Classically hard spots are defined as a localized increase in hardness produced during hot rolling of steel plate as a result of localized cooling. Although more prevalent in plate formed pipe, seamless pipe can be susceptible through poor controls in the mill resulting in material property variations including hardness.
Hard spots are often considered by pipeline operators as non-injurious stable features. Many hard spots that were created as early as the plate/pipe manufacturing process will reside in the pipeline for its life without any additional issues. However, the locally hardened material does pose an increased risk for potential cracking and hence can be injurious during operations, especially if additional threats exist such as the presence of hydrogen. Cracking can occur at hard spots that eventually grow over time eventually leading to pipe failure,
A European liquids pipeline operator, TRAPIL, wanted to assess the possibility of existing hard spot into one pipeline section . But the question was asked, how hard is hard? Also, when is a hard spot a hard spot and not just material variation? To detect hard spots in the line, the multiple data set (MDS) in-line inspection (ILI) platform was selected as the most appropriate solution. This platform enabled the assessment of hard spots beyond just the length, width and single hardness value.
Examples of hard spots found in other pipelines will be presented and how the tool can be used for the applications of identification of interacting features and complex hardness mapping for hard spot prioritization. The paper will also look at what was found both from the ILI and field and how these correlated in TRAPIL's pipeline and discuss the issues and importance of hard spots for the future integrity of European pipelines.