numerical analysis of jacket foundations

Numerical analysis of jacket foundations: a coupled shell-

beam approach.

Annabel Lush

MScR Student, Computational Mechanics Group,

School of Engineering and Computing Sciences.

Supergen Wind 2: 7th Training Seminar, 4th September 2013.

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Jacket Foundations

IMAGES - web

2 Images: www.ship-technology.com; www.rwe.com

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Foundation Design Competition

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Suction Bucket

Monopile

Twisted Jacket Gravity Structure

Suction Bucket Tripod

Images: www.energyengineering.co.uk

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Inward Battered Guide Structure

IBGS – a twisted-jacket foundation designed by Keystone Engineering.

• 20% CAPEX reduction.

• Compact design – 2x more jackets per transportation vessel.

• Supports turbines ≥ 5MW.

4 Image: www.keystoneengr.com

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IBGS Installation Process

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(I) (II) (III)

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Research Aims

I. Develop a bespoke analysis tool to model the IBGS, using the Finite Element Method.

II. Understand and improve the approach for coupling beam and shell finite elements to achieve a rapid analysis capability.

III. Understand the structural behaviour of the IBGS and highlight design modifications if appropriate.

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Preliminary Analysis

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To understand the general structural behaviour of the IBGS using beam elements.

Photo: www.carbontrust.com

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3D Beam Element

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Linear beam element: 3 translations and 3 rotations per node.

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Beam vs. Brick Elements

A cantilever with an end point load.

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Element type Beam Brick

No. of elements 1 100

No. of DOF 12 1212

Deflection (mm) - 0.1905 - 0.1905

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A B

C

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Combined axial/bending stresses (MPa) per load case:

Preliminary Analysis – Results (I)

Jacket-leg Fxx Fyy Fzz Mxx Myy Mzz

A 74 - 113 - 2 5 5 - 7

B - 133 - 76 - 2 3 3 - 7

C 63 121 - 2 - 4 - 4 - 7

A 142 59 - 4 3 - 1 - 22

B - 69 - 120 - 4 1 2 - 22

C - 69 123 - 4 - 1 2 22

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60° twist

0° twist

x

y

z

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12 Data: UpWind Report 2011 (Table 6.5)

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Combined axial/bending stresses (MPa) per load case:

Preliminary Analysis – Results (II)

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60° twist

0° twist

Jacket-leg Fxx Fyy Fzz Mxx Myy Mzz

A 15 - 11 - 24 47 - 58 - 7

B - 27 - 8 - 24 31 109 - 7

C 13 12 - 24 - 45 - 82 - 7

A 28 6 - 36 27 - 19 - 22

B - 14 - 12 - 36 14 48 - 22

C - 14 12 - 36 - 13 - 48 22

x

y

z

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Preliminary Analysis – Results (II)

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Total combined axial and bending stresses.

60° twist 0° twist

A - 38 B 74

C - 133

A - 16

B - 22

C - 77

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x

y

z

Pile height 5-60m

Jacket height 30m IBGS

height

Nacelle height

Water surface

Image: www.timesdispatch.com

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Preliminary Analysis – Results (III)

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60° twist

0° twist

Pile height (m)

L2-norm of nacelle

displacement (mm)

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Creating a more detailed model

Shell elements used to give detail of stresses at joints:

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MITC9 shell element: 3 translations (not shown) and 2 rotations per node (β1, β2).

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Coupling of beam and shell elements is enforced by:

• reducing the system to be solved, and

• developing relationships to link the shell and beam DOF.

Creating a more detailed model

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Coupled Analysis

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• To examine the stresses at each joint.

• To assess the influence of the jacket twist on these stresses.

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Observations

I. Removing the twist in the IBGS increases the stiffness under the analysed loads cases.

II. An efficient method is employed in the coupled analysis.

III. The analysis tool developed could be used to optimise other tubular structures.

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Further Research Directions

Extend analysis code to include:

• dynamic analysis,

• soil-pile interaction,

• effects of scouring, and

• greater detail around structural joints through using continuum elements on fillets.

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This research was conducted under the Dong Energy

Durham University Partnership, with supervision from

Dr William Coombs, Professor Jon Trevelyan and

Professor Roger Crouch (City University).

Email: a.m.lush@durham.ac.uk

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