Offshore Petroleum Production Systems

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Offshore Petroleum Production Systems - Brief History and Guide

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Offshore Petroleum Production Systems

  1. 1. Offshore Petroleum Production Systems (A brief history)
  2. 2. Brief History (Mid 19th century) Early large scale petroleum production Onshore with wooden derricks
  3. 3. Brief History (1900 ’s) Lakes (wooden piles) and Jetties California, Venezuela, Russia
  4. 4. 19th and early 20th Century Petroleum production characterized as opportunistic Shallow drilling (by today’s standards)  Recovery without significant enhancement  Somewhat inefficient
  5. 5. Brief History (1940 ’s-1950’s) First offshore developments Shelf development in the Gulf of Mexico New design environments – new challenges (deeper water, wind, wave and current, combined) Early steps in shallow water with wooden structures Quick evolution to steel tubular structures
  6. 6. Fixed Platform Components
  7. 7. Offshore Environment Global variability Wind, wave and current Current speed and direction varies with depth Wave height and period varies with direction Wind varies with height and direction A random environment defined by statistics, hindcasting and forcasting Mild Moderate Extreme
  8. 8. Brief History (1940 ’s1950’s) Fixed platform evolution required development of methods and procedures for: •Design •Fabrication •Installation •Maintenance
  9. 9. Fixed Platform Design Demand Vertical – weight & buoyancy Lateral – environmental Jacket bracing resists shear Legs and piles – Resist vertical loads and differential end loads that arise from overturning moments  Structural period increases with water depth
  10. 10. Brief History (1950 ’s -1980’s) Progressive development of steel jackets Deeper water – Greater environmental loads New field developments - Harsher environments Improved understanding of environment • Wind, wave and current • Ice • Earthquake • Geotechnical conditions Improved understanding of structural response through analytical methods (finite element methods) New installation methods (and bigger equipment)
  11. 11. Brief History (1950 ’s – 1980’s)
  12. 12. Brief History (1950 ’s – 1980’s)
  13. 13. Brief History (1950 ’s -1980’s) Steel and concrete gravity base structures as alternative to tubular jackets Internal storage of product Large process area (topside weight) Limited by water depth and seabed conditions
  14. 14. Gravity Base Structures (GBS)
  15. 15. Fixed Platform Design Sometimes the environment gets the better of us unanticipated severity
  16. 16. Fixed Platform Design Sometimes the environment gets the better of us understanding long term loading
  17. 17. Fixed Platform Design Sometimes we gets the better of ourselves
  18. 18. Adaptability of Steel Jackets Economic drives for a minimal structure in shallow water or for fields with limited production
  19. 19. Exploration – Jack up Platform Mobile – can be moved to different sites for exploration (drilling) Three or four legs with a hull that can be elevated (self elevating units)  May be supported on a mat or legs may be independent  Legs may be truss structure or cylindrical
  20. 20. Mat Supported Jack up
  21. 21. Limits of Jacket Design Water depth Platform size increases with water depth Construction becomes difficult Installation becomes more difficult These difficulties are the sure sign of increased cost and at some point, this becomes uneconomic So what are the alternatives to a fixed structure? At some water depth a jacket period will coincide with the peak period of the wave environment Not desirable for design as this leads to dynamic amplification
  22. 22. Compliant Towers Used in water depths of about 1000 ft to 2000 ft Structural period is designed to be greater than spectra peak (>15 sec) Compliant tower characteristics Articulated upper jacket Fixed lower jacket  May have guy lines
  23. 23. Floating Systems
  24. 24. Floating Systems
  25. 25. Floating Systems Common components for floating systems Hull form (TLP, Spar Semi-submersible, FPSO) Mooring system and anchors to keep hull on station Riser and flow lines to transport fluids between seabed and hull
  26. 26. Semi-submersible Hull Free floating hull Pontoons, columns and bracing Moored using catenary or taught mooring lines Anchors at base of mooring lines Vertical or catenary risers Y
  27. 27. Tension Leg Platform (TLP) Ballasted hull keeps tendons in tension Tension eliminates heave motion
  28. 28. Spars Vertical column floater First spars had solid hull 2nd generation truss hull 3rd generation cell hull Mooring system similar to semi sub
  29. 29. 2nd Generation Spar
  30. 30. Shipshape Hull
  31. 31. Choice of Hull Hull selection is combinations of: Company economics Field layout and production capacity Wet/dry tree and process requirements Reservoir layout Environment For large fields in international setting, politics
  32. 32. Hull Motions It is not feasible to hold a floating hull at a “fixed” position in the same way as a fixed platform The hull will response to waves in surge, sway, heave, roll, pitch and yaw – high frequency response Depending on mooring and riser systems, the hull will move in a “watch circle” slow drift or 2nd order motions
  33. 33. High Frequency Response Response Amplitude Operators (RAO’s)
  34. 34. Slow Drift Motion Caused by Second order wave loads Current loads on hull Wind loads on structure above waterline Slow drift motions have periods in the 100’s of seconds and motions of 100’s of feet (wave loads have periods of less than 20 seconds and motions less than 10 feet) Controlled by mooring lines (and risers)
  35. 35. Mooring Line Design Mooring line acts as a catenary Seabed end termination is a fixed location Vessel end termination moves with vessel In deepwater the line weight is controlled by using combinations of materials Anchor types include: •Drag embedment •Suction caissons
  36. 36. Anchor Types
  37. 37. Risers and Flow l ines Line Types •Steel line pipe •Unbonded flexibles •Composite pipe  Design conditions •Internal pressure (burst) • Hydrostatic collapse • Strength (survival) • Fatigue (operational)
  38. 38. Riser variants on the basic theme Different Riser Configurations
  39. 39. Catenary Analysis Applicable to risers, flow lines, umbilicals & mooring lines Equation of motion at a point on line F(t) – Static and dynamic forces on the system (self weight, buoyancy etc.) [c] – hydrodynamic (Morison’s eqn) and structural damping ma – hydrodynamic added mass
  40. 40. Catenary Analysis  Change in configuration with vessel motion
  41. 41. Riser End Connections Line pipe •Flexible joints •Taper stress joints
  42. 42. Riser End Connections Flexibles • Bend stiffeners • Bend restrictors (vertebrae)
  43. 43. Riser Buoyancy
  44. 44. Challenges in Floating Systems Floating system design still has areas where research is ongoing Riser Soil Interaction Complex fluid/riser/soil interaction After 25 years there is still no definitive solution  Line vortex shedding (VIV) Vortex shedding on risers and hulls generates significant fatigue loads We are only beginning to truly understand this phenomena
  45. 45. Integrated System Design System design covers all aspects of: •Topsides (structures and process) •Hull •Mooring system •Riser system •Subsea components Expensive – anywhere from $300M to $2B Design must cover all aspects of system life including installation and decommissioning
  46. 46. Installation Costs Vessel day rates – $200k to $1.5M Poor choice of equipment or installation schedule can be very costly Contracting strategy is important
  47. 47. The Future Petroleum production will continue in many areas of the world while product is in demand Demand will drive industry to areas with harsher environment This is the challenge for the future 50

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