# Petroleum Engineering 626 Offshore Drilling Leson 2 - Station ...

Oil and Offshore

Nov 8, 2013 (4 years and 6 months ago)

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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

Connected:

100 ft max

1,000’

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:

A
C
C
V
F
s
h
A
A
*
*
*
003388
.
0
2

Wind Force

Where:

yaw.

and

heel
both
ith
w

changes

area

This

.
ft

surfaces,

exposed

all

of

area

projected
A
ess
dimensionl

2,
-
3

Table

from
t
coefficien
height
C

ess
dimensionl

1,
-
3

Table

from
t
coefficien

shape

C
knots

velocity,
wind
V
lb

force,

wind

F
2
h
S
A
A

Table 3
-
1. Shape Coefficients

Table 3
-
2. Height Coefficients

(i) Wind Force
-

example

V
A

= 50 (wind velocity, knots)

C
h

= 1 (height coefficient)

C
s

= 1 (shape coefficient)

A = 50 * 400 (projected target area, ft
2)

A
C
C
V
F
s
h
A
A
*
*
*
003388
.
0
2

Then F
A

= 0.00338 * 50
2

* 1 * 1 * 50 * 400

F
A

= 169,000 lbf = 169 kips

(i) Wind Force
-

example

V
A

= 50 (wind velocity, knots)

1 knot = 1 nautical mile/hr

= 1.15078 statute mile/hr

A
C
C
V
F
s
h
A
A
*
*
*
003388
.
0
2

1 nautical mile = 1/60 degree = 1 minute

= 6,076 ft

Where:

A
V
C
g
F
2
c
s
c
c

4
2
c
2
c
s
c
ft
sec
*
lbft
1

g
ft

area,

projected
A
ft/sec

locity,
current ve
V
1)
-
3

(Table

t
coefficien

wind
the
as

Same

ess.
dimensionl

t,
coefficien

rag
d
C
lb

force,

drag
current

F
(ii) Current Force

lbf

F
c

= 1 * 1 * 2
2

* 30 * 400

F
c

= 48,000 lbf = 48 kips

(ii) Current Force
-

example

V
c

= 2 (current velocity, ft/sec)

C
s

= 1 (shape coefficient)

A = 30 * 400 (projected target area, ft
2)

A
V
C
g
F
2
c
s
c
c

(iii) Bow Forces:

L
0.332
T

for

4
2
2
bow
T
L
B
H

273
.
0
F

T = wave period, sec

L = vessel length, ft

H = significant wave height, ft

Where:

ft

draft,

vessel
D
ft

length,

beam

vessel
B
ft

length,

vessel
L
ft

height,

t wave
significan
H
lb

force,

wave
F
sec

period,

wave
T

Bow Forces:

L
0.332
T

for

4
2
2
bow
)
T
L
664
.
0
(
L
B
H

273
.
0
F

NOTE: Model test data should be used

when available

Beam Forces:

2D
B
0.642
T

for

4
2
2
beam
T
L
B
H

10
.
2
F

NOTE: API now has Recommended

Practices with modified equations

Beam Forces:

2D
B
0.642
T

for

4
2
2
beam
)
T
D
2
B
28
.
1
(
L
B
H

10
.
2
F

Figure 3
-
1. The catenary as used for
mooring calculations.

Floating Drilling: Equipment and
Its Use

The Mooring Line

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.

Stud keeps chain from collapsing

3” chain has breaking strength > 1,000 kips!

Wire

Dia.

Pitch

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
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

practical

(iii) Does not need anchor boats

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

WH
1

= WH
2

= WH
3

WH
1

= WH
3

WH
2

> WH
1
,

WH
3

W

H
1

H
2

H
3

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.

Acoustic Position Referencing

Diagram of controller operations.