Session: 09-01-11 Wind Energy: Cables 1

Paper Number: 124923

124923 - Investigation Into Sloshing-Induced Fatigue of Inter-Array Cables Inside a Fixed Offshore Wind Turbine Monopile Foundation 

There has been a worldwide action to combat effects of global warming in the past few decades. This has led to a great focus towards development of renewable sources of energy. Wind energy has emerged as one of the forerunners with onshore technology being matured over many decades. Offshore wind offers greater potential both in terms of higher wind speeds and overall availability, and, thus, several offshore fixed wind turbine (FWT) farms have come into operation over the last decade and there is great effort to reduce overall costs of electricity production to make it competitive. This has led to larger turbines (for better efficiency) and larger supporting structures. FOWTs are generally installed on a cylindrical monopile (MP) structure. With MPs reaching diameters of 9-12 m and FOWTs being installed further offshore, water motion of the internal free-surface (referred to as sloshing) inside the monopile can be relatively strong.

The aim of the present work is to perform a 3D sloshing analysis inside the monopile foundation (for a proposed wind farm) and study its effect on fatigue of inter-array cables (IACs). The sloshing analysis is accomplished using Computational Fluid Dynamics (CFD) for Reynold Averaged Navier Stokes (RANS) equations without accounting for the internal structures inside the monopile. Sloshing resonance periods are calculated using a theoretical formulation for cylindrical tanks. CFD simulations are performed for forced oscillatory motion of the monopile in surge direction at few selected sloshing resonance periods. It is then determined if sloshing is excited near the theoretically calculated resonance periods.  Quantities such as pressure and velocity on the internal free surface are estimated to understand effect of sloshing on internal structures, for example, IACs. Further, it is analysed if the sloshing-induced velocities can be important for fatigue of IACs. This is accomplished in a potential-flow based time-domain solver (Orcaflex) using a simplified setup for the IAC (excluding the monopile for simplicity). IAC is excited using regular waves (wave periods equal to forcing periods in CFD studies) with amplitudes such that the free-surface velocities are equal to sloshing-induced velocities. The IAC cross-section is modelled in detail using tailormade cable cross-section analysis tool UFLEX2D for extracting accurate stresses under varies bending and curvature combination from Orcaflex analysis at critical components. Accumulated fatigue is then calculated for the IACs over the operation period using in-house tool FFU with appropriate SN curves.  Cable fatigue contributed by sloshing is presented as final result.

Presenting Author: Henan Li Kongsberg Maritime AS

Presenting Author Biography: Principal Engineer at Kongsberg Maritime with 10+ years experience in SURF analysis and Masters specialization in Umbilicals

Authors:

Mohd Atif Siddiqui Kongsberg Maritime AS
Henan Li Kongsberg Maritime AS
Knut-Aril Farnes Equinor ASA
Steinar Midttveit Equinor ASA