Advantages of Membrane Technology

LNG Carriers Advantages of Membrane Technology

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Pressure Vessel Engineering

LNG vessel construction –

Advantages of membrane technology

The arrangement for containment of

cargo including, where fitted, a primary and

secondary barrier, associated insulation and

any intervening spaces, and adjacent

structure, if necessary for the support of

these elements. If the secondary barrier is

part of the hull structure it may be a

boundary of the hold space.

To contain LNG cargo at cryogenic

temperatures (-160 0C).

To insulate the cargo from the hull

structure.

The materials used for the hull structure

are designed to withstand varying degrees

of temperature. At temperatures below their

specified limits, these steels will crystallise

and become brittle. The materials used for

the containment system are required to

reduce the heat transfer from the hull

structure to minimise boil-off gas from the

cargo, as well as to protect the hull structure

from the effects of cryogenic temperatures.

MEMBRANE CARGO CONTAINMENT

The cargo containment system consists

of insulated cargo tanks encased within the

inner hull and situated in-line from forward

to aft. The spaces between the inner hull

and outer hull are used for ballast and will


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also protect the cargo tanks in the event of

an emergency situation, such as collision or

grounding.

The cargo tanks are separated from

other compartments, and from each other,

by transverse cofferdams which are dry

compartments.

The following description is of a Gaz

Transport GT96 double membrane system

design. Although the principal design

features will be similar in other systems, e.g.

Technigaz, there will be differences in

membrane construction and insulation

structure.

In the Gaz Transport GT96 design, the

inner hull, that is, the outer shell of each of

the cargo tanks, is lined internally with the

patent tank containment and insulation

system. This consists of the following:

1. A thin flexible membrane, called the

primary membrane, which is in contact

with the cargo. This is fabricated from

Invar and has a typical thickness of

0.7mm.

2. A layer of plywood boxes filled with

Perlite, called the primary insulation,

typically of approximately 230 mm

thickness.

3. A second flexible membrane similar to

the first one, called the secondary

membrane. Also of Invar and having a

typical thickness of 0.7mm.

4. A second layer of boxes, also filled with

Perlite, and in contact with the inner

hull, called the secondary insulation.

This layer is typically of approximately

300 mm thickness.

The tank lining thus consists of two

identical layers of membrane and insulation,

so that in the event of a leak in the primary

barrier, the cargo will be contained by the

secondary barrier. The secondary barrier is

only designed to contain any envisaged

leakage of cargo for a period of 15 days.

(IGC Chapter 1V 4.7.4). This system

ensures that all the hydrostatic loads of the

cargo are transmitted through the

membranes and insulation to the inner hull

plating of the ship.

Fig: Membrane design


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The function of the membranes is to

prevent leakage, while the insulation

supports and transmits the loads and, in

addition, minimises heat exchange between

the cargo and the inner hull. The secondary

membrane, sandwiched between the two

layers of insulation, not only provides a

safety barrier between the two layers of

insulation, but also reduces convection

currents within the insulation.

The primary and secondary insulation

spaces are maintained under a pressure-

controlled nitrogen atmosphere. The

pressure of nitrogen within the primary

space must never exceed the cargo tank

pressure, in order to prevent the membrane

from collapsing inwards. The insulation

design should ensure that:

1. The heat flow into the tank is limited to

such an extent that the evaporation, or

boil-off rate, is about 0.15% per day

based on sea surface temperature of

32 degrees and air temperature 45

degrees Celsius.

2. The inner hull steel does not attain a

temperature below its minimum design

value, even in the case of failure of the

primary barrier.

3. Any deflections resulting from applied

strains and stresses are acceptable by

the primary barrier.

In addition to the above, the insulation

acts as a barrier to prevent any contact

between ballast water and the primary

barrier, in the event of leakage through the

inner hull.

DETERIORATION OR FAILURE OF

INSULATION SYSTEM

The insulation system is designed to

maintain the boil-off losses from the cargo

at an acceptable level, and to protect the

inner hull steel from the effect of excessively

low temperature. If the insulation efficiency

should deteriorate for any reason, the effect

may be a lowering of the inner hull steel

temperature, i.e. a cold spot and an

increase in boil-off from the affected tank. If

necessary, increased boil-off gas may be

vented to the atmosphere via the vent riser

and gas heater. The inner hull steel

temperature must, however, be maintained

within acceptable limits to prevent possible

brittle fracture.

Thermocouples are normally distributed

over the surface of the inner hull, but unless

a cold spot occurs immediately adjacent to

a sensor, these can only serve as a general

indication of steel temperature. To date, the

only reliable way of detecting cold spots is

by frequent visual inspections of the ballast

spaces on the loaded voyage.


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In addition to failure of the membrane,

local cold spots can occur due to failure of

the insulation. While the inner hull steel

quality has been chosen to withstand the

minimum temperature likely to occur in

service, prolonged operation at steel

temperatures below 0°C will cause ice build-

up on the plating, which in turn will cause a

further lowering of steel temperature due to

the insulating effect of the ice. To avoid this,

heating coils may be fitted in the cofferdam

spaces, of sufficient capacity to maintain the

inner hull steel temperature at 0°C under

the worst conditions.

If a cold spot is detected, either by the

inner hull temperature measurement

system, or by visual inspection, the extent

and location of the ice formation should be

recorded. Small local cold spots are not

critical and, provided a close watch and

record are kept as a check against further

deterioration and spreading of the ice

formation, no further action is required. If

the cold spot is extensive, or tending to

spread rapidly, salt water spraying should

be carried out.

In the unlikely event that this remedy is

insufficient and it is considered unsafe to

delay discharge of cargo until arrival at the

discharge port, the final recourse will be to

jettison the cargo via a spool piece fitted at

the cargo liquid manifold, using a single

main cargo pump. This action should only

be taken after full consultation with the

Managing Office and relevant authorities.

INNER HULL INSPECTIONS (MEMBRANE

HULL CONTAINMENT)

It is a requirement that all spaces

around the cargo tanks are inspected at

least once in every six month period. To

meet this requirement the inner hull around

a nominated cargo tank is inspected from

the ballast tank, cofferdam, and whaleback

areas (including the whaleback areas

external to the ballast tank), each alternate

passage. This frequency ensures every

space is inspected within the required

period.

These inspections should commence

approximately 48 hours after a cargo is

loaded. The following points are to be

covered and recorded.

1. The position and temperature of cold

spots or absence of cold spots.

2. Condition of anodes.

3. Condition of paintwork - a reference

sheet is provided for this.

4. Extent of corrosion on both the inner

and outer hulls, particularly under the

suction strums, in the way of striking


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plates and behind heating coils in the

ballast and cofferdams.

5. Position and amount of sediment.

6. Any damage, fractures etc. Particular

attention to be paid to the external

portion of the inner hull for evidence of

fractures, and to the turn of the bilge

areas of the inner hull within the

midships section of the vessel.

7. Hydraulic or heating coil leaks and the

condition of scupper pipes. The duct

keel is to be inspected every six months

and must be well ventilated by fan

before entry. All spaces should be

inspected on the first cargo after a dry-

dock period. The void space around

each of the liquid domes should be

included in the inspection of the spaces

around the nominated cargo tank.

NOTE: It is a Classification requirement for

the granting of a valid Certificate of Fitness

for ships carrying liquefied gases in bulk

that routine cold spot inspections are carried

and recorded.

Fig: MEMBRANE DESIGN – GAS TRANSPORT TECHNIGAZ (GTT) –GT96

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Source : Scrapped from liquefiedgascarriers.com

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Advantages of Membrane Technology