Abstract:
In mature offshore hydrocarbon fields, an increasing number of aging assets have reached or exceeded design life, meaning fatigue is of growing concern. Digital twins incorporating a fatigue model are typically used to assess structural integrity and inform decisions on life of field extension. However, they rely on idealized wave models which cannot truly reflect the hydrodynamic loading, which leads to uncertainty and conservatism. The operator of the Ninian Southern Platform in the UK North Sea concluded that if strain could be monitored in situ at key subsea locations, they could develop a more accurate relationship between sea conditions and structural loading, which could be used to calibrate the asset fatigue model, providing enhanced understanding of the jacket integrity. We describe the development, validation, installation, and first field results from an instrument designed to meet that challenge. Retrofitting strain sensors to subsea structural members presents many technical challenges, as neither conventional strain gauges nor cabled connections are feasible in the harsh subsea environment. For the first deployment, the instrumentation required robust attachment to a 1.2 m diameter tubular member situated at 43 m depth, which would withstand 100-year metocean maxima. It needed a strain transfer mechanism for autonomous measurement of the smallest fatigue-inducing strain reversals on three quadrants of the tubular over a two-year period. Strains were to be continuously sampled and recorded at a sufficient rate to accurately capture the peak values. Additionally, accelerations in three axes and wave height derived from hydrostatic pressure were to be monitored. The whole package had to be installed by ROV, and needed to deliver periodic data downloads throughout the life of the instrument. At the heart of the instrument is a set of environmentally packaged strain transfer mechanisms. These engage with the surface of the structure through spring-loaded actuators that are deployed by the ROV during installation. They allow strains to be measured directly by the instrument’s data acquisition system. These mechanisms were validated and calibrated using specially designed test equipment. Prior to deployment, the complete instrument, including its magnetic attachment system, was tested on a sample pipe section. All instrument subsystems were proven, including a high-speed optical data link for through-water transfer of recorded data, and an acoustic communication system for status updates. The instrument was successfully deployed subsea in April 2021, and one complete and continuous data set has already been retrieved. The data shows that the instrumentation is able to accurately capture a range © 2023, Offshore Technology Conference.
