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Wednesday, 12 March 2014
16:30 - 18:00 Floating wind turbines
Science & Research  


Room: Llevant
Session description

The session covers design problems related to floating wind turbines and how current research is overcoming these hurdles through innovative platform concepts, experimental methods and mooring system analysis. In particular, technical and economic studies and analyses for three different novel concepts will be presented, including one vertical axis concept, a concrete platform design and a combined wind & wave energy device. In addition, a new methodology for experimental model testing with a focus on aerodynamics and control will be presented, as well as a detailed assessment on long-term mooring system loads.

Learning objectives

  • Learn about a novel experimental methodology for floating wind turbine aerodynamic and controller testing
  • Identify challenges and benefits of an innovative vertical axis concept
  • Understand the design and potential benefits and challenges of a concrete platform
  • Assess structural fatigue damage of a multi-modal wind/wave energy device
  • Learn about a new methodology capable of reproducing life cycle mooring loads
Lead Session Chair:
Denis Matha, University of Stuttgart, Germany

Co-chair(s):
Antoine Peiffer, Marine Innovation and Technology, United States
Feargal Brennan Cranfield University, United Kingdom
Co-authors:
Feargal Brennan (1) F P Athanasios Kolios (1)
(1) Cranfield University, School of Engineering, Cranfield, United Kingdom (2) Cranfield University, Cranfield, United Kingdom

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Feargal Brennan is Professor of Offshore Engineering, head of the Energy & Power Energy Division at Cranfield University and its Offshore Renewable Energy Group.
He has published over 150 papers in journals and conferences, he is the current chairman of the ISSC (International Ship and Offshore Structures Congress) Fatigue & Fracture Technical Committee, sits on the BSI committee for fatigue testing of metals, the Engineering Integrity Society (EIS) durability & fatigue committee, the IMechE Offshore Engineering Committee, the EPSRC peer review college and is co-editor of the international journal Fatigue & Fracture of Engineering Materials and Structures.

Abstract

Structural integrity considerations for the H2OCEAN multi modal wind-wave platform

Introduction

The H2OCEAN platform is a combined Wind-Wave Floating concept funded through an FP7 Project. This abstract introduces a methodology developed to assess the structural fatigue damage of such multi-modal device taking into account the stochastic loading regime that is subjected by wind, wave and machine loading. the approach allows inspection and maintenance planning against a target structural reliability.

Approach

Offshore Wind has evolved from a land-based activity and it is not surprising that there is still, to a large extent, separate onshore and offshore design cultures that are not always easily integrated. A whole-systems approach to design is one that considers all aspects of the design solution in an integrated manner. This includes not only the functional structural design requirements for operation, but design for manufacture, assembly, installation and ultimately design for controlled failure, inspection, maintenance, repair and decommissioning. Offshore & Marine Renewable Energy structures differ significantly from ships and oil & gas installations in a number of ways. A large proportion of structural loading is due to the machine and therefore not necessarily dominated by wind, current and wave action directly to the structure. This means that functional design requirements have distinctly different installation, operation and survival characteristics.

Our approach has been to adopt a reliability based probabilistic approach to structural integrity assessment taking into account the stochastic nature of input variables including uncertainty of material strength and fatigue resistance. In addition, it is important to design for life-cycle operation including consideration for inspection, maintenance and repair.

Fatigue is a local phenomenon and so it is imperative that the engineer can relate global structural response to a local condition and assess the critical fatigue prone areas of a structure understanding the concepts of structural redundancy and damage tolerance. The reliability based approach allows such calculations and assessments to take place and is based on methods developed in the offshore Oil & Gas industry over several decades.





Main body of abstract

Floating Wind Turbines are fast becoming an area for research but it is important to build on the work completed to date by many researchers working in this sector and the wider Offshore Oil & Gas industry over several decades.

For example, a review of the research concerning the development of offshore wind turbines revealed several dynamic models to describe the coupled aero-hydro-servo-elastic behavior of floating HAWTs. A simple approach based on Wayman (2006) and Wayman et al. (2006b) proposes a methodology that consists of the following steps:

• The body mass matrix of the wind turbine (including the rotor, the nacelle, the drive train and the tower) is added to the body mass matrix of the floating structure, rotor damping and restoring matrices, due to the aerodynamics and to gyroscopic effects, are added to the hydrostatic and hydrodynamic restoring and damping matrix of the floating structure, the influence of the mooring system is taken into account estimating a mean offset displacement, and in this state the linearized restoring coefficient are added to the global restoring matrix the aeroelasticity of the rotor is ignored

• Utilizing a frequency domain analysis, the aim of the dynamic optimization is to demonstrate that the Response Amplitude Operator (RAO) peaks of this coupled system do not overlap with the wave spectrum of the operational site, and therefore the wave response of the whole structure is minimized.

• This frequency domain linear analysis is not able to take into account transient loads and/or nonlinear dynamic characteristics, factors influencing the turbine analysis, and new, more advanced approaches that have been proposed (Jonkman, 2007; Henderson and Patel, 2003).

The H2OCEAN project has integrated a coupled VAWT and WEC Floating Platform to give the combined multi-dimensional dynamic response of such a multi-modal system. From this the following steps have been undertaken to develop a fatigue assessment methodology:

• Determine a “typical” annual loading case based on probability of occurrence of site specific environmental loading information including machine control strategy;

• Determine a load-local stress transfer function based on structural dynamic analysis of the integrated platform;

• Generate local (at critical locations) stress spectra by combining the loading spectra and structural dynamic stress transfer functions;

• From this to produce a weighted average stress range based on the chosen damage model and associated Rainflow cycle count from a generated time series using an Inverse Fast Fourier Transformation;

• Fatigue calculations can then be conducted using either an S-N or Fracture Mechanics approach.

By repeating the above steps and instead of using absolute inputs, parameters can be defined by probabilistic distributions based on an assessment of the uncertainty of each variable. A fatigue limit state can then be applied as a distribution of local stresses and material resistance to cracking as stochastic variables.

Global reliability can then be calculated taking into consideration the damage across the structure and the ability to inspect and repair critical members. A target reliability value can be set to ensure that repair and maintenance strategies are implemented if this reliability is seen to reduce below that required by the operator/certification authority and insurers.


Conclusion

H2OCEAN is a radical design for a multi-modal offshore wind-wave energy converter. The project involves European Partners from a broad range of disciplines working together to integrate complex engineering systems.

This abstract has presented a very high-level summary of the steps required to approach fatigue assessment in order that support structural designs can be optimized in terms of operation and maintenance. Over design will certainly result in prohibitively high costs and might not necessarily guard against structural fatigue; the other extreme can result in catastrophic failure and/or very high maintenance and repair costs. It is therefore imperative that we have available methodologies to allow such optimization.

In this abstract, the consideration of the global structural reliability of floating support structures of a coupled wind-wave platform has been discussed. After a review of the background literature concerning VAWTs, the available floating support structures concepts and fundamentals on structural reliability, limit states for stability check as well as failure of mooring lines have been considered and a methodology for the coupled dynamic response has been determined.

A framework for conducting a fatigue analysis of the integrated unit has been presented and the presentation will illustrate the implementation of the approach using both Fatigue and Fracture Mechanics damage models. The methodology does not prevent the use of new damage models and/or fabrication methods and the presentation will highlight the manner in which fatigue improvement methods can be implemented. A reliability framework for the approach allows the consideration of uncertainty associated with input variables.


Learning objectives
To integrate hydrodynamic response with fatigue analysis;
To provide a methodology for the consideration of probabilistic use of stochastic input variables in structural reliability calculations.



References
Wayman, EN (2006). "Coupled Dynamics and Economic Analysis of Floating Wind Turbine Systems." M.Sc. dissertation, Dept. of Mech. Eng., MIT, Cambridge, USA, June 2006.
Wayman, EN, Sclavounous, PD, Butterfiled, S, Jonkman, J, and Musial, W (2006b). "Coupled Dynamic Modeling of Floating Wind Turbine Systems." 2006 Offshore Technology Conference, Houston (TX), OTC 18287.