Pushing the Limits of Storage
John A. Schlosberg, Corban Energy Group, USA, presents a case study on the provision of an LNG storage solution for a power plant in equatorial Africa.
In this article, Corban Energy Group (CEG) takes on the challenge of a tight budget, a short schedule and difficult logistics to find a 14 000 m3 LNG storage solution for a new power plant in a developing area of equatorial Africa.
The challenge
Target gross storage capacity would suggest the possibility of using either site or factory built storage tanks. However, the schedule would allow less than 12 months from contract to commissioning. This alone would rule out a site-built, flat bottom tank. Even under ideal conditions, more time would be needed, and conditions here would not be ideal.
A larger cost saving was needed, and this focused attention on the selection of outer vessel material. For smaller tanks requiring thinner steel plate, ASTM A283 Grade C carbon steel is an accepted choice. However, the use of this economical material in pressure parts for pressure vessels is limited to a thickness of 16 mm by the ASME BPVC, Section VIII, Division 1, Part UCS-6(3). 1228 m3 tanks, with proportions optimised for this application, require an 18 mm thick outer shell.
This calculation appeared to require an upgrade to the more costly ASTM A285 Grade C carbon steel. However, a more critical reading of the ASME BPVC, Section VIII, Division 1, Part U-1(c)(2)(-h)(-1) suggested that the outer vessel of these tanks is not a pressure vessel and is not subject to the provisions of the ASME BPVC. This is because, among other things, the pressure at the top of the vessel does not exceed the design pressure and neither the internal or external pressure exceeds 15 psi. This interpretation was confirmed as correct by the cognizant ASME authorised inspector and inspector supervisor.
The result of the interpretation was a substantial saving in both material cost and in the expense of pneumatic testing that would otherwise have been required. It allowed the design approach to focus both on the function of the vessel as a container for the vacuum and expanded perlite insulation system, and on the strength of the vessel as a structural component.
Schedule
The schedule was helped by the selection of outer vessel material. The use of ASTM A283 Grade C carbon steel helped to shorten the procurement process, fabrication time and the non-destructive testing (NDT) schedule. Other methods were employed to help shorten the schedule. These included the following:
- Use of a manufacturing facility with enclosed and yard space sufficiently large to lay out the entire 12 tank order in production line sequence, with a minimum of interruptions to relocate work pieces as they move from one manufacturing operation to the next.
- Extensive use of sub-contract manufacturers to produce specialised subassemblies, including the spherical heads, trunnions and piping spools.
- Selective use of extended and double work-shifts to debottleneck certain phases of the work process, including marking, cutting, bending and fit up of steel plate.
- Close coordination with the ASME authorised inspector to avoid gaps in schedule around required inspections and QC tests.
All of these measures succeeded in shortening a process that customarily takes at least 12 months by almost six full weeks, the minimum time needed for shipping and installation.
Logistics
1228 m3 Type C tanks measure approximately 10 m wide, almost 9 m high and almost 31 m long. They weigh almost 317 t. Moving them from factory to port was a critical consideration. The solution was to move production into a suitable factory located in an industrial area approximately 1 km from the dock at the South Korean port city of Gunsan on the Yellow Sea.
Transport of oversize/overweight equipment from the factory to the dock is routinely accomplished using hydraulic transporters along a high bearing capacity roadway requiring no widening or wire lifting. Similar accommodations facilitated transport at the receiving port and from the receiving port to the project site.
Conclusion
1228 m3 Type C tanks measure approximately 10 m wide, almost 9 m high and almost 31 m long. They weigh almost 317 t. Moving them from factory to port was a critical consideration. The solution was to move production into a suitable factory located in an industrial area approximately 1 km from the dock at the South Korean port city of Gunsan on the Yellow Sea.
Transport of oversize/overweight equipment from the factory to the dock is routinely accomplished using hydraulic transporters along a high bearing capacity roadway requiring no widening or wire lifting. Similar accommodations facilitated transport at the receiving port and from the receiving port to the project site.