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

Paper Number: 124924

124924 - Fatigue Assessment and Resonance Effects in an Inter Array Cable Due to Global Dynamic Deflections in a Bottom Fixed Monopile Wind Turbine Foundation 

Within Offshore wind the bottom fixed monopile (MP) WTG foundation structure has been and still is the preferred solution for many operators where the soil conditions and water depths are suitable for the MP foundation concept. The industry standard solution for the inter array cables (IAC) is to pull the cable into the MP a few meters above seabed and keep it free hanging on the inside of the monopile until a hangoff level some 5-10 m above sea level.
In recent years the size of the turbines has increased significantly and at the same time the monopile foundation has been used in deeper waters. This increases the free hanging span of the IAC and also increases the deflection of the MP, causing forced motions into the IAC and increased potential for dynamic response and fatigue accumulation in the IAC is present.
The present work models the IAC hanging through MP using a potential-flow based time-domain solver (OrcaFlex) with imposed motion at cable hang-off location. The IAC is hanging from MP cable hang-off platform and via MP entry hole. The IAC responses (tension and curvature time series) are extracted for further computation of component stress time series. A relatively large load case matrix (2000+) with irregular wave approach is considered to reduce conservatism. The imposed motions are extracted from coupled modelling of MP and turbine structure considering both environmental loads and turbine motions. The IAC cross-section is further modelled in detail considering friction between components using a tailor-made cross-section analysis tool (Uflex2D) for extracting accurate stresses under various tension and curvature combinations. The computed stresses are non-linear and include the friction stick-slip effect.
Finally, cable fatigue is calculated using an in-house tool, Flexstress for Umbilicals (FFU), first by combining the irregular tension and curvature time series from the time-domain solver with the stress vs. curvature relationships from local cross-section analysis in an effective way to compute total stress time series for each component. FFU further performs Rainflow counting and calculates fatigue damage based on appropriate SN curves.
During fatigue assessment, it is also observed that IAC oscillates with high frequency which is considered as its mechanical resonance due to turbine motions. The cable resonance period coincides with one of the energy peaks of the input MP and turbine motion. The resonance effect is further investigated by this work and mitigation methods are discussed.

Presenting Author: HENAN LI Kongsberg Maritime AS

Presenting Author Biography: Henan Li is principal engineer at Kongsberg Maritime AS with 10+ years experience on various marine operation studies and SURF analyses. She has extensive experience in design and fatigue assessment for power cables and umbilicals.

Authors:

Henan LI Kongsberg Maritime AS
Carsten Arnholm Kongsberg Maritime AS
Mohd Atif Siddiqui Kongsberg Maritime AS
Knut-Aril Farnes Equinor ASA
Steinar Midttveit Equinor ASA
Louise Ankerstjerne Rolland Equinor ASA
Knut Nordheim Equinor ASA