# Petroleum Engineering 626 Offshore Drilling Leson 2 - Station Keeping

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```					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
operate.

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
calculations.
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
discussed.
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
Disconnected

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

Example

Water Depth
= 1,000 ft

Drilling: 50-60 ft              1,000’

Connected:
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
2
A
Wind Force
Where:

FA  wind force, lb
VA  wind velocity, knots
CS  shape coefficien t from Table 3 - 1,
dimensionless
C h  height coefficien t from Table 3 - 2,
dimensionless
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
2

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
2

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 
lbf
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

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

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
T

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
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.

Dia.

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
replaced
(iii) Check for excessive wear or
corrosion
Dynamic Positioning

Dynamic positioning uses thrusters
to keep the vessel above the wellhead.

Glomar Challenger used dynamic
positioning as early as 1968.

ODP uses 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

(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
H1
H2

H3

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

W
Acoustic Position Referencing

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

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.
Diagram of controller operations.

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