TIDAL TURBINE FOUNDATION OPTIMISATION

RAMBOLL ENERGYMOJO MARITIME

INTRODUCING RAMBOLL

• Ramboll is a leading engineering, design and consultancy company founded in Denmark in 1945

• Today, we employ more than 10,000 ambitious experts.

• Ramboll has a significant presence in Northern Europe, India, Russia and the Middle East

• With close to 200 offices in 20 countries we emphasise local expertise combined with a global knowledge-base.

INTRODUCING MOJO MARITIME

• Mojo provide Marine Operations Management and Consultancy Services to the Offshore Renewable Energy Sector.

WINDWAVE TIDE

TIDAL FOUNDATIONS

• Tidal foundations have huge challenges: uneven rocky seabeds, high current speeds, water turbulence and limited access.

• At present foundations are trials and so are not suitable for mass production. The next stage is a repeatable structure.

• Ramboll and mojomaritime are drawing on considerable experence in marine engineering from both oil and gas and offshore wind sectors to derive a solution.

CONTEXT OF PRESENTATION - FOUNDATION DESIGN AND INSTALLATION STUDY• Working on a 10MW array in NE Scotland.• Concept phase leading to reduction to 4 concepts.• Review of each concept leading to elimination two (Gravity base

and Duopod) and development of two (Monopile and Tripod).• Tripod selected for detail design.• Modelled and analysed ULS, ALS, FLS, SLS.

FOUNDATION DESIGN AND INSTALLATION “INTER-TWINED”

FOUNDATION DESIGN AND INSTALLATION“INTER-TWINED”

• Must be considered together, foundation design impacts installation – a major project cost driver.

• It goes beyond installation - the design of foundation also impacts the cost of other interventions:

Decommissioning · Cable connection · Turbine O&M

GBP/day TYPICAL VESSELS FOUNDATION DESIGNS

METHODOLOGY

150,000 Dynamic Positioning (DP) HLV Monopile Topside drilling O.hydro, Pulse, MCT

90,000 Moored Heavy Lift Vessel (HLV) Pin Piles or Subsea Drilling TGL

60,000 DP Offshore Construction Vessel Pre-installed pin piles

30,000 Jackup Barges Gravity Base Monoblock Open hydro, Voith

15,000 Moored Crane Barges Modular Atlantis, HSUK

5,000 Multicats Floating ?

FOUNDATION OPTIONS

• Gravity Bases• Moored• Seabed Engagement

Note that all of these require gravity at some stage!

GRAVITY BASES

GRAVITY BASE– STRUCTURAL CONSIDERATIONS 1 of 2

Option 1Floating Gravity Base

Option 2aStreamlined Modular

GBA

Option 2bTripod Modular GBA

Modular Gravity

• Sensitive to seabed slope, limit <10°•Less technical risk than floating GB•Lower potential for cost reduction at array scale• More expensive vessels required

Floating Gravity

• Sensitive to seabed slope, limit <10°•Novel Concept = High Risk• Minimal Vessel Requirements•High CapEx investment• Investment recovered for large array deployment

GRAVITY BASE– STRUCTURAL CONSIDERATIONS 2 of 2

Conclusion• Substantial mass required - expensive• Highest risk option• Minimal vessel requirements

GRAVITY BASES – INSTALLATION CONSIDERATIONS

Monoblock GBA

– Attractive solution, marine operations simple and quick

– Ideal for smaller scale devices and prototype deployment

– BUT… unlikely to be long term foundation solution:

– Limited in scalability– Little opportunity for cost reduction– Not applicable at sites with significant seabed slopes– Carbon footprint large for big lumps of steel/concrete– Long term reliability concerns

Modular GBA

Floating GBA12

MOORED

MOORED – INSTALLATION CONSIDERATIONS

• Still requires fixation by GBA or seabed engagement

• Potential application at some sites

• Obvious O&M benefits• But introduces some additional

considerations:• Dynamic export cable• Mooring spread vs. array layout• Dynamic platform• Failure modes• Needs Naval Architecture

Hydra tidal moored device

SEABED ENGAGEMENT

MONOPILE – STRUCTURAL CONSIDERATIONS

Design Conclusions• Pile Diameter at upper limit of drilling capability

• Simple design and proven construction

• Fatigue governs so ULS utilisation ratios are low

•Feasible and well researched concept

• Material thickness / diameters governed by fatigue

• Low ULS/fatigue life ratios

• Simple Fabrication, weld automation

• No fatigue sensitive joints

DUOPOD– STRUCTURAL CONSIDERATIONS

• The Duopod benefits from bi-directional flow, resulting in more axial load path rather than bending in a monopile.

• Reduction in pile diameter.• Can have problems with

alignment to flow.

TRIPOD–STRUCTURAL CONSIDERATIONS

•Similar to Duopod in the manner that axial load paths are set up.•Smaller pile diameter than monopile.•Additional leg reduces flow direction problem presented by the Duopod.•Weight reductions compared to monopile.•Angles dependant on turbine rotor exclusion zone.

JACKET–STRUCTURAL CONSIDERATIONS

•Additional weight savings over monopile.•Use in larger water depths.•Majority of forces in axial manner.•Reduction in pile diameter however multiple piles leading to installation complexity. •Angles dependant on turbine rotor exclusion zone.

JACKUPS AND TOPSIDE DRILLING 1 of 2

– Topside drilling requires stable platform such as a Jack up barge.

– BUT... successful application of Jack ups in tidal races is limited.

JACKUPS AND TOPSIDE DRILLING 2 of 2

– Jack ups in tidal races

• Out of class operation

• Possible stability/VIV issues

• Susceptible to weather downtime

• Depth limited

• Expensive day rates

• Restricted availability

Topside drilling from a DP vessel or moored barge? Not likely.

Bottom line: If “no” to jackups it’s “no” to topside drilling.

SUBSEA DRILLING 1 of 3

Post-install piles

• Drilling within foot sleeves of jacket held temporarily in place under gravity.

• Smaller diameter percussive drilling demonstrated by TGL

Pre-install pile(s)

• Drilling through template placed temporarily on seabed

• Monopile, Duopod, Tripod, Quadrapod etc. (or anchor piles for moored solutions)

22

SUBSEA DRILLING 2 of 3

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SUBSEA DRILLING 3 of 3

CONCEPT t = 0

DESIGN t = 3 months

BUILT & TESTEDt = 6 months

CONCLUSIONS

CONCEPT ELIMINATION PROCESSMore detailed study of risk, cost and schedule for 4 options

TIDAL FOUNDATIONS

• Every tidal foundation is unique due to site requirements and device needs.

• For an optimum foundation which is cost effective and efficient, every aspect has to be individually analyised.

CONCLUSIONS – INSTALLATION 1 of 4

• No “one size fits all” solution when considering various sites and devices:

SITE DEPTH may preclude the use of jackups

SEABED CONDITIONS overburden, voids and other drilling challenges

SLOPE on bottom stability by gravity, levelling for drilling

CURRENT VELOCITIES don’t forget forces~v2, best DP OCVs max 6kn

WAVE EXPOSURE impacts design and marine ops

ENVIRONMENTAL never under-estimate these factors

LOGISTICS/VESSEL AVAILABILITY muddle through with a barge?

CONCLUSIONS – INSTALLATION 2 of 4

• The cost (and technical ability) of intervention is the tidal energy sticking point

CONCLUSIONS – INSTALLATION 3 of 4

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0

5 TIDAL CURRENT (m/s)

0

1

2

3WAVE HEIGHT (Hs, m)

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5

10

15

20 WIND SPEED (m/s)

£0

£2

£4

£6

£8

£10CUMULATIVE COST OF n FOUNDATION INSTALLATION (£m)

EXISTING DP OCV

FUTURE VESSEL

CONCLUSIONS – INSTALLATION 4 of 4

• Fit-for-purpose vessels are required if the tidal energy industry is to be economically viable.

• Not just lower day rates on:• Charter• Fuel• Required crew

• Also compound savings from increased capability and shortened ops time:

• Fuel• Personnel• ROV/equipment hire• Reduced weather exposure

COST

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Tidal Energy Project Life Cycle CostImpact of Intervention Vessel

MOJO INSTALLER

EXISTING DP VESSELFUTURE VESSELEXISTING DP OCV

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