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 © 2006 by ASME

Integrated dynamic analysis of floating offshore wind turbines

Smoothed particle hydrodynamics method for fluid flows, towards industrial applications: Motivations, current state, and challenges

The application of control techniques to offshore wind turbines has the potential to significantly improve the structural response of these systems. A new simulation tool is developed that can be utilized to model passive, semi-active and active structural control systems in wind turbines. Two independent, single degree of freedom (DOF) tuned mass- damper (TMD) devices are incorporated into a modified version of the aero-elastic code FAST (Fatigue, Aerodynamics, Structures and Turbulence). The TMDs are located in the nacelle of the turbine model, with one TMD translating in the fore-aft direction, and the other in the side-side direction. The equations of motion of the TMDs are incorporated into the source code of FAST, yielding a more realistic system for modeling structural control in wind turbines than has previously been modeled. The stiffness, damping and commanded force of each TMD are controllable through the FAST-Simulink interface, and so idealizations of semi-active and active control approaches can be implemented. A parametric study is performed to determine the optimal parameters of a passive single DOF, fore-aft, TMD system in both a barge-type and monopile support structure. The wind turbine models equipped with TMDs are then simulated and the performance of these new systems is evaluated. The results indicate that passive control approaches can be used to improve the structural response of offshore wind turbines. The results also demonstrate the potential for active control approaches. Copyright © 2010 John Wiley & Sons, Ltd.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/we.426

Passive structural control of offshore wind turbines

Integrated Dynamic Analysis of Floating Offshore Wind Turbines

Linear Systems, Random Data, Spectra Zero-Memory Nonlinear Systems Bilinear and Trilinear Systems Nonlinear System Input/Output Relationships Square-Law and Cubic Nonlinear Systems Statistical Errors in Nonlinear Estimates Parallel Linear and Nonlinear Systems.

Nonlinear System Analysis and Identification from Random Data

Loads for use in the design of ships and offshore structures

This paper investigates the hydrodynamical aspects of a float­ ing two body system. The emphasis is geared to a side by side mooring of a FPSO vessel and a LNG carrier. The main focus is on the following issues:

/pdf/diffraction-and-radiation-of-waves-around-side-by-side-2bdeld6u17.pdf

Diffraction and Radiation of Waves Around Side-by-Side Moored Vessels

In this paper a comparison between model basin experiments and results of diffraction computations on side-by-side moored LNG carriers is presented. The computations are based on a new lid method in diffraction codes to suppress non-realistic high wave elevations between the two floating objects. This lid method was originally formulated by Chen (2005). In this method a damping value is added to the free surface by means of a damping parameter. Since no theoretical solution can be found to establish the required value of the damping parameter, model basin experiments have been performed to determine this value. However from the results of the model basin experiments it is shown that it is difficult to obtain one unique value of the lid damping (for the 4m or small gap). The way of tuning the damping value of the lid is crucial. Tuning the damping based on first order results, like motions or wave height RAO’s will lead to a much larger variation in the estimate of the second order sway wave drift force transfer function. INTRODUCTION LNG carriers are used to transport the cargo from the floating LNG-FP(S)O to the shore or from terminal to terminal. ownloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 02/03/2016 T Offshore floating LNG systems become a more and more viable economic solution. The need for an accurate assessment of the hydrodynamic performance of LNG offloading systems is an important issue. LNG production offshore give rise to the need for a remote offshore berthing location, without the benefit of coastal protection against wind, waves and current. Calculation of the wave elevations at the waterline of the two floating bodies is necessary to calculate the wave drift force quadratic transfer functions (QTF’s) using a pressure integration scheme. In the model test campaign the wave heights are measured at the centerline between the two vessels, effectively at the wall of the model test basin. Unrealistic wave heights at the centerline will obviously also indicate their effects at the waterline between the two vessels. Conventional near field calculations result in unrealistic free surface elevations in the gap. Far field estimates of the QTF’s do not apply on the individual bodies of a multi body diffraction calculation. The middle field equation technique, to avoid spurious effects in pressure integration techniques is described by Chen (2006). The complex hydrodynamics of two floating bodies in close proximity has been a research topic for over two decades. Two main issues are to be considered. 1.The resonance behavior of the waves in the gap between the two vessels tends to be overestimated using standard diffraction programs. E.g due to non-linear effects in the physics of the waves like breaking tend to reduce the extreme wave elevations. 2. Viscous effects, neglected in diffraction theory, may be more dominantly present at the bilges of the keel due to the resonant flow between the two floating bodies. In our approach we disregards also any viscous effects at the tank wall and the hull of the vessel. For the viscous flow effects the current set-up is not applicable, then a full representation of the two side-by-side 1 Copyright © 2007 by ASME erms of Use: http://www.asme.org/about-asme/terms-of-use Dow vessels is necessary. However, again this was neglected in our study. Different methods to reduce spikes in the QTF’s due to resonance effects have been reported. A suppression method based on the damping of generalized wave modes was given by Newman (2003). A simplistic approach to suppress the unrealistic wave motions was presented by Huijsmans et al. (2001) where a rigid lid was applied to the free surface between the two vessels. The formulation of this rigid lid approach is similar to the formulation to suppress irregular frequencies. A first attempt in applying the new damping lid method on multi-body hydrodynamics (Chen, 2005) was presented by Fournier et al. (2006). These studies showed that the lid method was effective in reducing the relative wave elevation and consequently in reducing the wave drift forces at the resonance peaks. In this case of a gap between the bodies of 25.m, it led to unique value for the damping of the lid both for the first order quantities as well as the second order wave drift forces. Also it was shown that no rationale could be found in establishing an a priori guess for the damping value for the lid. DIFFRACTION MODEL THEORY The damping lid method is based on the implementation of a damping force at the meshed free surface in between the two floating bodies (figure 1). This method can be implemented in standard diffraction theory programs. The theory behind the damping lid method, originally formulated by Chen (2005) is described briefly. Figure 1: The Free surface domain has been divided into F0, at which no damping is applied, Fi, the inner surface at which to suppress irregular frequencies and Flid where the boundary equation of the lid applies In the fluid a rotational free damping force is applied in the whole fluid domain as: F μ = ∇Φ As such the force can be resembled as a sort of a frictional force. Applying the rotation free properties leads to a formulation that only is affected by the free surface boundary condition. Here Φ is the velocity potential and μ is the damping parameter. nloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 02/03/2016 Te From Bernoulli it follows that the wave elevation at the free surface is: 1 1 1 2 t g g g ζ μ = − Φ − ∇Φ∇Φ − Φ At the free surface lid the boundary condition modifies to: ( ) 2 1 0 i z g φ ω ε φ ∂ − − = ∂ in which ω is the wave frequency and the non dimensional parameter ε is related to the damping parameter μ by: μ ε ω = The selection of a damping parameter, ε can be determined such that either the computed wave elevation and/or the wave drift force transfer function can be fitted to the measured ones. The free surface condition of the conventional approach, no-lid, the rigid lid and the damping lid, are summarized in table 1. From the adapted free surface condition it becomes clear that also a frequency dependent epsilon value could be chosen. This was not done in this study for practical reasons. Table 1: Free surface boundary equations In order to be able to validate the damping lid method, model basin experiments have been performed. Conventional Rigid lid Damping lid Rigid Semi permeable Undamped wave elevation No wave elevation Damped/tuned wave elevation 2 0 z g φ ω φ ∂ − = ∂ 0 n φ ∂ = ∂ ( ) 2 1 0 i z g φ ω ε φ ∂ − − = ∂ 2 Copyright © 2007 by ASME rms of Use: http://www.asme.org/about-asme/terms-of-use D MODEL BASIN SET-UP Dedicated model basin experiments on side-by-side mooring of two floating bodies were carried out with one LNG carrier model, representing a general design LNG (table 2). The LNG carrier was positioned parallel, close to the side wall of the basin. Table 2: Main particulars and stability data of the LNG carrier Main particulars and stability data of LNG carrier Designation symbol unit Length between perpendiculars Lpp m 274 Beam B m 44.2 Depth D m 25 Draught T m 11 Displacement weight ∆ ton 9775 9 Centre of gravity above keel KG m 16.1 Centre of gravity forward of st 10 LCG m -1.1 Transverse metacentric height GMt m 5.0 Roll radius of gyration kxx m 16.3 Pitch radius of gyration kyy m 70.1 Yaw radius of gyration kzz m 70.0 Waterdepth d m 37.5 Modelscale 1:50 The wall acts as a mirror to the hydrodynamics, simulating a LNG carrier with an alongside moored identical LNG carrier at twice the distance to the wall (figure 2). ownloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 02/03/2 Figure 2: Test set-up representing a two body set-up. ( • is wave probe) The wall represents the centerline of the gap between the two bodies. The close proximity can be recognized in figure 3, the mirror on the picture helps to explain the function of the quay. Figure 3: Model basin set-up Free floating (decay) tests, tests with the model moored in a soft mooring system and tests with the model rigidly fixed to a six component frame have been performed. The soft mooring system represents spring and brest lines; no fenders were modelled. The waterdepth was 37,5m and the model scale 1:50. A set of nine wave probes were located over the length of the gap close to the wall, approximately at the gap centerline in the two body analogy. The parallel position of the LNG carrier model to the basin wall restricts to head wave conditions. In the rigidly fixed setup two white noise wave spectra (WN), in soft mooring five operational sea states (O1-O5) have been generated with peak 3 Copyright © 2007 by ASME periods covering a large interval (5-20s). Regular waves with 016 Terms of Use: http://www.asme.org/about-asme/terms-of-use frequencies around the anticipated resonance frequency have been selected. Comparison of the results of the model basin experiments with the results of diffraction calculations in which the damping lid has been implemented, requires a numerical model set-up which resembles the set-up of the model basin experiments. A two body panel distribution of two identical LNG carriers with a 4 meter gap in between is composed. At the free surface of the parallel part in between the two floating bodies the lid has been introduced. Figure 4: Panel distribution, including the mesh of the lid, used in diffraction computations WAVE PATTERN ANALYSIS Wave heights in the gap have been measured during fixed model tests. Interesting observations have been made considering the wave RAO’s, since no vessel motions affect the wave motions in the gap. Different mechanisms of wave interaction seem to appear in the gap between the two bodies. Firstly superposition of the incoming and diffracted waves results in cancellation in nodes at certain frequencies. This cancellation is dependent on the ratio between the gap dimensions such as the width and the length and t

Advances in the Hydrodynamics of Side-by-Side Moored Vessels

Offshore wind energy appears to be on the verge of a phase of enormous expansion to becoming a significant source of electricity for a number of countries in northern Europe. Dozens of large projects are planned and a significant number now have planning permission and firm construction dates. The projects built to date have been in shallow seas, of up to around 20m in depth, in the North and Baltic seas, with the planned projects extending the range to new seas and greater depths. However, numerous challenges remain in the greater depths, in particular relating to the necessary size of the support structures, the resulting wave loads, handling equipment and natural frequencies. At one point, the inherent advantages associated with a floating support structure (of compliance due to the flexible attachment to the ground) will match the additional costs due to complexity and novelty. Questions of course remain under what conditions this will be (water depth, sea climate, distance to shore) and whether offshore wind energy can be proven to be economic under those conditions anyway

Rene´ Huijsmans

Papers

Loads for use in the design of ships and offshore structures

Loads for use in the design of ships and offshore structures

Diffraction and Radiation of Waves Around Side-by-Side Moored Vessels

Advances in the Hydrodynamics of Side-by-Side Moored Vessels

Floating windfarms for shallow offshore sites