Experiments with a weakly damped monopile, either fixed or free to oscillate, exposed to irregular waves in deep water, obtain the wave-exciting moment and motion response. The nonlinearity and peak wavenumber cover the ranges:
$\unicode[STIX]{x1D716}_{P}\sim 0.10{-}0.14$
and
$k_{P}r\sim 0.09{-}0.14$
where
$\unicode[STIX]{x1D716}_{P}=0.5H_{S}k_{P}$
is an estimate of the spectral wave slope,
$H_{S}$
the significant wave height,
$k_{P}$
the peak wavenumber and
$r$
the cylinder radius. The response and its statistics, expressed in terms of the exceedance probability, are discussed as a function of the resonance frequency,
$\unicode[STIX]{x1D714}_{0}$
in the range
$\unicode[STIX]{x1D714}_{0}\sim 3{-}5$
times the spectral peak frequency,
$\unicode[STIX]{x1D714}_{P}$
. For small wave slope, long waves and
$\unicode[STIX]{x1D714}_{0}/\unicode[STIX]{x1D714}_{P}=3$
, the nonlinear response deviates only very little from its linear counterpart. However, the nonlinearity becomes important for increasing wave slope, wavenumber and resonance frequency ratio. The extreme response events are found in a region where the Keulegan–Carpenter number exceeds
$KC>5$
, indicating the importance of possible flow separation effects. A similar region is also covered by a Froude number exceeding
$Fr>0.4$
, pointing to surface gravity wave effects at the scale of the cylinder diameter. Regarding contributions to the higher harmonic forces, different wave load mechanisms are identified, including: (i) wave-exciting inertia forces, a function of the fluid acceleration; (ii) wave slamming due to both non-breaking and breaking wave events; (iii) a secondary load cycle; and (iv) possible drag forces, a function of the fluid velocity. Also, history effects due to the inertia of the moving pile, contribute to the large response events. The ensemble means of the third, fourth and fifth harmonic wave-exciting force components extracted from the irregular wave results are compared to the third harmonic FNV (Faltinsen, Newman and Vinje) theory as well as other available experiments and calculations. The present irregular wave measurements generalize results obtained in deep water regular waves.