Out Of Straightness (OOS) survey is an investigation procedure aimed to acquire information about the vertical and horizontal configuration of offshore pipelines. The main objective of OSS survey is to provide:
Detailed design and during construction: a quantitative basis for assessment of global - eulerian buckling (upheaval and lateral buckling as results of axial loads due to restrained thermal expansion) in presence of out-of-straightness. This is the case of buried pipelines for which the straightness of the bottom of the trench is a factor.
During operation: verification of the presence of expected or unexpected buckles where bending may exceed the operational limits established for the material and pipe wall thickness, by the mechanical design. This is mainly the case of exposed pipelines for which unexpected lateral deflection can be the result of incidental third party interaction.
Global buckling is a significant design topic for HP/HT flowlines or export pipelines in the early stretch, where thermal expansion might be recovered by developing excessive bending. Therefore, the development of specific techniques to perform OOS survey as well as the analysis of the acquired data is getting increasingly crucial, since the outcome of this activity drives the decisions on mitigations needed for pipeline safe operation as per design. After a brief description of the OOS survey technology and engineering interpretative tools, a few operational applications are presented. External survey and in-line inspection outcomes are compared in correspondence of pipeline sections affected by global buckling. The effectiveness of the available technology and engineering tools is discussed.
