Exploring the Benefits of External Pressure Testing for Pipeline Integrity

Pressure testing is required for many types of pipelines and piping systems. These tests check that a channel or pressure vessel can withstand internal or external pressure without structural flaws permitting leakage.

Hydrotesting is widely used for post-construction integrity assessment of hazardous liquid and higher-stress natural gas pipelines (particularly where inline inspection tools are impractical). Mobile skid-mounted hydro testing equipment facilitates this type of assessment economically.

Detection of Corrosion

Detecting external corrosion can be challenging since thermal insulation has proven not to shield O&G pipelines from decay under insulation (CUI). However, a pressure test with non-conductive mineral oil can indicate the status of a corroded pipe location.

When an integrity assessment or information analysis identifies a covered segment as being susceptible to third-party damage, it requires the operator to evaluate other conditions, including implementing best practices for leak prevention, conducting cathodic protection surveys, reexamining damaged sections of the pipeline, increasing the duration of hydrostatic pressure tests and establishing shorter inspection intervals.

One method that is often used to mitigate these risks is direct assessment (DA). DA includes a process for evaluating three time-dependent threats to a covered pipeline segment’s integrity: internal corrosion, external stress corrosion cracking and limited longitudinal seam weld degradation. DA is not considered a primary evaluation method but may be implemented as a contingency with ILI or hydrostatic external pressure testing.

Detection of Cracks

Pipeline operators increasingly recognize the value of inline crack inspection tools to reduce costly hydrostatic test failures. However, some non-injurious defects with longitudinal orientation, such as elongated inclusions or laminations, can produce signals that look like those produced by cracks and could mislead the crack detection tool into identifying a potentially critical flaw.

This problem is particularly acute for older pipeline materials with manufacturing anomalies that produce crack-like indications when pressurized. In many cases, these defects require hydrostatic testing to a higher pressure to remove the long flaws capable of producing rupture. Still, they are more likely to fail when repressurized at the lower test pressure, creating a potentially expensive hydrostatic test reversal.

Inline crack inspection with the EMAT crack detection tool can help to mitigate this problem. By incorporating advanced signal processing, including logarithmic data compression and context-related B-scan visualization, the crack detection tool can distinguish at-risk defects from those unlikely to grow into critical flaws.

Detection of Leaks

Detecting leaks in oil and gas pipelines is essential to ensuring network safety. Several methods have been used to detect leaks, including internal inspection and monitoring using inline inspection tools, external sensing systems, and pipeline pigs.

External systems typically involve more sensors along the pipeline and rely on outward manifestations of a leak, such as pressure changes, temperature changes, or chemical spills. These systems are generally more expensive to install and operate but may offer better detection accuracy, especially for small leaks.

In a hydro test, water is pumped into the pipe at high pressure to evaluate its integrity. The test is conducted to ensure no pipe cracks before being put into service. The test is also used to identify cracks already developed in the pipeline.

Detection of Faults

During the inspection of pipelines, gas cylinders and boiler components, hydrostatic testing is often required to assess whether an item can be safely put back into service after repairs. This testing is performed by increasing the pressure within a vessel and measuring how much strain is applied before the rupture point is reached.

Unlike traditional testing frames, which can improve load and displacement control by adding material to the frame, a test system designed for pressurization cannot do this. This is because of the greater compressibility of testing fluids compared to the specimen and the large volume of testing fluid required to achieve equilibrium during the pressurization process.

This paper aims to demonstrate a new method for applying external hydrostatic pressure to buckling-critical shells that prevent catastrophic specimen failures while also improving the control of the deformation of the specimen during testing. This is achieved by using a high-pressure hose to connect the inside of the test specimen to a separate high-pressure pressure chamber. The loop has a fluid-release and cross-over valve that can break the connection between the specimen and the main pressure chamber.

*This is a collaboration post