Edo Vonk, VSL International, highlights the basic guidelines for inspection and maintenance of pre-stressed concrete tanks for LNG and LPG storage, emphasising the differences between the various systems. he use of pre-stressed concrete containments has proven to be a cost-effective, durable, and safe solution for liquid storage facilities. Their size can range from small (several metres in diameter) to very large (up to 100 m dia.), and their height varies depending on the application. The basic principle of pre-stressed concrete tanks relies on the tensile strength of steel tendons, which are tensioned against the circular concrete structure. This induces a uniform axial compression into the concrete cross-section under the hoop stresses of the tendon. While concrete itself is a material that exhibits relatively high compressive strength, it does not allow the transfer of significant tensile stresses. Under tensile stresses, the concrete will crack and lose its structural integrity, unless reinforced by steel reinforcement bars, as well as foregoing its leak tightness. Putting the concrete into compression by the use of pre-stressing for all prevailing service load-cases is an elegant solution that has demonstrated its efficiency and reliability for approximately a century. For LNG and LPG storage tanks, the concrete wall acts (in most cases) as the outer secondary containment designed to react the internal tank pressure in
accidental scenarios only, i.e., after failure of the primary steel alloy containment membrane. In addition, this outer secondary concrete structure provides a robust barrier against external abnormal loads (impact, explosions, etc.) and aggressive environments, in particular where exposed to industrial emissions or chloride-laden marine air. The concrete containment is hence a safety-critical element of any LNG or LPG storage facility, and tank designers, operators, and owners have to ensure that the concrete containment could fulfil its critical role throughout the life of the facility. Similar to any other pre-stressed concrete applications, in particular bridges, pre-stressing systems require regular inspection and maintenance in order to ensure their integrity throughout the service life of the structure, and the structure can be prone to hidden defects, in particular corrosion of the high tensile steel.
Basic guidelines for inspection and maintenance of pre-stressing systems
In this context, it is important to establish some basic guidance for the inspection and maintenance of pre-stressing systems in LNG and LPG storage tank applications. To the knowledge of the author, such guidelines are currently lacking for LNG and LPG storage tank applications, and it is therefore the objective of this article to present a snapshot of the state-of-the-art in the inspection of pre-stressing systems and discuss its application and relevance in the case of LNG and LPG storage tanks.
Corrosion of high-tensile steel
Pre-stressing systems use very high-strength, cold-drawn steel wires (up to approximately 2000 MPa tensile strength) with low ductility (below 5% elongation at rupture) and small cross-sections (typical wire cross-sections below 10 mm dia.) These steel wires can be potentially prone to failure by corrosion. While corrosion failure mechanisms of conventional structural steel are mainly governed by gradual section loss, the failure mechanism of high-tensile cold-drawn steels are typically controlled
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