ORE Supergen Challenge

Workshop (Offshore Wind)

Offshore renewables structures -

foundation challenges and solutions

Trevor Hodgson, BSc, MBCS, CITP, Ceng

(late replacement for Peter Larkin)

Grosvenor Hotel, London

16th - 17th October 2017

Trevor HodgsonDegree in Civil Engineering

Chartered Engineer

1977 to 2002 : WS Atkins, UK

Transportation and bridge engineering

Offshore oil and gas (fixed, floating, drilling structures)

Steel, aluminium and concrete construction

Software development (ASAS and AQWA)

Nuclear and renewable industries

2002 to 2007 : Galbraith Consulting Limited, UK

Offshore oil and gas (fixed, floating, drilling structures)

Software development (in-house FEA pre- and post-processors)

2007 to present : Atkins Energy, UK

Offshore Structural Integrity Management

Steel and concrete oil and gas platforms

Offshore renewable energy (wind, wave and tidal)

WTG monopile, jacket, floater and concrete gravity substructures

2001 to 2006 : Visiting Professor, Universities of Glasgow and Strathclyde

Atkins have been involved

with the design of Offshore

Wind Turbine Generator

substructures since 2002

Prior to that time, Atkins

have had 40 years

experience of Offshore Oil

and Gas Structure design

worldwide

This experience is spread

over many types of WTG

foundations: steel and

concrete; fixed and floating

Atkins are also very active

in wind farm substructure

design, having completed

some 18 OSP designs

Offshore Wind Turbine

Substructure Design

Others

● Turbine Manufacturer Foundation Concept Design

● Turbine Manufacturer Jacket Sensitivity Study

● OWA Concept Study

25 LEMS26 Beatrice OTM27 Windfloat Floating28 Dounreay Floating29 Hornsea OSP30 Bard 131 Blyth GBFs

25

26 Types of work undertaken:

• Geotechnical site survey

planning, interpretation and

foundation design

• Concept, FEED and Detailed

Design

• WTG Foundations

• OSPs

• OTMs

• Met Masts

• Floating Wind

• Asset Management

• Integrity Management

No Locations

● WTS Foundation Concept Design

● WTS Jacket Sensitivity Study

● OWA Concept Study

27

31

29

30

28

Other Countries

● Portugal

● Norway

● USA

● Taiwan

● China

Monopile Detailed Design

5

The detailed design of 67 Monopile substructures

for the Dudgeon Wind Farm in varied ground

conditions, including weathered chalk

Currently working on Triton Knoll Detailed Design

Jacket Design ExperienceFEED of 67 Galloper offshore Wind Farm and

FEED of suction bucket jackets at Dudgeon

Detailed Design of 84 jacket substructures

(and 2 OTMs) for the Beatrice offshore wind

farm

Floating Wind Turbine Structures

Concept, FEED and Detailed Design of Floating

Structures for support of offshore wind turbines to various

concepts and in different materials. Complete system

design including appurtenances, moorings and ballasting.

2008 - Three Column Wind Turbine

2011 – WindFloat Prototype

2013 – VolturnUS

2014 – Full Scale WindFloat

2015 – Hywind Installation

2016 – Dounreay Tri (Hexicon)

2016 – Kincardine (KOWL)

Owners Engineer for the design and construction of 5

concrete wind turbine substructures for the Blyth site

• Constructed in the Neptune dry-dock in Newcastle

• Design and construction methodology driven by the size

of this dry-dock (gate 32m wide), as well as ground and

metocean conditions

• Over 1,800 m3 of concrete per foundation

• Over 500 tonnes of steel per foundation for concrete

reinforcement

• Over 600 tonnes for each of the steel shafts

Access

platform

Airtight

platform

J-Tube

External

concrete walls

Upper

shaft

Concrete roof

Field weld

Lower

shaft

Internal walls

Concrete slab

Gravity Based Structures

Geotechnical Involvement

Desk

studies

Concept

engineering

Site investigation

& laboratory testingDetailed understanding

of ground conditions

Foundation

design

Consultancy

Ground

modelling

Geohazards

GIS

Soil parameter definition

?

CONCEPT FEED DETAILED DESIGN CONSTRUCTION, INSTALLATION,

STRUCTURE LIFE

Challenges and Solutions• Holistic Wind Farm Design

• Geotechnical Considerations

• Bespoke vs Clustered Design

• Design Integration

• Secondary Steel and Appurtenances

• Fatigue Design Improvements

• Fabrication Efficiency

• Transportation and Installation Issues

• Monitoring and Design Feedback

Holistic Approach – Virtual Wind Farm

Multi-Level Data in the VWFMetocean Model

Generic Data

Calibrated Data

Location Data

Bathymetry Model

Simple Data

Point Data

Location Data

Soil Model

Descriptive Data

Intermittent Data

Location Data

Cable Model

Simple Data

Intermediate Data

Optimised Data

Financial Model

Simple Discount

Intermediate Model

Complex Model

Incentive Model

None

UK Based Only

Worldwide Models

O & M Model

Simplistic Model

Complex Model

Optimised Model

WTG Model

Experience Based

Concept Design

FEED Design

OSP Model

Experience Based

Concept Design

FEED Design

Wind Yield Model

Mean Wind Speed

Derived Data

CFD Data

CAPEX Model

Fabrication Data

Installation Vessels

Site-Wide Costs

18 October 2017 1313

Interpretation of geophysics profiles

Desk study, geological info

Geotech. data , logs & lab test results

Data integration & analysis

3-D Ground model Terrain unit map Soil parameters

Geotechnical Issues

Geotechnical Design ConsiderationsGeotechnical data available in stages

• Progressive confirmation of design?

• Not the most efficient process

Site wide study of pile response

• Identify relative stiffness of piles (stick up / soil)

• Define bounding conditions (upper and lower bounds)

Seabed variability and uncertainty

Pile driving design

• Drivability, strength, fatigue, buckling, contingency for refusals

Designs based on worst case (bookend approach)

• Not efficient for the design of most locations

Bespoke vs Clustered DesignClustering principles:

• As much similarity as possible across site

– Despite water depth variation over site

and significant soil variability

• Consistent upper structure & Transition

Piece, common foot print for single standard

jacket piling template and seafastening

• Design and fabrication efficiency and

• but at cost – design must be for the worst

case across the cluster/site

Bespoke Design:

• Greater weight efficiency can be achieved,

but is this an improvement on clustering

• What is the optimum balance?

Design of structure in distinct “clusters” with

variable pre-piling stick-up at mudline

Example of Clustering and Design Efficiency

Cluster 1

Cluster 3

Cluster 2

Main design at bounding locations

per Cluster

Design must cover other

bounding locations for site

Check on

intermediate

cluster by

interpolation

Wind

Wave & current

Largemoment

Design Integration• Design efficiency depends on the integration of wind and

wave loading

• Traditional approach is still based on onshore turbines

where the interface is at the base of the tower

• The substructure designer is presented with a “fait

accompli”, the tower design is frozen

• Greater design efficiency could be offered by integration of

the substructure and tower

• Design loading is also developed based on the wind first

principle, wave loading is related to it

• GBFs, larger diameter monopiles and parts of jackets are

increasingly dominated by wave effects, not so much wind

• Design improvements may be offered by integrated, wave-

first design

• Tower design different to monopiles, more efficient, lessons

to learn?

Secondary Steel and Appurtenances

Boat landing design

• Single or dual boatlandings?

• Vertical or inclined?

• Fixed or replaceable?

• What orientations?

• What impact criteria (ULS and ALS)?

• Prevent damage to primary steel?

• Design standardisation required

Other design issues

• Is the current interface level optimum?

• Provision of more facilities into tower?

• Requirements differ from project to project

• Standardisation of appurtenances?

• Better corrosion protection design

Wind and

wave loads

Stress in leg,

induced

sympathetic

strain in J-

Tube

Loading

induced in

supports

Stab-in

Future

flanges

Loading

induced in

supports

Stab-in

Fatigue Design ImprovementsFatigue is normally the key driver in WTG support

structure design

• Rules and guidance typically based on oil and gas structures

and loading (not axial in chord)

• Update of empirical Stress Concentration Factors for WTGs?

• Bespoke Stress Influence Functions based on FEA required

for design efficiency, but slows design

• Ongoing large scale joint tests under way for development of

SN curves

Significant Axial

Chord Forces

Lower

magnitude but

still important

brace axial

and bending

loads

Fabrication EfficiencyDesign for specific fabricator

or keep options open?

Options for construction of jacket:

• Vertical construction and assembly

• Horizontal construction in shed

• Subsequent upending to vertical

Options for member sizes:

• Standard or fabricated sections?

• Preferred rolled sizes differ

• D/t limits for rolled sections

Options for welded assembly:

• Point to point or nodal construction?

• Automated node welding available?

• Single or double-sided joint welds?

• Location of closure welds in legs?

Vertical Assembly Upending to Vertical

Image: Smulders

Designing an efficient structure for one fabricator is not necessarily efficient for others

Transportation and Installation IssuesVertical transportation and lift

• Preferred if lift vessel hook heights permit

• Care with barge stability and jacket design stresses

• Onerous seafastening design

• More efficiency / automation needed

Horizontal transportation and upending

• Required if hook height insufficient

• Option more expensive than vertical transportation

Other Issues

• Simple Noble Denton transport criteria conservative

• Based on oil and gas, better guidance for WTGs?

• Pile driving fatigue prevents attachment of appurtenances on

monopiles

• Blue hammer technology?

Vertical Transportation

Upending

from

Horizontal

to Vertical

Rotation

frame

Monitoring and Design FeedbackProject Scopes

Reassessment of the adequacy of existing structural designs based on monitoring results, understanding of real-world structural response

22

Lessons Learnt

• Long term shaft performance from monitoring data

• Pile-soil gapping impact on monopile performance

• The importance of natural frequency of monopiles to turbine loading

• The potential conservatisms in conventional fatigue design

• Need to compile database across the industry and incorporate findings into codes

• Integration with turbine monitoring

Design Data

Recorded Data

Summary• Holistic Wind Farm Design

• Geotechnical Considerations

• Bespoke vs Clustered Design

• Design Integration

• Secondary Steel and Appurtenances

• Fatigue Design Improvements

• Fabrication Efficiency

• Transportation and Installation Issues

• Monitoring and Design Feedback