Approach to Include Load Sequence Effects in the Design of an Offshore Wind Turbine Substructure

Dragt, R. C. (Netherlands Institute for Applied Scientific Research (TNO)) | Allaix, D. L. (Netherlands Institute for Applied Scientific Research (TNO), Eindhoven University of Technology) | Maljaars, J. (Keppel Verolme BV) | Tuitman, J. T. (Keppel Verolme BV) | Salman, Y. | Otheguy, M.



Fatigue is one of the main design drivers for offshore wind substructures. Using Fracture Mechanics methods, load sequence effects such as crack growth retardation due to large load peaks can be included in the fatigue damage estimation. Due to the sequence dependency, a method is required that represents the sequences of loads in the design or maintenance procedures.

This paper presents a methodology to deal with this challenge. First, a framework is presented for coupling between the design load cases and the Fracture Mechanics methods, resulting into the requirements for loads and load sequences. Second, a 2-stage Markov Chain Monte Carlo model is presented which is able to create realistic loading sequences based on measurement data. The method is elaborated for fluctuating wind loads.


One of the main design drivers for Offshore Wind Turbine (OWT) substructures is fatigue. Current standards (e.g.; DNVGL, 2016; IEC, 2009) specify a wide range of operational conditions, such as normal operation, parked condition and fault conditions, and the environmental conditions (combinations of wind, wave and current conditions) that are to be included in the fatigue assessment. Every single combination is used as input for a time domain simulation, which results in a stress history at given (hot)spots in the structure. Prescribed Stress Concentration Factors (SCF) are used to account for structural details. Standards prescribe that a 1-hour period is simulated (by either simulating 6 times a 10-minute realization or a 1-hour realization) per load combination. The total analysis requires several thousands of individual 1-hour long time-domain simulations, each typically containing several thousands of stress cycles.

The stress history is used to estimate the fatigue damage in a structural detail during its intended lifetime. Fatigue damage is referred to as the utilization of the total fatigue capacity of a structural detail expressed in terms of life. For every stress history, the number of occurring cycles for each stress range are counted using the Rainflow Counting method. The appropriate SN-curve is selected from the design standard and used to determine the number of cycles to failure for each stress range. The damage contribution per range of stress cycles is defined as the ratio between the occurring number of cycles and the number of cycles per stress range. The damage contributions of all stress cycles are summed in agreement with the damage accumulation rule of Palmgren-Miner in order to arrive at the total fatigue damage. The result is multiplied by a Design Fatigue Factor (DFF) in order to arrive at a certain probability of failure, which amongst others accounts for difficulties encountered during inspection or repair of a specific detail and for the risk of structural failure (DNVGL, 2016). The final result is the estimated design fatigue damage for this particular hotspot, for a particular environmental and operational condition.