Performance of Anchors for Floating Offshore Windfarms

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1 Geotechnical Engineering for US Offshore Wind Infrastructure Boston April 25, 2016 Performance of Anchors for Floating Offshore Windfarms Charles Aubeny Texas A&M University

2 Background: Multi-Line Attachment at Anchor

3 Moorings to Floaters Moorings to Floaters Attachment Ring Anchor Line/Chain Plate Stiffeners Caisson Shell Caissons & Piles Plate Anchors

4 Consideration of Anchor Alternatives Ideal anchor characteristics: Suitable for variable & complex soil profiles Adaptable to catenary & taut moorings (horizontal & inclined loading) Amenable to precise positioning in array

5 Piles & Caissons Driven (courtesy JD Murff) Dynamic (deaguiar et al, 2009) Suction ( courtesy E Clukey)

6 Drag Embedded Plates VLA Mode DEA Mode Shank Drag Embedment Anchor, DEA (courtesy JD Murff) Vertically Loaded Anchor, VLA

7 Direct Embedment Plate Anchors Pile Driven, PDPA NAVFAC (2011) Dynamic, DEPLA O Loughlin et al (2014) Suction, SEPLA (courtesy R Wilde, Intermoor)

8 Performance Considerations for Anchors 1. Soil profile constraints 2. Vertical load capacity 3. Precision of positioning 4. Installation cost 5. Efficiency 6. Performance under sustained loading 7. Potential loss of embedment 8. Other (anchor-specific)

9 1. Soil Profile Constraints Considerations: soil type + heterogeneity

10 2. Vertical Load Capacity Vertical (TLP) Stiff Clay, Sand (Catenary) Inclined (taut, catenary) Soft Clay (taut) Caissons/Piles Drag Embedded Anchors

11 3. Precision of Positioning Horizontal positioning to within 0.3m Caissons/Piles Drag Embedment

12 4. Installation Cost Inexpensive Costly Driven Piles Offshore Pile Driving from Jack-up Rig

13 5. Efficiency F max F max Anchor mass concentrated at depth Soil Strength Gradient 1.6 kpa/m Caisson 18m x 3.6m F max = 5565 kn Plate 4.3m x 6.7m F max = 8630 kn Bearing more efficient than friction W = 650 kn F max / W = 8.6 W = 214 kn F max / W = 40 (Can double for horizontal loading)

14 Other aspects of efficiency: required vessel trips required vessel size required equipment

15 6. Behavior under Sustained Loading Static Load a. Pore pressure redistribution/loss of suction >25% capacity reduction for caissons Pore Water Flow Dissipation of Suction b. Creep effects should keep sustained load < 30% monotonic (Doyle, 2013)

16 Anti-Symmetric Pore Pressure Distributions Applied Load H Pore Water Flow Densification Load V Dissipation Of Suction Dissipation Of Suction Densification Pore Water Flow Consolidation movements still an issue!

17 7. Potential Loss of Anchor Embedment 1 Counter-measures Control 1, 2 & 3 or Higher safety factor 3 2 Uplifting => loss of embedment => brittle failure Diving => increased capacity after failure

18 Driven Piles Soft-stiff clay, sand, soft rock/heterogeneous Any load angle Precise positioning Costly installation Low efficiency Noise requires mitigation

19 Dynamically Installed Piles Soft-stiff clay, sand/heterogeneous Any load angle Inexpensive installation Some uncertainty in positioning Limited experience in stiff clay & sand Efficiency: heavy but compact Suitability for multi-line uncertain

20 Suction Caissons Soft-stiff clay, sand/homogeneous Any load angle Precise positioning Simple installation Low efficiency Partial capacity loss under sustained loads Trenching around mooring chain

21 Drag Embedded Anchors - DEAs (excluding soft clays) Stiff clay, sand, soft rock Inexpensive installation Horizontal loading only (catenary) Moderate uncertainty in positioning May be impacted by scour

22 Vertically Loaded Anchors (VLAs) Homogeneous soft clay Inclined loading (<40 o ) Inexpensive installation High uncertainty in positioning Brittle failure at high load angles

23 Pile Driven Plate Anchors: Hammer Soft-stiff clay, sand/heterogeneous Any load angle Precise positioning Costly installation Brittle failure at high load angles Noise requires mitigation

24 Pile Driven Plate Anchors: Vibratory/Jetted Soft-stiff clay, sand/heterogenous Any load angle Precise positioning Reduces issues with noise Costly installation Brittle failure at high load angles Limited experience in energy industry

25 Dynamically Embedded Plate Anchors (DEPLAs) Soft clay/homogeneous* Any load angle Moderate uncertainty in positioning Inexpensive installation Brittle failure at high load angles Relatively immature technology * flying wing may work in sand

26 Suction Embedded Plate Anchors (SEPLAs) Soft clay only Any load angle Precise positioning Intermediate installation costs Brittle failure at high load angles

27 Concluding Comments Wide range of soil profiles + any load angle Pile driven plate anchors Driven piles Dynamically installed piles (multi-line uncertain) Uniform soft clay soil profiles + any load angle Suction caissons, SEPLA, DEPLA VLA (positioning possible issue) For catenary mooring systems DEAs (positioning possible issue) Suction caissons

28 Acknowledgments NSF M Sanjay Arwade, Don DeGroot (Umass Amherst) Melissa Landon (UMaine) Senol Ozmutlu & Leopoldo Bello (Global Maritime)

29 Questions?