Offshore Petroleum Production Systems - Brief History and Guide

1. Offshore Petroleum
Production Systems
(A brief history)

2. Brief History (Mid 19th
century)
Early large scale petroleum production
Onshore with wooden derricks

3. Brief History (1900 ’s)
Lakes (wooden piles) and Jetties
California, Venezuela, Russia

4. 19th and early 20th Century
Petroleum production characterized as opportunistic
Shallow drilling (by today’s standards)
 Recovery without significant enhancement
 Somewhat inefficient

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. Fixed Platform Components

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. Brief History (1940 ’s1950’s)
Fixed platform evolution required development of
methods and procedures for:
•Design
•Fabrication
•Installation
•Maintenance

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. 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. Brief History (1950 ’s – 1980’s)

12. Brief History (1950 ’s – 1980’s)

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. Gravity Base Structures (GBS)

15. Fixed Platform Design
Sometimes the environment gets the better of us unanticipated severity

16. Fixed Platform Design
Sometimes the environment gets the better of us understanding long term loading

17. Fixed Platform
Design
Sometimes we gets the better of ourselves

18. Adaptability of Steel Jackets
Economic drives for a minimal structure in shallow water or for fields with limited
production

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. Mat Supported Jack up

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. 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. Floating Systems

24. Floating Systems

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. 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. Tension Leg Platform (TLP)
Ballasted hull keeps tendons in tension
Tension eliminates heave motion

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. 2nd Generation Spar

30. Shipshape Hull

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. 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. High Frequency Response
Response Amplitude Operators (RAO’s)

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. 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. Anchor Types

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. Riser variants on the basic
theme
Different Riser Configurations

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. Catenary Analysis
 Change in configuration with vessel motion

41. Riser End
Connections
Line pipe
•Flexible joints
•Taper stress joints

42. Riser End
Connections
Flexibles
• Bend stiffeners
• Bend restrictors (vertebrae)

43. Riser Buoyancy

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. 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

47. Installation Costs
Vessel day rates – $200k to $1.5M
Poor choice of equipment or installation schedule can be very costly
Contracting strategy is important

48. 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