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  1. 1. Introduction to Spar Buoy Platforms An Overview of Design Aspects atNational University of Civil Engineering Hanoi, Socialist Republic of Vietnam Presented by Dr. Joko H. Widjaja
  2. 2. Presentation Outlines • Industrial Codes and Standards • Types of Spar Buoy Platforms in the World • Prototypes of Spar Buoy Platforms (AIT Research) • Kikeh Spar Buoy Platform in Asia (Arup and Technip) Technip) • Design Philosophy and Methodology • Spar Buoy Platform - Structural Configuration; Models • Weight Management and Hydrostatic Stability • Environmental Conditions: Criteria, Parameters and Loads • Design Methods • Response Motions • Vortex Induced Vibration (video) • Summary of findings
  3. 3. Industrial Codes and Standards• API RP 2FPS ‘Recommended Practice for Planning, Designing and Constructing of Floating Production Systems’, 1st Ed., 2001/2011 2001/• API RP 2SK ‘Recommended Practice for Design and Analysis of Station Keeping Systems for Floating Structures’, 2nd Ed. 1996, 2011 1996,• ISO 19901-7 ‘Station Keeping Systems for Floating Offshore Structures and 19901- Mobile Offshore Units’, 2005• NORSOK Standard N-001 ‘Structural Design’, 2004 N-• API RP 2A WSD ‘Recommended Practice for Planning, Designing and Constructing of Fixed Offshore Platforms’, Errata and Supplement 3, 21st 21st Ed., 2007• AISC ‘Allowable Stress Design’, 9th Ed., 1988• API Bull 2V ‘Design of Flat Plate Structure’ 3rd Ed., 2004• API Bull 2U ‘Bulletin on Stability Design of Cylindrical Shells’ 3rd Ed., 2004• API RP 2T ‘‘Recommended Practice for Planning, Designing and ‘‘Recommended Constructing of Tension Leg Platforms’2nd Ed. 1997; 3rd Ed. 2011 Platforms’2 1997;
  4. 4. Spar Buoy PlatformsSource: Mustang, 2010
  5. 5. Types of Spar Buoy Platform Classic- Classic-Spar Truss -Spar Cell -Spar
  6. 6. AIT Research – Spar Buoy Platforms Prototype Spar Buoy Platforms (Source: Chana S. AIT Thesis)
  7. 7. AIT Research – Spar Buoy Platforms Morpeth Seastar - Spar Platform Morpeth Seastar TLP (Source : Adisak K. AIT Thesis) (Source: Atlantia Offshore)
  8. 8. Malaysia Kikeh Spar Buoy PlatformSource: PETROMIN 14, April 2006 14,
  9. 9. Design PhilosophyTo avoid Resonant Motion by properly designing the platform naturalperiod in view of environmental frequency content 1 1 Response controlled: Fixed Pltf DAF Floating Pltf Stiffness Damping Mass Type of analysis Quasi- Quasi-static Dynamic Dynamic TS/TW DAF = x/xs Design storm H Waves 1 1-yr Scatter Waves Swells 1 TS / TW Dynamic response characteristics of a structure Tw (sec) Typical spectral wave of a sea state
  10. 10. General ConsiderationsA. As per US Coast Guard or ABS: HYDROSTATIC STABILITY• Intact and Damage Stability: Metacentric height > 1m above COGB. As per API RP 2FPS: STRUCTURAL• Project Phases: construction, loadout, transportation, installation, loadout, drilling, in- in-place and decommissioning• System condition (Intact and Damaged Conditions): Dropped Object, Boat Impact, etc.• Environmental Events: extreme enviromental. extreme load and extreme enviromental. motion events• Structural Reserve Structural Strength• Safety: Blast Overpressure and Fire Hazard, Escape Route with Muster Area, etc.• Air Gap: minimum clearance of wave cress and BOS of Topside at any roll position• Interface with other systems: with mooring system and risers• Water tight compartment of hull for ballasting• Corrosion Protection: corrosion allowance and cathodic protection• Vortex Induced Vibration (VIV): use of helical strake
  11. 11. Design Methodology In-place analysis: Transportation: Time or Frequency -Wet Tow Domain - Dry tow Fatigue Design Vortex Induced Vibration (VIV) Assessment Installation of Hull: - Heavy Lift Vessel - Self-installed from barge or wet tow Self- Installation of Topside: - Heavy Lift Vessel - Float-over method Float-
  12. 12. Design Procedure Environmental data Topsides design Data input Structural Model Hull design Mooring line design Selection of Wave Theory ANSYS AQWA A Computer Model
  13. 13. Analysis Procedure A Hydrostatic analysis Static/ No (Free Floating) dynamicHydrodynamic analysis stability? OK In-place analysis: Time or Frequency • Offset displacement Domain • RMS Acceleration No Serviceability? OK No In-place Results UC<1?
  14. 14. Spar Buoy Platform – Structural Configuration Helical Source: PETROMIN 14, April 2006 14, Strakes • Topside • Hull: - Hard Tank w/ Helical Strakes - Mid Section (Stiffened cell or Truss with Heave plates) - Soft Tank Source: PETROMIN 14, April 2006 14, • Mooring System • Center Well (Moon-pool) (Moon-
  15. 15. Prototype Spar Buoy Models Single Hull 3 - Hulls 4 - Hulls
  16. 16. Prototype Spar Buoy Platforms – Case Study 200 m water depth 400 m water depth 600 m water depth
  17. 17. Parametric Dimension of Spar models Prototype Spar Buoy Platform Models Parameters Single Hull 3- Hull 4-Hull Topside Load (MT) 2,929 2,929 2,929 Topside Area (m2) 600 600 600 Free Board (m) 10 10 10 Cell Spar Diameter (m) 20.5 3@10 4@10 Cell Height (m) 60 60 60 Truss Structure Height (m) 60 60 60 Soft Tank Height (m) 5 5 5 Structure Weight (MT) 1,288 1,790 2.038 Sand Ballast (MT) 3,150 8,891 7,007 Water Ballast (MT) 5,773 10,606 7,297 Total Weight (MT) 13,170 24,216 19,271
  18. 18. Weight Management and COGBase weight = Self-Weight + Topside payload Reserve Buoyancy = Buoyancy capacity – Base weight > min % as shown below • 15% of base weight for intact hull • 5% of base weight for damage hullMooring Tension Buoyancy Force
  19. 19. Hydrostatic Stability - Philosophy Self-Weight + Topside payload • Location of COG and COB • Meta Centric Height (GM) from COG ABOVE COB ( Pendulum action) • Reserve Buoyancy (overboard)Mooring Tension Buoyancy Force
  20. 20. Reserve Buoyancy – zero trim Items Single Hull 3-Hulls 4-Hulls Topside Load (MT) 2,929 2,929 2,929 Structure Weight (MT) 1,288 1,790 2.038 Ballast Sand (MT) 3,150 8,891 7,007 Vertical Force Component of 8 x 25.89 = 12 x 25.89 = 8 x 25.89 = Mooring Lines (MT) 207.13 310.68 207.13 Trial Ballast Water (MT) 16,186.86 10,295.32 16,374.86 Total weight 23,760 24,216 28,555 COG (m) from Chart Datum -54.575 -77.091 m -67.192 Buoyancy Volume (m3) 23,181.5 23,625 27,859 Displacement (MT) 23,181.5 23,625 27,859 COB (m) from Chart Datum -46.546 -60.346 -53.960 COB to COG (m) 8.029 16.745 13.232 Corrected Ballast Water (%) 69.00 % 43.80 % 58.07 % Reserve Buoyancy (%) 12.70% 14.96% 16.92%
  21. 21. Hydrostatic Stability Y X Fh Fh Fv Fv I ìN é ü V î n=1 ë ë ( û ) GM = BM ± GB = + íå ( Fn ) H écos q - (qm )n ù yn + ( Fn )V xn ùý / (W sinq ) ± GB ûþ
  22. 22. Typical Restoring Moment ofSpread Mooring Line System The Restoring Moment (kN-m) of 8 Spread Mooring Lines 4.500E+08 4.197E+08 4.000E+08 3.500E+08 3.000E+08 2.333E+08 2.371E+08 2.500E+08 1.838E+08 2.000E+08 1.388E+08 1.500E+08 1.253E+08 1.342E+08 9.590E+07 1.000E+08 6.106E+07 5.000E+07 0.000E+00 0.000E+00 0 10 20 30 40 50 60 70 80 90
  23. 23. Hydrodynamic Stability Free Floating Offshore Structures Structural heave period Structural roll, pitch period Dynamic stability criteria 0.707 > Ts/Tw > 1.414
  24. 24. Hydrodynamic StabilityStation Keeping system for Floating Offshore PlatformsSystem heave periodSystem roll periodSystem pitch period
  25. 25. Strength Design of Spar Buoy Platforms
  26. 26. Prototype Spar Buoy Models Single Hull 3 - Hulls 4 - Hulls
  27. 27. Gravity and Environmental Loads Self-Weight + Topside payload Steady/Gust Wind Wave + Current Mooring Tension Buoyancy Force
  28. 28. Environmental Design Criteria• API RP 2FPS, 1st Ed., Section 2.4.5: extreme 100-year sea state 100- for FPS category 1 for life-time > 5 years life-• API RP 2SK, 2nd Ed.: extreme 100-year sea state for permanent system 100-• API RP 2T, 2nd Ed.: extreme 100-year sea state 100-Normal environmental sea state: appropriate return period storm (10 (10years)Sea State Design Parameters:• Steady and Gust Wind• Wind driven Waves• Wind-driven, tidal and circulation (oceanic) current Wind-• Tide and Water Level• Joint Probability Statistics: wind, wave, swell, tide and current
  29. 29. Design Wave Heights Direction (from) Design wave N NE E SE S SW W NW 10-year return period (Tropical storm/Typhoon) – South Vietnam (100 09’ N Latitude) Maximum wave height (m) 11.5 11.5 8.5 6.0 3.6 4.9 5.5 6.6 Associated period (s) 8.2 11.6 8.6 12.9 12.8 8.7 8.1 6.9 10-year return period (Tropical cyclonic storm) – Myanmar (140 08’ N Latitude) Maximum wave height (m) 8.0 8.2 8.6 9.0 9.6 10.1 9.6 9.0 Associated period (s) 8.6 8.7 8.6 9.1 9.4 9.9 9.4 9.1 10-year return period (Typhoon storm) – Thailand (100 10’ N Latitude) Maximum wave height (m) 6.16 6.46 6.07 5.39 4.68 4.22 4.22 4.61 Associated period (s) 5.76 5.78 5.75 5.69 5.63 5.59 5.59 5.62 100-year return period (Tropical storm/Typhoon) – South Vietnam (10 09’ N Latitude) 0 Maximum wave height (m) 17.7 17.7 13.1 9.3 5.5 7.6 8.5 10.2 Associated period (s) 12.2 12.2 10.8 9.5 7.8 8.8 9.2 9.9 100-year return period (Tropical cyclonic storm) – Myanmar (140 08’ N Latitude) Maximum wave height (m) 13.8 14.1 14.4 15.3 15.6 15.6 15.6 14.5 Associated period (s) 11.3 11.4 11.6 11.9 12.0 12.0 12.0 11.6 100-year return period (Typhoon storm) – Thailand (100 10’ N Latitude) Maximum wave height (m) 12.16 13.22 12.43 11.03 9.57 8.64 8.64 9.44 Associated period (s) 6.31 6.40 6.33 6.21 6.07 5.98 5.98 6.06
  30. 30. Waves and Swells at Gulf of MotamanMyanmar Swell Wave Height (m) Wind-driven Waves Frequency (0.1 Hz) Wave Scatter Diagram
  31. 31. Location of Andaman Sea
  32. 32. Schematic Forces on a Spar Buoy Platform Wind Force Buoyancy Force Mooring Tensions Current Force GravityForce Wave Force
  33. 33. Environmental LoadsMorison Equation or Diffraction Theory Slender member (Morison Equation) • Hydrostatic force • Morrison wave force Large body (Diffraction Theory) • Hydrostatic force • Steady drift force 1st order wave excitation • Mean drift wave force 2nd order wave excitation • Slow varying wave force Source: DNV-RP-C205: Environmental condition and loads
  34. 34. Hydrodynamic Force Modifiers • Apparent Wave Period (AWP) • Load factor increase of 5% due to anodes • Wave kinematic factor taken 0.88 for extreme storm wave • No current blockage factor considered
  35. 35. Directional Current Speed Profiles Direction (from) Current speed (m/s) N NE E SE S SW W NW 10-year return period (Tropical cyclonic storm) Surface (0% of water depth) 0.7 0.9 0.8 0.6 0.7 1.2 1.1 0.6 25% of water depth 0.5 0.7 0.6 0.4 0.5 0.9 0.9 0.4 Mid-depth 0.4 0.6 0.5 0.3 0.4 0.8 0.7 0.3 75% of water depth 0.4 0.5 0.4 0.3 0.4 0.7 0.7 0.3 1m above seabed 0.2 0.3 0.3 0.3 0.2 0.4 0.4 0.2 100-year return period (Tropical cyclonic storm) Surface (0% of water depth) 1 1.1 1 0.8 1 1.4 1.3 0.9 25% of water depth 0.6 0.8 0.7 0.5 0.6 1 1 0.5 Mid-depth 0.4 0.6 0.5 0.3 0.4 0.8 0.8 0.3 75% of water depth 0.4 0.5 0.5 0.3 0.4 0.8 0.7 0.3 1m above seabed 0.2 0.3 0.3 0.2 0.2 0.5 0.4 0.2
  36. 36. Directional Reference Wind Speeds Design wind Speed(m/s) Remarks 10-year return period (Tropical cyclonic storm) 1-hour mean wind 19 Weibull Distribution 10-minute mean wind 21 Ditto 1-minute mean wind 23 Ditto 3-second gust wind 26 Ditto 100-year return period (Tropical cyclonic storm) 1-hour mean wind 32 Weibull Distribution 10-minute mean wind 35 Ditto 1-minute mean wind 40 Ditto 3-second gust wind 45 Ditto Direction E NE N NW W SW S SE (from) 0 45 90 135 180 225 270 315 Factor 0.7 0.7 0.75 0.65 0.85 1 0.95 0.7
  37. 37. Equation of Motion for Floating OffshoreStructuresRelative velocity model with Morison environmental laod shows the inclusion of entrained mass andhydrodynamic damping Simplified velocity model which ignores hydrodynamic damping term CH for stationary cylinder
  38. 38. Random Waves - Steady Mean (1st order) (1& Low Frequency Waves (2nd order) (2The wave and current induced force for moored floating objects can only be determined fromflow pressure field over the entire wet area of floating object.From Bernoulli equation. the dynamic pressure is related to the squared velocity of flow particles.The squared velocity can be expressed as the summation of a constant and oscillatory termswhich, respectively, represent steady mean wave and low frequency wave.
  39. 39. Linear Diffraction WaveVelocity potential is considered as the sum of incident wave and diffracted (scattered)wave which should satisfy the Laplace equation and be subjected to boundary conditionsBoundary Conditions
  40. 40. Wave Forces Wave pressure field and force on wetted area (S) Where: ni is unit vector in i direction
  41. 41. Mooring Line System Mooring tension Mooring extension
  42. 42. Spread Mooring Line Systems Single Hull -8 3-Hull -12 4-Hull -8
  43. 43. Non-Non-linear Material Property of Mooring Lines 1.60E+07 y = 0.0032x5 - 1.1564x4 + 148.36x3 - 7176.8x2 + 242287x 1.40E+07 synthetic fiber rope 1.20E+07 1.00E+07 P(e) (N) 8.00E+06 6.00E+06 4.00E+06 2.00E+06 y = - 1.156x4 + 148.3x3 - 7176.x2 + 24228x 0.00E+00 0.00E+00 2.00E+01 4.00E+01 6.00E+01 8.00E+01 1.00E+02 e
  44. 44. Suction Piles (Source: http:www.ead.anl.gov)
  45. 45. Drag Anchors
  46. 46. Stiffness Matrix of Spar Buoy PlatformHydrostatic stiffness of Free Floating Structures Surge - X Sway - Y Heave - Z Roll - RX Pitch- RY Yaw- RZ Aw = cut water plane area xcg , ycg , zcg = position of Center of water plane area to Center of Gravity x,y,z = distance from the center of Aw with local axis X’. Y’ and Z’ ρsw = mass density of seawater g = gravitational acceleration
  47. 47. Stiffness Matrix of Spar Buoy PlatformSingle taut mooring line stiffness in global coordinate system X Y Z Translation Matrix from Mooring Tie-in point to COG
  48. 48. Stiffness Matrix of Spar Buoy PlatformMooring line stiffness in Global axis (Spar Buoy Coordinate Systemwith Origin at COG)Global System stiffness can be determined using superpositionconcept as shown below.
  49. 49. Force Equilibrium of Spar Buoy Platforms Surge - X Sway - Y Heave - Z Roll - RX Pitch - RY Yaw - RZ
  50. 50. FLOWCHART - ANSYS AQWA Data input AQWA solver Output Free floating Geometry AQWA-LINE static stability Environmental parameter Moored floating AQWA-LIBRIUM static stabilityMooring tension and spread Frequency AQWA-FER domain responses Time history AQWA-NAUT responses
  51. 51. Natural Period of Free Floating SparBuoy Models
  52. 52. RAO of Heave Motion – 400m WD 400m 4-Hull 3-Hull Single Hull
  53. 53. RAO of Roll Motion – 400m WD 400m 3-Hull 4-Hull Single Hull
  54. 54. RAO of Pitch Motion – 400m WD 400m 3-Hull 4-Hull Single Hull
  55. 55. Response Motion – 200m W.D 200m Allowable Criteria (Operation Case) Translation Displacement =10m Rotation Angle = 5 Degree Acc. = 0.07g = 0.686 m/s^2 Allowable Criteria (Storm Case) Translation Displacement =14m Rotation Angle = 10 Degree Acc. = 0.15g = 1.471 m/s^2
  56. 56. Response Motion – 400m W.D 400m Allowable Criteria (Operation Case) Translation Displacement =20m Rotation Angle = 5 Degree Acc. = 0.07g = 0.686 m/s^2 Allowable Criteria (Storm Case) Translation Displacement =28m Rotation Angle = 10 Degree Acc. = 0.15g = 1.471 m/s^2
  57. 57. Response Motion – 600m W.D 600m Allowable Criteria (Operation Case) Translation Displacement =30m Rotation Angle = 5 Degree Acc. = 0.07g = 0.686 m/s^2 Allowable Criteria (Storm Case) Translation Displacement =42m Rotation Angle = 5 Degree Acc. = 0.15g = 1.471 m/s^2
  58. 58. Maximum Mooring Line Tensions Tension (MT) • Pre-Tension is recommended less than 20% of MBL
  59. 59. Weight of the Cell-Spar Buoy Platforms Cell- Weight of structure 2,500 2,000 1,500 1,000 500 0 Model 1 Model 2 Model 3 (ton) (ton) (ton) weight of structure 1,288 1,790 2,038
  60. 60. Acceptance Criteria Asset (Spar Buoy Platform) phase In-place Platform condition Intact 10-year sea state for operating storm case Environmental condition 100-year sea state for extreme storm case Number of wave attack direction 8 Maximum mooring tension (API RP 2SK) 60% of MBL 5% of water depth for operating storm condition Maximum offset 7% of water depth for extreme storm condition Maximum heave motion ± 4.8m 4 degrees for operating storm condition Maximum pitch/roll motion 7 degrees for extreme storm condition 0.315 m/s2 for 8 working hours RMS heave acceleration 0.210 m/s2 for 16 working hours 0.140 m/s2 for 24 working hours Ref.: HOE Recommendation (Bea, Gregg, Hooks, Riordan, Russell, & Williams),
  61. 61. Design Basis – Hull and Topside STR•API RP 2A WSD, 21st Ed. Errata and Supplement 3, 2007, for tubular members• AISC ASD 9th Ed.. 1988, for non-tubular members• API Bull 2V, 3rd Ed., 2004, for plating design• API Bull 2U, 3rd Ed., 2004, for shell design
  62. 62. Summary of Findings• Center of Buoyancy is above Center of Gravity, use trussed Spar buoy platform type or to ballast soft tank• Reserve Buoyancy for intact condition >15% of base weight• COG shall be at close at possible to center line of platform• Structural configuration shall be symmetric as possible• Use double wall hull for upper part of hull against boat impact• Use heave plate to compensate heave motion (heave period is the lowest period of Spar platform and close to sea state / swell period)• Mooring lines shall be light, strong, non-linear properties, non- corrosive, durable, less maintenance, etc.; to be symmetric arrangement in use (Spread Mooring system)• Mooring line vertical stiffness insignificantly affects heave period• Roll and pitch angles and displacement response for three hull (cell) model are commonly larger than single- and four-hull models due to most probably by non symmetric structural configuration and spread mooring pattern
  63. 63. Thank youAny question? question?

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