Research on deformation evolution mechanism of monopile-seabed system in offshore wind farm based on UWFBG

• UWFBG enables depth-resolved monitoring of monopile strains • Deformation evolution of the monopile–seabed system is classified into four stages • Early-warning curve is established by combining strain data with soil parameters The deformation behavior of offshore wind turbine monopiles under long-term environmental loading remains insufficiently understood, largely due to the absence of continuous in-situ measurements capable of resolving depth-dependent pile–soil interaction. This study addresses this gap by investigating whether distributed strain monitoring can capture the progressive deformation mechanisms and instability characteristics of monopile–seabed systems. Long-term in-situ field monitoring was conducted on two full-scale offshore wind turbine monopiles in an offshore wind farm in the Yellow Sea using ultra-weak fiber Bragg grating (UWFBG). The results demonstrate that the monopile–seabed system exhibits a systematic and staged deformation evolution, which can be consistently classified into four representative stages, reflecting a progressive transition in pile–soil interaction regimes. In particular, the gradual rightward migration of the strain–depth profile within the embedded section is identified as a distinctive precursor to pull-out–type instability, providing a deformation-based indicator that complements conventional rotation-based criteria. These findings provide new field-based observational evidence for staged monopile deformation and contribute to a more mechanistically grounded evaluation of monopile stability under long-term environmental loading.