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LNG Vapour and LNG liquid leakage into Barrier Spaces


Under normal operations the IBS and IS barrier spaces are continually swept with nitrogen.
在正常操作下,IBS 和 IS 屏障空间不断被氮气扫过。

A vapour leakage will be detected by the gas sampling analyser

Liquid leakage into an IBS space could develop slowly over a period of days or hours, or may develop suddenly if a major failure of the membrane occurs.

3.Failure of Primary Barrier

A major failure in the primary membrane, allowing liquid into the inter barrier space, will be indicated as follows:

A rapid increase in the methane content of the affected space.

A rise in pressure in the inter barrier space nitrogen header, accompanied by continuous venting to atmosphere.

Low temperature alarms at all temperature sensors in the insulation below the damaged cargo tank.

A decrease in inner hull steel temperatures.

Gas alarms in IBS at 30% by Vol
IBS 中的气体警报率为 30% (按 Vol)

Likely lifting of the Primary Barrier Space (cargo tank) relief valves.

If a major failure of the membrane occurs, liquid from the tank will flow into the inter barrier space until the levels in both compartments are equal.

Immediately connect the portable liquid level meter to the IBS at the first sign of a liquid leak to assist in determining the height size of the leak by recording the IBS level increase rate, IBS pressure and the cargo tank level pressure.
在液体泄漏的第一个迹象时立即将便携式液位计连接到 IBS,通过记录 IBS 液位增加率、IBS 压力和货舱液位压力来帮助确定泄漏的高度大小。

If the liquid level increases to the cargo tank level before the tank can be pumped out, then extreme care must be taken to protect against over pressurisation of the membrane during the discharge of the damaged tank.

LNG liquid in the IBS can be removed by either gravity drainage back into the cargo tank as the tank is pumped out, or if that does not work, then by using the IBS drainage system (As per Cargo Manual)
当油箱被抽出时,可以通过重力排水回货舱中去除 IBS 中的液化天然气液体,或者如果这不起作用,则使用 IBS 排水系统(根据货物手册)

Whenever Gas or LNG is detected in IBS (Inter Barrier Space):
每当在 IBS(屏障间空间)中检测到气体或液化天然气时:

Permit nitrogen to continuously flow through the IBS and ensure stable LEL gas concentration in the IBS.
允许氮气持续流过 IBS,并确保 IBS 中稳定的 LEL 气体浓度。

Adjust the flow rate as necessary so that the gas concentration remain less than 10% LEL (desired) or below the maximum value of 30% LEL on both the laden and ballast voyages.
根据需要调整流速,使气体浓度在满载和压载航行中保持小于 10% REL(所需)或低于 30% LEL 的最大值。

Verify and maintain the pressure in the insulation space (IS) at a minimum of 2 mb above the pressure in the IBS of the affected tank to prevent LNG vapour from passing into the IS.
验证并保持绝缘空间 (IS) 中的压力至少比受影响储罐 IBS 中的压力高 2 mb,以防止 LNG 蒸汽进入 IS。

4.Vapour leakage

Leakage of Methane into to the Primary insulation space present NO immediate danger to the tank of the vessel.

On NO96 system, Primary and Secondary barriers are constructed of 0,7mm thick Invar membrane and are liquid and vapour tight.
在 NO96 系统上,初级和次级屏障由 0.7 毫米厚的殷钢膜构成,并且是液态和蒸汽密封的。

The vapour concentration in each primary insulation space is recorded daily to detect any such small but steady change.

Indicators of serious leakage

A sudden rise in percentage of methane vapour in on einsulation space:

Any porosity in primary barrier weld allow passage of methane vapour into the primary insulation space. Amount of this vapour should be kept on minimum by using nitrogen purging.

If fracture occurs below the level of liquid in the tank, the vapour concentration will increase rapidly and suddenly.

If fracture is above the liquid level, the concentration will exhibit a fluctuating increase.

An increase of pressure in one Primary Insulation Space:

A fracture above the liquid level in cargo tank will allow a direct flow of vapour into the primary insulation space. This flow will vary according to the pressure in the tank.

Fracture below liquid leve in cargo tank, resulting in a small amount of liquid vaporising as it passes through the fracture will cause increase in pressure (600:1).
货舱中液体裂隙下方的裂缝,导致少量液体在通过裂缝时蒸发,将导致压力增加 (600:1)。

Small quantity of liquid, which enters in barrier space from any source, vaporises and will have the same effect.

Liquid leakage

In the event of mechanical damage or overpressure of the primary barrier space, a failure of the primary membrane of the cargo tank could occur.

Primary marrier space will be filled with LNG in a time proportional to size and location of the membrane failure and the height of the LNG in the cargo tank.

Liquid lekage may develop slowly over period of hours or days or it may occur suddenly.

7.Liquid leakage indications

Gas detection alarm in IBS

Rise pressure in the effecte Primary Barrier Space

Likely lifting of the Primay Barrier Space relief valves
可能解除 Primay Barrier Space 安全阀

Frost of the exaust pipe from the IBS at the vapour dome
来自蒸汽穹顶 IBS 的 exaust 管道结霜

Confirmed by a drop in the recorded temp. of the bottom sensors in the secondary barrier

A general lowering of the inner hull steel temperature

If any two of above events occur, imediately segregate the gas in PBS from the others and vent it to the atmosphere and maintain the pressure below the IBS releif valves set point 1.0 kPag.
如果发生上述任何两种情况,请立即将 PBS 中的气体与其他气体隔离,并将其排放到大气中,并将压力保持在 IBS 释放阀设定点 1.0 kPag 以下。

8.Liquid leakage indications In secondary barrier space

Pressure in Secondary barrier space should be increased to prevent gas contanination from Primary barrier space.

In case the leak is so severe that pressure in SBS cannot maintain above that in PBS than contaminated tank Secondary barrier space should be isolated from other SBSs by closing nytrogen supply valve at the end of the tank.
如果泄漏非常严重,以至于 SBS 中的压力无法保持在高于 PBS 中的压力,则应通过关闭储罐末端的 nytrogen 供应阀将二级屏障空间与其他 SBS 隔离开来。

Hull heating flow rate to be increased as soon as temperature of SBS or inner hull are observed to be dropping.

NO 96 Membrane punching device
NO 96 膜冲孔装置

Messenger system – NO96 Barrier punch
信使系统 – NO96 阻隔冲床

Punch to be Installed on Float gauge pipe

Specially designed punch about 30 kg weight

Float attached to messenger line wmarked for refrence

2.Messenger system – NO96 Barrier Punching device
2.信使系统 – NO96 屏障冲孔装置

Punching device is inserted in to the tank Float Gauge Standpipe after removing float gauge and allowed to gravity fall through the LNG liquid in the pipe (liquid must be there)

The bottom of the pipe is fitted with a split perforated base allow the Messenger to penetrate through to the membrane.

Messenger puch hole of 50mm diameter through the primary membrane and box cover.

3.When to use punching device?

If the cargo tank were pumped out with a head of liquid remaining in the IBS, severe damage to the membrane would result.

For this reason it is necessary to intentionally puncture the primary membrane when the damaged tank is being pumped out.

Pumping of the tank must be limited so tank level decreases slower than 0.4m/h
必须限制储罐的泵送,使储罐液位下降速度低于 0.4m/h

Approval from CLASS/GTT/Owner has to be obtained before dropping device.
在丢弃设备之前,必须获得 CLASS/GTT/Owner 的批准。