Conference

“Deeper Water Offshore Wind”

Pros & Cons of various

foundation design &

installation methodologies

Dr.-Ing. Marc Seidel

Leading Expert

REpower Systems SE

2

Introduction and overview

Topics for this conference

Conference Programme:

Is there an optimum type of foundation for deeper, more hostile waters?

Which are the best technologies and is standardisation possible?

Investigating the most cost-effective foundation installation methodologies

available and being developed today – new technologies, speed installation,

project optimisation, cost reductions

Analysing the design and installation constraints and structural integrity

issues related to foundation loads, dynamic loads and the increased

weights of larger turbines in deeper waters

3

Substructures for deeper waters

Substructures – Some options

Currently employed substructure types:

Jackets

Gravity Base Structures (GBS)

(Tripods, Tri-Piles)

Promising novel types:

Keystone Twisted jacket

Universal foundation suction bucket

4

Project experience with jackets

Beatrice Windfarm

Demonstrator (2)

Thorntonbank

Phase 1, 2 & 3

(48 jackets)

alpha ventus (6)

Ormonde (30)

6 Projects with jackets:

- 63 jackets installed

- additional 72 in fabrication

Nordsee Ost (48)

Bremerhaven (1)

5

Advantages of jacket substructures

Advantages of jacket substructures for offshore wind turbines

High structural stiffness: The turbine behaves nearly like an onshore turbine,

virtually no wave-induced vibrations.

Light-weight: Compared to other structures, jackets are the lightest. This is

beneficial for material cost and installation.

Potential for large supplier base: Jackets are not complicated and don’t need

large wall thicknesses. Potentially many suppliers (with sufficient space) can

build them.

Water depth: Jackets can be used for a large range of water depths – from

about 20m to (at least) 60m.

Site conditions: Jackets can be used in nearly all conditions. Waves can be

very high and soil conditions are much less relevant as for a monopile.

6

Project experience with jackets

Ormonde – Pre-piling

Source: http://www.gaga1.be/EN/Projects_post.html?postId=61

Methodology significantly

improved compared to

alpha ventus

Time required for pre-

piling: 1,67 day per

location, ex weather

Maximum speed: One

location in 24h

In practice all piles were

driven during 2,5 months

Further improvements for

Thorntonbank

7

Project experience with jackets

Ormonde – Jacket installation

Photo: http://www.foundocean.com/webpac_content/global/documents/more/Case%20Studies/Case%20Study%20-%20Ormonde%20Offshore%20Wind%20Farm.pdf

Nr. of days jacket lifting 31

Nr. of days topside lifting 2

Nr. of days additional work 11

Nr. of days wait for others 20

Nr. of days WOW 26

Nr. of days total 90

Jacket installation: Less than one day

per jacket lifting operation for HLV

Rambiz!

8

Bremerhaven prototype

Bremerhaven Prototype

REpower-owned design

All structural calculations performed in-house – no external consultants

Newly developed Transition Node

Very light and slender construction: Transition node: 48t Jacket tubulars: 206t Castings: 61t Total: 315t (Weights are without internals, secondary items, etc.; weights for tubulars will increase for offshore applications)

9

Bremerhaven prototype

Advantages of cast design

Only simple circumferential welds

Quality control can be automated and is more reliable

Cast elements have a high fatigue and ultimate capacity

Nodal angles can be varied on a broader scale

10

Gravity Base Structures

Pro

Concrete is cheap

No noise emission

Offered by a number of experienced European construction companies

Well known proven concrete technology

Highly resistant to damage by salt water

Maintenance costs are low

Fully removable

Contra

New approach for deep water

Complex permission - legal issues

Applicable to limited water depth depending on location

Detailed soil investigation is required

Applicability depending on sea bed / soil conditions; suitable for soils with high bearing capacity only (dredging to some extent possible)

Seabed preparation normally required

Scour protection is normally needed

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Gravity Base Structures

Thorntonbank Phase 1

12

Gravity Base Structures

Strabag Serial System

13

Gravity Base Structures

Strabag Serial System

14

Gravity Base Structures

Gifford/BMT/Freyssinet concept (http://gbf.eu.com/)

Installation of pre-assembled, pre-commissioned WT with foundation: - Lower weather risk? - Early revenue? What about cable installation??

Purpose-built transport and installation barge

Seabed preparation?

15

Gravity Base Structures

Seatower design (http://www.seatower.com)

Combined steel / concrete solution

Steel parts prefabricated and transported to construction site

Lower part consists of steel skirt and concrete body – constructed and casted at the construction site

Installation up to HS = 2.0m with standard tugs

Concrete injected in void below bottom slab and structure ballasted with sand

16

Gravity Base Structures

Gravitas (Arup, Hochtief, Costain) design (http://www.gravitasoffshore.com)

Claims to minimise seabed preparation “by accommodating existing seabed slopes and surface sediments“

Skirt variants to suit seabed soil conditions

Self-buoyant, installed with standard tugs

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Gravity Base Structures

Where are the limits?

Main influencing factors:

Soil conditions

Wave climate (heights, periods, directionality)

„Allowable“ weight & size for chosen installation method

Logistics and capability to produce, store and install

Guesstimates:

North Sea, typical soil with dense sands: - app. 45-55m

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Promising novel concepts

Keystone “Twisted jacket”

Advantages:

Few members, few welds

Small “guide structure”

No under-water pile driving

Low weight

Most of the weight is in the cheap piles

Disadvantages:

Pile splices required

Several grouted connections

Two different pile sizes

Inclined pile driving

Noise mitigation difficult due to complex structure

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Promising novel concepts

Keystone “Twisted jacket”

Design exercise for a German North Sea project:

REpower 6M turbine, 126m rotor diameter

40m water depth

Extreme wave 20.8m

Sandy soils

Model built in ANSYS ASAS(NL), based on Keystone SACS model

20

Promising novel concepts

Keystone “Twisted jacket” – Met mast under tow on Kiel channel

21

Promising novel concepts

Keystone “Twisted jacket” – Met mast under tow on Kiel channel

22

Promising novel concepts

Universal foundation

Advantages:

No piling noise issues

Leveling can be achieved during installation process

Quick installation process

Simple decommissioning

Disadvantages:

Relatively complex steel structure at seabed (difficult to inspect)

Fabrication cost and weight?

Large wave loading due to large diameter structure

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Standardisation

Is standardisation possible?

Jackets:

Standardisation only possible for construction principles

Difficulties for more general standardisation are variations in water depths, differences in loading and pile capacities (footprint)

GBS:

Standardisation of bottom part possible if ground conditions sufficiently homogenous

Variation in water depth can be easily accommodated

Universal foundation / Twisted jacket:

Standardisation probably similarly difficult as for standard jacket

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Critical points in the design and design process

Load simulations – wind turbine and substructure

Fle

x 5

A

NS

YS

AS

AS

(NL

)

Responsibility of the external designer

FE model of the jacket – can contain wide range of standard finite elements

Input format is precisely defined by REpower, such that external models can be directly used within our environment without any further modification

This has worked very well for four projects already

REpower‘s tool for aeroelastic simulation

28 Degrees of Freedom, 6 for the substructure

Turbulent wind field

Controller behaviour, electrical system

Very fast and efficient tool

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Critical points in the design and design process

GBS / Keystone / Universal

Excitation of global vibrations by

waves in fundamental mode

significant

Misaligned waves may cause large

fatigue loads in support structure

Detailed consideration of wind-wave-

misalignment is required

Soil data most important parameter

for load simulations (stiffness and

damping contribution)

Conventional jacket

Stiff jacket structure prevents global

vibrations to be excited

Wind-wave-misalignment completely

meaningless!

Only local (quasi-static) wave loads

on jacket and appurtenances must

be considered

Soil properties do not have

significant influence on the design

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Summary

Summary

Currently jackets are the most mature option for deeper water

Several GBS options are offered to the market – all of them are specific to

one supplier

Key factors are fabrication and logistics – may be attractive depending on

the project specific conditions

Promising new concepts are the “Keystone jacket” and the “Universal

foundation”

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REpower Offshore Engineering