Rankine Lecture 2014 Imperial College, 19 March 2014

Transcription

1 Rankine Lecture 2014 Imperial College, 19 March 2014 Interactions in Offshore Foundation Design Prof. Guy Houlsby Department of Engineering Science, University of Oxford

2 Summary Part 1 Installation of jack-up units Part 2 Performance of jack-up units Part 3 Foundations for offshore wind turbines Rankine Lecture

3 Part 3: Foundation for offshore wind turbines Why offshore renewables? Challenges and solutions for offshore turbine foundations Conventional, unconventional and completely novel solutions Rankine Lecture

4 Loads on an offshore turbine foundation V H V H V 2 M H V 1 H 1 H 2 V Rankine Lecture S

5 Water depth (m) Foundation type related to size and depth Beatrice Beatrice 40 Monopiles Most future developments? London Array Teesside North Hoyle Blyth Past developments Thanet Barrow Robin Rigg Walney 2 Gabbard Walney Sheringham London Lynn Dowsing Lincs Gunfleet Rhyl Ormonde Gunfleet 3 Scroby Kentish Burbo Turbine power (MW) 6 7 Rankine Lecture

6 Foundations for offshore turbines Conventional: monopiles cyclic loading Unconventional: suction caissons why? challenges: installation, tension capacity Novel: screw piles solution to the tension problem Rankine Lecture

7 Monopiles photo: Anholt Offshore Wind Farm Oil and gas photo: Ciscon Length: 30m - 80m Diameter: 1m - 2m L/D approx Offshore wind monopile Length: approx. 30m Diameter: 4m to 6m L/D approx. 5 to 7 Rankine Lecture

8 PISA PROJECT Lead partner: Delivery team: Partners: PISA = Pile Soil Analysis Rankine Lecture

9 Cyclic loading tests Reaction Frame Mass Motor Mass Rankine Lecture 2014 LeBlanc, Houlsby and Byrne (Géotechnique, 2010) 9 Mass

10 Approximately 100,000 cycles 1000 cycles 9000 cycles cycles data supplied by Abadie Rankine Lecture

11 Stiffness increases with load cycles One-way load cycles z b = 0.20 z b = 0.27 z b = 0.40 z b = 0.53 Increasing amplitude Rankine Lecture 2014 LeBlanc, Houlsby and Byrne (Géotechnique, 2010) 11

12 Accumulated rotation Increasing amplitude z b = 0.53 z b = 0.40 z b = 0.27 z b = 0.20 static k 0.31 N Rankine Lecture 2014 LeBlanc, Houlsby and Byrne (Géotechnique, 2010) 12

13 Effect of cycle type T b T c One-way cycling static T b T c N 0.31 Symmetric cycling M R M Rankine Lecture 2014 LeBlanc, Houlsby and Byrne (Géotechnique, 2010) 13 M R M

14 Pressure differential W Flow Suction caissons Installed by: 1. Self weight 2. Suction Advantages: Less expensive equipment for installation No pile driving noise Flow photo: Universal Foundation A/S Rankine Lecture

15 Main issues for suction caissons Can they be installed? OK except: Very stiff or fissured clays Very coarse-grained soils Layered and other nonhomogeneous soils Wind and wave Tensile capacity Cyclic loading Tension Rankine Lecture

16 Capacity on tensile loading (sand) Rankine Lecture

17 Screw piles Small diameter shaft (D) Large diameter helical plates (D p ) Installed by twisting motion from hydraulically driven torque-motor Some downward vertical load helps installation Rankine Lecture

18 Screw piles Onshore: Used regularly for light construction Quick and easy to install Offshore: Why? Tension capacity Silent installation Torque measurement helps confirm capacity Challenges: Scale up to much larger sizes and capacities Develop installation equipment photograph: FLI Rankine Lecture

19 Key Dimensionless Groups Geometry: D p /D, s/d p, N Capacity clay: V/(s u D p2 ) sand: V/(g D p3 ) T V or V t Installation (T = torque) clay: T/(s u D p3 ) sand: T/(g D p4 ) D p s Key ratios: VD p /T, V t /V (not V/T as often currently used onshore) D Rankine Lecture

20 Summary data of screw pile experience (model tests and onshore) Source Test type Soil V t D p /T V t /V Min Mean Max Tsuha et al (2010) Centrifuge Sand Rao et al (1991) Laboratory Soft Clay 0.64 Sakr (2009) Field Oil Sand Livneh and El Naggar (2008) Field Clayey Silt Ghaly et al (1991) Laboratory Sand Cerato and Victor (2009) Field Layered soil Perko (2009) Various Various (implied) Tensile capacity x Diameter / Torque Rankine Lecture

21 Independent plates Compressive capacity Envelope Rankine Lecture

22 Independent plates Tension capacity Envelope Rankine Lecture

23 Pile Tip Depth (m) Compression and tension capacity Total Bearing Load, kn Minimum - Compression Independent - Compression Interacting - Compression Tension Rankine Lecture

24 Pile Tip Depth (m) Dimensionless torque ratio 0 Torque Ratio, V t D p /T Rankine Lecture

25 Pile Tip Depth (m) 0 Tension/compression capacity ratio Tension/Compression capacity ratio, V t /V 0 0,2 0,4 0,6 0, Rankine Lecture

26 Maplin Sands Lighthouse (1838) Foundation designed by Alexander Mitchell 9 screw piles into sand 1.2m (4 ft) diameter 0.125m (5 inch) shaft diameter 7m (22 ft) depth below mudline Operated till 1931 illustrations provided by Alan Lutenegger Rankine Lecture

27 Whether this broad spiral flange, or Ground Screw, as it may be termed, be applied to the foot of a pile to support a superincumbent weight, or be employed as a mooring to resist an upward strain, its holding power entirely depends upon the area of its disc, the nature of the ground into which it is inserted, and the depth to which it is forced beneath the surface. The proper area of the screw should, in every case, be determined by the nature of the ground in which it is to be placed, and which must be ascertained by previous experiment. Mitchell On Submarine Foundations, 1848 Rankine Lecture

28 Conclusions from Part 3 Offshore wind will be a key element of the UK s energy mix Larger structures in deeper water will see a transition from monopiles/monopods to multiple footing structures We need innovative solutions to drive costs down: helical piling is an old solution to a new problem Rankine Lecture

29 Centre for Doctoral Training in Renewable Energy Marine Structures Structures by Cranfield and Geotechnics by Oxford Training leaders with high level technical expertise Cohorts of at least 10 graduate students per year for 5 years University based PhD and industry based EngD students 4-year research degrees with taught components > 15 companies have pledged involvement (inc. Arup, Atkins, Centrica, DNV, EDF, e.on, Fugro, GE, HRW, Mojo Maritime, Qinetiq, RES, Skanska, Tata Steel, RWE) R E M S but we need more Contact byron.byrne@eng.ox.ac.uk for more information Rankine Lecture