Mitigation of In-line Tee Assembly Rotation

The Challenge

During an ROV survey performed 3 years into operation of the pipeline it was discovered that an In-line Tee Assembly (ITA), used for future connections, had unexpectedly rotated from its as-built position. The cause of the rotation was due to unexplained lateral buckling either side of the ITA which has not bee predicted during design. There was client concern that if left untreated, the rotation would increase and exceed what was allowable and prevent any future tie-in. Crondall Energy was tasked to match the surveyed ITA response through finite element analysis (FEA), preform integrity checks to the pipeline and and also investigate solutions to mitigate further rotation.

What We Did

• Performed a muti-year survey review to investigate the changes to the pipeline over time.

• Developed a FEA model of the pipeline including the ITA structure supported by its mud-mat.

• Through analysis, matched the observed buckling behavior of the pipeline around the ITA resulting in rotation

• Performed integrity checks of the pipeline in its buckled condition.

• Determine the required restraint of the mitigation to prevent further buckling/rotation of the ITA.

• Develop a concrete mattress solution and model its effectiveness through FEA

• Develop an installation plan for the recommended mitigation.

Key Results

✅ Root Cause Identified and Validated: Finite Element Analysis (FEA) successfully reproduced the observed lateral buckling behaviour and resulting rotation of the In‑line Tee Assembly (ITA), providing confidence in the understanding of the pipeline response.

✅ Mitigation Solution Proven: A concrete mattress restraint solution was designed, analysed, and demonstrated through FEA to prevent further buckling and rotation of the ITA.

✅ Future Field Development Enabled: The recommended mitigation ensured the ITA remained within allowable limits, preserving the option for future tie‑in and increased field output.

Impact

Crondall Energy’s work provided the operator with confidence to continue safe operation of the pipeline both before and after mitigation, underpinned by a rigorous assessment of integrity and structural response. A simple and cost‑effective solution was developed to address a complex and unanticipated pipeline behaviour, avoiding the need for extensive modification or intervention. Crucially, the mitigation preserved the ability to connect future infrastructure to the ITA, safeguarding planned field development and maximising the long‑term value of the asset.