WHITEPAPER

INTEGRATED ANALYSIS OFFLOATING WIND TURBINES

Sesam whitepaper - Integrated analysis offloating wind turbineReference to part of this report which may lead to misinterpretation is not permissible.

No.

Date

Reason for Issue

Prepared by

Verified byApproved by

1.0

2016-05-13

First issue

Chuang, Jenny

Berg-Jensen, Jan

Berg-Jensen, JanZhenju

HenrikHenrikChuang, JennyZhang, Fan Joe

Vada, Torgeir

2.0

2018-02-22

Second issue

ZhenjuKirkhornDate: January / 2018
Prepared by DNV GL – Digital Solutions © DNVGL AS. All rights reservedThis publication or parts thereof may not be reproduced or transmitted in any form or by any means,including copying or recording, without the prior written consent of DNV GL AS2

TABLE OF CONTENTS
1EXECUTIVE SUMMARY ..................................................................................................... 42

INTRODUCTION .............................................................................................................. 5

3

MODELLING IN SIMA ....................................................................................................... 8

3.1

Locations and environment conditions

83.2

Body

93.3
3.3.1
3.3.2

Slender system
SupernodeLines

1112134

CALCULATION PARAMETERS .......................................................................................... 14

4.1

Static analysis

144.2

Dynamic calculation

155

DISPLAY AND POST-PROCESSING RESULTS ..................................................................... 166REPORT ....................................................................................................................... 197SUMMARY .................................................................................................................... 198REFERENCES ................................................................................................................ 2031EXECUTIVE SUMMARY

Offshore wind is a young technology and experiencing fast development. As the industry evolves,
offshore wind farms are being built further from the coast and in deeper waters to achieve a steadier andstronger wind resource, leading to a big potential for the floating wind turbine market.
Engineers are facing great challenges to ensure successful numerical modelling and analysis, amongwhich the most critical part is to have the integrated system simulated in one numerical tool, taking into
account the coupling effects between different objects, such as wind turbine, floating support structure,mooring and cables, and obtaining reliable results.
Sesam’s module Sima (including Simo and Riflex) overcomes this difficulty, providing our users with areliable and efficient solution(1).

Why choose Sesam?
▪

Complete floating wind turbine system could be simulated in one model, including blades, nacelle,
tower line, and floater and mooring system, etc.

▪

Coupling effects between aerodynamic, hydrodynamic and structural response could beconsidered, which ensures more realistic analysis compared to other tools.

▪

It is straightforward to combine a variety of environmental loads, such as wind, wind wave, swellwave and current.

▪

Efficient time domain algorithm saves a large number of engineering hours.

▪

Engineers can easily post-process results based on the powerful in-built post-processor. Thisavoids the need to interface with another program.

▪

The user-friendly interface ensures good visualization, easy modelling and error check.

In this paper, you will find all the necessary information about how to model and analyse a floating windturbine in Sesam.

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2INTRODUCTION

The offshore wind turbine industry only began doing research in the mid-1990s. It remains a hot topic
around the world. According to a report by the European Wind Energy Association(2), wind will be theleast-cost option for building excess electricity capacity almost universally by 2025.
Compared with onshore wind, offshore wind has many advantages:••••

Large areas available at a low priceNo noise and visual impacts
Higher wind velocities, less turbulent wind
Feasible transportation and installation

These advantages are the real engines to drive offshore wind turbine development.
Different types of sub-floaters are chosen depending on the water depth. In general, if the water depth
is more than 50 meters, it is preferred to use floating wind turbines(2).

Fig. 2.1 Offshore wind floaters
Currently, three main kinds of floaters are adopted by the industry:•Spar buoy: A very large cylindrical buoy stabilizes the wind turbine using ballast. The centre ofgravity is much lower in the water than the centre of buoyancy. This provides good righting
behaviour when the structure is heeling.

In 2009, Statoil installed the world’s first large scale grid-connected floating wind turbine by using thiskind of floater, Hywind from Norway, with a 2.3 MW Siemens turbine.
A pilot park will be established with five 6MW Hywind turbines on the coast of Scotland. This will be theworld’s largest floating wind farm, also owned by Statoil.
•Tension leg platform: A very buoyant structure is semi-submerged. Tensioned mooring lines areconnecting the platform with anchors on the seabed to add buoyancy and stability.

5•Semi-submersible: Combining the main principles of the two previous designs, a semi-submerged
structure is designed to reach the necessary stability.

One of the successful designs is WindFloat from Portugal. There are three primary advantages to theWindFloat foundation: Firstly, it has good stability performance. Secondly, the assembly of the wholesystem is done on shore to avoid dynamic weather influence. Lastly, its shallow draft ensures convenienttransportation.

This document aims at providing the best practice to guide our users on:••••Modelling a floating wind turbine
Assigning locations and environmental conditions, necessary hydrodynamic properties
Carrying out a real-time domain coupling dynamic analysis
Post-processing the result data and making a comprehensive reportThree modules in Sesam, including GeniE, HydroD and Sima, are used with seamless interfaces.
GeniE is used to model the geometry of the floater for the hydrodynamic analysis with potential theory.
FEM file including wet surface meshes is used for this purpose. Sima is using the geometric files (OBJ,GDF, FEM) for visualization.
HydroD calculates the hydrodynamics of the shell hull to get coefficients such as added mass, damping,wave force transfer function and motion transfer function, etc. These hydrodynamic data will be
imported into Sima.
Sima will take the input from GeniE and HydroD. The wind turbine and mooring system will be createdinside Sima. Both static and dynamic analyses will be done in Sima.
The following two figures illustrate a spar buoy floating wind turbine and a semi-submersible typefloating wind turbine model in Sima.

Fig. 2.2 Spar buoy type floating wind turbine in Sima

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Fig. 2.3 Semi-submersible type floating wind turbine in Sima

73

MODELLING IN SIMA

A complete floating wind turbine system can be modelled inside Sima. In general, it consists of threemajor parts: locations and environment conditions, body and slender system. Details for each element
will be explained as following.

Fig. 3.1 Elements for modelling

3.1 Locations and environment conditions
Location includes the general information about physical properties for an actual location. Acceleration ofgravity, water density, air density, water depth, etc. can be defined there. Position, size and colour forsea surface and flat bottom can also be defined for visualization.
Sea environment setting is quite straightforward. Wave, swell, current and wind can all be included
together from the user interface. Both regular and sea spectrum can be defined in Sima with any
combination of frequency, heading and wave height.
Sima also provides numerical wave generation. Users can create their own wave spectrum with any
combination of frequencies and headings.
Multiple options for wind spectrum are available, as shown in Fig. 3.2.
Current profile and velocity will vary with different levels relevant to sea surface and propagating
direction.

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Fig. 3.2 Wind spectrum in Sima

3.2 Body
Different types of bodies can be defined in Sima. Body can be large volume type with 6 or 3 degrees offreedom, small volume body with 3 translational degrees of freedom, or prescribed motion type.
In this example, three large volume bodies are created, which are spar, hubmass and nacelle. Each bodyhas a slender system connection.
Spar is a slender, cylinder-type floater submerged into water. See from the highlighted part in Fig. 3.3.
As a large volume body its 6-DOF motions are simulated in time domain.

Fig. 3.3 Spar type floater

9Hydrodynamic data regarding kinetics and radiation data (as shown in Fig. 3.4) can be imported byreading Sesam interface files from HydroD/Wadam, which can be modified (editing the values, add ordelete) after importing into Sima.
Sima also provides the option to include drag force from viscous effect. This is done by creating slenderelements attached to the body. Linear drag and quadratic drag coefficients can be specified.

Fig.3.4 Hydrodynamic data for Spar

Fig.3.5 First order transfer function for spar

Hubmass is an important component for wind turbine. It connects the shaft line, tower line and blades.
Correct mass information is important for rotation purpose of the wind turbine.
Nacelle is located right behind the hub. Since nacelle is above sea surface, only mass information andwind coefficient are relevant.
Relative positions of hubmass and nacelle are shown in Fig. 3.6.

Fig. 3.6 Nacelle and hubmass modelling10

3.3 Slender system
Modelling of the slender system should start from supernode, element, segment and line. At last, thewhole wind turbine is created by combining these components. Fig. 3.7 shows the basic topology of theslender system.
• SUPERNODE: Branching points or nodes with specified boundary conditions.
• LINE: Suspended structure between two supernodes.
• SEGMENT: (Part of) line with uniform cross-section properties and element length.
• ELEMENT: Finite element unit.

Fig. 3.7 Slender system terms
Slender system takes a large part of the floating wind turbine model. Blades of the turbine, electricallines, shaft lines and mooring lines should all be modelled with slender system. Fig. 3.8 shows the wholesystem.

11Fig. 3.8 Slender system

3.3.1 Supernode
We have a certain amount of supernodes in our example. Two main cards should be set up forsupernodes: position and boundary condition. There are four types of boundary conditions: free, fixed orprescribed, slaved, fixed relative to orientation. Any of six DOF including translation and rotation can beset up with free or fixed.
Setting up correct boundary condition for all supernodes is challenging work. Please see Fig. 3.9 for thedetailed boundary information on all the supernodes.

Fig. 3.9 Supernode boundary condition

123.3.2 Lines
Lines are created by connecting two supernodes. There are in total 12 lines covering mooring lines, bladelines, electrical lines, tower line and shaft line. See line table in Fig. 3.10.

Fig. 3.10 Table of Lines
The process of creating lines in Sima is quite clear and easy to follow.

Fig. 3.11 Properties for Line
Cross section: ‘Generic axisymmetric pipe’ and ‘double symmetric cross section’ cross sections havebeen used here. Details including cross section property, stiffness property, axial stiffness, bending
stiffness, torsion stiffness, hydrodynamic force coefficients and aerodynamic force coefficients, etc. canall be specified here.
Cross section of the blades has air foil shape. Different air foil shapes can be assigned to different crosssections from the tip to the base. Sima provides elaborated user interface for detailed design. Air foil
geometry is given by X and Y coordinates in two dimensions. Air foil characteristic is defined by assigning
drag coefficient, lift coefficient and moment coefficient at different attack angles for different Reynolds
numbers.
One line can be divided into any number of segments; each segment can have its own cross section
property and number of elements. An element can be clearly seen from the user interface if you zoom inclose enough. This is good for modelling work and checking the quality of element length.
The component represents the elementary description of the mechanical properties. A flex joint has been
applied in our example at one end of the shaft line. This is for flexible connection between shaft andtower line.

13Wind turbine: The wind turbine can be created by composing body, shaft line, tower line, wind loadoption and blades together. The controller is used to set up the data for engine and control the rotational
speed of the blades. Fig. 3.12 and Fig. 3.13 show more details.

Fig. 3.12 Wind turbine components

Fig. 3.13 Controller data

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CALCULATION PARAMETERS

4.1 Static analysis
Static configuration is the starting point for time domain dynamic calculation Five loading sequences areincluded in static analysis.
Volume forces: Presents weight and buoyancy of the slender system.
Boundary change: Identification of supernode for boundary change

14Specified displacement: supernode with specified rotation will be rotated to static equilibrium positionand line ends will be moved from stress free position to static equilibrium position.
Body force: Force from floater
Current forces: Forces from the current
Number of load steps, max iterations and accuracy for iterations can be defined for each load sequence.
Static configuration after static analysis can be seen from Fig. 4.1.

Fig. 4.1 Static configuration

4.2 Dynamic calculation
The dynamic responses of floating wind turbine in designed environmental load condition are of the maininterests, including the nonlinear coupling effects between the floater and slender system.
Simulation length and time step should be setup carefully, especially considering the high frequency
wind loads on the turbines. Multiple options for storage of results are also provided to avoid too large
database with unconcerned signals. Time history of displacement, force response, acceleration,curvature and wave kinematics etc. can be stored for different bodies, lines, segments or nodes, forpost-processing.
Fig. 4.2 shows how to set up the displacements storage for upper node for tower line. It is easy to add
or delete any parts of the slender system for storage purpose.

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Fig. 4.2 Displacement and Force storage for tower line

5DISPLAY AND POST-PROCESSING RESULTS

Any of the stored results can be used for post-processing. Sima has powerful post-processing abilities, as
shown below some examples:••••••••Direct simulation results to plots or file output
Filtering of results
Arithmetic operations
Spectral analyses
Statistical operations, distributions etc.
Code check
Fatigue analysis
OthersAn easy example is demonstrated here to capture the local peaks for the signals in Fig. 5.1 and Fig. 5.2.

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Clearly information for all these local peak responses will be very easy to observe and understand how
the system is behaving under oscillating environmental loads. This is useful to predict and prevent
excessive response happens.
A flowchart needs to be created in the post-processor as shown in Fig. 5.1. Then, by double-clicking the‘plot’ icon, results will be shown in Fig. 5.2.

Fig. 5.1 Flowchart for capturing maximum points for signals

Fig. 5.2 Capturing local maximum points for force at upper node of tower
Another example below is aimed at showing more possibilities in Postprocessing. Axial forces of top
segments of three mooring lines can be plotted (Fig. 5.3.1). Damaged fatigue is also calculated withdefined SN curve (Fig. 5.3.2). Moreover, Rayleigh distribution with peaks can also be done to obtain
extremes of each top segments with probability of non-exceedance (Fig. 5.3.3).

Fig. 5.3 Postprocessing of axial forces of top segments of three mooring lines

17Fig. 5.3.1 Axial force of top segments of three mooring lines

Fig. 5.3.2 Damaged fatigue for top segments for three mooring lines

Fig. 5.3.3 Rayleigh distribution of peak responses for mooring line 118

6REPORT

A report can be generated from Sima using multiple kinds of data both inside Sima and from external
resources. Users have a full control of the report organization and content by defining sections andinserting data resources such as text, image, plot, tables, formula and so on.

Fig. 6.1 Report generator in SimaFig 6.2 shows an example of report template. Three sections are created. Section 1 includes a model
description and an image of floating wind turbine. Section 2 contains a plot of floater wave drift force, atable of spar structure mass and a snap short of static configuration. Section 3 aims at showing axial
forces for three mooring lines. After editing the structure and content of the report, a Word document
will be easily generated by clicking the icon ‘Generate report’.

Fig. 6.2 An example of report template

7SUMMARY

Sima has the capability to take the whole system of the floating wind turbine into account and running
coupling analysis in a reliable and efficient way.
If readers want to learn the best practice from this whitepaper, please kindly contact our support team
via software.support@dnvgl.com.

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8REFERENCES/1/ Zhang, F. FLOATOVER ANALYSIS. Whitepaper from DNV GL Software, 2015.
/2/ Arapogianni, A; Genachte, A; et.al. Deep Water-The next step for offshore wind energy. A report bythe European Wind Energy Association - July 2013

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