Scour detection using fiber optical strain sensing technology

Activiteit: Talk or presentation at a conference

Description

Most offshore wind turbines (OWTs) rely on the monopile foundation concept, and scour formation around the monopile is a well-known issue. Scour has a negative impact on the entire offshore wind turbine system by reducing natural frequencies, which can increase resonant behavior and thus ultimately reduce the life expectancy. Given its impact, the industry monitors the formation of scour either by executing recurring surveys or indirectly by monitoring changes in natural frequencies. The former offers only intermittent observations, while the latter offers a continuous assessment but lacks further information regarding the size of the actual scour hole.
As part of the BOPTIC project, this contribution discusses the feasibility of an alternative strategy for detecting scour by monitoring changes in subsoil conditions. The monitoring strategy investigated here is based on strain sensing technology, specifically fiber Bragg grating (FBG) and optical frequency domain reflectometry (OFDR). FBGs have several advantages over conventional strain gauges, including a good resistance to corrosion and faster installation time, and have been successfully used on a multitude of pile instrumentation projects. OFDR, on the other hand, is a novel technology that has gained traction owing to its high sensitivity and spatial resolution. While FBGs only offer measurements at discrete points along a fiber line, OFDR has the potential for a continuous measurement along the fiber line. Nonetheless, its applicability to the offshore wind industry has yet to be investigated.
An experimental campaign was carried out on a small-scale model of a driven monopile foundation in sand to see if strain measurements recorded by optical sensors could detect scour formation. The monopile was instrumented with FBGs and a continuous fiber for OFDR above and below ground level, a load cell, temperature sensors, and displacement sensors. The monopile bending moment profile was inferred from the measured strains using the Euler-Bernoulli theory after performing monotonic lateral tests for various scenarios with varying local scour depths. Moments calculated from optical sensors at ground level were very close to the applied moment measured by the load cell. Furthermore, measurements were compared to numerical results and found to be in very good agreement. Experimental results showed that strain measurements are sensitive to scouring even in the case of shallow scour hole formation (0.2D). The potential benefits of the high spatial resolution provided by OFDR technology are also highlighted through a real-world FBG-instrumented monopile foundation in a Belgian wind farm.
Periode24 mei 2023
EvenementstitelWESC 2023
EvenementstypeConference
LocatieGlasgow, United Kingdom
Mate van erkenningInternational