Reducciones en la sucesión «mortis causa»

Key

Nitrogen From Secondary Insulation Spaces

Nitrogen From Damaged Primary Insulation Spaces

Interbarrier Space

When a membrane leak is stopped or the tank itself has been gas freed, it is necessary to gas free the insulation space affected.

The gas freeing is carried out in an identical manner to the initial inerting described in 4.3.2, by evacuating and refilling with nitrogen. Care must be exercised with the vapour evacuated from the insulation space to avoid forming a combustible mixture with air. The evacuation procedure may have to be repeated two or more times to reduce the gas concentration to an acceptable level

(0.2%).

The operating procedure is as follows: (see figure 5.8.2a) • If the insulation spaces of the non affected tanks are under their normal operating conditions, increase their pressure up to 8mbar g.

• Isolate the primary and secondary insulation spaces of the non affected tanks from the primary and secondary pressurisation headers by closing the valves 541, 531, 928 or 511 and 542, 532, 522 or 512. • Connect the insulation spaces of the affected tank with the pressurisation headers by opening the corresponding valves.

• Evacuate both the primary and secondary spaces of the affected tank as described in 4.3.1. It is assumed that the membrane damage, if large, is temporarily made tight.

• Refill with nitrogen, either produced from the ship’s generators or supplied from shore, both spaces as described in 4.3.2.

• If the hydrocarbon content is not reduced sufficiently, repeat the cycle; of evacuation and nitrogen filling. • The hydrocarbon content of the primary space is

checked after each nitrogen sweep. It is measured with a portable gas detector, or with the inboard gas detection equipment or from samples taken at the sample connections provided on deck and analysed with shore equipment.

• During the gas freeing of a contaminated insulation space, and particularly when welding work for membrane repairs are necessary, the following points must to be carefully met:

• Once the insulation space is gas freed and before any hot work, the gas mixture flowing through the leak is checked by means of a portable gas detector. When three samples, carried out every 15

0.2%, cutting and welding can proceed.

• The nitrogen in the insulation spaces must be replaced by air if large areas of membrane are to be worked upon. The hydrocarbon content of the barrier spaces must be checked daily, with the vacuum pump discharge analysed.

• As the safety depends on the analysis of the gaseous mixture contained in the insulation space, it is essential that the portable gas detectors to be selected according to the nature of the mixture and the accuracy required for the final hydrocarbon content. • For measuring the residual content of hydrocarbon

in air, a gas detector with a range 0-100% of the

lower flammability limit (LFL), or preferably with a range 0-10% LFL, is acceptable.

• Due to the low residual hydrocarbon content, the gas analyser should be one which is normally used to measure non combustible mixtures.

• If the above portable detector is not accurate enough to measure low hydrocarbon contents, a sample is taken from the vicinity of the leak and analysed with the ship’s gas detection equipment previously re-calibrated.

5.8.3 Damage to Primary Insulation Space - Emergency Discharge of LNG

In the event of an accidental break in the primary membrane of a tank, the primary insulation space will be filled with LNG in a time proportional to the size of the break.

A break in the membrane will be signalled by:

• the gas detection alarm, not immediately after the break, but few minutes later when the insulation space is sampled by the gas detection system; • a drop in temperatures of the secondary barrier; • a rise in pressure in the tank and in the primary

insulation space with the possibility of lifting the insulation space safety valves.

In this event, it is necessary to immediately segregate the damaged insulation space from the others and to vent it to atmosphere.

Leakage of LNG into the primary insulation space also lowers the temperature of the hull surrounding the tank, and requires additional heating in the cofferdams.

Insulation Space

• The damaged primary insulation space is isolated from the others and vented to atmosphere via valves 543, 533, 523 or 513.

• Shut the gas detection sampling valve at the damaged space.

• Check the pressure in the damaged space and adjust the bypass valve to reduce the pressure below safety valves lifting point, 10mbar g.

• Check the heating in both cofferdams aft and forward the tank.

• At the first opportunity, the damaged tank should be emptied and gas freed and the primary space gas freed.

• If the tank is to remain out of service for one or more voyages before repairs, the tank should be filled with inert gas and shut in at a slight overpressure (about 100mbar g).

• Depending on the size of the break in the membrane, the primary insulation space (after gas freeing) may either be left in communication with the tank and isolated from the other spaces, or be connected with the pressurisation system as for normal service.

5.8 Insulation and Barrier Systems - Page 2 Issue: 1

5.8.4 Primary Insulation Space Drainage - Barrier Punch System Issue: 1 Drain Valve Flexible Hose Reducing Valve Nitrogen Bottle Nitrogen Connection

Tank Bottom/ Primary Barrier Base of Cargo Tank Trellis Structure

Support for Equipment on Trellis Structure Bellows Support Arm Expansion Tube Stripping Pump Cable Conduit

Primary Insulation Space Secondary Insulation Space

Duct Keel Nitrogen Piping Diaphragm LNG Liquid Bellows Perforations

SITUATION NORMAL BARRIER PUNCH SYSTEM

IN OPERATION