Reducciones en la transmisión «inter vivos». Normas del Estado

Punch Systems

Punching Device for the Membrane

A punch diaphragm, fitted below the pump tower in each cargo tank, permits the punching of an opening in the primary membrane. This operation will be necessary in the event that damage to the membrane has permitted LNG to accumulate as a liquid in the primary insulation space.

If the cargo tanks were pumped out with a head of liquid remaining in the primary space, severe damage to the membrane would result. For this reason, it is necessary to intentionally puncture the primary membrane when the damaged tank is pumped out, and to pump the tank slowly enough to enable the level of the liquid imprisoned in the insulation space to fall at the same rate as the tank without overpressurising the membrane.

The device punches a 50mm dia opening at the bottom of the tank. Liquid from the side walls will drain out through the opening. Liquid in the bottom portion of the insulation space must be removed by evaporation during warmup.

Use of the punching device is an extreme measure. It

floods the insulation space with LNG, and requires that the tank be gas freed and entered in order to replace the punched diaphragm.

The operation procedure for using the punching device is as follows: (see figure 5.8.4)

• The USE of the punching device depends on the following indications of liquid in the primary space. • If liquid is indicated by all the temperature sensors

up to the level of the sensor 5 A/B, the membrane should be punched at the start of the pumping operation.

• If liquid is indicated by all the temperature sensors in the bottom and by either the sensor 9 A/B or 8 A/B or 7 A/B or 6 A/B, the membrane should be punched when the LNG level in the tank is 4 metres above the level of either the sensor 9 A/B or 8 A/B or 7 A/B or 6 A/B.

• If liquid is indicated by all the temperature sensors in the bottom and not by the sensor 6 A/B, the membrane should be punched when the LNG level in the tank is at the level of the sensor 6 A/B. • If liquid is indicated only by some of the

temperature sensors in the bottom, it is evidence that a head of liquid is not present in the side walls, and the membrane need not be punched.

pumping rate corresponding to a liquid level fall of 0.4m/h.

• The punching device is operated as follows: (see figure 5.8.4)

• Connect the portable nitrogen flask with the attached pressure reducer to the punch connection at the liquid dome of the damaged tank;

• Close the equalising valves between the punch piping and the well of the emergency level.

• Open the valves at the hose connection and on the nitrogen flask, and apply full pressure from the reducing valve (12 bars).

• After about one minute close the valves, disconnect the nitrogen flask, and reopen the equalising valves to the well of the emergency level.

• After the tank has been gas freed and repaired, the punching diaphragm will be replaced by welding a new one. After re installation of the punch device, PURGE all lines and the internal bellow with nitrogen at low pressure (150mbar) to avoid actuation of the system.

• Before cooling down of the tank, open the equalising valves to the well of the emergency level. These valves should remain blocked open at all times when the tank is in service to avoid the inadvertent actuation of the punching device.

Notes: - If a primary membrane has been punched or

damaged to such an extent that the primary insulation space is in free communication with the tank, it is not possible to pull a vacuum on the space without pulling a vacuum on the tank. At 10mbar below atmospheric pressure, the tank safety valves will open and admit air to the tank. - With damage of this type, the cargo tank should be gas freed and inerted, but not filled with air until the insulation space is gas freed.

- The insulation space should be gas freed by sweeping the inert gas from the tank through the damaged barrier, or by sweeping with nitrogen from the pressurisation system, or by combination of the two.

- The vacuum pumps may be used in this situation to assist the sweeping with nitrogen or inert gas, to reduce the pressure created in the insulation space by evaporation of the imprisoned LNG or to maintain the space pressure lower than the tank pressure when the tank is opened.

5.8 Insulation and Barrier Systems - Page 3 Issue: 1

5.9.1a Ventilating the Ballast Tanks Issue: 1 Removable Watertight Manhole Cover FWD (View Outboard)

Illustration 5.9.1a Ventilating the Ballast Tanks

Removable Watertight Manhole Cover From Cofferdam / Duct

Keel Supply Fan

Removable Watertight Manhole Cover Removable Watertight Manhole Cover

5.9.1 Ventilating a Double Hull Ballast Tank

Ventilation of ballast tanks is necessary to ensure that the atmosphere inside the tank is safe before entry can take place. The oxygen content in the tank may be low, for example, due to the effects of corrosion.

The SNAM regulations for tank entry must be complied with. A permit to work must be completed prior to entry and adhered to.

The double hull ballast tanks can be ventilated via the duct keel and cofferdam forced ventilation system. The ballast tank to be inspected must first be deballasted via the main and stripping line. Warning notices must then be posted in the CCR and ECR that the ballast pumps are isolated and should not be started. Once empty, there are two man hole covers on deck that are removed (two per tank), warning notices and guard rails are arranged around the openings. Entry can now take place, tank entry procedures being followed for this action. In the duct keel there are two man hole covers for each double hull ballast tank, one in a forward and one in an aft position. Removal of these covers will now allow the forced ventilation for the duct keel and cofferdam to flow through the ballast tank and ventilate (see illustration 5.9.1a ).

See Section 6 Inner Hull Inspection Routes.

! WARNING

The spaces to be inspected must be thoroughly ventilated before entry as there is the possibility of very low oxygen content due to corrosion of the steelwork.

5.9 Ventilation of Ballast and Trunk Void - Page 1 Issue: 1