deepwell or submersible pumps must be used for cargo discharge. Pipelines to cargo tanks must be taken through a cargo tank dome which penetrates the deck.
Where ships are fitted with a reliquefaction plant, this is located in a compressor house on deck. Adjacent to the compressor house is an electric motor room which contains the machinery for driving the reliquefaction compressors. The electric motor room and compressor room must be separated by a gastight bulkhead (see Figure 3.7).
Дата добавления: 2018-02-28; просмотров: 603; Мы поможем в написании вашей работы! |
Мы поможем в написании ваших работ!