Challenges Involved in Design, Construction and Inspection of Full Containment Refrigerated LPG Storage Tanks

Abstract Saudi Aramco has been a major player in the LPG (Propane and Butane) market since the 1960s and the company continues to be the source of benchmark pricing in the industry.[9] A variety of refrigerated storage tank concepts have been developed to support international export of LPG products. Currently the industry has shifted to the use of safer full containment tank systems which have the capability to contain liquid and vapor release. Effective implementation of new tank systems has the potential of improving safety at company facilities. The benefit of the full containment storage tank type is attractive given that the industry is focused on safety and environmental protection. However, the implementation of this technology is challenging due to the complexity of the design and construction of these tanks. This paper explores overcoming the challenges involved in the design and construction of full containment tanks in refrigerated LPG service. The paper provides a case study of a current Saudi Aramco project being constructed for a new installation of LPG storage tanks at a size of one million barrels for both propane and butane services. Based on recent project execution, identification of a qualified engineering firm who has good experience in this application is critical to the success of the design effort. Design of full containment tanks requires extensive knowledge of specialized standards and specifications. Often internal company standards are insufficient to guide the design team. Tank vendors are reluctant to provide front end engineering support prior to award of the tank job. However, the tank vendors are the most knowledgeable of the design requirements and typically have their own engineering procedures and specifications. This significantly restricts the number of qualified vendors for these tanks. There are several auxiliary systems that are required as part of the overall full containment tank system design such as foundation heating system and the submersible pumps. These auxiliary systems have specialty vendors and specification that add to the complexity of the design process. Prior to construction, detailed geotechnical surveys and soil improvement must be completed to avoid settlement of the full containment tank. The refrigerated LPG service requires low temperature materials which in some cases have few available vendors. The erection of the steel for the tank is taken in stages where the outer shell is constructed first, followed by the raising of the roof through air pressure and then construction of the inner tank. The tanks require large platforms for the inspection and maintenance of submersible pumps and process control equipment including a Jib crane for removal and installation due the lack of side penetrations. The workflow of the welded plates is critical to ensure minimization in the number of lifts and welding at heights. Inspection of the welds is accomplished through the use of radiographic testing for the bottom plates and ultrasonic testing for the shell and roof. Pressure testing of the inner tank is accomplished through hydrotest with chemically treated seawater utilizing temporary nozzles on the shell of the inner tank. The outer tank is pressure tested via pneumatic testing. The interstitial space between the tanks is filled with an expanded perlite insulation with installation accomplished through vendor specialty equipment. Finally, the training of the operations team is crucial to the successful commissioning of these tank systems. Support personnel receive specialized training at vendor facilities and require site visits to similar installations to ensure safe operation.