Finn Gunnar Nielsen

Integrated Dynamic Analysis of Floating Offshore Wind Turbines

Two different simulation models for integrated dynamic analysis of floating offshore wind turbines are described and compared with model scale experiments for the Hywind concept for floating offshore wind turbines. A variety of both environmental conditions and wind turbine control schemes are tested. A maximum power control strategy is applied for wind velocities below the rated wind speed for the wind turbine, while a constant power control strategy is achieved by controlling the rotor blade pitch for wind velocities above rated wind speed. Conventional rotor blade pitch control for wind velocities above rated wind speed introduces negative damping of the tower motion. This results in excitation of the natural frequency in pitch for the tower and may lead to unacceptable tower motions. Active damping of the undesirable tower motions is obtained by an additional pitch control algorithm based on measurement of the tower velocity.Copyright

Wave synchronizing crane control during water entry in offshore moonpool operations - experimental results

A new strategy for active control in heavy-lift offshore crane operations is suggested by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimizing variations in the relative vertical velocity between payload and water using a wave-amplitude measurement. Wave synchronization is combined with conventional active heave compensation to obtain accurate control. Experimental results using a scale model of a semi-submerged vessel with a moonpool shows that wave synchronization leads to significant improvements in performance. Depending on the sea state and payload, the results indicate that the reduction in the standard deviation of the wire tension may be up to 50%.

Importance of Control Strategies on Fatigue Life of Floating Wind Turbines

Exploitation of wind energy at deep-waters locations requires floating wind turbine foundations. Several floating wind turbine foundation concepts are reported in the literature, and a common challenge is to make a low cost foundation with acceptable motion characteristics. In order to analyze the fatigue life of floating offshore wind turbines, the coupled action of wind, waves, current and blade pitch control strategy must be considered. State-of-the-art computer programs for motion analysis of moored offshore bodies, Simo-Riflex from Sintef Marintek, are coupled to a state-of-the-art aero-elastic computer program for wind turbines, Hawc2 from Riso National Laboratory. The wave loads on the body may include wave diffraction and radiation loads as well as viscous forces. The mooring lines are modelled using cable finite elements with inertia and drag forces. The wind load on the rotor is based on common rotor aerodynamics including corrections for skew inflow and relative motion caused by large displacement and large tilt and yaw rotations of the rotor. Conventional wind turbine control strategies lead to wind-induced loads that may amplify or damp the motions of the floating wind turbine. The first case is a result of the blade pitch control strategy above rated wind speed for the wind turbine, and can result in large resonant motions that will reduce the fatigue life of the floating wind turbine significantly. The latter case implies energy extraction from the waves. This paper addresses the importance of control strategies on fatigue life for a given floating offshore wind turbine. A fatigue life time comparison between a conventional blade pitch control strategy and an estimator based blade pitch control strategy show that the fatigue life of floating offshore wind turbines can be significantly increased by use of alternative blade pitch control strategies.Copyright

Comparative study on airgap under floating platforms and run-up along platform columns

Present engineering practice for computing airgap (i.e. the clearance between waves and deck) on floating platforms relies on very simplified approaches. However, recently several new codes for computation of nonlinear wave diffraction have been developed. To obtain a state of the art of the capabilities of these methods, test cases related to airgap under a floating platform and run-up along platform columns have been defined. Several organizations have been invited to apply their numerical tools to compute airgap and run-up for the defined test cases. The results from the comparisons are summarized and compared to experimental results. The study has been part of the work of ISSC2000 Committee 1.2.

Assessment of VIV Induced Fatigue in Long Free Spanning Pipelines

Current design practice for free spanning pipelines is to allow free spans as long as the integrity with respect to potential failure modes are checked and found acceptable. The case study for Ormen Lange (OL) pipelines planned in the deep waters of the Norwegian Sea is associated with a large number of very long free spans, which requires significant intervention work if based on the state-of-practice acceptance criteria. The design philosophy of the state-of-the-art design code DNV-RP-F105 “Free Spanning Pipelines” is applied in combination with the experience gained from dedicated OL model tests. Updated project specific design guidelines with multi-mode behavior, typical for OL long free spans, is taken into account and an updated Cross-Flow (CF) response model has been developed. An approach to select the In-Line (IL) mode excited by CF response is suggested. Methods for combining stresses from multiple active modes have been proposed and tested, for both IL and CF Vortex Induced Vibrations (VIV). Fatigue analysis has also been performed on the stress series measured in the model tests and this has been successfully used to verify and validate the presented computational procedure. Uncertainty in the model test based fatigue estimates has been assessed and sensitivity studies have been carried out. Reasons for deviations and potential problem areas for long free spanning pipelines have been identified.Copyright

Wave synchronizing crane control during water entry in offshore moonpool operations

A new strategy for active control in heavy-lift offshore crane operations is suggested, by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimization of variations in the relative vertical velocity between payload and water using a wave amplitude measurement. Wave synchronization is combined with conventional heave compensation to obtain accurate control. Experimental results using a scale model of a semi-submerged vessel with a crane and moonpool shows that wave synchronization leads to significant improvements in performance. Depending on the sea state and payload, the results indicate that the reduction in the standard deviation of the wire tension may be up to 50%.

VIV Response of Long Free Spanning Pipelines

Vortex induced vibration (VIV) of free spanning pipelines in current is considered. In standard VIV estimation, one mode of oscillation is considered only. Increasing the length of the span, several modal shapes may be excited. Further, due to the sag effect of a long free span, the dynamic properties in vertical and horizontal direction of the span are different. This causes a much more complex VIV response pattern for long free spans than for short spans. The observed VIV response of long free spans during model testing is discussed. Hypotheses that may explain the observed behaviour are presented. Also a format of new design principles for long free spans is outlined.Copyright

Coupling Between In-Line and Transverse VIV Response

Most present-day programs for engineering prediction of VIV response are based on experimental lift and mass coefficients derived from 2D tests, where the cylinder has been constrained in in-line direction. The present paper describes a preliminary investigation into the coupling between in-line and transverse VIV response. A simple pendulum test set-up was used to study how the in-line response influenced the transverse one. For reduced velocities above 5, when the in-line oscillation frequency was exactly twice the transverse frequency, it was found that the transverse response was significantly reduced when allowing the cylinder to oscillate in both directions simultaneously, compared to the case when it was constrained in-line.Copyright

Non-linear wave forces on a fixed vertical cylinder dueto the sum frequency of waves in irregular seas

A method for calculating second order forces on a vertical, large diameter circular cylinder is reported.Force spectra at sumfrequencies and difference frequencies as well as the first order force spectrum are calculated as a result of irregular waves generated from a Pierson-Moskowitz wave spectrum. Non-viscous fluid is assumed. Forces at sumfrequencies are compared to first order forces with focus on the wave period rangefrom 3 to 5 s, in which many deep water platforms have their first natural period. The calculations are also compared to model scale measurements performed in a wave flume at theNorwegian Hydrodynamic Laboratories in Trondheim.

MEAN DRIFT FORCES ON A SLOWLY ADVANCING VERTICAL CYLINDER IN LONG WAVES

Based on the far field momentum formulation, a potential flow analysis is derived for the case of low forward speed in the long wavelength limit. The well known mean drift force at zero forward speed is recovered, together with an expression for the wave drift force damping. Results are obtained showing the significance of this damping effect in comparison with a viscous damping term. The analysis also sheds light on a simplified theoretical procedure that has been proposed for predicting low frequency damping of arbitrary three dimensional bodies.