NO.3 CARGO TANK
Part 5 Emergency Procedures
Illustration 5.1a Temperature Monitoring System
3A/B
1A/B
2A/B
7A/B
10A/B
11A/B 12A/B
13A/B 14A/B
9A/B 6A/B
4A/B(Fwd)
5A/B(Aft)
8A/B(Fwd)
Key
Secondary Barrier Space Temperature TE 1A/1B
TE 2A/2B TE 3A/3B TE 4A/4B TE 5A/5B TE 6A/6B TE 7A/7B TE 8A/8B TE 9A/9B TE10A/10B TE11A/11B TE12A/12B TE13A/13B TE14A/14B
Port Forward Lower Key
Inner Hull and Cofferdam PT 15A
PT 16A/B PT 17A PT 18A PT 19A PT 20A PT 21A PT 22A PT 23A PT 24A PT 25A PT 26A PT 27A PT 28A PT 29A
Forward Bulkhead Forward Bulkhead Forward Bulkhead Forward Bulkhead Forward Bulkhead
Aft Bulkhead Aft Bulkhead Aft Bulkhead Aft Bulkhead Aft Bulkhead
Centre Aft Bottom Port Forward Upper Starboard Aft Upper Centre Aft Top Centre Top
29A 28A
23A 22A
26A 21A
25A 24A
18A 20A
16A/B
15A
19A
27A Aft Aft
Aft Aft
Aft
Fwd Bulkhead
Port Starboard Port Starboard
CentreTop
Starboard Aft Upper Port Forward Upper Forward Bulkhead Aft Bulkhead Port Forward Upper Starboard Aft Upper Forward Bulkhead Starboard Aft Lower Starboard Aft Bottom Pump Tower Base Support Centre Bottom
Port Aft Bottom
Part 5 Emergency Procedures
Introduction
All tests carried out on the primary insulation membrane have shown that a fatigue fracture in the membrane will not extend.
Fatigue fractures in the primary insulation membrane are generally small and will pass either vapour only, or a sufficiently small amount of liquid, which will vaporize as it passes through the fracture.
It is possible, however, that a larger failure of the membrane could occur, allowing liquid to pass through and eventually gather at the bottom of the primary insulation space.
5.1 Vapour Leakage
A small leakage of vapour through the membrane may not be readily obvious.
However, indications are likely to be:
A sudden rise in the percentage of methane vapour in one primary insulation space. Some porosity in the primary barrier weld will allow the presence of methane vapour in the primary insulation space. The amount of this vapour should be kept to a minimum by the nitrogen purging.
If a fracture occurs in the primary insulation barrier below the level of the liquid in the tank, the vapour concentration will increase slowly and steadily.
If the fracture is above the liquid level, the concentration will exhibit a fluctuating increase.
The vapour concentration in each primary insulation space is recorded daily, to detect any small and steady change.
An Increase in Pressure in One Primary Insulation Space
A fracture above the liquid level in a 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.
A fracture below the liquid level in a cargo tank, resulting in a small amount of liquid vaporizing as it passes through the fracture, will cause a small increase in pressure (any small quantity of liquid which enters the primary insulation space, then vaporizes, will have the same effect). This increase is dependent upon the height of liquid above the fracture and the pressure in the tank.
Temperature Variation
No temperature change will be obvious, unless the fracture is in the immediate vicinity of the sensors below the cargo tank.
Leakage of methane vapour into the primary insulation space presents no immediate danger to the tank or vessel. As much information as possible concerning the fracture and leak should be obtained and recorded.
Determine whether the leak is increasing as follows:
a) After the leak is detected (and without changing the flow of nitrogen to the primary insulation space), record the gas concentration and primary space temperatures every hour for eight hours.
b) Then, if necessary, adjust the flow of nitrogen to maintain the gas concentration below 30% (vol %) and record the gas concentration and primary insulation space insulation temperatures every four hours.
c) In conjunction with the above, record all pressure changes occurring in the cargo tank and primary insulation space.
Illustration 5.2a Barrier Punch
MESSENGER HEAD OF MESSENGER
BOTTOM VIEW
TOP VIEW
B B
Detail 1
At -163
125 / T.B.
At -100
100 / T.B.
At 21
50 -0~+5/T.B.
Level Tank Bottom
Primary Box
See C.C.S. Drawing Insulation 0.5 Stainless
Steel
a=4 a=4
135 60
65
DETAIL 1
B-B
Sliding pad HD-PE
Screw C HC M 3-14
To be screwed after
sliding pad erection 2 at 45
14 9
R10
Endless Screw Disengaging Lever
Enlarged Sectional View Showing Barrier Punch Messenger
Inspection Plate
Reduced Thickness Material
Split Casing
5.2 Liquid Leakage
General
In the event of a serious ship collision or grounding or primary barrier overpressure, a failure of the primary barrier of a cargo tank could occur. The primary and secondary barriers are constructed of 0.7 mm thick Invar membrane and are liquid and vapor tight. If the primary barrier did fail, then its primary barrier space (PBS) will be filled with LNG in a time proportional to the size and location of the membrane failure and the height of the LNG in the cargo tank. Liquid leakage into the PBS may develop slowly over a period of days or hours, or may occur suddenly with one or more of the following indications:
- Gas detection alarm
- Rise in pressure in the effected PBS - Likely lifting of the PBS relief valves
- Confirmed by a drop in recorded temperatures in the bottom temperature sensors in the secondary barrier space (SBS)
If any two of the above events occur, immediately segregate the gas contaminated PBS from the others and vent the damaged PBS to the atmosphere to maintain the pressure at about 4 mbar (6 mbar below the PBS relief valves 10 mbar set point).
Increase the set pressure of the SBS service header from its normal 2~3 mbar set point to 6 mbar. This higher pressure in the SBS should prevent gas contamination from the PBS, should the secondary barrier not be completely tight.
If the leak is so severe that the pressure in the SBS cannot be maintained above that in the PBS, then isolate the SBS of the contaminated tank from the other SBSs by closing the nitrogen supply valve to the SBS at the after end of the tank.
Stable gas concentrations in the SBS up to the 30% LEL (1.5% by volume) alarm set point are allowed by DNV and the other Classification Societies for a GTT (Gaz Transpot Technigaz) type 96.2 cargo containment system.
As a precaution, immediately remove the flow cartridge and spring from the dynamic auto balancing valves on the effected tank to permit the glycol to flow at a higher rate to the coils in the cofferdam and around the liquid dome. See Illustration 2.3b “Cofferdam Heating System”. Increase the hull heating flow rate surrounding the effected tank as soon as the temperatures in the secondary barrier space or inner hull are observed to be dropping (colder).
Notes !
- Report any membrane leak immediately to the HSE and Operations Department of the Head Office.
- If the damaged cargo tank is to remain out of service with the other tanks in use for one or more voyages before repairs are to be made, the tank should be filled with inert gas and shut in at a pressure of about 100 mbar. Throttle OPEN the manual vent valve (at forward transverse PBS header) from the PBS of the damaged tank as necessary to maintain the PBS between 2 and 4 mbars.
- Depending on the size of the break in the membrane the damaged PBS (after gas freeing) may either be left in communication with the tank and isolated from the other PBSs or be connected into the rest of the barrier space system as for normal service.
Segregate and Vent the Damaged PBS
On the damaged tank, ensure that the PBS stays segregated from the SBS:
a) SHUT the nitrogen supply valve to the PBS at the after end of the tank.
b) OPEN first the small manual vent valve on the forward transverse PBS header of the tank to try and control the pressure in the PBS of the damaged tank at 4 mbar. If that valve is not able to vent sufficient gas, then slowly throttle OPEN the large manual vent valve as needed to maintain the pressure in the PBS at about 4 mbar. Throttle the small manual vent valve as needed for fine control.
c) Log the PBS and SBS gas detection readings in the Cargo Log. If no gas is detected in the SBS, leave its nitrogen supply valve to the SBS OPEN.
On each intact tank, keep the valves set up as normal:
a) Log the gas concentration on the PBS and SBS in each tank on an hourly basis initially until the extent of the leakage to the damaged tank can be determined.
b) If the gas concentration in the intact tank PBS and SBS are not changing, then leave the nitrogen supply valves to those spaces for the other tanks unchanged.
c) If the gas concentration in any of the intact tank PBS or SBS is increasing then immediately SHUT the nitrogen supply valve to the SBS of the damaged tank.
At the first indication of gas in the SBS, immediately isolate the damaged tank SBS from the other SBSs by shutting its nitrogen supply valve on the after end of the tank.
Check the pressure in the PBS and open the bypass vent valves as necessary to maintain the pressure about 4 mbar (6 mbar below the 10 mbar set point of the relief valves).
Check the hull heating for the ballast tanks surrounding the damaged tank and operate as necessary.
Barrier Punching Device
A punching device weighing 30 kg is stored in the engine room spares for punching a hole through the membrane in the bottom of the tank. The
“Messenger” punching device is inserted into the cargo tank Float Gauge Standpipe on trunk deck and allowed to gravity fall through the LNG liquid in the Standpipe. The shape of the Messenger is designed to prevent it from turning sideways or over during the fall to the bottom of the tank. The bottom of the Standpipe is fitted with a split perforated base to allow the Messenger to penetrate through to the membrane. The Invar membrane directly beneath the Standpipe is fitted with a thin diaphragm and the plywood insulation box cover is thinner than normal. This will allow the Messenger to punch a clean hole of about 50 mm diameter through the primary membrane and box cover. This operation will be necessary only in the event that damage to the membrane has permitted LNG to accumulate as a liquid in the PBS and rise up the lower chamfer and sidewalls of the PBS.
The height of the LNG liquid in the walls of the PBS could eventually reach a height of about 0.5 meter above that in the cargo tanks due to the tank pressure being about 100 mbar higher than that in the PBS. If the cargo tank were pumped out with a head of liquid remaining in the PBS, 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. The tank must be pumped slowly enough to enable the level of the liquid imprisoned in the PBS to fall at the same rate as the level in the cargo tank to prevent over pressurizing the membrane.
The use of the punching device is an extreme measure. It floods the PBS with LNG and likely will result in the relief valves protecting that space to open and possibly remain open for some time until the space is cooled down to cryogenic temperature.
! Caution
Before using the punching device, notify company headquarters of all the circumstances and obtain their approval before proceeding.
The punching device should be used when pumping out the damaged cargo tank, ONLY if at least one of the following gives definite indication of liquid in the PBS:
a) If liquid is indicated by all six of the bottom and lower chamfer thermocouples and by any of the four thermocouples located above the mid-height, the membrane should be punched at the start of the pumping operation. See Illustration 5.1a “Temperature Monitoring System”.
b) If liquid is indicated by all six of the secondary barrier thermocouples in the bottom and on the lower chamfer but not on the two thermocouples located at about mid-height, some liquid must be considered present in the sidewalls, and the membrane should be punched when the tank level decreases to one-half full.
c) If liquid is not indicated by all four of the bottom thermocouples and by none of the lower chamfer or mid-height thermocouples that is evidence that a head of liquid is not present in the sidewalls, and it is NOT necessary to use the punch device.
d) If the after the membrane has been punched, great care must be taken to ensure the liquid in the cargo tank is not pumped at a rate faster than what the level in the tank walls can gravity drain back into the tank. This is necessary to prevent localized over pressurize which will severely damage the membrane in those areas. The calculated rate by Gaz Transport is a maximum decrease in tank level of 0.4 meters per hour.
Warning
When a membrane has been punched, the tank pumping rate must be limited so that the cargo tank level decreases at or slower than 0.4 meters per hour.
After the Messenger punches the hole in the membrane and box cover, it must be removed to permit the liquid in the sidewalls to gravity drain out through the hole as the cargo tank level slowly decreases during the pumping out of the tank. The remaining liquid trapped in the PBS can be removed only by evaporation during the warming up of the cargo tank.
Operating Procedure for Inserting and Dropping the Messenger To activate the punching device, refer Illustration 5.2a “Barrier Punch”