Offshore pile design according to international practice

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In this webinar, industry leading organizations present:

- Learnings from project Borkum West 2, one of German´s most advanced offshore wind projects
- The challenges of the piling design and results of the geotechnical investigation
- Recommendations and observations about potential hazards or obstruction during the foundation installation

Register for free here:
http://www.web2present.com/upcoming-webinars-details.php?id=116

Offshore pile design according to international practice

  1. 1. Offshore pile design:International practice Dr David Cathie Cathie Associates
  2. 2. Outline Oil and gas industry has a lot of experience in developing both small and large offshore projects. Practical solutions are available today for safe design of tripod structures  Pile design in the offshore industry  International standards and methods  Pile resistance (capacity) methods – API, CPT  Pile driveability  Pile driving monitoring  Piled tripods for wind converters – key issues  ConclusionsOffshore pile design : International practice
  3. 3. Tallest offshore piled structure Bullwinkle, GOM Bullwinkle platform: Bullwinkle piles: 529m high 28 x 84”OD, 165m long 412m water depthOffshore pile design : International practice
  4. 4. Offshore oil & gas industry  10,000+ platforms worldwide  ~99% piled jacket structures Location Ground conditions Gulf of Mexico Normally consolidated clay Offshore Brazil West Africa Middle East Carbonate soils, sands, calcarenite Australia Far East Loose to medium dense sands, soft clays North Sea Medium dense to very dense sands, very soft to hard claysOffshore pile design : International practice
  5. 5. Pile sizes – piling hammers  Typical pile OD: 1.2 – 2.4m (1.8 – 2.4m in N.Sea)  Typical length: 40 – 100m  Pile hammers:  90-150 kJ hydraulic hammers for typical “small” piles  600kJ or more for large piles  Put in table with rated energy IHC range Menck MHU range is similarOffshore pile design : International practice
  6. 6. Pile design in the offshore industry  Industry is risk adverse, and highly cost conscious  Consequences of structural failure leading to shutdown are very high, and unacceptable  Consequences of installation delays are very high, and unacceptable (production delayed, cost overrun)  Innovation is seen as risky and must have very high cost- benefit  Reliability of pile design is very high (no failures reported). Belief that methods are conservative to very conservative. No account taken of ageing effects.Offshore pile design : International practice
  7. 7. International Standards  API RP2A – WSD 29th edition, 2000  API RP2A – LRFD, 1st edition 1993  DNV classification notes No. 30.4, 1992 (based on API 1987)  ISO 19902:2007 Fixed steel offshore structures (based on API)  API RP2A – WSD 29th edition, 2000, errata and supplement 3, 2007, provides the Commentary on CPT-based methods for pile capacity (C6.4.3c)Offshore pile design : International practice
  8. 8. API pile design approach  Pile capacity/resistance methods  Main text API 93 method  CPT methods in commentary in 2007 edition  Axial/lateral response  Cyclic loadsOffshore pile design : International practice
  9. 9. API Main text methodShaft resistancef = K σv’ tanδf <= flimOffshore pile design : International practice
  10. 10. API 2007 - CPT methods  Motivation  Research programs  Key features  Database  Industry acceptanceOffshore pile design : International practice
  11. 11. API 2007 CPT-based pile resistance  Motivation  Improve reliability and reduce conservatism  Based on more fundamental understanding of pile behaviour  Practical method capturing basic mechanics of driven pile  Direct use of CPT results in silica sand  Research Programs  Euripides (started 1995) in Eemshaven, Netherlands: dense to very dense sands  Pile load tests in Dunkirk (dense to very dense marine sands) – CLAROM site  Pile load test in Labenne (loose to medium dense sand), LCPC site.Offshore pile design : International practice
  12. 12. Key features of CPT methods  Direct use of cone resistance (qc) to determine radial stress (σ’rc)  Effect of distance from pile tip  “friction fatigue” or degradation during driving as pile progresses  Unit shaft resistance based on residual soil-pile friction angleOffshore pile design : International practice
  13. 13. CPT methods – database  ICP database: 20 open-ended tubular piles in sand  Length: 2m to 47m  Diameter: 0.07m to 2.0m (average 0.65m)  Range of Dr at tip: 57-96%  UWA database: 32 open-ended tubular piles in sand  Length: 5.3m to 79.1m  Diameter: 0.36m to 2.0m (average 0.73m)  Range of Dr at tip: 15-100% (average 68%)  Range of Dr along shaft: 23-100% (average 74%)Offshore pile design : International practice
  14. 14. CPT method – application by Shell Dynamic SRD [MN] Dynamic SRD [MN] 0 4 8 12 16 20 0 4 8 12 16 0 0 5 ICP API 20 10 15 40 20 25 ICP 60 API m m w w a a o e o e n n B B P P S S ] [ r ] [ r t t f f l i l i 30 35 80 Overy (2007) The Use of ICP Design Methods for the Foundations of Nine Platforms installed in the UK North Sea, Int. Offshore Site Investigation and Geotechnics ConferenceOffshore pile design : International practice
  15. 15. CPT Method – industry acceptance  Adopted by Shell UK in 1996 for requalification of a number of North Sea platforms (Overy, 2007)  Pile length: 26m to 87m  Diameter: 0.66m to 2.13m  Variable soil conditions  Adopted by API, 2007 as a “recent and more reliable method ...considered fundamentally better..”  But qualified by offering 4 alternative methods  Should be used only by qualified engineersOffshore pile design : International practice
  16. 16. API pile design approach  Axial/lateral response  T-Z/Q-Z and P-Y standardised approach  Mainly based on research in 1980’s  Cyclic loading  Axial  Axial and lateral effects uncoupled  Long (=flexible) piles can experience capacity degradation in clay soils (due to strain softening)  Wave loading rate effect may compensate for degradation.  Lateral  Cyclic effects included by softening P-Y response near seabed and reducing peak lateral pressures  Methods proposed are guidelines only (but everyone uses them)Offshore pile design : International practice
  17. 17. Pile driveability - SRD  Soil resistance during driving (SRD)  Alm and Hamre (2001) method  Database 18 installations, 1.83 – 2.74m OD, up to 90m penetration, MHU 1000-3000, IHC S-400, S-2300  Key feature: degradation of shaft resistance as pile passes, calibrated to databaseOffshore pile design : International practice
  18. 18. Pile driveability – wave equation  Wave equation (SRD v Blow count)  GRL WEAP – same quake, damping soil model as used by Alm & Hamre  Blow count v depth  Pile acceptance criteriaOffshore pile design : International practice
  19. 19. Pile driveability prediction SOIL RESISTANCE TO DRIVING (MN) 100 BLOWS PER 0.25 METRE MHU 500T (Eff. = 80%), 40m penetration 0 25 50 75 100 0 50 100 150 200 250 0 MHU 500T (Eff. = 95%), 40m penetration 0 MHU 800S (Eff. = 80%), MHU 500T (Eff. = 80%), 20m penetration Best Estimate SRD Best Estimate SRD 75 MHU 500T (Eff. = 95%), 20m penetration MHU 800S (Eff. = 80%), High Estimate SRD High Estimate SRD 10 10 MHU 800S (Eff. = 95%), Best Estimate SRD MHU 800S (Eff. = 95%), 50 High Estimate SRD 20 20 25 M G O C N A V D R E S L T ) ( I 30 30 M W M W O O A N A N D D U U B R P B R P S E S E L T L T ) ( ) ( I I 0 0 50 100 150 200 250 40 BLOWS PER 0.25 METRE 40 SRD v Depth SRD v Blow count Blow count v DepthOffshore pile design : International practice
  20. 20. Pile driving monitoring  Purposes  Confirming pile resistance during driving, or after set-up, SRD  Correlate SRD with calculated static pile resistance for the site.  Establish reliable pile acceptance criteria (blow count) based on a calibrated wave equation & SRD model  Monitor the stresses at pile top and to correlate with risks of tip buckling (when driving in rock)Offshore pile design : International practice
  21. 21. Pile driving monitoring  Instrumentation  Pile instrumentation consists in installing strain gauges and accelerometers at pile top Operationally, the offshore environment is extremely challenging. It requires specific experience and extreme precautions for data of good quality Many attempts have resulted in failureOffshore pile design : International practice
  22. 22. Pile driving monitoring - underwater Under-water monitoring requires specific equipment Risks of mechanical damage and electrical instability require specific operational proceduresOffshore pile design : International practice
  23. 23. Pile driving monitoring – signal matching G-OCTOPUS C126 UW PDM OLOWI PILE DRIVING ANALYZER ® PDA OP: ACR-MHA Version 2009.098.061 PILE C1 JACKET WHT-A BN 3/828 30/07/2009 18:23:25 80000  Signal matching (CAPWAP/TNOWAVE) 7.16 EMX 845.1 kN-m kN m/s E2E 828.6 kN-m CSI 229.6 MPa F  Iteratively modifying V numerical soil model until the a EF2 848.7 kN-m E2F 843.1 kN-m calculated reflective wave matches the measured waveEV2 877.5 kN-m RMX 18448 kN DMX 30.3 mm DFN 14.0 mm B w o 12 lo N. 63 8 0 .0 00 kN LE 39.970 m AR 2759.57 cm^2 W pM u sd W pCt u p EM 207413 MPa 51.2ms SP 77.5 kN/m3 15.60 ms WS 5123.0 m/s Measured upward and 2 6 .7 66 EA/C 11173 kN-s/m 80000 downward waves 80000 15 LP 25.750 m 45 ms kN kN F12 A2 6 L/c WD WU - 6 67 26 . F1: [898W] 132.7 (1) F2: [904W] 130.2 (1) A2: [29997] 1035 gs/v (1) Measured and calculated upward waves - 0 00 80 . 51.2msOffshore pile design : International practice
  24. 24. Pile driving monitoring – data example Driving Data Capwap resultOffshore pile design : International practice
  25. 25. Pile driving monitoring – accuracy and limitations  Generally within 10-15% of static tests  Best agreement in sedimentary soils (sand, clays)  Agreement depends on set-up time and failure criteria for static test  Limitations  Cannot accurately differentiate between tip and shaft resistance near the base  Cannot define exact distribution of shaft resistanceOffshore pile design : International practice
  26. 26. Piled tripods for wind converters – key issues  Tripod foundation response very different from monopile  Structural dynamics of tripods  Insensitive to lateral stiffness  Axial stiffness related to pile penetration/capacity (for stiff piles) so natural frequencies insensitive also to detailed pile design  Cyclic axial loads much more important than cyclic lateral  Allow generously for scour – not a design problem with tripods  Need practical solutions to design foundations today!Offshore pile design : International practice
  27. 27. Offshore platform/tripod loading Moment loading at mudline for a monopile is translated into axial pile loading for a tripodOffshore pile design : International practice
  28. 28. Tripod and monopile loads Bending moment at mudlevel during 50 year severe sea state 350000 Tripod Pile Monopile 300000 250000 200000 150000 100000 50000 0 -50000 0 20 40 60 80 100 120 140 M m N o n b e k s -100000 r t ] [ -150000 -200000 time [s] Axial (vertical) force at mudlevel during 50 year severe sea state 10000 Monopile Tripod Pile max compression Tripod max tension 5000 0 0 20 40 60 80 100 120 140 -5000 -10000 -15000 M m N o b e k c s -20000 r ] [ f -25000 -30000 time [s]Offshore pile design : International practice
  29. 29. Tripod - pile head deflections Extract of displacement time history from 50 yr extreme event covering governing ULS peak load Extreme deflections: Axial: 5mm Lateral: 34mmOffshore pile design : International practice
  30. 30. (Geotechncial) advantages of tripods/quadripods  Not sensitive to uncertain soil parameters (operational soil modulus)  Not sensitive to scour assumptions  Lateral pile deflections are restrained by structure stiffness  Main cyclic loads transmitted as axial loading  Offshore oil & gas industry has strong preference for multiple leg structures – almost exclusively builds 3, 4 or 8 legged piled platforms for offshore operations  Other structures require specific conditions to be cost- effectiveOffshore pile design : International practice
  31. 31. Research – tripod foundations  Must not delay design and procurement process  Solutions must be adopted today even if research to confirm or improve methods continues in parallel  Solutions are available today  May be conservative but based on oil & gas experience  Use pile driving monitoring to confirm capacity during installation  Use structural monitoring to confirm eigenfrequencies and foundation stiffness in different conditions  Not essential today but research desirable to provide improved (less conservative) methods of design i.e. reduce development costsOffshore pile design : International practice
  32. 32. Research – tripod foundations Priority Topic Why? 1 Axial pile stiffness at working Key for accurate structural loads dynamics 2 Lateral pile behaviour Not critical design issue for tripods subject to cyclic loads but little is known (fixed head, many low level load cycles) 3 Lateral pile stiffness at 2nd order importance for structural working loads dynamics, and for cyclic axial pile capacity 4 Axial pile capacity under Most previous research low level cyclic loads concentrated on higher levels of cyclic axial load 5 Effect of ageing on pile Ageing is known to increase pile response resistance and stiffness but the mechanisms are not understoodOffshore pile design : International practice
  33. 33. Conclusions  International oil & gas industry has long and successful track record with piled structures  Offshore industry is conservative and risk adverse (high costs involved in all marine work)  New CPT methods of pile design have been introduced recently because of the recognition that the earlier API methods were over conservative in some circumstances (e.g. dense sand)  Cyclic loading is handled within (API) design methods for wind/wave loads for jacket or tripod structures  Tripod solutions for wave converters are very robust and insensitive to variations in foundation conditionsOffshore pile design : International practice

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