Ørsted’s suction bucket jackets: observations from … · one of the solutions: the suction...

ØRSTED’S SUCTION BUCKET JACKETS:Observations from Borkum Riffgrund 1 and 2

DGFGeotechnical Engineering in Offshore Wind

Avi ShonbergIn collaboration with Michael Harte, Amin

Aghakouchak, Cameron S. Brown, Miguel Pacheco Andrade and Morten A. Liingaard

February 7th 2019

▪ Introduction and background

▪ Results from Borkum Riffgrund 1

-Inclinations

-Vertical forces

-Drainage characteristics

▪ Borkum Riffgrund 2 installation

-Summary

-Challenges

▪ Summary and conclusions

2

▪ In 2013, Ørsted saw the need for a

foundation solution which could

accommodate future challenges

▪ What are the future challenges for offshore

foundations?

- Deeper waters

- Bigger turbines

- Noise restrictions during installation

▪ One of the solutions: the suction bucket

jacket!

- Tripod jacket supported on suction buckets

- A ‘prototype’ SBJ was designed and

installed at Borkum Riffgrund 1 in 2014

- 20 SBJ’s were installed at Borkum

Riffgrund 2 in 2018

3

Bigger loads

▪ Borkum Riffgrund 1 (BKR01) offshore wind farm:

- German sector of North Sea (38 km off the island of Borkum)

- 312 MW windfarm

- 78 no. Seimens 4MW wind turbine generators (WTG) with 60 m blades

- 77 monopile foundations, 1 suction bucket jacket (SBJ)

- Suction bucket dimensions:

o Diameter: 8 m

o Skirt length: 8 m

▪ Soil and site conditions at SBJ location:

- Water depth: 24.4 m LAT

- Predominantly sandy

- Potential silt layers between 3 m and 6 m below sea floor (bsf)

4

Germany

Germany

UK

Denmark

Netherlands

BelgiumSource: Wikipedia

0 m bsf

2.9 m bsf

5.6 m bsf

SAND Dr =100 % FC = 5 – 10 %

SAND, some layers of silty sand to sandy silt. Dr = 70 % FC = 5 – 20 %

SAND Dr = 90 % FC = 5 – 15 %

▪ For this study, measurements utilised are:

- Forces through the clean section

- Inclination at the transition piece (TP)

- Pore water pressure

▪ Bucket BC is the most heavily instrumented

▪ For this study, data from September 2014 to Jan 2016 is utilised.

- The turbine was installed in January 2015

5


▪ Results

- Inclinations

-Vertical forces




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▪ During standstill only periods, inclination is observed to:

- Be very small

- Vary slightly with significant events (wind turbine installation)

- Have a significant reduction in scatter after first power (less

standstill)

▪ For all monitoring data, inclination vs wind speed is plotted

- Maximum short term inclinations correlate with the rated wind

speed (12-13 m/s) of the Siemens 4 MW WTG.

- As wind speed increases past the rated wind speed, loads

reduce and inclination reduces

- Scatter is a function of wind direction

- Data cloud at low inclinations represents non operational

times (dead load on the foundations)

7


▪ Results

- Inclinations

-Vertical forces




8

9

Static conditions

(standstill periods)

Load applied

(from ‘west’)

Load applied

(from ‘east’)

BC AC AB

Not to scale

Load and direction

10

BC

AC

AB


▪ Results

- Inclinations

-Vertical forces


▪ Borkum Riffgrund 2 installations


11

12

Theory:

During very short load durations (e.g. a

wave), all load is transferred from the clean

section to the PWP and therefore excess

PWP is generated

i.e. fully undrained conditions

During longer loading durations (e.g.

variable winds over an hour period), some

excess PWP can dissipate

i.e. drained (or partially drained) conditions

Pressure

beneath lid

Pressure at inside

of skirt tip

Pore pressures were measured at two positions

,cs

13

Pressure

beneath lid

FPWP

Pressure at inside

of skirt tip

FWP,2

,cs

Fz

Fz = Fz, cs / Alid

14

If Fz ≠ FWP,2 a gradient inside the bucket

exists (i.e. (partially) drained conditions )

Pressure

beneath lid

Pressure at inside

of skirt tip

FWP,2

Equipotential lines

If Fz = FWP,2 all load is transferred from the clean section

to the PWP (i.e. fully undrained conditions)

14

Pressure

beneath lid

Pressure at

inside of skirt

tip, FWP,2

Equipotential lines

,cs ,cs

Fz Fz

After integrating :

▪ 64% of vertical pressure observed in PWP response beneath lid

▪ 42% of vertical pressure observed in PWP response at skirt tip

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Note: 10 minute averages

Vertical pressure = Force measured in clean

section converted to an equivalent pressure

across the lid area)

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High frequency = undrained response

Fz = Force measured at lid

FWP = PWP measured beneath lid (converted to a force)

High frequency = undrained response

Low frequency = partially drained response

with a return to steady state after 100 s

Generating excess PWP Dissipating excess PWP

17

Gain

(/

100)

OR

If Fz = FWP, all load is transferred from the clean

section to the PWP

(i.e. fully undrained conditions)

DATA FROM ONE DAY

Reduced ‘coherence’

• As the waves pass over the bucket, the static

head pressure outside the bucket changes

and the coherence between input load and

excess pore water pressure decreases

• Shows up as energy being added to the

excess pore water pressure response in this

frequency range

18

Gain

(/

100)

OR

Drainage characteristics are different for the

different buckets:

• Bucket AB has higher compressive load

compared to Bucket BC (on average)

• Loading direction (and therefore mean load)

has an effect on the drainage observed

ALL DATA

19

Gain

(/

100)

OR

Drained Partially

DrainedUndrained

46 mins 2.5 mins

When combined for all data, the drainage

characteristics of this particular soil profile

show that:

• Most high frequency loading is undrained

(up to about 2 minutes)

• The drained/undrained ‘boundaries’

vary with loading direction (and

therefore mean load)

ALL DATA


▪ Results

- Inclinations

-Vertical forces


▪ Borkum Riffgrund 2 installations


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▪ Taking the next step with suction buckets for an offshore windfarm project

▪ 20 SBJs installed

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BKR01 BKR02

▪ Some facts from the BKR02 SBJ installation

o All 20 structures installed within an 8 week

period

• First installation on June 2nd, 2018

• Installation completed on July 30th, 2018

▪ Some important new measurements

o Flow through the pump (and therefore

through the soil)

o Soil plungers

▪ 2 structures with full

monitoring setup

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▪ Wind farms are getting bigger – with bigger turbines, deeper waters and larger

loads. SBJ presents an attractive alternative.

▪ The BKR01 SBJ is a heavily monitored, unique foundation concept installed in

predominantly non-cohesive materials

▪ The BKR01 results provide an insight into the behaviour of a full scale structure:

-The BKR01 SBJ is stable and inclinations are as expected (low)

-Loads distribute as expected

- Using the PWP measurements, we can assess the drainage response

-The drainage response is more undrained than expected

-The drainage response is dependent on the loading direction

▪ BKR02 installations were very successful

-20 SBJs installed to target depth

-Monitoring of 2 structures at BKR02 will provide significant further insight

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Ørsted’s suction bucket jackets: observations from … · one of the solutions: the suction...

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