DECK SEAL

Deck Water Seal – Design, construction and purpose in Inert Gas (IG) system on board ships

The Deck Water Seal

The deck water seal is the main safeguard against the reverse flow of gases from the distribution system to the IG plant. 

SOLAS Regulation:

SOLAS Chapter 11-2, Regulation 62, provides the requirements for the non-return devices installed in the inert gas main of a typical system.

Regulation 62 requires that the inert gas system is equipped with two non-return devices, one of which shall be a water seal, to prevent the return of hydrocarbon vapour to the gas-safe spaces.

In addition, Regulation 62 requires that the arrangement of the deck seal shall be such that it will prevent the backflow of hydrocarbon vapours and will ensure the proper functioning of the seal under operating conditions.

The IMO “Guidelines for Inert Gas Systems”, Sections 3.6 and 3.7, indicate the design considerations, and provide details on the function, of various types of deck water.

TYPES OF DECK SEAL:

1. Dry type deck seal.
2. Semi dry deck seal.
3. Wet type deck seal.

The picture below shows the construction of a wet type deck water seal and its functioning. Basically it consists of a chamber semi-filled with water and two pipes for inlet and outlet of flue gases while another two small pipes denote inlet and outlet for sealing water. There is a demister pad to remove water droplets from gas. The operation of this device is pretty simple and the two diagrams shows conditions where the inert gas is flowing from the plant to the distribution area and the right hand side showing a condition where back pressure tends to push cargo gases into the IG system and is prevented by the water seal.

Semi Dry Type Deck Water Seal

The construction of this type of seal as well as the functioning under both conditions is shown in the diagram below. The main difference with the previous type of seal is that it uses venture action to draw water when there are chances of back flow of the gases thus reducing if not completely eliminating water carry over to the cargo tanks.

Dry Type Deck Water Seal

This seal totally eliminates any water carry over and uses automated valve control to deliver water to the seal in case there is any back flow but the only disadvantage is that if automation system fails then there is a danger of blow back of cargo gases.

Just keep in mind that only one of these seals is used, and in addition to this deck seal there is a mechanical non return valve for extra safety in the pipeline as well. We will study about the design considerations of non-return devices used in the IG system in our next article.

GAS CARRIER

A gas carrier (or gas tanker) is a ship designed to transport LPG, LNG or liquefied chemical gases in bulk.

Gas Tanker Basics – Definitions

The International Maritime Organization (IMO), for the purposes of its Gas Carrier Codes, has adopted the following definition for the liquefied gases carried by sea:

1. Gas

Liquids with a vapour pressure exceeding 2.8 bar absolute at a temperature of 37.8°C

2. Boiling Point

The temperature at which the vapour pressure of a liquid is equal to the pressure on its surface (the boiling point varies with pressure)

3. Cargo Area

That part of the ship which contains the cargo containment system, cargo pumps and compressor rooms, and includes the deck area above the cargo containment system. Where fitted, cofferdams, ballast tanks and void spaces at the after the end of the aftermost hold space or the forward end of the forwardmost hold space are excluded from the cargo area.

4. Cargo Containment Systems

The arrangement for containment of cargo including, where fitted, primary and secondary barriers, associated insulations, interbarrier spaces and the structure required for the support of these elements.

5. Gas-Dangerous Space or Zone

A space or zone within a ship’s cargo area which is designated as likely to contain flammable vapour and which is not equipped with approved arrangements to ensure that its atmosphere is maintained in a safe condition at all times.

6. Gas-Safe Space

A space on a ship not designated as a gas-dangerous space.

7. Hold Space

The space enclosed by the ship’s structure in which a cargo containment system is situated.

8. Interbarrier Space

The space between a primary and a secondary barrier of a cargo containment system, whether or not completely or partially occupied by insulation or other material.

9. MARVS

This is the abbreviation for the Maximum Allowable Relief Valve Setting on a ship’s cargo tank — as stated on the ship’s Certificate of Fitness.

10. Primary Barrier

This is the inner surface designed to contain the cargo when the cargo containment system includes a secondary barrier.

11. Secondary Barrier

The liquid-resisting outer element of a cargo containment system designed to provide temporary containment of a leakage of liquid cargo through the primary barrier and to prevent the lowering of the temperature of the ship’s structure to an unsafe level

12 Tank dome

It is not permitted for a cargo pump room to be placed below the upper deck, nor may cargo pipelines are run beneath deck level; therefore, deep well 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.

Different Types of Gas Tanker Ships

Gas carriers can be grouped into five different categories according to the cargo carried and the carriage condition. These are as follows:

1. Fully pressurised ships
2. Semi-pressurised ships
3. Ethylene ships
4. Fully refrigerated LPG ships
5. 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 deep sea routes.

1. Fully pressurised ships

• Fully pressurised ships are the simplest of all gas carriers. 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.

Advantages of fully pressurized tankers:

i) They are built with ordinary grades of steel as the cargo is carried at ambient temperature and no insulation is required
ii) no reliquefaction plant is required
iii) operations are simpler

Disadvantages

i) Due to their shape, the use of underdeck space cannot be optimised
ii) high design pressure requires considerable tank wall thickness, with consequent increase in displacement weight and cost

2. Semi-pressurised ships

• Semi-pressurised ships are similar to fully pressurised ships in a context 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.

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 are 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’.

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:—

1. Independent tanks with single hull but double bottom and hopper tanks.

2. Independent tanks with double hull.

3. Integral tanks (incorporating a double hull).

4. Semi-membrane tanks (incorporating a double hull).

• For this class of ship, the tank itself is a Type ‘A’ prismatic free-standing unit capable of a Maximum working pressure of 0.7 barg. 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 100,000 m3. There are relatively few fully refrigerated ships between 55,000 m3 and 70,000 m3.
• 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
• 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, inside ballast tanks.

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.

The containment systems on these ships are mainly of four types:

1. Gaz Transport membrane

2. Technigaz membrane

3. Kvaerner Moss spherical — independent Type ‘B’ , and

4. IHI SPB Tank — prismatic

• 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.
• Most LNG carriers have steam turbine propulsion plants.

CARGO CONTAINMENT SYSTEMS

A cargo containment system is an overall arrangement for containing cargo including, where fitted:

• A primary barrier (the cargo tank),
• Secondary barrier (if fitted),
• Associated thermal insulation,
• Any intervening spaces, and
• Adjacent structure, for the support of these elements.

For cargoes carried at temperatures between -10°C and -55°C the ship’s hull may act as the secondary barrier and in such cases, it may be a boundary of the hold space. The basic cargo tank types utilized on board gas carriers are in accordance with the list below:

Independent tanks

Independent tanks are completely self-supporting and do not form part of the ship’s hull structure. Moreover, they do not contribute to the hull strength of a ship. As defined in the IGC Code, and depending mainly on the design pressure, there are three different types of independent tanks for gas carriers: these are known as Types ‘A’, ‘B’ and ‘C’.

Type ‘A’ tanks

Type ‘A’ tanks are constructed primarily of flat surfaces. The maximum allowable tank design pressure in the vapour space for this type of system is 0.7 barg; this means cargoes must be carried in a fully refrigerated condition at or near atmospheric pressure (normally below 0.25 barg).

Figure below shows a section through this type of tank as found on a fully refrigerated LPG carrier. This is a self-supporting prismatic tank which requires conventional internal stiffening. In this example the tank is surrounded by a skin of foam insulation. Where perlite insulation is used, it would be found filling the whole of the hold space.

The material used for Type ‘A’ tanks is not crack propagation resistant. Therefore, in order to ensure safety, in the unlikely event of cargo tank leakage, a secondary containment system is required. This secondary containment system is known as a secondary barrier and is a feature of all ships with Type ‘A’ tanks capable of carrying cargoes below -10°C.

For a fully refrigerated LPG carrier (which will not carry cargoes below -55°C) the secondary barrier must be a complete barrier capable of containing the whole tank volume at a defined angle of heel and may form part of the ship’s hull, as shown in the figure. By this means appropriate parts of the ship’s hull are constructed of special steel capable of withstanding low temperatures. The alternative is to build a separate secondary barrier around each cargo tank. The IGC Code stipulates that a secondary barrier must be able to contain tank leakage for a period of 15 days.

On such ships, the space between the cargo tank (sometimes referred to as the primary barrier) and the secondary barrier is known as the hold space. When flammable cargoes are being carried, these spaces must be filled with inert gas to prevent a flammable atmosphere being created in the event of primary barrier leakage.

Type ‘B’ tanks

Type ‘B’ tanks can be constructed of flat surfaces or they may be of the spherical type. This type of containment system is the subject of much more detailed stress analysis compared to Type ‘A systems. These controls must include an investigation of fatigue life and a crack propagation analysis. These tanks may be able to withstand pressures up to 2 barg. The most common arrangement of Type ‘B’ tank is a spherical tank as illustrated in Figure 3.2. This tank is of the Kvaerner Moss design. Because of the enhanced design factors, a Type ‘B’ tank requires only a partial secondary barrier in the form of a drip tray. The hold space in this design is normally filled with dry inert gas. However, when adopting modern practice, it may be filled with dry air provided that inerting of the space can be achieved if the vapour detection system shows cargo leakage. A protective steel dome covers the primary barrier above deck level and insulation is applied to the outside of the tank. The Type ‘B’ spherical tank is almost exclusively applied to LNG ships; seldom featuring in the LPG trade.

A Type ‘B’ tank, however, need not be spherical. There are Type ‘B’ tanks of prismatic shape in LNG service. The prismatic Type ‘B’ tank has the benefit of maximising ship hull volumetric efficiency and having the entire cargo tank placed beneath the main deck. Where the prismatic shape is used, the maximum design vapour space pressure is, as for Type ‘A tanks, limited to 0.7 barg.

Cross Section View of Type B Tank Ship

Type B Tank being installed into ships hold

Spherical Tank IMO Type B

Type ‘C’ tanks

Type ‘C’ tanks are normally spherical or cylindrical pressure vessels having design pressures higher than 2 barg. The cylindrical vessels may be vertically or horizontally mounted.

This type of containment system is always used for semi-pressurised and fully pressurised gas carriers. Type ‘C’ tanks are designed and built to conventional pressure vessel codes and, as a result, can be subjected to accurate stress analysis. Furthermore, design stresses are kept low. Accordingly, no secondary barrier is required for Type ‘C’ tanks and the hold space can be filled with either inert gas or dry air.

In the case of a typical fully pressurised ship (where the cargo is carried at ambient temperature), the tanks may be designed for a maximum working pressure of about 18 barg. For a semi-pressurised ship the cargo tanks and associated equipment are designed for a working pressure of approximately 5 to 7 barg and a vacuum of 0.5 barg. Typically, the tank steels for the semi-pressurised ships are capable of withstanding carriage temperatures of -48°C for LPG or -104°C for ethylene. (Of course, an ethylene carrier may also be used to transport LPG.)

Figure below shows Type ‘C’ tanks as fitted in a typical fully pressurised gas carrier. With such an arrangement there is comparatively poor utilisation of the hull volume; however, this can be improved by using intersecting pressure vessels or bi-lobe type tanks which may be designed with a taper at the forward end of the ship.

A Type C Tank being installed.

A ship under construction in shipyard with IMO Type C Tanks

Membrane tanks (membrane – 0.7 to 1.5 mm thick)

The concept of the membrane containment system is based on a very thin primary barrier (membrane – 0.7 to 1.5 mm thick) which is supported through the insulation. Such tanks are not self-supporting like the independent tank & an inner hull forms the load-bearing structure. Membrane containment systems must always be provided with a secondary barrier to ensure the integrity of the total system in the event of primary barrier leakage. The membrane is designed in such a way that thermal expansion or contraction is compensated without over-stressing the membrane itself.

There are two principal types of membrane system in common use

1. Gaz Transport membrane system
2. Technigaz membrane system

Both named after the companies who developed them and both designed primarily for the carriage of LNG.

Initially, the Moss system was more popular, but higher Suez toll fees due to their higher gross tonnage made Moss vessels less attractive for trades involving the Suez Canal. Recently Moss has staged a comeback and currently, there are about 30 Moss vessels on order against 100+ membrane vessels. A fourth LNG containment system joined the ranks of the large marine LNG cargo tank designs in the early 1990’s; the Japanese IHI SPB (Self-supporting Prismatic shape IMO type-B) system. With only two orders for LNG carriers in the 1990’s, this system seemed to be inaccessible due to its high price. However, in 2014 four vessels were ordered with the SPB system, bringing it back as a credible alternative to the membrane systems and the Moss system.

Membrane systems

Technigaz designed a membrane type atmospheric LNG containment system with a corrugated stainless steel primary membrane supported by wooden boxes filled with insulation material. A secondary cryogenic barrier, also supported by wooden boxes filled with insulation material provides containment of the cryogenic cargo in case the primary membrane develops a leak. The characteristic corrugations in the primary membrane allow for the shrinkage of metal under cryogenic temperatures. This design, identified as the Mk I was soon superseded by improved versions and is currently available as the Mk III series from a number of shipyards in Korea and Japan. Soon the Mk V series will be going into production, which replaces the current Triplex secondary barrier with a corrugated stainless steel secondary barrier.

Compatriot Gaztransport designed a rather similar system consisting of a primary membrane supported by insulation in plywood boxes and a secondary membrane, also supported by insulation in plywood boxes. This system was called the No 88 system and featured a primary and secondary membrane of a steel alloy with a negligible contraction coefficient. Improvements have been over the years and the current system is the No 96, which is being used in LNG carriers under construction in Korea and China.

After years of competition, Gaztransport and Technigaz merged to form GTT, which has been developing and promoting both membrane type containment systems in parallel. GTT has licensed these systems to all major LNG carrier builders around the world. The main advantage of the membrane type containment systems is their prismatic shape, which allows these systems to use the space available within the hull of the LNG carrier to a very high degree. With the cargo tanks recessed deep inside the hull under a low trunk deck, membrane type LNG carriers do not need a high deck house to have good visibility. This results in the typical “squat” silhouette of this type of vessels. In France, GTT proposed membrane type LNG fuel tanks for the proposed newbuilding ferry for Brittany Ferries. Unfortunately this project was put on hold for the time being for non-technical reasons.

Both membrane systems have one traditional weakness; their vulnerability for sloshing damage. Sloshing is the motion of the LNG cargo in the tanks as a result of the motion of the vessel due to the effect of waves and wind. In certain circumstances, waves occur in the LNG cargo which upon impact on the tank walls can cause damage to the primary barrier and the boxes supporting the primary membrane. To counter the risk of sloshing damage, GTT advises the operators of membrane ships to operate their ships with tank levels of more than 90% or less than 10%. For applications that require part load operations, such as LNG Floating Storage and Regas Units (LNG FSRU), membrane systems with specially reinforced boxes have been developed.

Gaztransport Designed Membrane Tank featured a primary and secondary membrane of a steel alloy with a negligible contraction coefficient.

GTT-technology membrane tank model fabricated by Conrad Shipyard.

Cross Section of a Technigaz showing each layer

Technigaz designed a membrane type LNG containment system with a corrugated stainless steel primary membrane supported by wooden boxes filled with insulation material.

The Moss system

The Moss spherical LNG containment system does not have these sloshing issues. Its aluminium spheres have sufficient structural strength to withstand LNG wave impact due to the interaction between the cargo and the ship’s motion. The Moss system doesn’t need a full secondary barrier like the membrane system; there is only a small drip tray below the spheres to catch any liquid leaking. The design philosophy behind the Moss system is that the tank should be designed to be strong enough so that cracks should not develop in the tanks over the lifetime of the vessel. The structural strength of the containment system is exactly the reason why old Moss vessels are very popular candidates for conversion to LNG FSRU’s or even floating LNG production plants.

View from inside of Tank

View from inside the hold of ship

The only true disadvantage of the Moss vessels is the fact that the containment system has a very low hull space utilization rate. The sheers are mounted on the deck of the vessel by way of an equatorial ring, which means that half the sphere protrudes above the deck. While this makes for the characteristic silhouette of the Moss carrier, it also necessitates a high deck house to ensure adequate line of sight from the bridge. The low hull space utilization means that a Moss carrier has a higher GT rating than membrane carriers of similar cargo capacity, which translates in higher port and fairway dues and higher tonnage taxes.

 The SPB system

The fourth LNG cargo containment system, the IHI (now JMU) SPB system manages to combine the advantages of the membrane system and the Moss system and addresses the disadvantages of both systems too. The prismatic shape of the tanks ensures a high hold space utilization rate and a low air draft, while the solid aluminum construction with a centerline bulkhead and transverse swash bulkheads reduces liquid motion in the tanks and minimizes the risk for sloshing damage, even in part load conditions. The high price of this system originally prevented wide spread adoption but in 2014 JMU, the successor to SPB designer IHI, secured orders for tanks for four 165,000 m3 LNG carriers and it has been addressing the only true disadvantage of this system; its price tag. With possible licensing overseas, the SPB system could become a very serious contender in the LNG containment system arena. In Japan, JMU has already carried out a study with a shipyard into the feasibility of SPB tanks as LNG fuel tanks.

   
A SPB Tank designed by IHI Japan                                

“IHI-SPB” (Self-supporting, Prismatic Shape, IMO type B) is Japanese own technology developed by IHI Group while the competing technologies used for LNG floaters including LNG carriers by Japanese and Korean shipyards are imported from Europe (Norwegian Moss technology and French Membrane technology). “IHI-SPB” has unique features of “No Sloshing” which enables any level loading of LNG inside the tank at offshore, “Flat upper deck” which enables installation of the topside plants on upper deck, “Less and Easy Maintenance”, etc. and is most suited to use in the LNG floaters including FLNG and FSRU required to stay offshore and operate safely for long years without dry docking.

 Integral tanks

Integral tanks form a structural part of the ship’s hull and are influenced by the same loads which stress the hull structure. Integral tanks are not normally allowed for the carriage of liquefied gas if the cargo temperature is below -10°C. Certain tanks on a limited number of Japanese-built LPG carriers are of the integral type for the dedicated carriage of fully refrigerated butane.

Internal insulation tanks

Internally insulated cargo tanks are similar to integral tanks. They utilise insulation materials to contain the cargo. The insulation is fixed inside ship’s inner hull or to an independent load-bearing surface. The non-self-supporting system obviates the need for an independent tank and permits the carriage of fully refrigerated cargoes at carriage temperatures as low as -55°C. Internal insulation systems have been incorporated in a very limited number of fully refrigerated LPG carriers but, to date, the concept has not proved satisfactory in service.

CHEMICAL TANKER OVERVIEW

A chemical tanker is a type of tanker ship designed to transport chemicals in bulk. As defined in MARPOL Annex II, chemical tanker means a ship constructed or adapted for carrying in bulk any liquid product listed in chapter 17 of the International Bulk Chemical Code.[1] As well as industrial chemicals and clean petroleum products, such ships also often carry other types of sensitive cargo which require a high standard of tank cleaning, such as palm oil, vegetable oils, tallow, caustic soda, and methanol.


Oceangoing chemical tankers range from 5,000 tonnes deadweight (DWT) to 35,000 DWT in size, which is smaller than the average size of other tanker types due to the specialized nature of their cargo and the size restrictions of the port terminals where they call to load and discharge.


Chemical tankers normally have a series of separate cargo tanks which are either coated with specialized coatings such as phenolic epoxy or zinc paint, or made from stainless steel. The coating or cargo tank material determines what types of cargo a particular tank can carry: stainless steel tanks are required for aggressive acid cargoes such as sulfuric and phosphoric acid, while 'easier' cargoes — such as vegetable oil — can be carried in epoxy coated tanks. The coating or tank material also influences how quickly tanks can be cleaned. Typically, ships with stainless steel tanks can carry a wider range of cargoes and can clean more quickly between one cargo and another, which justifies the additional cost of their construction.

Classification

In general, ships carrying chemicals in bulk are classed into three types:

Type 1

Type 1 ship is a chemical tanker intended to transport Chapter 17 of the IBC Code products with very severe environmental and safety hazards which require maximum preventive measures to preclude an escape of such cargo.

Accordingly, a type 1 ship should survive the most severe standard of damage stability and its cargo tanks should be located at the maximum prescribed distance onboard from the shell plating.

The quantity of cargo required to be carried in ship < 1,250 m3 in any one tank.

Tank locations :

Cargo tanks shall be located at the following distances inboard – MEAN OF DOUBLE BOTTOM HEIGHT TO THE BASE LINE OR MOULDED LINE (ABOVE THE BOTTOM PLATE).

From the side shell plating, not less than the transverse extent of damage (B/5 or 11.5m whichever is less), and from the moulded line of the bottom shell plating at centerline, not less than the vertical extent of damage (B/15 or 6 m whichever is less), and nowhere less than 760 mm from the shell plating. This requirement does not apply to the tanks for diluted slops arising from tank washing.

Type 2

Type 2 ship is a chemical tanker intended to transport Chapter 17 of the IBC Code products with appreciably severe environmental and safety hazards which require significant preventive measures to preclude an escape of such cargo.

The quantity of cargo required to be carried in ship < 3,000 m3 in any one tank.

Tank locations :

Cargo tanks shall be located at the following distances inboard – MEAN OF DOUBLE BOTTOM HEIGHT TO THE BASE LINE OR MOULDED LINE (ABOVE THE BOTTOM PLATE).

From the moulded line of the bottom shell plating at centerline, not less than the vertical extent of damage ( B/15 or 6m whichever is less), and nowhere less than 760 mm from the shell plating. This requirement does not apply to the tanks for diluted slops arising from tank washing. 

Type 3

Type 3 ship is a chemical tanker intended to transport Chapter 17 of the IBC Code products with sufficiently severe environmental and safety hazards which require a moderate degree of containment to increase survival capability in a damaged condition.^[2] Most chemical tankers are IMO 2 and 3 rated, since the volume of IMO 1 cargoes is very limited.

There is no filling restriction for chemicals assigned to Ship Type 3 Cargo Tank.

Tank locations :

Cargo tanks shall be located at the following distances inboard – MEAN OF DOUBLE BOTTOM HEIGHT TO THE BASE LINE OR MOULDED LINE (ABOVE THE BOTTOM PLATE).

No requirement

Hazards of carrying noxious liquid chemicals at sea

Toxicology and associated hazards onboard chemical tankers

Toxicity is the ability of a substance, when inhaled, ingested, or absorbed by the skin, to cause damage to living tissue, impairment of the central nervous system, severe illness or, in extreme cases, death. The amounts of exposure required to produce these results vary widely with the nature of the substance and the duration of exposure to it. ....

Hazards of vapour given off by a flammable liquid while carrying at sea

Vapour given off by a flammable liquid will burn when ignited provided it is mixed with certain proportions of air, or more accurately with the oxygen in air. But if there is too little or too much vapour compared to the air, so that the vapour-and-air mixture is either too lean or too rich, it will not burn. ....

Reactivity of various noxious liquid chemicals

Self-reaction: The most common form of self-reaction is polymerisation. Polymerisation generally results in the conversion of gases or liquids into viscous liquids or solids. It may be a slow, natural process which only degrades the product without posing any safety hazards to the ship or the crew, or it may be a rapid, exothermic reaction evolving large amounts of heat and gases. .....

Most corrosive chemicals carried onboard chemical tankers

Acids, anhydrides and alkalis are among the most commonly carried corrosive substances. They can rapidly destroy human tissue and cause irreparable damage. They can also corrode normal ship construction materials, and create a safety hazard for a ship.....

Posoning hazards

The poison is a very toxic substance which when absorbed into the human body by ingestion, skin absorption, or inhalation produces a serious or fatal effect. Poison may enter the human body orally, by inhalation, or by skin contact. After being absorbed by the body it may affect certain organs or give a general poisonous effect. Lately the cancerogene effects of some industrial chemicals have been noticed. This has led to significant reductions of hereto accepted TLV- values in many countries.

General precautions onboard chemical tankers

Additional precautions for specific cargoes are necessary and should also be incorporated in the ship’s cargo handling procedures....

Mooring precautions onboard chemical tankers

The consequences of a chemical tanker ranging along a jetty or breaking away from a berth could be disastrous, especially during a cargo transfer involving multiple different chemicals. Correct and sufficient mooring is therefore of the utmost importance.

Berth precautions onboard chemical tankers

If an unauthorised craft comes alongside or operates in an area which may create a danger, it should be reported to the port authority and, if necessary, cargo transfer operations should cease. .....

Cold weather countermeasures, avoiding electric storms

During cold weather, precautions should be taken to prevent equipment and systems from freezing. Attention should be given to pneumatic valves and control systems, fire lines and hydrants, steam driven equipment, cargo heating systems, pressure/vacuum valves etc......

Restriction on using radio equipments and other mobile devices in cargo working areas

During medium and high frequency radio transmissions significant energy is radiated, which can create a danger of incendive sparking by inducing an electrical potential in unearthed steelwork.

Securing cargo tank lids and required safety precautions

Improper closing and sealing of cargo tank hatches can be a major cause of cargo contamination. A properly closed and sealed tank hatch/opening will prevent sea water ingress and maintain a positive pressure Nitrogen blanket in the ullage space. ....

Means of access (gangways or accommodation ladders) safety precautions

Emergency towing-off wires ( fire wires) ,Ship’s readiness to move Deckhouses and superstructures safety precautions .....

Precautions against static electricity

Static electricity is generated by friction that occurs between different materials during relative motion. Electrostatic charges can then accumulate in materials which are poor conductors of electricity or which are good conductors but are insulated.....

Cargo tank entry safety precautions

On chemical tankers the entry of personnel into cargo tanks is a more common practice than on oil tankers as a result of the requirement for inspections between grades etc; despite this, it is essential that the necessary checks are conscientiously made and recorded prior to entry in order to ensure the safety of personnel, enclosed space rescue equipment must be made ready for immediate use.