Prediction of Extreme Tensions in Mooring Lines of a Floating Offshore Wind Turbine in a 100-Year Storm

Abstract

Floating offshore wind turbines (FOWTs) contribute to an emerging green energy technology, by exploiting higher and consistent wind speeds above the ocean. There are several challenges facing the design of mooring system of FOWTs, including installation costs, stability of light-weight minimalistic platforms, and shallow depths (50–300m). The extreme tension in mooring lines of a light displacement platform in shallow-water is dominated by snap loads. This is because light pre-tension requirements in the line may be insufficient to prevent the mooring lines from being exposed to wave motion induced slack and shock events. In this paper, we present a comparative analysis of a semi-submersible based FOWT exposed to a 100-year storm condition, based on model test data and numerical simulations of well-known industry standard software. The data was obtained from a 1/50th-scale FOWT with the wind turbine modeled after the NREL 5MW wind turbine. The software, OrcaFlex, was used for numerical simulations of the mooring system. NREL’s FAST software was coupled to OrcaFlex to obtain aerodynamic loads along with hydrodynamic load for FOWT analyses. The numerical simulation of the moored FOWT in a 3-hour storm was executed in both the frequency-domain and the time-domain to determine the dynamic behavior of the platform and mooring system, respectively. Snap–type impact events were observed in both test data and numerical simulation. Tension maxima were fitted into extreme value distributions and comparisons are made between simulated and measured data. It is seen that snap events follow a different exceedance probability distribution compared to the cycle-to-cycle tension maxima.

Digital twins of the mooring line tension for floating offshore wind turbines to improve monitoring, lifespan, and safety

“…Fatigue damage of MoLs has been investigated by (Li et al 2018) using an Artificial Neural Network (ANN), where they observe the significant influence of the environmental conditions (waves, winds, and currents) on the tension bands. Additionally, (Hsu et al 2015(Hsu et al , 2017 demonstrate the potential benefits of using DDMs for FOWT MoLs when scaling health monitoring tools site-wide.…”

“…In the literature, it is possible to find a large body of work addressing MoL failures (Ma et al 2013;Davidson and Ringwood 2017;Brown et al 2005;Al-Solihat and Nahon 2018;Aranha and Pinto 2001;Bhinder et al 2015;Borg et al 2014;Hsu et al 2014Hsu et al , 2015Jin and Kim 2018;Qiao et al 2020;Maroju et al 2013;de Pina et al 2013;Prislin and Maroju 2017;Jaiswal and Ruskin 2019;Li et al 2018). The primary mechanical failure mechanisms in mooring systems are extreme load and fatigue, both of which are functions of the axial tension (Davidson and Ringwood 2017).…”

“…In particular, while at static level, a quasi-static model can be in very good agreement with multibody or finite-element models. However, when considering a stochastic load condition (e.g., random waves from a wave spectrum), the dynamic tension oscillation may be overestimated, since the inertia of the lines and, more importantly, the hydrodynamic viscous damping loads acting on the mooring lines are not captured (Hall et al 2014). The multibody mooring model (also called the "lumped mass approach") is able to capture the effects of the line inertial loads and the hydrodynamic viscous forces acting on the mooring lines, and it is also suitable to estimate the extreme and oscillatory tension loads at a preliminary design stage, since it is characterised by a sufficient accuracy but still within an acceptable computational cost (Cevasco et al 2018).…”

“…The multibody mooring model (also called the "lumped mass approach") is able to capture the effects of the line inertial loads and the hydrodynamic viscous forces acting on the mooring lines, and it is also suitable to estimate the extreme and oscillatory tension loads at a preliminary design stage, since it is characterised by a sufficient accuracy but still within an acceptable computational cost (Cevasco et al 2018). The finite-element method is suggested for very accurate analyses, but this is usually reserved for a limited set of load cases/simulations, typically at an advanced design stage, due to its substantially higher computational costs (Cevasco et al 2018;Hall et al 2014).…”

Improving Extreme Anchor Tension Prediction of a 10-MW Floating Semi-Submersible Type Wind Turbine, Using Highly Correlated Surge Motion Record

“…Determining the extreme values of the mooring system requires effective methods to deal with the extreme analysis problem. Hsu et al (Hsu et al, 2015) presented a comparative analysis in predicting the extreme mooring line tensions of an FWT exposed to a 100-years storm condition between model test and numerical simulations. The importance of the snap events to the mooring line tensions was investigated based on the exceedance probability analysis, and results showed that normalised snap loads tend to a constant 3, which can contribute to determining the safety factor of mooring lines.…”