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Petroleum Engineering 626 Offshore Drilling Leson 2 - Station Keeping


									Petroleum Engineering 406

       Lesson 9b
     Station Keeping
         Station Keeping

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

The ability of a vessel to maintain
position for drilling determines the useful
time that a vessel can effectively

Stated negatively, if the vessel cannot
stay close enough over the well to drill,
what good is the drilling equipment?
  Station Keeping - cont’d

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
      Station Keeping

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

3. The riser is disconnected from the
wellhead and the BOPs, and the
vessel can be headed into the seas.
     Station Keeping - cont’d


Water Depth
    = 1,000 ft

Drilling: 50-60 ft              1,000’

    100 ft max
 Environmental Forces Acting
    on the Drilling Vessel

       (i)    Wind Force

       (ii)   Current Force

       (iii) Wave Force

These forces tend to displace the vessel
The Station Keeping System

Must be designed to withstand the
             environmental forces

 Two types:
  – Mooring System (anchors)
  – Dynamic Positioning
          (i) Wind Force

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

 FA  0.003388V * Ch * Cs * A
            Wind Force

 FA  wind force, lb
 VA  wind velocity, knots
 CS  shape coefficien t from Table 3 - 1,
 C h  height coefficien t from Table 3 - 2,
 A  projected area of all exposed
       surfaces, ft 2 . This area changes
      w ith both heel and yaw.
Table 3-1. Shape Coefficients
Table 3-2. Height Coefficients
  (i) Wind Force - example

  FA  0.003388VA * Ch * Cs * A

 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
   (i) Wind Force - example

  FA  0.003388VA * Ch * Cs * A

 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
         (ii) Current Force

          Fc              2
                   g c Cs Vc A
Where:   Fc  current drag force, lb
         C s  drag coefficien t, dimensionless.
               Same as the wind coefficien t
               (Table 3 - 1)
         Vc  current velocity, ft/sec
         A  projected area, ft   2

                 lbft * sec2 
         g c  1             
                         4   
                      ft     
(ii) Current Force - example

          Fc            2
                 g c Cs Vc A
Vc = 2 (current velocity, ft/sec)
Cs = 1 (shape coefficient)
A = 30 * 400 (projected target area, ft2)

   Fc = 1 * 1 * 22 * 30 * 400
   Fc = 48,000 lbf = 48 kips
         (iii) Bow Forces:

for T  0.332 L
                                      2   2
                      0.273 H B L
             Fbow                4

T = wave period, sec
L = vessel length, ft
H = significant wave height, ft

    T  wave period, sec
    F  wave force, lb
    H  significan t wave height, ft
    L  vessellength, ft
    B  vesselbeam length, ft
    D  vesseldraft, ft
           Bow Forces:

 for T  0.332 L
                                 2   2
                          0.273 H B L
                Fbow 
                         (0.664 L  T)   4

NOTE: Model test data should be used
       when available
        Beam Forces:

 for T  0.642 B  2D
                                2   2
                         2.10 H B L
               Fbeam           4

NOTE: API now has Recommended
   Practices with modified equations
         Beam Forces:

for T  0.642 B  2D
                              2   2
                        2.10 H B L
           Fbeam 
                     (1.28 B  2D  T)   4
Floating Drilling: Equipment and
       The Mooring Line
              Its Use

Figure 3-1. The catenary as used for
       mooring calculations.
The Mooring Lines Resist the
   Environmental Forces
          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.
          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.
  Station Keeping, Continued

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

Typical Mooring Patterns for Non-
    Rectangular Semis
 Typical Mooring Patterns for Ship-
Like Vessels and Rectangular Semis
Typical 8-line Mooring Pattern
               Figure 3-15.
          Chain Nomenaclature.

Stud Link Chain                 Pitch      Wire

         Stud keeps chain from collapsing
    3” chain has breaking strength > 1,000 kips!
    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
(iii) Check for excessive wear or
      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.
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

(iii) Does not need anchor boats
Disadvantages of Dynamic Positioning

(i) High fuel cost

(ii) High capital cost (?)

(iii) Requires an accurate positioning
       system to keep the vessel above the

Usually an acoustic system - triangulation
Fig. 3-23. Simple position-referencing system


WH1 = WH2                 WH1 = WH3
    = WH3                WH2 > WH1 , WH3

Acoustic Position Referencing

To understand the operating principles
of acoustic position referencing, assume
     1. The vessel is an equilateral
     2. The kelly bushing (KB) is in
        the geometric center of the
Acoustic Position Referencing

   3. The hydrophones are located
      at the points of the triangular
   4. The subsea beacon is in the
      center of the well.
   5. No pitch, no roll, no yaw and
      no heave are permitted.
Diagram of controller operations.

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