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Problem of partial loading of cargo tanks & filling limits -LNG carrier guideline

Within a range of tank filling levels, the natural pitching and rolling movement of the ship at sea, and the liquid free-surface effect, can cause the liquid to move within the tank. It is possible for considerable liquid movement to take place, creating high impact pressure on the tank surface. This effect is called “sloshing” and can cause structural damage.

Sloshing is a problem which effects membrane constructed tanks. Independent containment systems such as the spherical Moss design and the IHI prismatic designs are not subject to the same sloshing impacts. Partial loading at any tank filling level is inherent in the design of Moss design tanks, giving them distinct advantages over membrane containment systems, when handling spot trades and offshore loading/unloading.

This has taken on an even greater importance with operators seeking the operational flexibility of partial cargo loading in combination with the growing preference for membrane-type containment systems.

LNG is carried at approximately minus 160 degrees Celsius. As the low-filling condition produces progressive waves known as hydraulic jumps, partially-loaded carriers can exhibit high dynamic loads.

As a consequence, sloshing due to partial filling must be examined very carefully. Characteristics unique to LNG, namely low temperature, compressibility of entrapped gas, hydrodynamic interaction between liquid and containment system, and dynamic material characteristics challenge the vessel’s strength and may require additional reinforcement of critical areas. These areas are the insulation system, tank structure or the pump tower which serves as the cargo handling connection to the hull, and the base support structure.

The sloshing motion in an LNG tank at the low-filling level is quite different from that experienced at high filling levels

When the tank motion is large, the front of the “hydraulic jump” (when the motion within the tank causes the liquid to create a wave action) becomes steeper, developing a breaking wave. If the hydraulic jump hits the bulkhead before breaking, a large impact can occur The uniform velocity of the hydraulic jump also results in a large drag force on the lower part of the pump tower and it’s supporting system

Sloshing impact occurs when there is a sudden change in the wetted surface due to liquid motion in the tank. In a partially filled compartment, a wider area on the tank wall is vulnerable to the sloshing impact of the cargo

Design modifications and improvements

The high dynamic loads and impact sloshing pressure on the insulation system and tank structure in membrane-type vessels are major concerns.

From the experience gained on the first LNG ships put into service and from a large number of model tests and computer analyses since, there have been numerous design improvements, to counter the sloshing impact.

The height of “chamfer” at the topside was increased and the insulation boxes at the tank top were reinforced to withstand the sloshing impact in the fully laden condition. There has also been a considerable improvement in the design and construction of the membrane and supporting insulation structures.

LNG vessels normally operate in a fully laden condition or with a minimum of cargo (heel) during the ballast voyage. In a fully laden condition the typical filling level is greater than 95% of the tank height, and in ballast condition less than 10%. The current design (tank insulation and scantlings) is effective in preventing sloshing impact loads when the vessel is carrying heel only.

The ship’s cargo tanks are designed to limit the impact forces and the safety margin has been considerably enlarged. New tank designs are reasonably free from any sloshing risk. However, operators should always be aware of the potential risks to the cargo containment system and also on the tank equipment due to sloshing.

Tank filling limits

Classification Societies, GTT and Marintek, carried out a series of model tests to investigate the effects of sloshing in partially filled prismatic LNG tanks. As a result of the tests, the following precautions should be taken to avoid damage due to sloshing
  1. CARGO TANK LEVELS: The first precaution is to maintain the level of the tanks within the required limits i.e.:
    N.B. The Certificate of Fitness for the Carriage of Liquefied Gases, may presently show the lower limits as being 10% of tank length, this will be amended by Class in due course.

  2. SHIP’S MOVEMENT: The second precaution is to try to limit the ship’s movement, which would generate sloshing in the tanks. The amplitude of sloshing depends on the condition of sea (wave pattern), the trim and the speed of the ship. Often a minor alteration of course may change the ship motion considerably, particularly at high speed, and this may have a significant effect on sloshing

The above limits will be stated within the ship specific Cargo Operating Manual, and will generally be included in the Conditions of Carriage section of the International Certificate of Fitness for the Carriage of Liquefied Gases in Bulk.

Reference temperature and cargo tank filling limits

Chapter 15 of the IGC Code gives requirements for maximum allowable loading limits for cargo tanks. The maximum limit as per IGC Code is 98% of the tank volume at the reference temperature. Some administrations will allow for a greater tank volume, typically 98.5%. This takes into account the expected boil-off of vapours from the cargo tanks during the loaded voyage.

The specific criteria for each vessel will be found on the certificate of fitness, and generally refer to the Reference Temperature. This filling limit should never be exceeded. When shutting tanks off, allowance must be made for the amount of cargo, which will still be loaded in the time taken for the tank loading valves to close.

In the case of cargo tanks on fully refrigerated ships, the Gas Codes envisage relief valves set to open only marginally above the vapour pressure of the cargo at the maximum temperature it will reach over the whole cycle of loading, transportation and discharge. The loading limit must also be such that, if a surrounding fire occurs, the tank will not become liquid-full before the relief valve opens.

The maintenance of the cargo tank pressures below the MARVS is generally controlled, by the use of the boil-off vapours, as fuel for shipboard use or in a waste heat system. This system may be used at all times, including time in port and while manoeuvring, provided that a means of disposing of excess energy is provided, such as a steam dump system.

When a cargo vapour pressure / temperature control is provided as above, the Reference Temperature means the temperature of the cargo upon termination of loading, during transportation, or at unloading.

All vessel staff must however be aware that although unlikely a significant rise in temperature of the cargo, may give rise to an increase in the volume of the tank contents above the stated maximum filling limits. In practice, during the cycle of cargo on board, there is generally only a very small change in the cargo temperatures and additionally the volume of cargo decreases as boil-off occurs.

Related Information:

  1. LNG tank leaks and immediate action by gas carriers

  2. Leaks from a Loading Arm due to Tidal or Current Effects

  3. Minor or major leaks from LNG tanks

  4. Risk of Overfilling of Cargo Tank during Loading

Gas cargo containment systems - primary barrier (the cargo tank),secondary barrier, thermal insulation and more

Discussion prior to cargo transfer in liquefied gas carrier

Safety checklist for gas carrier

Tanker Cargo Operations Logbook

Connecting Bonding Cable

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