Petroleum Engineering 406

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Petroleum Engineering 406
Lesson 9b Station Keeping
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Station Keeping

• Environmental Forces
• Mooring
• Anchors
• Mooring Lines
• Dynamic Positioning

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Station Keeping

• The ability of a vessel to maintain position for
  drilling determines the useful time that a vessel
  can effectively operate.
• Stated negatively, if the vessel cannot stay
  close enough over the well to drill, what good is
  the drilling equipment?

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Station Keeping - contd

• Station keeping equipment influences the
  vessel motions in the horizontal plane. These
  motions are surge, sway, and yaw. Generally,
  surge and sway are the motions that are
  considered.
• Yaw motion is decreased by the mooring system
  but is neglected in most mooring calculations.

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Station Keeping

• When investigating or designing a mooring system,
  the following criteria should be considered

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Operational Stage

• 1. The vessel is close enough over the well for
  drilling operations to be carried out. This
  varies between operators, but is usually 5 or 6
  of water depth. Later, other criteria, based on
  riser considerations, will be discussed.

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Non-operational but Connected

• 2. The condition from the operational stage up
  to 10 of water depth. Drilling operations have
  been stopped, but the riser is still connected to
  the wellhead and BOPs.

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Disconnected

• 3. The riser is disconnected from the wellhead
  and the BOPs, and the vessel can be headed into
  the seas.

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Station Keeping - contd

• Example
• Water Depth
• 1,000 ft
• Drilling 50-60 ft
• Connected
• 100 ft max

1,000
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Environmental Forces Acting on the Drilling Vessel

• (i) Wind Force
• (ii) Current Force
• (iii) Wave Force

These forces tend to displace the vessel
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The Station Keeping System

• Must be designed to withstand the
  environmental forces
• Two types
• Mooring System (anchors)
• Dynamic Positioning

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(i) Wind Force

• The following equation is specified by the
  American Bureau Shipping (ABS) and is
  internationally accepted

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Wind Force

• Where

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Table 3-1. Shape Coefficients
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Table 3-2. Height Coefficients
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(i) Wind Force - example
VA 50 (wind velocity, knots) Ch 1
(height coefficient) Cs 1 (shape
coefficient) A 50 400 (projected target
area, ft2)
Then FA 0.00338 502 1 1 50 400
FA 169,000 lbf 169 kips
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(i) Wind Force - example
VA 50 (wind velocity, knots) 1 knot 1
nautical mile/hr 1.15078 statute mile/hr
1 nautical mile 1/60 degree 1 minute
6,076 ft
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Where
(ii) Current Force
lbf
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Fc 1 1 22 30 400Fc 48,000 lbf 48
kips
(ii) Current Force - example
Vc 2 (current velocity, ft/sec) Cs 1
(shape coefficient) A 30 400 (projected
target area, ft2)
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(iii) Bow Forces
T wave period, sec L vessel length, ft H
significant wave height, ft
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Where
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Bow Forces
NOTE Model test data should be used when
available
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Beam Forces
NOTE API now has Recommended Practices with
modified equations
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Beam Forces
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The Mooring Line
Floating Drilling Equipment and Its Use
Figure 3-1. The catenary as used for mooring
calculations.
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The Mooring Lines Resist the Environmental Forces
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Station Keeping
1. In shallow water up to about 500 feet, a
heavy line is needed, particularly in rough
weather areas. 2. Chain can be used (but may not
be advisable) to water depths of about 1,200
feet. 3. Composite lines may be used to 5,000
feet.
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Station Keeping

• 4. Beyond about 5,000 feet, use dynamic
  positioning
• 5. Calm water tension should be determined
  to hold the vessel within the operating offset
  under the maximum environmental conditions
  specified for operation.

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Station Keeping, Continued

• 6. Once the riser is disconnected, the vessel
  heading may be changed to decrease the
  environmental forces on the vessel.

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Station Keeping
Typical Mooring Patterns for Non-Rectangular
Semis
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Typical Mooring Patterns for Ship-Like Vessels
and Rectangular Semis
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Typical 8-line Mooring Pattern
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Figure 3-15. Chain Nomenaclature.
Stud Link Chain
Wire Dia.
Pitch
Stud keeps chain from collapsing 3 chain has
breaking strength gt 1,000 kips!
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Chain Quality Inspection
Chain quality needs to be inspected periodically,
to avoid failure (i) Links with cracks should
be cut out (ii) In chains with removable studs,
worn or deformed studs should be
replaced (iii) Check for excessive wear or
corrosion
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Dynamic Positioning

• Dynamic positioning uses thrusters instead of
  mooring lines
• to keep the vessel above the wellhead.
• Glomar Challenger used dynamic positioning as
  early as 1968.
• ODP uses dynamic positioning.

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Advantages of Dynamic Positioning

• (i) Mobility - no anchors to set or retrieve
• - Easy to point vessel into weather
• - Easy to move out of way of icebergs
• (ii) Can be used in water depths beyond where
  conventional mooring is practical
• (iii) Does not need anchor boats

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Disadvantages of Dynamic Positioning

• (i) High fuel cost
• (ii) High capital cost (?)
• (iii) Requires an accurate positioning system
  to keep the vessel above the wellhead.
• Usually an acoustic system - triangulation

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Fig. 3-23. Simple position-referencing system
H1
H2
H3
WH1 WH2 WH3
WH1 WH3 WH2 gt WH1 , WH3
W
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To understand the operating principles of
acoustic position referencing, assume that 1.
The vessel is an equilateral
triangle. 2. The kelly bushing (KB) is in
the geometric center of the
vessel.
Acoustic Position Referencing
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3. The hydrophones are located at the
points of the triangular vessel. 4. The
subsea beacon is in the center of the
well. 5. No pitch, no roll, no yaw and no
heave are permitted.
Acoustic Position Referencing
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Diagram of controller operations.