Offshore Floating Wind Turbine: A Multi-Objective Design Optimization
Approach for Floating Support Structures
Meysam Karimi, Bradley Buckham, Curran Crawford
Department of Mechanical Engineering - University of Victoria
Victoria, B.C. Canada
Introduction
Results
Offshore floating wind turbine technology is growing rapidly but it is still at the
transient stage from research studies to MW scale prototypes. The primary
offshore wind technologies have been put into operation in shallow waters using
fixed-bottom foundations. Investigations show that usage of offshore wind turbines
are most appropriate in deep waters due to wave and wind characteristics, sea
bed properties, and visual pollutions1. In such areas, a floating structure for wind
turbine which is held by mooring lines allows harvesting wind energy when the
water depth exceeds 50 m.
Single-Body Platforms: two conventional types of floating platforms for offshore wind turbines are Tension-Leg
and Spar buoy platforms. The design points evaluated by optimizer in terms of platform cost and nacelle
acceleration of the wind turbine.
Discussions
In order to have a cost effective design with high performance, it is necessary to
evaluate the floating structure by optimization schemes. The approach taken into
account for this optimization problem is to provide a list of the most promising
floating support structures for NREL 5 MW wind turbine2 that can then use as
initial design points for more detailed design processes. In this way, the optimal
design may provide new insights into the conventional and unconventional floating
platforms.
 Looking at the optimal design points, one can see that the Tension-Leg platforms
with taut vertical mooring lines are the most optimal designs among all the
supporting structures.
 The only region to find a design point at the lower cost and the same performance
with TLPs is available at the σ ∈ [0.24,0.28] for Semi-submersible designs with one
central cylinder and an array of three outer cylinders.
 The results show that Semi-submersible platforms are more stable and cost
effective than Spar buoy design configurations.
 Among the multi-body structures, it seems that Semi-submersibles with 4 floats are
the best option below a cost of $6M.
(a) Tension-Leg Platform
(b) Spar buoy Platform
Multi-Body Platforms: the design case for this part is modeling Semi-submersible platforms. This multi-body
structure is created by a main inner cylinder with an array of vertical cylinders around the inner part. The Pareto
fronts for two types of Semi-submersible platforms represent the optimal design candidates in terms of cost and
performance of the system.
(c) Semi-submersible Platform
Methodology
To achieve a design optimization that captures a full range of floating platforms, a
parametric design scheme is required3. This study, therefore, is divided into four
main components :
 A fully coupled frequency-domain
dynamic model to evaluate the
response and behavior of design
points.
 A multi-objective Genetic
Algorithm with design constraints
to manage the exploration of the
design space4.
New Variables
 Environmental conditions
including water depth, wind
speeds, sea states, and
frequency range of encounter
waves.
I appreciate Dr. Curran Crawford, and Dr. Bradley Buckham for their supports during
my PhD program, and I am also thankful of Pacific Institute for Climate Solutions for
their financial supports providing a unique opportunity for conducting this research.
Design Variables
Start
Form Design
Geometry
Design
Constraints
Variation of
Variables
Stop
Optimization Scheme
Design is Unsatisfactory
 A support structure
parameterization scheme that
provides platform geometry and
mooring systems for three
design concepts.
Acknowledgment
Hydrodynamic
Analysis
Design
Evaluation
Design is Satisfactory
References
[1] Wayman E. Coupled dynamics and economic analysis of floating wind turbine systems. MIT;
2006. Ph.D. thesis.
[2] Jonkman, J., Butterfield, S., Musial, W., and Scott, G., Definition of a 5-MW Reference Wind
Turbine for Offshore System Development, Technical Report, National Renewable Energy
Laboratory, 2009.
[3] Hall, M., Mooring Line Modelling and Design Optimization of Floating Offshore Wind
Turbines, MSc thesis, University of Victoria, 2013.
[4] Arora, J.S , Introduction to Optimum Design, Elsevier Academic Press, 2nd edition, 2004.
ISBN 0-12-064155-0.