A major concern for the driving of monopiles is structural fatigue introduced by dynamic loading during the installation of the monopile. If the energy loads needed to drive the pile into the soil are above a certain threshold they can introduce fatigue to the steel structure. metal fatigue can dramatically decrease the lifespan of a foundation and change the economy of offshore wind investments. This article discusses the impact of two of the most common methods used for the installation of monopiles - vibratory and impact driving - on pile fatigue.
 
The risk associated with pile fatigue is considered so critical to the expected life time of the pile that it is mitigated by increasing the monopile’s wall thickness. Structural engineers ensure maximum stress values that cause such damage are below the pile’s ultimate tensile stress limit or the yield stress limit modifying wall thickness to ensure loading capacity. Wall thickness in return determines pile weight which in turn decides crane size hammer size vessel size material cost and total project cost.
 
Impact hammering
A single blow of an impact hammer is said to age a pile by one week. Therefore design institutes typically over-spec the pile’s wall thickness to ensure large safety margins to resist fatigue damage caused by dynamic loads due to impact driving. The energy transmission during impact hammering is calculated by counting blows and tallying up the energy of each blow. Driving fatigue damage is assessed by using best estimates of driving resistance coupled with wave equation analyses to calculate blow counts and stress ranges respectively.