The pump tower is a vital part of the GT96 LNG Membrane System, located at the aft end of the ship and designated as one of the GT96 Special Locations. This area is critical for the safe and efficient loading and unloading of liquefied natural gas (LNG) from the vessel.
Other GT96 Special Locations include the cofferdam, the water ballast tanks, and the void spaces adjacent to the cargo tanks. These areas require special attention and monitoring to ensure the integrity of the membrane system and the safe transportation of LNG. Understanding the importance of these locations is crucial for the safe and efficient operation of the GT96 LNG Membrane System.
Special Locations
No. 1 Cargo Tank
In the forward area of a ship, where the parallel mid-body transitions to the narrower forebody, GTT membrane systems are able to be configured to the variable geometry dictated by this area. This allows a greater amount of cargo to be carried within any given hull form.
The key component in enabling this to take place is a patented pillar/post, which firmly anchors a strake of membrane at its diminished end. (See Figure 1). When looking down on #1 tank, the shape is trapezoidal versus square or rectangular like the other tanks.
Figure 2 shows a pillar ready for inspection.
The bolted plates are used to minimize distortion of the collar due to welding.
Figures 3 shows the placement of the pillars on the inner hull.
These are placed at measured interval along the upper and lower dihedrons of the tank. These pillars are the terminus points for each longitudinal strake or run of membrane. The flange on the mid-portion of the pillar is the end point for the secondary barrier membrane.
Figure 4 shows pillars in relation to secondary or insulation space boxes.
Figures 5 shows secondary barrier in place around pillars.
Figure 6 shows pillar with primary attachment flange. The holes in the flange accommodate jack bolts, which hold primary or interbarrier perlite insulation boxes in place.
Figures 7 and 8 show the interbarrier boxes held in place by the pillars.
The primary barrier will then be attached to the boxes in the same manner as other tanks.
Figure 9 shows finished primary barrier of #1 cargo tank prior to vacuum testing.
Each of the longitudinal membrane (bottom not seen here) terminates at a pillar.
Liguid Dome
The liquid dome is one of the last openings of a cargo tank to be closed up. It is of sufficient size to allow installation of the pump tower it as lowered into the tank. The insulation box, which fits the liquid dome, is the largest single box in a GT96 Membrane System Installation Protocols for LNG ContainmentNO.96 system.
Insulation for liquid dome top consists of a large assembly of plywood sections, with compartments packed with precisely cut, rigid, closed-cell foam blocks. Stainless brackets are used for reinforcing corners and other potentially weak areas of the box.
Once completed, the plywood bottom cover is fitted with invar sheet over its entire surface.
Photos on the following pages illustrate the fabrication and installation of one box.
Figure 10 shows box during early stage of fabrication. Surface shown is top or upper side.
Figure 11 shows liquid dome box being turned so that reinforcing clips can be applied and chambers can be filled with insulation.
Figure 12 shows typical metal bracket used for reinforcing corners and other areas.
Figure 13 shows chambers and foam insulation. Note the precision with which the foam is cut.
Figure 14 shows close-up of chambers filled with foam. Note the detail in that even the smallest space receives insulation.
Figure 15 shows finished liquid dome insulation panel in invar shop prior to installation of invar sheeting to bottom surface (now shown facing up).
Figure 16 shows complete liquid dome insulation box.
Pump Column/Tower Supports
Each Independent Cargo Tankscargo tank has a liquid dome located near the ship’s centerline at the aft part of the tank. The combined steel structure of the trunk deck and inner deck in way of the liquid dome provide the strength to support the hanging weight of the pump tower, which is a tripod mast made of stainless steel piping (304L), suspended from the liquid dome structure and maintained in position at the bottom of the tank by a sliding bearing to allow for thermal expansion or contraction depending on the tank environment.
The tripod mast consists of the main discharging pipes and emergency pump well, in the form of a three-legged lattice structure and is used to support the tank access ladder and other piping and instrumentation equipment.
The instrumentation includes temperature and level sensors, independent high level alarm sensors and cargo pump electric cables. The two main cargo pumps are mounted on the base plate of the tripod mast, while the stripping/spray pump is mounted on the pump tower support. An emergency pump column, float gauge column and the filling line are also located in the liquid dome.
Figure 17 shows a typical pump tower supports set on pallet prior to placement on vessel.
Figure 18 shows the underside of support with strengthening brackets.
Figure 19 shows workers position the pump tower support on tank top.
The tank top has been precisely marked/punched, ground clean of all coating and cleared of any debris.
Figure 20 shows leveling jigs needed to assure that support surfaces are in correct plane and are at the correct height above tank top.
The light band seen at the top of the broad base of the support is a layer of plywood. This provides significant insulating value without seriously affecting the strength of the support. The base is filled with glass insulation as well.
Figure 21 shows pump tower support welded in place and undergoing dye penetrant testing.
Note the specific marking(s) to denote tank and orientation. The lowest two flanges are for attaching Comprehensive Framework: Primary & Secondary Barrier Testing Protocols on LNG Tankerssecondary and primary membranes. The top flange is for attaching the cargo-stripping pump.
Figure 22 shows pump tower support with completed secondary barrier.
Primary space insulation boxes will now be added and primary membrane welded to the support in the same manner and the secondary membrane.
Vapor Dome
A vapor dome is located near the geometrical center of each cargo tank ceiling.
Each vapor dome is provided with:
- A vapor supply/return line to supply vapor to the tank when discharging, vent vapor from the tank whilst loading and also vent the boil-off when the tank contains cargo.
- Spray line arrangement for cool-down purposes.
- Two pressure/vacuum relief valves set at 2,4 kPa absolute, and -1 kPa absolute, vacuum, venting to the nearest vent mast riser.
- Pick-up for pressure sensors.
- Liquid line safety valves exhaust.
Figure 23 shows special lift designed to allow people to inspect and work in vapor dome.
The vapor dome on older membrane vessels is located in the forwardmost area of the tank. Also, where newer vessels essentially have a pipe or tube for a vapor dome, the older membrane vessels have a square trunck fully as complex as the liquid dome.
Testing and Evacuation of Primary Barrier
Checks are identical or similar to those performed on secondary barrier. They include:
- Vacuum test.
- NDT – Dye Penetrant.
- Visual – for flatness, the surface of the boxes is inspected prior to installing the membrane. After the membrane has been installed and the space evacuated, it is then checked for protrusions and other “point” type irregularities.
When all work on the membranes have been completed, special overall or global tests are conducted to assure the tightness of both the secondary and primary barriers. Theses tests are conducted in the following manner:
- SECONDARY BARRIER
- PRIMARY BARRIER
The secondary space is pumped down to -20 kPa while the primary space is maintained at a slight vacuum -10 kPa. The rate of decay of the vacuum is monitored and plotted.
The primary and secondary spaces are simultaneously pumped down to 20 kPa whilst the primary membrane is subjected to tank atmospheric pressure on one side. Because the pressure in both spaces is equal, any vacuum decay will have to come from the primary barrier.
Fabrication and Installation of Pump Column/Tower
The Pump Column or Tower is a tripod mast made of stainless steel piping (304L), suspended from the liquid dome structure. It is held in position at the bottom of the tank by a sliding bearing to allow for thermal expansion or contraction.
The tripod mast consists of the main discharging pipes and emergency pump well, in the form of a three-legged lattice structure and is used to support the tank access ladder and other piping and instrumentation equipment.
Initially, the pump column is fabricated horizontally to facilitate easy set of jigs and other fittings.
Figure 24 shows a pump column prior to outfiiting and installation of pumps, cable, etc.
DSME now has special pump tower assembly and outfitting buildings – located on the LNG outfitting quay – where, in one, the initial fabrication is done horizontally. Once that stage has been completed, the building is literally rolled back; the pump column is picked up by a crane and then stood vertically and set into in a new (vertical) outfitting building.
Figure 25 shows the pump tower assembly and outfitting buildings.
Figure 26 shows assembled pump tower lattice.
Figure 27 shows further outfitting in progress. Pumps, and other items will be added in the vertical outfitting building.
Figure 28, taken inside vertical outfitting building, shows cargo pump installed on pump tower.
Once the pump tower has been completed, the doors, which extend the full height of the building, will be opened and the tower picked up vertically and inserted into the cargo tank via the liquid dome.
Subsequent photos show the transiting of the pump tower from the Vertical Outfitting Building to its designated vessel.
Figure 29 shows upper and side doors of Vertical Outfitting Building opened and finished pump tower ready for lifting by crane (left).
Figure 30 shows pump tower transiting from building and being inserted into No. 2 cargo tank of designated vessel.
Note temporary platforms installed on tower to facilitate final installation.
Figure 31 shows tank bottom and mating pump tower sliding connection. This is configured to allow for contraction and expansion of pump tower as it is cooled and warmed up.
- The Society of International Gas Tanker and Terminal Operators (SIGTTO). Liquefied Gas Handling Principles on Ships and in Terminals (LGHP4) / 4th Edition: 2021.
- The international group of liquefied natural gas importers (GIIGNL). LNG custody transfer handbook / 6th Edition: 2020-2021.
- American Gas Association, Gas Supply Review, 5 (February 1977).
- ©Witherby Publishing Group Ltd. LNG Shipping Knowledge / 3rd Edition: 2008-2020.
- CBS Publishers & Distributors Pvt Ltd. Design of LPG and LNG Jetties with Navigation and Risk Analysis / 4th Edition.
- NATURAL GAS PROCESSING & ITS ENERGY TRANSITION ROLE: LNG, CNG, LPG & NGL Paperback – Large Print, November 14, 2023.
- American Gas Association, Gas Supply Review, 5 (February 1977).
- The Society of International Gas Tanker and Terminal Operators (SIGTTO). Ship/Shore Interface / 1st Edition, 2018.
- Department of Transportation, US Coast Guard, Liquefied Natural Gas, Views and Practices Policy and Safety, p. IV-3.
- Department of Transportation, US Coast Guard, Liquefied Natural Gas, Views and Practices Policy and Safety, p. IV-4.
- 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).
- 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).
- Federal Power Commission, «FPC Judge Approves Importation of Indonesia LNG».
- OCIMF, ICS, SIGTTO & CDI. Ship to Ship Transfer Guide for Petroleum, Chemicals and Liquefied Gases / 1st Edition, 2013.
- 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.
- Office of Technology Assessment LNG panel meeting, Washington, D. C., June 23, 1977.
- Socio-Economic Systems, Inc., Environmental Impact Report for the Proposed Oxnard LNG Facilities, Safety, Appendix B (Los Angeles, Ca.: Socio-Economic Systems, 1976).
- «LNG Scorecard», Pipeline and Gas Journal 203 (June 1976): 20.
- Dean Hale, «Cold Winter Spurs LNG Activity»: 30.
