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Membrane Sheet Welding Procedure for LNG Containment Systems

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Membrane tank installation begins with meticulous surface preparation. The tank substrate must be clean, dry and free of any contaminants, protrusions or imperfections that could compromise the membrane liner. Careful layout and positioning of the membrane sheets are crucial, ensuring proper overlap at seams and precise detailing around fittings and penetrations. Secure temporary fastening prevents shifting during welding and maintains proper alignment for a leak-proof membrane tank installation.

Welding during membrane tank installation requires specialized equipment and adherence to specific temperature and pressure settings. Trained operators should employ consistent welding speed and pressure to create strong, homogenous seams, vital for long-term tank performance. Thorough post-weld inspection, including visual examination and potentially non-destructive testing, ensures seam integrity and the overall success of the membrane tank installation.

Membrane Erection

General

Figures 1 and 2 show the main elements that constitute the corrugated membrane.

Membrane Elements
Fig. 1 Membrane Components

Tank Installation: Bonding, Insulation and Final Quality CheckThe main principle for the membrane installation is the alignment and continuity of corrugations between the various bulkheads of the tank.

Membrane Panels
Fig. 2 Membrane Sheets

Corrugation continuity is fulfilled by a marking on the anchoring strip before the membrane erection.

Border Membrane Sheet Definition

As soon as the reference wedges are installed, the dimensions of the border membrane sheets can be determined by means of template to measure onboard the actual dimension between the level of the steel corner and the bulkhead axis. This dimension is used to calculate the position of the anchoring strip axis of the last membrane sheet according to the position of the steel corner. (See Figure 3.)

Border Dimensions
Fig. 3 Border Membrane Sheet Layout

Border membrane sheet shape can also be determined easily after panel installation by measuring the actual dimension on board between corner panels and border panel.

However this measuring will be carried out after panel erection to control the pre-defined dimension of the border membrane sheets.

Membrane Sheet Positioning on Insulating Panels

In order to control the positioning of the membrane sheet at each stage of the installation process, it is necessary to mark on the anchoring strips the intersection corner of membrane sheets.

Read also: Best Practices for Gas Tank Installation and Cargo Tank Insulation

In order to ensure the continuity of the corrugations, it is necessary, first of all, to mark out the reference axis on each bulkhead as it was done on the inner hull for panel installation.

These reference axes will be marked-out on the perpendicular anchoring strip lines, which are close to the geometrical axis of each bulkhead.

This operation starts on each corner by using a hinged template and marking of the junction line between two adjacent bulkheads. The maximum off-set between anchoring strip axis and junction line is equal to 6 mm. (See Figures 4, 5 and 6.)

Sheet Corner Marking
Fig. 4 Marking Membrane Sheet Intersections
Dihedral Axis Marking
Fig. 5 90-Degree Strip Axis Marking
135° Strip Axis Marking
Fig. 6 Layout Marking for Anchoring Strip Reference Axis (135° Dihedral)

The axes are marked at the crossing location and the perpendicularity is checked. The maximum allowable deviation of the square angle is 1 mm per 1 meter. (See Figure 7.)

Check Strip Axis for Membrane
Fig. 7 Verifying Anchoring Strip Axis for Membrane Installation

After checking the main axis positioning, each intersection corner of membrane sheets will be markedout on anchoring strips in reference to these axes. (See Figure 8.)

Corner Marking
Fig. 8 Membrane Sheet Corner Marking

Marking of the Membrane Sheet Limit Line on Corner Panel

After erection of the corner panels, the limit line of the membrane sheets will be marked on the steel corner with a template.

No membrane sheet limit will be marked on the side of corner panel of the transverse bulkhead along side the chamfers. In these areas, the membrane dimension is fixed and the overlap variable.

At the junction between transverse bulkhead and chamfer, it should be verified that the pad crossing axis of the border panels are within design tolerances and dimension bracket. (See Figures 9 and 10.)

Membrane Corner Limits
Fig. 9 Membrane Limit on Steel Corners (DR1 and DR2)
Bulkhead Membrane Arrangement
Fig. 10 Transverse Bulkhead Membrane Sheet Layout

Installation Sequence Concept

Membrane sheet installation is typically carried out from top to bottom and is based on the following:

  1. WELDING:
  2. The welding process (automated or hand) gives the better results in the horizontal up positions. For angled and vertical bulkheads, individual membrane sheets are placed progressively from top to bottom.

  3. SCHEDULE:
  4. The most advantageous schedule is to commence the membrane installation on “A” wall (tank top) so as to include liquid and gas dome.

  5. CLEANLINESS:
  6. Maximum cleanliness can be better maintained by starting at top of the tank.

Regardless of scheduling, it is better to start the installation of membrane sheets on the bottom before all the bulkheads are completed because it takes time to lift up the scaffolding legs.

Installation Sequence Organization

On the distribution drawings the erection sequence will be clearly noted. According to these drawings it will be noted on each location the identification number of each sheet to make the control easy. (See Figure 11.)

Bulkhead Membrane
Fig. 11 Membrane Drawing for a Single Bulkhead

Membrane Sheet Tack Welding – Tolerances for the Fabrication of the Membrane Sheets and Overlap Principle

The main tolerances are:

  • +/- 0,5 mm for the corrugation distance;
  • +/- 2 mm for the corrugation distance to the free end;
  • +/- 1 mm for the length of the sheet joggle;
  • +/- 2 mm for the length of the corner piece leg;
  • +/- 1 mm for the corner piece joggle length.

(See Figure 12.)

Sheet and Angle Tolerances
Fig. 12 Main Tolerances for Sheets and Angles

The top piece of an overlap joint is always joggled. (See Figure 13.)

Component Overlap
Fig. 13 Interlocking Principle of Components

Joggle length and overlap length depend of the type of components, but in no case less than 10 mm.

Although the installation procedure takes into account the aforementioned principles, a certain elasticity in the corrugation design of membrane sheets compensates for slight deviation from design installation tolerances.

Tack Welding

1) PREPARATION

The sheets and steel corners must be very clean: no dust, no marks of glue of oil are allowed.

The protective film sheet is to be completely removed on the back side and partially removed at the periphery of top in way of the overlap joints.

2) POSITIONING OF THE FIRST SHEET, NO JOGGLED BORDER

The first sheet is positioned in one corner of the bulkhead/face with two sides overlapping the 8 mm steel corner pieces and two sides on the anchoring strips. (See Figure 14.)

Initial Membrane Install
Fig. 14 First Membrane Sheet Installation, Non-Joggled Edge

The sheet corner, which is located on the anchoring pieces (on the crossing of two anchoring strips) is call the free corner (corner no 1) and is first positioned with accuracy with a centering template according to the junction axis on the anchoring strips.

Then the two corners adjacent to the free corner (corner No 2 and No 3) are aligned on the junction axes by means of an alignment template for large and small corrugation and the relative position according to the limit lines of sheet is checked. The border of the sheet must be in the tolerances + 8 mm, – 10 mm (+ 8 mm, – 15 mm on DR3).

If the sheet is out of tolerance, it is rejected or a special angle piece will be provided.

Once this checking is satisfactory, the tack welding of the sheet may start. At first the corner 1 is tacked, then the corrugation pitch is checked with a comb template, and corner 2 is tacked. The same operation is performed for corner 3 and then for corner 4. (See Figure 15.)

Corrugation Gauge
Fig. 15 Corrugation Pitch Verification Using a ‘Comb’

The use of the comb to confirm the pitch of the corrugation is essential to assure the continuity of the corrugation.

3) TACK WELDING ON STAINLESS STEEL CORNER AND ANCHORING INTERMITTENT WELDING ON ANCHORING STRIPS

The tack welding is used to fix the sheet on the steel corner before welding and the anchoring intermittent welding is used to fix the sheet on the anchoring strips.

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After completion of the tack welding the corners are welded, using the sequence wide tacking, close tacking and continuous welding.

See Figures 16 through 22 for tack welding and welding sequences.

Initial Membrane Sheet Welding
Fig. 16 Securing the First Membrane Sheet with Welds
Tack Welding 1,2 mm to 8 mm Corner
Fig. 17 Tack Welding a 1,2 mm Sheet to an 8 mm Steel Corner
1,2 mm Sheet Intermittent Welding
Fig. 18 Intermittent Welding 1,2 mm to Strip
Welding Procedure
Fig. 19 Order of Welding Operations
Welding Sheet Corners
Fig. 20 Principles of Sheet Corner Welding
Welding Order
Fig. 21 Sequence of Welding Operations
Sheet Corner Welding
Fig. 22 Principles of Sheet Corner Welding

4) PRECAUTIONS TO BE TAKEN

No weld is to be made on the fixation rivets of anchoring strips. The rivets are made of aluminum. Aluminum Welding Techniques: Advanced MethodsAluminum dilution of weld may lead to fracture.

No gap is to be left between sheet and anchoring strips or steel corner during welding.

Alternate weld sequence along side sheet edge to avoid distorsion of sheet and corresponding deformation of the corrugation pitch due to weld shrinkage.

The comb should be used to confirm the pitch.

5) POSITIONING OF THE SECOND SHEET AND OTHER SHEET WITH 1 JOGGLED BORDER

After checking the intermittent welds and corner welds, the second sheet is laid overlapping on the previously installed sheet. (See Figure 23.)

Joining Joggled Membrane
Fig. 23 Joining Techniques for Second Membrane Sheet with Joggled Border

Corner No 1, the free corner, is first positioned with accuracy by means of the centering template as described in the previous paragraph.

Corner No 2 is aligned on the junction axis by means of the alignment template and the relative position according the limit line of the sheet is checked. (See Figure 24.)

Corner Sheet Welding
Fig. 24 Corner Welding Principle for Sheets and 2 mm Anchoring Strip

Once this checking is satisfactory, the tack welding of the sheet can start. At first, the free corner is tacked. Then the corrugation pitch is checked with the comb on the edge 1 and 2 (free edges) and corner No 2 is tacked by using the alignment template in line with the junction axis. (See Figures 25 and 26.)

Overlap Method
Fig. 25 Overlapping Assembly Method
Wide-Space Sequence
Fig. 26 Sequence of Wide-Space Overlap Tack Welding

Corners 3 and 4 are tacked by using an accurate pitch template.

During tack welding, special attention is to be paid to reduce as far as possible the gap (< 0,3 mm) between the overlap pieces at the corrugation feet. If necessary, a clamping device may be used.

6) POSITIONING OF THE FIRST SHEET OF THE SECOND LINE 1 JOGGLED BORDER

After checking of the intermittent welds and corner welds, the sheet is laid by overlapping on the previously installed sheet.

The corner No 1, the free corner, is first positioned with accuracy by means of the centering template. (See Figure 27.)

Steel-Backed Corner Welding
Fig. 27 Corner Welding Principle for Sheets and 8 mm Steel Corner

The corner 3 is aligned on the junction axis by means of the alignment template, and the relative position according to limit line of sheet is checked.

Once the checking is satisfactory, the tack welding of the sheet can start. At first, the corner No 1 is tacked, then the corrugation pitch is checked with a comb on the edge D3 and corner 3 is tacked using the alignment template inline with the junction axis.

Corners 2 and 4 are tacked welded by using an accurate pitch template. (See Figures 28 and 29.)

Tack Welding Seam
Fig. 28 Anchoring Tack and Intermitent Welding of Storage Tank Liner Edge
Corner Seam Weld
Fig. 29 Corner Welding Third Sheet

7) POSITIONING OF THE SECOND SHEET OF THE SECOND LINE, 2-JOGGLED BORDERS

After checking the intermittent welds and corner welds, the sheet is laid by overlapping on the previously installed sheets. (See Figure 30.)

Joggled Border Weld
Fig. 30 Anchoring Tack Weld

The corner 1, the free corner, is first positioned with accuracy by means of the centering template and the corrugation overlapping of the other corners are checked.

Once this checking is satisfactory, the tack welding of the sheet can start. At first the corner 1 is tacked.

Then the corners 2 and 3 are tacked by using an accurate pitch template in line with the junction axis.

Corner 4 is finally tacked when it is sure that the two perpendicular corrugation are perfectly applied on the previous sheets. (See Figure 31.)

Fourth Line Seam
Fig. 31 Fourth Sheet Seam

Figure 32 is a photograph showing the four corners of membrane sheets after welding.

Welded Corners
Fig. 32 The Membrane Sheet Corners After Welding

8) POSITIONING OF CLOSING SHEET ALONGSIDE CORNER PANEL, 2 JOGGLED BORDER

After checking of the intermittent welds and corner welds, the sheet is laid by overlapping on the previously installed sheets. (See Figure 33.)

Corner Seam Welding
Fig. 33 Side Corner Panel Seam

At this stage, the sheet must be centered between the two border membrane sheets and the pitch is checked on each side with the pitch template. The possible gap in + or – must be balanced on each corner between all the corrugation by compression or extension of the sheet to be installed. The joggled border of the sheet must be inside the tolerance +/- 6 mm according the junction axis.

Once this checking is satisfactory, the tack welding of the sheet can start by the corners 1 and 2.

Then corners 3 and 4 are tacked on the steel corner.

9 POSITIONING OF CLOSING SHEET, 3 JOGGLED BORDERS

The same previously mentioned principle applies. (See Figure 34.)

Joggled Border Weld
Fig. 34 Tack and Intermit Seam

10 POSITIONING OF THE LAST CLOSING SHEET, 4 JOGGLED BORDERS

After checking the intermittent welds and the corner welds, the sheet is laid by overlapping on previously installed sheets. (See Figure 35.)

Joggled Weld
Fig. 35 4 Joggled Borders Welding

With pitch tools, the sheet is adjusted in order to mesh all the sheet corrugations as well as possible with the already installed sheets.

Once the checking is satisfactory, the tack welding of the sheet may start by corner 1, 4, 2 and 3.

Membrane Sheet Wwlding Procedure

The membrane sheet welding can be divided in two parts:

  • The welding of the border sheets on the steel corners.
  • The welding of the sheet overlap joint (sheet to sheet).

1) WELDING OF THE 1,2 MM SHEET ON 8 MM STEEL CORNER

This weld can be carried out either manually or automatically by means of a welding machine (in fact automatic manual process).

In case of manual welding it is necessary to first carry out close tack welding then, continuous welding can be performed.

Excellent fusion is required for this welding because of the transmission of the stresses from the membrane edges to the steel corners.

Once this welding is carried out on each side of a corner panel, the installation of angle pieces may start.

2) OVERLAP WELDING OF 1,2 MM SHEETS

This weld can be carried out either manually or automatically by means of a welding machine (in fact automatic manual process).

In case of manual welding it is necessary to first carry out close tack welding then, continuous welding can be performed.

3) ANGLE PIECES AND OTHER CLOSING OVERLAP PIECES

Most of the angle pieces and other closing overlap pieces are of fixed dimensions. They can be installed on board without any adjustment in normal condition. However the minimum of overlapping is to be checked before welding (usually 10 mm).

Only angle pieces on transverse bulkheads adjacent to the chamfers must be adjusted
according to actual dimension.

The different closing pieces are the angle pieces (90°, 135° and similar, dog leg, end cap, salient angle piece). (See Figure 36.)

Chamfer Angle Measurement
Fig. 36 Determination of Chamfer Angle

4) DEFINITION OF ADJUSTABLE ANGLE PIECE AND INSTALLATION PRINCIPLE

Definition of angle pieces adjacent to chamfers will be carried out at the step of marking of membrane sheet limit line. At this time angle piece dimension will be measured for fabrication.

At first, pieces to be covered by angle piece will be checked. No protrusion is allowed and grinding will be carefully carried out if necessary.

Then proper bearing and overlapping of the angle piece has to be checked. After these operations the tack welding can start.

During this operation, the angle piece will be clamped with adequate tool to insure a good bearing of the radius in the corner. Tack welding is then performed following an approved sequence, wide tacking and close tacking. As for sheet overlapping, particular attention will be paid at the corrugation fit up. After tacking the welding is carried out.

The proper welding sequence is corner radius first, then corrugation and after flat area. (See Figures 37, 38 and 39.)

Angle Installation
Fig. 37 Angle Piece Setup
Angle Piece Mounting
Fig. 38 Angle Piece Tack Welding
Welding Process Example
Fig. 39 Welding Sequence Demonstration

General Welding Control

Main control items can be summarized as follow:

  • During tack welding, quality of the spots and also proper fit up of the sheets, maximum gap 0,3 mm.
  • Intermittent welding on anchoring strips: size and position, no weld on the rivet.
  • Automatic weld to be carefully visually checked.
  • Manual weld to be carefully visually checked. Note that individual welders are subject to monthly re-qualification testing and automatic machines are to be tested every day before starting work.

The membrane will be checked also outside the welded area especially on the lower chamfers and on the bottom where the fall of material or tool can damage the membrane sheets. Protection is to be applied.

Shoe protection is to be worn when walking of the membrane.

Figure 40 is showing plasma welding of pieces of membrane.

Plasma Seam Welding
Fig. 40 Welding of Membrane Panels

Figures 41 and 42 show the installation of the membrane sheets over the insulation.

Membrane Install
Fig. 41 Membrane Sheet Placement over Insulation
Insulation Cover
Fig. 42 Insulation Covering with Membrane Sheets
Author
Author photo - Olga Nesvetailova
Freelancer
Literature
  1. The Society of International Gas Tanker and Terminal Operators (SIGTTO). Liquefied Gas Handling Principles on Ships and in Terminals (LGHP4) / 4th Edition: 2021.
  2. The international group of liquefied natural gas importers (GIIGNL). LNG custody transfer handbook / 6th Edition: 2020-2021.
  3. American Gas Association, Gas Supply Review, 5 (February 1977).
  4. ©Witherby Publishing Group Ltd. LNG Shipping Knowledge / 3rd Edition: 2008-2020.
  5. CBS Publishers & Distributors Pvt Ltd. Design of LPG and LNG Jetties with Navigation and Risk Analysis / 4th Edition.
  6. NATURAL GAS PROCESSING & ITS ENERGY TRANSITION ROLE: LNG, CNG, LPG & NGL Paperback – Large Print, November 14, 2023.
  7. American Gas Association, Gas Supply Review, 5 (February 1977).
  8. The Society of International Gas Tanker and Terminal Operators (SIGTTO). Ship/Shore Interface / 1st Edition, 2018.
  9. Department of Transportation, US Coast Guard, Liquefied Natural Gas, Views and Practices Policy and Safety, p. IV-3.
  10. Department of Transportation, US Coast Guard, Liquefied Natural Gas, Views and Practices Policy and Safety, p. IV-4.
  11. Federal Power commission, Trunkline LNG Company et al., Opinion No. 796-A, Docket No s. CP74-138-140 (Washington, D. C.: Federal Power Commission, June 30, 1977).
  12. Federal Power Commission, Final Environmental Impact Statement Calcasieu LNG Project Trunkline LNG Company Docket No. CP74-138 et al., (Washington, D. C.: Federal Power Commission, September 1976).
  13. Federal Power Commission, «FPC Judge Approves Importation of Indonesia LNG».
  14. OCIMF, ICS, SIGTTO & CDI. Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases / 1st Edition, 2013.
  15. Federal Power Commission, «Table of LNG imports and exports for 1976», News Release, June 3, 1977, and Federal Energy Administration, Monthly Energy Review, March 1977.
  16. Office of Technology Assessment LNG panel meeting, Washington, D. C., June 23, 1977.
  17. Socio-Economic Systems, Inc., Environmental Impact Report for the Proposed Oxnard LNG Facilities, Safety, Appendix B (Los Angeles, Ca.: Socio-Economic Systems, 1976).
  18. «LNG Scorecard», Pipeline and Gas Journal 203 (June 1976): 20.
  19. Dean Hale, «Cold Winter Spurs LNG Activity»: 30.

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