Fleet-scale evidence and a testable coupled mechanism for radial-raceway spalling in an offshore 5 MW three-row roller main bearing

Abstract. Standard rating-life calculations predicted sufficient durability, yet premature radial outer-raceway spalling recurred in a 203-turbine offshore 5/6.2 MW fleet. By 11 August 2026, eight bearings had been removed after 26–53 months and two additional units were stopped awaiting replacement. All eight dismantled bearings exhibited damage within the stationary 3–9 o’clock lower radial load zone, whereas axial-row damage was limited. Fleet-scale teardown evidence for offshore three-row cylindrical-roller main bearings is rarely reported in the peer-reviewed wind-energy literature. The present dataset therefore provides an unusual opportunity to establish a repeatable circumferential damage pattern and to organize competing mechanisms within an explicit evidence hierarchy. Roller-end breakage, gearbox-side cage contact, a measured full-power inner-to-outer-ring temperature difference of about 12 °C, and partly obstructed grease inlets provide mutually consistent, but individually non-unique, evidence for load redistribution and lubrication-access effects. Grease screening of 198 turbines identified Cu ≥ 5000 ppm in 21 units (10.61 %), Fe ≥ 5000 ppm in 10 units (5.05 %), and the combined condition Cu > 10000 ppm and Fe > 5000 ppm in five units (2.53 %); these thresholds are used only as project-specific risk flags, not as transferable diagnostic cut-offs. A nominal Hertz calculation gives a baseline maximum roller load of about 109 kN and contact pressure of 0.997 GPa. System FEA predicts 0.067° maximum raceway misalignment; across the 100 mm roller length this corresponds to an end-to-end geometric offset of about 0.12 mm, providing an order-of-magnitude structural basis for testing a moderate 20–50 % edge-pressure amplification range (Kedge = 1.20–1.50), while not constituting a calibration of Kedge. The combined evidence is most consistent with accelerated rolling contact fatigue in which the fixed load zone controls damage location, while thermo-structural load redistribution, restricted grease replenishment, debris and local case-condition variability can reduce fatigue margin. The principal contribution is therefore fleet-scale field evidence and an evidence-ranked, falsifiable engineering framework for offshore main-bearing failure assessment, rather than identification of a unique root cause or a new life-prediction equation.