Geotechnical Engineering for
US Offshore Wind Infrastructure
Boston
April 25, 2016
Performance of Anchors for
Floating Offshore Windfarms
Charles Aubeny
Texas A&M University
Background:
Multi-Line
Attachment
at Anchor
Moorings
to Floaters
Moorings
to Floaters
Attachment
Ring
Plate
Stiffeners
Anchor
Line/Chain
Caisson
Shell
Caissons & Piles
Plate Anchors
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
Piles & Caissons
Driven
Dynamic
(courtesy JD Murff)
(deAguiar et al, 2009)
Suction
( courtesy E Clukey)
Drag Embedded Plates
VLA Mode
DEA Mode
Shank
Drag Embedment Anchor, DEA
(courtesy JD Murff)
Vertically Loaded Anchor, VLA
Direct Embedment Plate Anchors
Pile Driven, PDPA
NAVFAC (2011)
Dynamic, DEPLA
O’Loughlin et al (2014)
Suction, SEPLA
(courtesy R Wilde, Intermoor)
Performance Considerations for Anchors
1.
2.
3.
4.
5.
6.
7.
8.
Soil profile constraints
Vertical load capacity
Precision of positioning
Installation cost
Efficiency
Performance under sustained loading
Potential loss of embedment
Other (anchor-specific)
1. Soil Profile Constraints
Considerations: soil type + heterogeneity
2. Vertical Load Capacity
Vertical
(TLP)
Stiff Clay, Sand
(Catenary)
Inclined
(taut, catenary)
Caissons/Piles
Soft Clay
(taut)
Drag Embedded Anchors
3. Precision of Positioning
Horizontal positioning
to within 0.3m
Caissons/Piles
Drag Embedment
4. Installation Cost
“Inexpensive”
“Costly”
Driven Piles
Offshore Pile Driving
from Jack-up Rig
5. Efficiency
Fmax
Fmax
Anchor mass concentrated
at depth
Soil
Strength
Gradient
1.6 kPa/m
Caisson
Plate
18m x 3.6m
4.3m x 6.7m
Fmax = 5565 kN
Fmax= 8630 kN
W = 650 kN
W = 214 kN
Fmax/ W = 8.6
Fmax/ W = 40
(Can double for horizontal loading)
Bearing more efficient
than friction
Other aspects of efficiency: required vessel trips
required vessel size
required equipment
6. Behavior under Sustained Loading
Static Load
a. Pore pressure redistribution/loss of suction
>25% capacity reduction for caissons
Pore
Water
Flow
b. Creep effects
should keep sustained load < 30% monotonic
(Doyle, 2013)
Dissipation
of
Suction
Anti-Symmetric Pore Pressure Distributions
Applied Load H
Densification
Load V
Pore Dissipation
Of
Water
Suction
Flow
Dissipation
Of
Suction
Densification
Pore
Water
Flow
Consolidation movements still an issue!
7. Potential Loss of Anchor Embedment
1
Counter-measures
Control 1, 2 & 3
or
Higher safety factor
2
3
Uplifting => loss of embedment => brittle failure
Diving => increased capacity after ‘failure’
Driven Piles
• Soft-stiff clay, sand, soft rock/heterogeneous
• Any load angle
• Precise positioning
• Costly installation
• Low efficiency
• Noise requires mitigation
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
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
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
Vertically Loaded Anchors (VLAs)
Homogeneous soft clay
Inclined loading (<40o)
Inexpensive installation
High uncertainty in positioning
Brittle failure at high load angles
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
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
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
Suction Embedded Plate Anchors
(SEPLAs)
Soft clay only
Any load angle
Precise positioning
Intermediate installation costs
Brittle failure at high load angles
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
Acknowledgments
NSF M1503234
Sanjay Arwade, Don DeGroot (Umass Amherst)
Melissa Landon (UMaine)
Senol Ozmutlu & Leopoldo Bello (Global Maritime)
Questions?