Recommended site investigation practice for offshore energy systems Don J. DeGroot, Sc.D., P.E. University of Massachusetts Amherst Amherst, MA, USA Presentation is based on the paper by Don J. DeGroot (UMass Amherst), Tom Lunne (Norwegian Geotechnical Institute) and Tor Inge Tjelta (Forewind-Statoil): "Recommended best practice for geotechnical site charaterisation of offshore cohesive sediments." Invited Keynote Paper. Proceedings of the 2nd International Symposium on Frontiers in Offshore Geotechnics. Perth, Western Australia, Nov. 2010.
Offshore Geotechnical Engineering 1. Site investigations 2. Soil structure interaction: foundation systems, pipelines 3. Instrumentation and monitoring 4. Geohazards assessment
From US Minerals Management Services
Frigg TCP2 platform North Sea, UK
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Emerging Offshore Infrastructure – wind farms
from www.hornsrev.dk Musial et al. (2006)
Forewind Project • • • • • •
Dogger Bank Zone area = 8660 km2 water depth = 18–63 m 9 GW to 13 GW ≈ 10% UK electricity l i i > 2000 turbines
Denmark
From Barthelmie et al. (2009)
From Forewind
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Tidal Power
• TidGen, water depth = 15 to 20 m • OCGen, > 25 m water depth • Piles or suction caissons
from Ocean Renewable Power Co.
Pipelines: Thousands of miles Pipeline-soil interaction: - Soil investigations (upper 0.5 meter is critical) - Pipe seabed friction - Trenching systems - Pipe settlement
From BOEMRE
From NGI
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Seabed cables
From image.guardian.co.uk
Offshore environments Barents Sea: ice loading
Beaufort Sea: ice loading, ice gouging Relict permafrost Gulf of Mexico
Gulf of Mexico: highly irregular seabed topography, overpressures
From NGI From NGI
North Sea/Norwegian Sea: heavy seas, massive submarine landslides Caspian Sea: over pressures, mud volcanoes, high salinity, fissured soils
Newfoundland: icebergs
Malaysia: gas blow outs West Africa
West Africa: rapid sedimentation, very weak sediments, surficial crust
NW Shelf
Australia: calcareous soils
- non-hydrostatic pore pressures - gas exsolution - shallow gas and gas hydrates - low effective stress state - temperature changes - deep to ultra deep water
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0
0
Beaufort Sea (Young 1986)
5 Site W Seabed
40
Depth (m)
Dep pth below sealevel (m)
Site W, non-gouged Site T, gouged
10
15
Site T Seabed
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West Azeri (van Paassen & Gareau 2004)
Linear regression Site W data
20
0
200
400
600
80 120 Caspian sea water
160
NaCl saturation 200 0
100 200 Salinity (g/kg)
300
800
'p at 1oC [kPa]
Offshore Nigeria (Ehlers ett al. (Ehl l 2005)
West Azeri (Allen et al. 2005)
Offshore Geohazards Tsunami impact moderate ((deep water))
Tsunami impact severe
Wave generation
Mud volcano
Submarine landslide Debris flow Gas hydrates or free gas
Earthquake
Underground blowout
After NGI
Overpressure
Gas chimney
Diapirism Doming
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Storegga Submarine Landslide - in location of low seafloor slope - about 8000 yrs BP - slide volume = 3500 km3 - rapid sedimentation, over pressures, earthquake, trigger, retrogressive sliding
3500 km3 = 4x1011 ready mix concrete trucks
From Bondevik et al. (2003)
Troll A Platform - 470 meter height, tallest structure ever moved by man - Built in a fjord and towed 200 km into North Sea in 1996
From Statoil
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Focus of presentation - Geotechnical engineering site investigations - In situ and laboratory testing - Transitional, deepwater , and ultra deepwaters (for offshore wind, transitional = 30 to 60m, deep > 60m) ((for oil & g gas: deep p > 200 to 300 m,, ultradeep p > 1500 m;; investigations are approaching 3,000+ m, > 4000 psi)
- Therefore, cohesive soils
Integrated approach via multidisciplinary geo-teams Multibeam SWATH Bathymetry
3D Seismics + HR 2D
Geological model
Site Characterization
Soil investigations - field tests - sampling - lab. testing
Design properties From NGI
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Consolidation behavior
Clay soil behavior - stress (geologic) history - anisotropy - rate effects, temp effects - stiffness degradation
Stiffness Behavior Shear behavior
Clay soil behavior - anisotropy
1f ( = 0°)
TC
su(CAUC) = 0.28'p
1f ( = 45 ± 15 15°)) DSS
su(DSS) = 0.23'p
1f ( = 90°)
TE
su(CAUE) = 0.17'p DeGroot et al. (2010)
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Clay soil behaviour – rate effects 0.40
su/ 'vm
0.35 0.30
Sheahan et al. (1996)
CK0UC
1.5
OCR 1
0.25
4
0.20
CPTU, TV 1 FC Sec.
1.4
2
1.3
8
0.15 0.01
0.1 1 10 100 . Axial Strain Rate, a (%/hr)
su 1.2 su0 1.1
1.5
Days
3
qin/qin(ref)
10 1.0 1.0
Log tf
DeJong et al. (2006)
0.5 0.01
4
0.9
2 FVT, UUC
Hr.
Min. CK0U Log strain rate Ladd and DeGroot (2003)
T-bar 0.1
v/d (s-1)
1
10
Clay soil behavior – remoulded shear strength Troll clay Sensitivity = s /s = 2 to 12 u ur
Thixotropic hardening 10 Fits: y = y0 + ax/(b + x)
su(FC) [kPa a]
8 6 4
su fall cone
2
Fit to fall cone data
Gvh [MPa]]
0 4 3 2 Gvh bender elements
1 0
Fit to bender element data
0
DeGroot et al. (2010)
10
20
30
40
Time (days)
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Design parameters - clays Offshore geotechnical design: - Randolph et al. (2005) - Andersen et al. (2008)
Triaxial Extension
Triaxial Compression DSS Triaxial Extension
DSS
2
From NGI and COFS
Offshore wind turbine platforms
Jacket structure
M Monopile il - 6 m and greater diameter
From Wind Energy The Facts
Gravityy platform p
Tripod structure
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from Landon et al. (2010)
Design parameters - clays 1. Effective stress state – u0 is the challenge 2. Stress (geologic) history = yield stress or preconsolidation stress ('vy = ( 'p ) and corresponding OCR = 'p/ /'v00 = YSR) 3. Undrained shear strength (su) anisotropy 4. Stiffness (e.g., Gmax, G(), M) 5. Flow parameters (cv, kv) 6. Remoulded and reconsolidated 7. Cyclic and dynamic properties
Most critical: 'v0, 'p, su
How to best determine (accurately) these soil properties??
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In Situ Testing Enterprise CPT FVT
T-bar
Ball
The Soil Sampling Enterprise 1. Drilling
5. Storage
2. Sampler
6. Extraction
3. Processing
7. Specimen Prep.
4. Transportation
8. Testing
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Site Characterization
CPT
In Situ Testing
Laboratory Testing
Requires Empirical Correlations
Requires Good Quality Samples
T-bar
FVT
Correlate & Corroborate
Design Parameters - 'p, su, M, M G G, etc
Offshore SI deployment modes Drilling mode: borehole advanced using rotary drilling from vessel (= vessel based drilling) or seabed system (= seabed based drilling) Non-drilling mode: advance of tools from seabed (i.e., no borehole drilling)
Critical issues: 1. Dynamic positioning 2. Depth accuracy for in situ testing and soil sampling is critical, For vessel based system use of "hard-tie" heave compensation 3. Set down conditions – for seabed systems, minimize disturbance and imposed seabed stresses
ROV
Strout & Tjelta (2007) 26/xx
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In Situ Testing – Seabed Frames
Neptun
Roson – A.P. van den Berg
GEO CPT ROV
IFREMER Penfield
Benthic GeoTech PROD
Benthic GeoTech: Portable Remotely Operated Drill (PROD) - 260 meters of drill tools
From Benthic GeoTech
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In situ testing - recommendations
CPT
Tools 1. CPTU - main recommended tool. Best tool for soil profiling soil behavior type, profiling, type estimating 'p and su, can measure Vvh (Gvh), u(t). But problems persist with T-bar Ball friction sleeve + want hydrostatic compensation. ……… Lunne (2010) CPTU'10 2. T-bar and Ball - especially for very soft sediments and shallow test depths, p , best option p for soil shear degradation, can do variable rate testing
FVT
3. FVT - remains a good tool for estimating su and sur but CPTU and full-flow more cost effective and give continuous profiles
Example CPTU in NE Massachusetts Boston Blue Clay - Newbury, MA Significant variations in qt, fs and u2 with depth Stiff, high OCR CLAY Crust Sensitive, soft CLAY Dissipation Test Increasing silt content Interbedded Layers, Silt, Clay, Sand
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su degradation from full flow penetrometers
qrem DeJong et al. (2010)
SI for West of Duddon Sands Offshore Wind Project • area = 66 km2 • water depth ≈ 24 – 30 m • 139 turbines (approved for 108 after SI), ≈ 400 MW (roughly similar size as Cape Wind)
Nominal site investigation plan: • 139 CPTU (35 to 40 m below seabed) • 39 boreholes with sampling (40 to 60 m depth) • possible drilling and downhole CPTU (40 to 60 m d th d depth; depending di on fifirstt sett CPTU CPTUs)) • plus additional CPTU (shallow) and vibrocores (shallow) for substations, cable routing.
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In Situ Testing: methods and data interpretation 1. Deck-to-deck zero readings 2. Long continuous push strokes for CPTU, full-flow 3. Full-flow - measure push and extraction, cyclic at selected depths 4. Be very specific with N factors for conversion qnet to su = qnet/N, e.g., Nkt,CAUC, NT-bar,suave, etc. 5 General vs site specific N factors 5. See: Lunne et al. (2010) – Canadian Geotechnical Journal DeJong et al. (2010) – Geotechnical Testing Journal Low et al. (2010) - Géotechnique
In situ pore pressure - critical to conduct of reliable site charaterisation program 'v0 - very difficult challenge → but exciting developments continue Genesis: "under-consolidated" vs post deposition equilibrium changes
West Azeri field (Allen et al. 2005)
Workshop Report: Sheahan & DeGroot. (2009). Seabed sediment pore pressure: genesis, measurement and implications for Design/Analysis
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In situ pore pressure - direct measurement options 1. CPTU & Piezoprobes – dissipation testing 2. Piezometers – predrilled borehole or push-in piezometer
Target layer
Flemings et al. (2008)
Strout & Tjelta (2007)
In situ pore pressure - recommendations Piezometers via long term monitoring is the only reliable method to obtain direct measurement of equilibrium in situ pore pressure Cost is not trivial and installation has often been challenging but new solutions keep being developed, e.g., Luva Investigation (Tjelta & Strout 2010) - 1300 m water depth - used PROD system - very accurate depth control - 20 to 100 bml - t ~ 4 to 5 hrs for 20 m, - 100% success rate
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Sampling and sample quality Poor sampling – soft clays
In situ condition
Lab condition Ladd and DeGroot 2003
Sampler design - recommendations - appropriate sampler geometry - truly stationary piston relative to seabed - steady rate of penetration - real time measurement of penetration & underpressure d b below l piston i t
After Lunne and Long (2006), Lunne et al. (2008)
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Sampler design - recommendations Drilling mode (vessel or seabed): - thin walled piston sampling with favorable geometery Non-drilling mode (seabed based) - samplers such as DWS - gravity sampler with piston fixed relative to seabed, e.g., STACOR - gravity sampler but piston fixed relative to vessel e.g., Kullenberg - gravity sampler without fixed piston Deep water sampler (DWS)
Assessment of sample quality - recommendations 1. No definitive method to determine quality relative to the "perfect sample". 2. Ideally want an a priori non-destructive method for quantification of sample quality (x-raying (x raying is non non-destructive destructive but not quantitative)
76 mm tubes
3. The current "gold" standard for rating of sample quality = volumetric strain (v or e/e0) during laboratory 1-D reconsolidation to estimated in situ stress state ['v0, 'h0] (i.e., NGI method; Lunne et al. 1997, 2006)
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Shallow seabed properties How to characterize the upper few decimeters of the seabed?? - major application = flowlines, pipelines, conductors, etc. - in situ effective stress ~ 2 to 3 kPa at 0 0.5 5 m depth - inadequacy of lab testing and conventional in situ testing
Side-scan sonar image of lateral buckling (from Burton et al., 2008) Lateral buckle model showing contact pressure (in Pa) along pipe (from Burton et al, 2009)
Shallow seabed properties - recommendations Collect box cores and test immediately after retrieval to the vessel - common size 0.5 m x 0.5 m x 0.5 to 1 m height
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WND
East coast USA
Luva
Laboratory testing 1. Classification and index testing 2. Index strength tests (e.g., FC, UUC, TV, etc.) 3. "Advanced" laboratory tests (e.g., CRS, Triaxial, DSS, etc.)
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Problems with Index strength testing (FC, TV, PP, UUC, etc.) -
unknown effective stress state significant influence of sample disturbance highly variable (and often fast) shear rates how account for anisotropy
Net result = highly scattered, often unreliable, data 2 UUC TV
1 0 Ele evation (m)
-1 -2 -3 -4 -5 -6 -7
0
10
20
30
Undrained Shear Strength (kPa)
CH Clay Nigerian Swamp
Harrison Bay, Alaska
Laboratory testing - recommendations 1. Reliable determination of design parameters: - 1-D CRS test for stress history, compressibility & flow behavior - Consolidated-undrained (CU) tests (e.g. CAUC, DSS, CAUE) for measurement of stress-strain-strength behavior and anisotropy 2. Essential to evaluate sample quality (e.g., e/e0 at 'v0) 3. Always evaluate su profiles in context of stress history data 4. Remediation of sample disturbance
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Repeatable and reliable test procedures 1. Develop project specifications with an understanding of appropriate procedures and standards to specify. 2. Clear communication among all parties and use of proper QA/QC programs. 3. Universal adaptation of the proposed ISO standard on Marine Soil Investigations will provide a valuable reference framework. framework
In Sum - offshore geotechnical engineering offers major challenges and fantastic opportunities for development of innovative solutions
Terrestrial Site Investigation Solutions Offshore Site Investigations
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Acknowledgements 1. Colleagues and students at: − UMass Amherst − Norwegian Geotechnical Institute (Tom Lunne, Lars Andresen) − Statoil-Forewind (Tor Inge Tjelta) − Centre for Offshore Foundation Systems, University of Western Australia (Mark Randolph) − MIT (Charles Ladd and John Germaine). 2 US National Science Foundation 2.
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