The present work addresses the results of the qualification testing of a fibre optic instrumented pre-tensioned bundle pipe-in-pipe (PIP) section, to provide a real-time integrity tool for pipeline bundle health monitoring. The aim of the testing is to allow the pipeline bundle system to be a cost-effective solution for subsea developments, unlocking high pressure/high temperature (HPHT) fields rated up to 220°C and providing thorough life assurance of the pipeline bundle system performance and integrity.
In line with Subsea 7's pipeline bundle research and development programme, the most recent trial at Subsea 7's Wester Pipeline fabrication facility, North East of Scotland was carried out on a 170m long bundle PIP section to demonstrate the capability of the fibre optics to detect different thermal and mechanical loads. The test commenced with a pressure strength test at 287bar, followed by three pre-tensioning cycles using a jacking system. The bundle PIP section then undertook two cycles of combined HPHT application. In addition, a 300mm high boulder bend was applied to the pipeline at pressure and temperature to verify that the compressive loads are too low for buckling to occur. The applied loads and the bundle PIP responses have been monitored in real-time using fibre optics and strain gauges for verification, in the presence of a third party. The test results were successful and are presented and discussed herein.
It has been found that the monitoring solution can be used to measure pre-tensioning construction load cycles accurately by detecting and recording strain levels up to approximately 1600με at the maximum pre-tensioning load with a high resolution. It can also monitor combined operational loads: pre-tensioning tension/compression, boulder case bending, thermal expansion and pressurisation up to 193bar at temperatures up to 220°C. The bundle PIP section endured all those loads safely and the fibre optic recorded data was in line with that recorded by the strain gauges, as well as the theoretical and FEA model results. Furthermore, the result dataset shows high consistency and repeatability at different loading combinations.
The value of this monitoring tool appears in the associated low cost and minimal risk in the use of the fibre optic technology to provide continuous real-time health monitoring and pinpoint the location on the pipeline bundle, at which an abnormality is present. Additionally, the fibre optic monitoring, demonstrated herein, is used to ensure the accurate input of the pre-tensioning into the pipeline bundle, allowing its use for HPHT fields.