Many decisions that are made at the design stage of engineering can have a major impact on safety and cost during the operational stages. This is especially true of pipe supports. Corrosion under pipe supports (CUPS) is a growing problem and a particular threat to pipework integrity, as Laza Krstin, Principal Consultant at ABB Consulting, explains.
For many operating companies, extending the operational life of production installations beyond their original design life is a key business objective. However, for ageing assets, corrosion under pipe supports (CUPS) presents a particular threat to the integrity of pipework. Failure at supports has the potential to lead to major loss of containment, and such instances are difficult to repair online (eg by clamping). Inspection to determine pipe condition is difficult and remediation presents many practical difficulties. The extensive (system-wide) nature of the problem means safety implications, potential disruption to production and the cost of dealing with CUPS can be significant.
A recent CUPS incident illustrates the safety and business impact. A leak caused by corrosion in a 40-year-old steel pipe was found to be the direct cause of an explosion and fire in May 2009 at a US oil refinery. The explosion happened when flammable gas escaped from a 10-inch pipe, which had corroded and developed a leak. The corrosion occurred in a small section of piping because moisture was trapped between the pipe and a metal sleeve on which it was supported. The explosion shook homes for miles around the refinery. Fortunately, no one was hurt in the blast and resulting blaze, which burned for nearly 24 hours. The refinery operator said that customer demand didn't justify the cost of rebuilding the production unit.
In recognition of the severity and potential consequence of CUPS, the Energy institute (El) initiated a CUPS project in 2013. ABB Consulting was commissioned to produce a guide to help operating companies understand the threat from CUPS. The guide is being developed in collaboration with HOIS (an industry forum for discussing inspection issues and developing improved inspection technology for applications in oil and gas, administered by ESR Technology), who are providing a section in the guide covering the latest non-destructive testing (NDT) techniques.
The El document aims to help operating companies to better understand the range of integrity threats from CUPS; to help determine an effective inspection and maintenance strategy for managing CUPS, including advice on NDT techniques; and to understand how to reduce the risks of CUPS. The guidance is applicable to pipework on topsides of offshore installations, onshore plants, and the topsides/above ground portions of pipelines.
CUPS is relevant to two initiatives launched by the UK's Health and Safety Executive (HSE) aimed at promoting awareness and management of the risks associated with ageing plant - KP4 for offshore installations and the COMAH (Control of Major Accident Hazards) Ageing Plant programme for onshore assets.
Damage mechanisms The greatest concern is that corrosion develops in areas that are difficult to inspect so pipe wall loss is undetected. The features and shape of the support, its material of construction, and a number of other factors affect the type and rate of deterioration. The main contributory factors include:
- Moisture (rain, fire water deluge testing, etc) and contaminants (eg chlorides from sea water) which increase the likelihood and rate of corrosion.
- The design of most pipe supports results in crevices, which allow moisture to collect.
- The pipe operating temperature affects the corrosion rate. The support can act as a 'heat sink' on 'hot' pipework, resulting in the pipe wall local to the support being cooler than the bulk of the piping system, which may encourage both internal and external corrosion.
- Protective coatings should be carefully selected for the expected conditions, and correctly applied. However, coatings tend to deteriorate over time.
- Thermal expansion and contraction of the piping can damage the coating by fretting, or cause it to crack and disbond.
The main focus for the CUPS guide is corrosion of carbon steel, the most commonly used material of construction for pipework and pipe supports. However, where austenitic stainless steels are used, there are different concerns - such as chloride induced stress corrosion cracking (particularly in marine and coastal environments) where the pipe is above 60°C or pitting corrosion in the presence of chlorides below 60°C. Duplex steels and other materials may also be vulnerable to CUPS.