Stress corrosion cracking (SCC) is associated with pipelines made of susceptible steel operating in harsh corrosive environments under high stress. Saudi Aramco adapted the use of EMAT and UTCD in-line inspection (ILI) tools to locate, identify and size potential SCC defects, which revealed a significant number of cracks. All pipelines found with cracks have been in service for more than 30 years and tape coated. Many of these pipelines operate in subkha soil (alternatingly wet), indicating that the threat of SCC, which is time dependent, is persistent; requiring implementation of long-term mitigations.
The number of ILI-reported crack features varied from one to tens of defects per pipeline segment for the majority of the pipelines. Yet, some pipelines have been reported to have hundreds to thousands of cracks. Although these reported cracks are not deep enough to affect the near-term integrity, the cracks could potentially grow to become critical. Along with conducting ILI, Saudi Aramco investigated the use of hydrotests as a mitigation measure for pipelines found to contain SCC. Hydrotests are thought to complement the ILI program and ensure that the probability of premature pipeline failure is minimal.
Saudi Aramco conducted a study that entailed the evaluation of the benefit of conducting hydrostatic pressure testing as an SCC mitigation method for pipelines with reported SCC.
The assessment of hydrotests as a mitigation measure includes developing a set of survival probability matrices for each pipeline to determine which SCC defects would fail a hydrotest as a function of the test pressure and pipe properties.
A statistical analysis of the ILI and field results, is used to predict the number and dimensions of the estimated defect population within a pipeline. When this estimated defect population is combined with the survival matrices, a prediction of the expected number of failures during a hydrostatic pressure test can be estimated. The remaining defect population after a hydrotest is also predicted.
The effectiveness of the hydrostatic pressure test is taken as the ratio of features that failed in the hydrotest, divided by the total number of features that existed before the hydrotest. With the effectiveness of a hydrotest defined, it is possible to compare two assessment methods, such as ILI and hydrotesting when developing line-specific mitigation plans.