Papers & Projects


OMAE2014-23587Snap Loads on Mooring Lines of a Floating Offshore Wind Turbine Structure

Authors: Wei-Ting Hsu (, Krish Thiagarajan, Matthew Hall, Michael MacNicoll, Richard Akers.

There are a number of design challenges facing the mooring systems of floating offshore wind turbine (FOWT) platforms in an offshore environment. Some unique aspects of the FOWT industry should be considered when examining applicability of established offshore mooring practices. Important among these are: economy and cost effectiveness; light weight minimal platforms; and water depths ranging from 50 – 300 m. A lighter displacement platform in shallow water, supported by lines with light to moderate pre-tension can result in a higher probability of slack line events and hence snap loads during re-engagement. Such loads can result in shock on the line material and considerably reduce the fatigue life. Such events have the potential to occur in various sea states, and not necessarily limited to extreme conditions. These conditions will be dependent on structure resonant motions, which are influenced by wind loads and moments, wave conditions and mooring line properties. Model tests of typical concepts for FOWT reported in literature have shown occasional slack line episodes. This paper is a review of literature on snap load occurrence in marine applications, including lifting and lowering operations, ROV and diving bell operations. This paper presents a case study of a FOWT. Special focus is on mooring systems which are affected by impact load conditions. Criteria are reviewed and consequences are documented.

OMAE2014-23638Viscous Damping Effects on Heading Stability of Turret-Moored Ships“,

Authors: Razieh Zangeneh (, Krish Thiagarajan, Raul Urbina, Zhigang Tian.

Tankers used for offshore oil production and storage are kept in station by turret mooring systems, enabling the vessels to weathervane in the direction of the dominant environmental loads. These passive weathervaning systems have been observed in model experiments to be ineffective in swell-dominated long wave conditions. Over a range of wavelengths from 0.6 < ?/L < 2 (L – ship length), the vessel was observed to lose heading control in head sea condition, due to a pitchfork bifurcation that is initiated at a critical wavelength of 0.73L. A notable feature of poor heading stability is the existence of a stable equilibrium at a large heading angle (50 – 60) with respect to the direction of oncoming waves. With lack of heading control, the ship motions, principally roll, can increase thus affecting onboard operations. Time domain analysis conducted with no added viscous damping shows reasonable agreement with experimental data for the final heading angle. Further numerical tests reported in a previous paper by the authors showed that small to moderate viscous damping in sway and yaw did not alter the finalTime domain analysis conducted with no added viscous damping shows reasonable agreement with experimental data for the final heading angle. Further numerical tests reported in a previous paper by the authors showed that small to moderate viscous damping in sway and yaw did not alter the final heading, while the role played by viscous damping in other modes (heave, roll and pitch) needed further investigation. This paper reports on a parametric study on the heading stability of a turret moored tanker using time domain tools. Viscous damping is systematically varied in different modes of motion and its effect on final heading equilibrium is assessed. It is shown that effects of pitch damping are stronger than heave or roll, and can eliminate heading instability altogether.

OMAE2014-24497  Performance Specifications for Real-Time Hybrid Testing of 1:50-Scale Floating Wind Turbine Models

Authors: Matthew Hall (, Javier Moreno, Krish Thiagarajan

This paper presents performance requirements for a real-time hybrid testing system to be suitable for scale-model floating wind turbine experiments. In the wave basin, real-time hybrid testing could be used to replace the model wind turbine with an actuation mechanism, driven by a wind turbine simulation running in parallel with, and reacting to, the experiment. The actuation mechanism, attached to the floating platform, would provide the full range of forces normally provided by the model wind turbine. This arrangement could resolve scaling incompatibilities that currently challenge scale-model floating wind turbine experiments.

In this paper, published experimental results and a collection of full-scale simulations are used to determine what performance specifications such a system would need to meet. First, an analysis of full-scale numerical simulations and published 1:50-scale experimental results is presented. This analysis indicates the required operating envelope of the actuation system in terms of displacements, velocities, accelerations, and forces. Next, a sensitivity study using a customization of the floating wind turbine simulator FAST is described. Errors in the coupling between the wind turbine and the floating platform are used to represent the various inaccuracies and delays that could be introduced by a real-time hybrid testing system. Results of this sensitivity study indicate the requirements – in terms of motion-tracking accuracy, force actuation accuracy, and system latency – for maintaining an acceptable level of accuracy in 1:50-scale floating wind turbine experiments using real-time hybrid testing.

Research Paper: J. Offshore Mech. Arct. Eng. 137(1) “Nonlinear Pitch Decay Motion of a Floating Offshore Wind Turbine Structure”

Authors: Krish Thiagarajan, Raul Urbina, W. Hsu

Model tests were conducted on three generic floating wind turbine systems in 2011 and reported in a series of papers at the 31st Ocean, Offshore, and Arctic Engineering Conference in 2012. These tests were conducted at the MARIN facility in The Netherlands, by a consortium of universities, government research organizations, and industry. As part of the testing program, decay tests in platform pitch were conducted with and without wind forcing. It was found that for spar and semisubmersible type structures, resonant pitch motion was damped due to wind in storm sea conditions. The nonlinear decay motion of a floating wind turbine platform is modeled using a one degree-of-freedom nonlinear oscillation equation about a mean offset angle. Attention is paid to the turbine thrust coefficient and its variability with respect to oncoming flow speed, which in turn is affected by the structure pitch motion. The equation of motion reveals that the mean offset position has an important role in the stiffness, damping, and consequently the natural period of pitch motion. Several important dimensionless parameters are introduced. The paper discusses a simple thrust model for an offshore wind turbine (OWT) based on rudiments of blade element theory. Using the simplified thrust coefficient formulation, the increase in platform pitch damping due to wind is formulated. Experimental data reported from prior tests described above show good agreement with the theoretical model.