GT96 Membrane System Installation Protocols for LNG Containment

The GT96 Membrane System is a cutting-edge technology designed for the safe containment and transportation of Liquefied Natural Gas (LNG). Installation of this system requires meticulous attention to detail and adherence to strict guidelines to ensure optimal performance and safety. The process begins with thorough preparation of the ship’s hull, followed by the precise installation of insulation layers and the primary and secondary membranes.

Key considerations during installation include maintaining a clean and controlled environment to prevent contamination, ensuring proper alignment and sealing of all components, and conducting rigorous quality control checks at each stage. Specialized training for installation teams is essential, as is the use of approved materials and tools. Regular inspections and testing throughout the installation process help guarantee the system’s integrity and compliance with international safety standards for LNG carriers.

GT96 Membrane System Installation

Introduction

Figure 1 shows a typical cross-section of a prismatic membrane type LNGC.

This cross-section is common to both variations of membrane systems. Along the inner hull, precise measurements will be taken and used to determine the construction parameters, such as shim thickness, mastic thickness, etc., of the membrane containment system.

Figure 2 shows a cross-section GT96 containment system installed.

The important difference between GT96 and other containment systems is that is has a complete, identical, fully redundant secondary barrier.

Figure 3 shows the containment system with relation to the ship in general. Of particular interest are the box girders, or side tunnels that run the length of the containment system and constitute a major strength member for the vessel.

Figure 4 shows a simplified illustration of GT96 containment system as installed on flat areas. It is important to remember that the insulation boxes are not bonded to the hull – as in MARK III System: Hull and Deck Components for Marine VesselsMARK III – but rather are fastened to the hull with thread stud, nuts and washers.

GT96 membrane is welded to tongue pieces which are fitted to grooves in the box covers, as opposed to being welded directly to the panels as is MARK III.

Figure 5 shows details of method of fastening insulation boxes to hull starting with coupler, and progressing up to final washers and nuts.

Figure 6 shows further detail of GT96 containment system with regard to fastening boxes to hull and installation of soft insulation between the boxes.

Figure 7 depicts corner construction of GT-96 containment system. Of particular note is the Invar Tube (circled), which enables the transition through the corners.

Figure 8 depicts corner construction in way of dihedrons.

Figure 9 provides clearer detail of corner construction of GT-96 containment system.

Hold Planarity and Coupling Installation

Inner hull planarity is a key factor in the construction of a membrane containment system. Planarity or flatness is checked at the plate stage, sub-assembly stage and finally the block stage.

At the block stage, a straightedge, of approximately 3 meters length, is held against the surfaces to be measured and checked with taper gauges to assure that the inner hull deviation is less than 6 mm. (See Figure 10).

Once flatness is assured, couplings are attached using a specially developed, magnetic-base welding machine, which holds the coupling against the hull and moves its electrode in a circular motion around the coupling. Thus guaranteeing a high quality weld. (See Figure 11).

Of interest to class is the overall flatness, also checked by Owner, and quality of coupling welds.

Often, bad welds can be traced back to an individual machine, which is then taken out of service for repair and recalibration.

Couplers are installed at the block stage of construction. Laying out of anchor points is done with snapped chalk lines at carefully measured intervals. When blocks are joined the basic laying out of the tank has been completed. (See Figure 12).

Construction of Insulation Boxes

For GT96, special Maple/Birch plywood is imported from Finland. Attempts to obtain less expensive sources resulted in inferior quality. Plywood is shipped in sheets and cut to maximize usable. (See Figures 13 and 14).

For vessels built at DSME, the boxes are manufactured in two different facilities:

• An-Jeong – Standard size boxes – reinforced, non-reinforced – for primary and secondary insulation spaces, as well as special shaped boxes (triangular boxes) for corners of tanks against slopes.
• DSME shipyard – Specialty boxes such as those used in corners and invar tubes. (See Figure 15).

An-Jeong, the assembly lines are set up to facilitate rapid, error-free assembly of the boxes. Production is set up in six parallel process lines. This allows for simultaneous production of primary and secondary space boxes for three vessels. (See Figure 16).

After the box bottom, sides, and partitions have been assembled, perlite insulation is added. During the filling process, the box is vibrated in order to settle the perlite. The ultimate goal is to provide the optimum amount of perlite that:

1. will not settle further and leave a void space in the box;
2. will provide maximum insulation value.

In Figure 17 an empty box has been cleaned and sent on its way to the perlite chamber where it will be filled and vibrated. (See Figure 18).

After perlite filling, filled weight is taken, recorded, covers secured in place, bow is labeled, palletized, protected with wrapping. (See Figure 19).

Installation of Boxes to the Inner Hull

In the cargo hold, mastic is applied as “ropes” to the boxes according to computer programs for that exact box and intended location. (See Figure 20).

This mastic compensates for slight surface irregularities in the surface of the inner hull and allows the box to uniformly bear the weight of the membrane and cargo.

After the mastic is applied, it is covered with strips of craft paper. (See Figure 21.)

This allows the mastic to conform to the inner hull without adhering to it. In this manner, the boxes and inner hull can “flex” without causing damage.

The boxes are attached to the inner hull by means of threaded studs, which screws into the coupler sockets. Square washers, which bear against the wooden “cleats” or “ears” on each box are held in place with hex nuts, which are torqued in place.

Figure 22 shows temporary block used to hold the box in place until the mastic can set.

Once set, adjacent boxes can be installed and the hex nut permanently torqued.

Figures 23 shows the threaded stud, square washer, and hex nutwith locking plate permanently installed.

The closed cell foam, thermal sleeve which fits over the threaded stud can also be seen in between the two wooden tabs.

Figure 24 shows clear view of closed cell foam thermal sleeve, which fits over the threaded stock.

Once the secondary boxes are installed, their surfaces are checked with a straight edge to assure that adjacent box surfaces are level with an another, and that no damage from installation is observed which might damage invar membrane. (See Figure 25).

Installation of Invar Tubes in Corners

The corners of GT96 tanks require specially configured pieces called Invar Tubes. Essentially, these are perlite filled insulation boxes clad with invar sheeting. The invar is drilled to form flanges to accommodate attachment to unclad boxes. (See Figure 26).

These tubes form the transition between the bonding strips, soft insulation and unclad boxes on the inner hull and Comprehensive Framework: Primary & Secondary Barrier Testing Protocols on LNG Tankersthe primary and secondary barrier membranes. (See Figure 27).

This combination of the invar tubes and unclad boxes forms a strong unit structure, which facilitates liquid-tight integrity of the attached membranes while still possessing the ability to flex with ship’s movement.

Installation of Secondary Barrier and Insulation Space

In addition to the coupler bases, special stainless steel anchor bars (See Figure 28) are affixed to the inner hull.

Soft insulation and specially shaped perlite filled boxes are affixed to the inner hull prior to the installation of the invar tubes.

Special clamps or jigs are used to hold the various components (See Figure 29) in place until they can be secured and the invar tubes installed.

The invar tubes need their own special clamps to hold them in place until they can be screwed to the unclad insulation boxes. (See Figure 30).

Figure 31 shows workers installing tongue pieces into the insulation space boxes.

The material for the tongue pieces is brought into the tanks in rolls and formed on location where it is to be installed.

Figure 32 shows end view of T-slot routed into box cover to accommodate the invar tongue piece.

The tongue piece is bent at a 90° angle to prevent it from pulling loose. (See Figure 33).

Figure 34 shows tongue piece in place with membrane fitted; ready to be set and welded.

Like the tongue pieces, membrane is brought into the tanks in rolls and formed in-situ. (See Figure 35).

Owing to the size of the tanks, there really is no other way to handle the membrane without causing extensive damage.

Once set in place on the insulation boxes, the membrane is held in place with temporary wires fitted in holes in the tongue pieces. (See Figure 36).

Once the membrane is set in place, a thorough inspection for defects, and other irregularities is conducted prior to welding.

It should be noted that invar is not stainless steel and that special care is taken in the handling, working and inspection of the membrane. In particular, contact by bare hand is discouraged. Invar comes with a protective coating that is cleaned off when all work is finished.

Just prior to welding of the membrane to the tongue pieces, special toggle clamps are used to tension the tongue piece and to push the membrane tightly against the insulation boxes. (See Figure 37).

The majority of the straight-run welding is done by a special machine which clamps the membrane and tongue pieces tightly as it welds them and rides along them at the same time. (See Figure 38).

Corners and other areas that are not accessible by machine, welding is done manually. Unlike the welding done by the machines, which does not bear directly against the boxes, the manual welding is often directly against the boxes and is separated only by a thin strip of glass fiber cloth. Because of this, the welding is done in a carefully stepped, or controlled sequence. The main purpose is to avoid any significant heat build-up, which could cause a fire in the insulation boxes.

The welding starts as a series of tacks spaced approximately 25 mm apart. Then subsequent tacks are spaced more closely. In the end, the bead is done continuously for short lengths as seen in Figure 39.

In the corners, special clamps are used to maintain alignment – as can be seen in Figure 40.

When Membrane Sheet Welding Procedure for LNG Containment Systemsthe welding has been completed, the seams are subjected to NDT to determine any welding defects. (See Figure 41).

The goal of the testing is to determine and correct defects before the entire barrier/membrane is given a vacuum test. As can be seen from the photo at right, the manual welding in the corners is quite extensive and time consuming. The studs protruding from the secondary membrane are for attaching the primary space insulation boxes.

Testing and Evacuation of Secondary Barrier

When the membrane is fully installed, it has a loose, billowy appearance. (See Figure 42).

This appearance will change when the insulation space is evacuated for testing.

After evacuation of the insulation space, the membrane is actually pulled flat against the boxes with such force that the spaces between the boxes can be clearly seen. (See Figure 43).

The vacuum is pulled down to approximately 0,3 bar absolute and the vacuum pumps are then shut off.

Assuming a tight barrier, the vacuum reading should not change to any appreciable degree.

During the test, the barrier is given another thorough inspection for flatness and for defects. (See Figure 44).

Installation of Insulation Boxes for Primary or Interbarrier Space

The studs seen protruding from the secondary barrier membrane are used for securing the primary insulation space or interbarrier boxes. The studs, shown in Figure 45 during a weld test, are actually screwed into a rectangular flat washer affixed to the secondary box studs.

As can be seen in the photo at right, the bottom threads of the stud are screwed into the rectangular washer by using a wrench across the flats on the upper part of the shank. The threads on either end are different to facilitate correct installation. The collar shown at right is a sample of membrane used for welding tests. The cutouts are a means to visually inspect the weld.

Shown in Figure 46, is the rectangular washer in to which the stud gets threaded.

The washer is secured from rotating by two allen-head cap screws, which are tack welded once tightened.

Holes are punched in the secondary membrane to align with the threaded portion of the washer.

Studs are fabricated with a consumable skirt that, when welded to the secondary membrane using an autogenous weld process, seals the penetration through the secondary membrane.

Once the secondary barrier has been completed, primary space insulation boxes and other components can be installed.

Figure 47 shows the plywood spacers that fit over and are fastened to the stud in the insulation space (via center hole).

Once fastened, the flat washer that accommodates the stud, which holds the primary boxes in place, is screwed to it (via the end holes). Using wooden components provides good insulation and excellent resilience.

Primary space/interbarrier space boxes are differently sized from their secondary space counterparts and actually overlap them. This overlap actually strengthens the containment system in the same manner as staggering bricks makes for a stronger wall.

Read also: Cargo containment system of gas vessel

The primary boxes are attached in much the same manner as the secondary boxes. That is, with nuts and washers which engage tabs on the individual boxes. (See Figure 48).

Note the slots cut into the boxes to accommodate the tongue pieces and flanges of the secondary barrier. The temporary shim is used to prevent the weight of the box from bearing against the tongue piece.

Figure 49 shows details of corner treatment in way of the invar tubes.

As with the regular boxes, there are slots to accommodate the tongue pieces and flanges of the secondary barrier.

Figure 50, shows primary boxes with a batt of loose insulation installed.

The insulation is composed of 5 different layers sandwiched together for optimum thermal efficiency.

Figure 51 shows details in upper corner and dihedron.

The empty spaces will be filled with specially cut insulating batts and closed cell foam where appropriate.

Installation of Primary Barrier

The primary barrier is installed in much the same manner as the secondary barrier, but without the studs protruding through. That is, tongue pieces are inserted into the box covers, and membrane is formed to fit to the tongue pieces.

Figure 52 shows the beginning of installation of primary membrane.

Note the glass fiber cloth behind the butt ends, this provides insulation during the welding process. The vertical lines on the membrane are temporary securing wires inserted through the holes in the tongue pieces. They hold membrane in position until lock weld process.

Figure 55 shows primary barrier and tongue piece.

Again, note the glass fiber cloth between membrane and cover of box.

Figure 55 shows primary barrier positioned in place and ready to be clamped and welded.

Figure 55 shows the clamps used to tension the tongue piece and position the membrane against the boxes.

The actual welding of the primary barrier is accomplished in the same manner as with the secondary barrier; using the same machines as depicted earlier. Figure 56 shows manual welding taking place on the primary barrier.

Author

Olga Nesvetailova

Freelancer

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