  Floating Offshore Wind Turbines (FOWTs) are more prone to suffer from faults
and failures than bottom-fixed counterparts due to the severe wind and wave
loads typical of deep water sites. In particular, mooring line faults may lead
to unacceptably high operation and maintenance costs due to the limited
accessibility of FOWTs. Detecting the mooring line faults is therefore
critical, but the application of Fault Detection (FD) techniques has not been
investigated yet. In this paper, an FD scheme based on a wave-excited linear
model is developed to detect in a reliable way critical mooring line faults
occurring at the fairlead and anchor ends. To reach the goal, a linear model of
the FOWT is obtained by approximating the wave radiation and incident wave
forces. Based on this model, an observer is built to predict the rigid rotor
and platform dynamics. The FD scheme is thus implemented by comparing the
Mahalanobis Distance of the observer prediction error against a probabilistic
detection threshold. Numerical simulations in some selected fault scenarios
show that the wave-excited linear model can predict the FOWT dynamics with good
accuracy. Based on this, the FD scheme capabilities are demonstrated, showing
that it is able to effectively detect two critical mooring line faults.
