dr. david igoe delivering the most progressive, reliable ... challenges... · engineers ireland /...

Engineers Ireland / ICE Presentation 2017 – David Igoe

Delivering the most progressive, reliable, and efficient geotechnical designs across a wide variety of situations.

Geotechnical Challenges in Offshore Engineering

Dr. David IgoeBE PhD CEng MIEI

BE in Civil Engineering (UCD 2005)PhD in Offshore Pile Design (UCD 2009)Postdoctoral Researcher (UCD 2010 – 2015)Head of Offshore at GDG (2016 – Present)


Introduction

Key Geotechnical Challenges Monopiles

Jacket Structures

Gravity Base Foundations

Research Projects PISA (Pile Stability Analysis)

Pile Ageing

DemoGravi3

Other Projects

Conclusions

Overview


Introduction


Offshore Wind


Offshore Wind

European Offshore Wind Installations up to 2015 (EWEA)


Offshore Wind

Economics

Pre 2015, average price >€100 / MWh

July 2016 - Borssele 1&2 wind won at €72 per MWh (Dong Energy)

Oct 2016 - Kriegers Flak won at €60 per MWh (Vattenfall)

Dec 2016 – Borssele 3&4 won at €54.5 / MWh (Van Oord / Shell)

March 2017 -


Offshore Wind

Economics

Pre 2015, average price >€100 / MWh

July 2016 - Borssele 1&2 wind won at €72 per MWh (Dong Energy)

Oct 2016 - Kriegers Flak won at €60 per MWh (Vattenfall)

Dec 2016 – Borssele 3&4 won at €54.5 / MWh

March 2017 – First Subsidy-free offshore wind bid – Dong Energy


Offshore Wind –Geotechnical Engineering Challenges

Foundation Design

Monopiles Lateral Loading

Cyclic Loading

Tripods / Jacket Structures Pile Axial Loading

Ageing

Cyclic Loading

Gravity Base Shallow Foundations

Cyclic Loading


Monopile Design

Traditional Monopile Design

Lateral Pile Response typically using 1D FE ‘p-y’ approach


Monopile Design

Traditional Monopile Design

Lateral Pile Response typically using 1D FE ‘p-y’ approach

API ‘p-y’ curves derived from small diameter (0.6m diameter), slender pile tests –Not suitable for monopiles typically >4m

API approach thought to be conservative for monopiles

Implemented in 1D FE model software (e.g. LPile / OasysALP)


Monopile Design - ULS

Monopile Design – ULS case

When large extreme load hits structure - Structural and Soil Integrity Checks. Including scour corrosion and cyclic degradation due the environmental and turbine loading.

Ensure deflection/rotation does not become excessive (<2 degrees)

Pile Length Check – Change in length should not significantly effect mudline rotation – 10% criteria


Monopile Design - SLS

Monopile Design – SLS case

Ensure permanent accumulated rotation due to cyclic loading does not exceed specified tolerance – typically 0.5 degrees

Hettler approach

Values of t and Neq are calculated from rainflowcounting of a BSH storm event

0.16 < t < 0.22 (Seed et al.)

Neq typically <15


Monopile Design – FLS & DLS

Monopile Design – FLS & DLS case

Structural fatigue checks - Materials within structure to last beyond specified design life. Fatigue check performed using linearized ‘p-y’ springs

Should include driveability analysis – Stresses and blowcounts from driveability analysis used in structural fatigue checks

Dynamic check to ensure natural frequency of structure lies outside excitation frequency bands to avoid resonance.


Monopile Research – PISA project

PISA Overview

Led by Dong Energy and funded through the Carbon Trust Offshore Wind Accelerator program

Academic Work Group involving Oxford, Imperial College and UCD

Total Budget ≈ £3m over 2.5 years. Started in August 2013

Project aim was to develop a new design methodology for laterally loaded piles for the offshore wind industry

Two Pronged Approach Campaign of Monopile field tests

3D FE modelling of field tests and parametric study



DL1

DL2 DM1DM2

DM3

DM4

DM5

DM6DM7

DM9

DR1

DS1-421m

12m

PISA Overview

Field testing began with pile installation in October 2014. Final load testing completed 24th July 2015.

Two test sites with 14 pile tests performed at each site



z

PL

PH

AL

AH

STICK UP INCLINOMETERS - SUI

LOAD CELL - LCL

RAMSTROKE

DISPLACEMENT TRANSDUCERS - LVDT

FIBRE OPTICS - FOS

INCLINOMETERS - IPI

EXTENSOMETERS - IPE

PIEZOMETERS - PZT

M/H

D

L


Monopile Research – Blessington Tests

Blessington Monopile Research

15 Piles - Lateral Static Load Tests

3 Piles - Lateral Cyclic Tests


Monopile Research – RWE Vibro Tests

Other Monopile Research

RWE Vibro Project

5 x 4.5m Diameter Pile Tests

Comparison between vibrated and impact driven installation

Vibrated Installation can:

Reduce time of installation

Reduce noise and environmental impact to sealife

Reduce Cost


Monopile Design Optimisation - GDG



Foundation Design


Cyclic Loading


Pile Ageing


Cyclic Loading


Tripods / Jacket structures – Pile Axial Loading and Ageing

Axial Pile Design

Jacket structures reasonably well known – challenge for design optimisation

Jackets piles primarily resist overturning moments through ‘push-pull’ (tension and compression) axial loading

API main text approach uses traditional earth pressure (Beta) approach in sands and total stress (alpha) approach in clays for estimating shaft friction.

Database research indicate CPT based method preferable (ICP, UWA, NGI, Fugro) – offer better reliability and more optimised design

ICP, UWA, NGI, Fugro methods included in API design guide commentary section and are in theory preferred method

ICP method has been used by shell UK for the past 10 years



Pile Ageing

Primary item of debate in various parts of the offshore engineering community (OTC, API/ISO meetings, ISFOG 2015)

Shaft capacity of piles seen to increase by up to 250% over 120 days after installation

Initial testing performed by Imperial College. Later tests by UWA, UCD and Norwegian Geotechnical Institute (NGI) show similar findings.

0

0.5

1

1.5

2

2.5

3

1 10 100 1000

No

rmal

ise

d C

apac

ity,

Q/Q

ICP

Time after Driving (Days)

Blessington

Dunkirk

EURIPIDES

Hound Point

Padre Island

I-880

ICP Intact Ageing

Characteristic



Pile Ageing

In theory, up to 50% on pile length can be saved allowing for 90 days ageing

Typically piles are driven first

After >30 days jacket structures / Tripods are installed

After >90 days Turbines are installed

Max loading will occur a minimum of 90 days after pile installation

Huge potential for cost and steel savings

Further Research required for savings to be realised

0

10

20

30

40

50

60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Req

uir

ed P

ile L

engt

h (

m)

Location No.

Pile Required Length (10-days) Pile Required Length (90-days)

0%

10%

20%

30%

40%

50%

60%

70%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Per

cen

tage

Sav

ing

Location No.



Foundation Design


Cyclic Loading


Ageing

Cyclic Loading


Cyclic Loading


Gravity Bases – GDG

DemoGravi3

H2020 Full Scale Demonstrator Project

Novel Tripod Gravity Base Structure

Self-Buoyant during transport

Ballasted during installation

Relies on Ballasted Weight to prevent sliding and overturning

Currently under fabrication

To be installed – Sep / Oct 2017


Gravity Bases – GDG research

DemoGravi3

GDG responsible for Geotechnical Design

Detailed analysis of cyclic pore pressure response

Advanced 3D FE analysis and analytical checks



Gravity Base Design

Bearing Capacity

Settlement / Differential

Sliding Change in design guidance DNV (2014)

Pre 2014 – Hd < Vd . tan(phi) < 0.4

Post 2014 - Hd < r. Vd . tan(phi)

R is roughness parameter which is 1.0 for soil – soil or <1 for soil structure



Conclusions

Offshore Wind Industry undergoing significant expansion over the comes decade

Costs related to offshore wind have reduced dramatically over the past 12 months

Ongoing research into offshore foundations starting to be realised by industry – design optimisation

Significant growth expected in offshore renewables and need for talented engineers across Europe

Interesting, Innovative and Exciting area for Engineers


Engineering a sustainable future

The End – Thanks For Listening!

dr. david igoe delivering the most progressive, reliable ... challenges... · engineers ireland /...

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