Dynamic Response of a Large-Diameter Monopile Considering 35-Hour Storm Conditions

Abstract

As a part of the assessment of foundation resistance for monopiles, several offshore wind standards prescribe symmetric 35-hour (or 42-hour) storm sequences in terms of wind speed and significant wave height. The temporal evolution of the peak period is not specified explicitly in the standards, despite the fact that large monopile wind turbines are sensitive to the wave period. In the present work, the storm sequences according to the standards are first compared to hindcast data for intermediate water depth locations in the North Sea. An alternative storm sequence is proposed based on the hindcast data, and possible values of the peak period evolution are proposed for the standard models. The responses of a 10 MW monopile wind turbine are then computed for both the standard and proposed sequences using a time domain aero-hydro-servo-elastic code coupled to a macro element model for the soil-structure interaction. The resulting mudline load cycles are then compared for the different storm sequences.

Cyclic Capacity of Monopiles in Sand under Partially Drained Conditions: A Numerical Approach

“…A one-hour peak history (including the maximum loads) during a 35-hour storm load sequence at a water depth of 30 m in the North Sea (Bachynski et al 2019) is considered. The significant wave height, peak wave period and wind speed at the location are based on hind-cast data.…”

“…The main purpose of this paper is to demonstrate the effect of cyclic loading on the capacity of a large diameter monopile foundation in a uniform dense sand under partially drained and undrained conditions for the DTU 10-MN reference wind turbine in the North Sea, subjected to a representative peak storm history (Bachynski et al 2019). The lateral displacement of the monopile foundation (used to check monopile capacity in this work) is calculated by PDCAM.…”

Specification Document for OC6 Phase II: Verification of an Advanced Soil-Structure Interaction Model for Offshore Wind Turbines

“…The output time series should have a length of 90 s (corresponding to 10 wave cycles). LC 4.2 studies the response of the system for a Pierson-Moskovitz wave spectrum with a significant wave height ( ) of 1.25 m and peak-spectral wave period ( ) of 5.5 s. LC 4.3 is representative of a storm condition in the North Sea for a 30-m water depth site (Bachynski et al 2019). LC 4.3 is based on a JONSWAP wave spectrum with = 7.60 m, = 8.6 s and peak-enhancement factor (γ) equal to 5 according to the IEC (2009).…”