Subsea assets are subject to both low-frequency and high-frequency motions, driven by environmental loading, operational conditions and system dynamics. It is important to understand and to be able to measure both types in order to understand their impact on your subsea assets.

Low and slow…
Low-frequency motions are typically large-amplitude, slow movements caused by factors such as vessel motions, mooring and riser dynamics, thermal expansion and contraction of pipelines, seabed settlement or long-period currents. Examples could include pipeline walking, lateral buckling, slow cyclic riser movement at touchdown zones, wellhead displacement and gradual movement of pipeline end terminations (PLETs) or structures under sustained loading.

These can be measured using either USBL or LBL positioning techniques as the motions are large allowing for time-of-flight measurements. Alternatively, displacement sensors like linear variable differential transformers are designed to be coupled to the structure.

Fast and furious…
High-frequency motions, by contrast, are smaller-amplitude but rapid vibrations caused by turbulent flow, vortex-induced vibration (VIV), wave-induced excitation, rotating equipment. Examples include VIV on pipeline free spans, flow-induced vibration (FIV) in jumpers and short-period dynamic response on risers or jumper connections.

These are typically measured using an inertial measurement unit (IMU) and in some cases a strain measurement to correlate motion data with a stress calculation leading to a fatigue estimation.

The importance of understanding both
Understanding both motion regimes is critical for subsea integrity monitoring, as low-frequency motion is often associated with fatigue accumulation, while high-frequency motion can drive accelerated fatigue damage and localised wear.

You can find out more about subsea integrity monitoring in another of our knowledge base articles here.