Grace Robert. Advanced Blowout and Well Control
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296
Advanced
Blowout
and
Well
Control
4p
=
Lifting force,
Zbz
Dpt
=
Piston diameter, inches
0,
N,
phyd
The determination
of
the snubbing and lifting force
is
illustrated
=
Outside diameter of piston rod
in
jack cylinder, inches
=
Number
of
active jack cylinders
=
Hydraulic pressure needed on jacks
to
snubflift, psi
in
Example
6.5:
Snubbing
Pressure
Lift
Pressure
~~ ~~
Figure
614
-
Hydmulic
Snubbing
Jack
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Well
Control
297
Example
6.5
Given:
A
hydraulic snubbing unit Model
225
with four jack cylinders.
Each cylinder
has
a
5-inch diameter bore and
a
3.5-inch diameter
piston rod. The
maximum
hydraulic pressure is
2500
psi.
Required:
1.
The snubbing force,
E,,&
at
the
maximum
pressure
2.
The lifting force,
E;;ip,
at the
maximum
pressure
Solution:
1.
The snubbing force at
2500
psi
is given by Equation
6.7:
Cmb
=
0.7854(2500)(4)(5*
-
3.52)
F,rmb
=
100,139
Zbf
Calculate the lifbg force at
2500
psi
using
Equation
6.8:
2.
2
I&
=
0.7854P,,NcDpst
I&
=
0.7854(
ZOO)(
4)(
52)
&
=
196,350
Zbf
The hydraulic pressure required
to
snub
or
lift
in
the hole can be
calculated by rearranging Equation
6.8.
298
Advanced
Blowout
and
Well
Control
Example
6.6
illustrates
the
determination of the hydraulic
pressure required for a specific lifting or snubbing force:
Examule
6.6
Given:
The same hydraulic snubbing
unit
as
given in Example
6.5.
The
hydraulic jacks have
an
effective snubbing area of
40.06
in2
and
a effective lifting
area
of
78.54
in2.
Required:
1.
The hydraulic jack pressure required
to
produce a
snubbing force of
50,000
lbs.
2.
The hydraulic jack pressure required to produce a lifting
force of
50,000
Ibs.
Solution:
1.
The hydraulic pressure required for snubbing is
determined by rearranging Equation
6.7:
c
50000
=
0.7854(52
-
3S2)4
50000
40.06
4-
=-
pshyd
=
1248
psi
2.
The hydraulic pressure required for lifting is determined
by rearranging Equation
6.8:
Special Services
in
Well
Control
299
68
'h~=
0.7854Dps:N,
(6.10)
50000
-
'w
-
0.7854(
52)4
=
637
psi
Table 6.1 is a listing
of
the dimensions and capacity
of
snubbing
units
normally utilized.
Pipe
Buckling
Calculations
After determining the required snubbing force,
this
force must be
compared with the compressive load that the work string can support
without buckling. Pipe buckling occurs when the compressive force
placed on the work string exceeds the resistance of the pipe
to
buckling.
The smallest force
at
which
a
buckled shape is possible is the critical
force. Buckling occurs first
in
the
maximum
unsupported length
of
the
m70rk string, which is usually
in
the window area
of
the snubbing
unit
if
a
window guide
is
not installed.
When
the work string is subjected
to
a
compressive
load,
two
types
of
buckling may occur. Elastic or long-column buckling occurs
along the major
axis of
the work string. The pipe bows out from the
center line of the wellbore
as
shown in
Figure 6.15a. Inelastic or local-
intermediate buckling
occurs
along the longitudinal
axis
of
the work string
as
shown in Figure 6.15b.
300
Advanced Blowout
and
Well Conirol
Table
6.1
Dimensions and Capacities
of
Snubbing
Units
Effective Lift
Area
(in2)
I
50.27
Lifting Capacity at
150,796
3000
txi
Obs)
Effective Regenerated Lift
28.27
Area
(in2)
Block Speed
Up
(fpm)
Bore through
Unit
(in)
8
28
1
Stroke (in1
I
116
225
I
340
I
600
I
4 4 4
5.0
6.0 8.0
3.5 4.0 6.0
78.54
113.10 201.06
235,619
339,292 603,186
40.06
62.83 87.96
120,166
188,496 263,894
38.48
I
50.27
I
113.10
I
115,440 150,810 339,300
280
I
178
I
137
I
8,500
9,600 34,000
8,750
8,750 11,000
As
illustrated in Figure
6.16,
the buckling load is
a
function
of
the
slenderness ratio.
In
order to determine the type
of
buckling which may
occur in the work string
,
the column slenderness ratio,
S,,,
is compared
to
the effective slenderness ratio,
S,,
of
the work string.
If
the effective
slenderness ratio,
S,,
is greater than the
column
slenderness ratio,
S,
(S,
>
S,),
elastic or long-column buckling will occur.
If
the
column
slenderness ratio,
S,,
is
greater
than
the
effective slenderness ratio,
S,
(S,
>
S,),
inelastic or local-intmnediate buckling will occur. The
column slenderness ratio,
S,,
divides elastic and inelastic buckling.
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301
Figure
6
15
Buckling
Load -Function
of
Slenderness
Ratio
0
50
100
150
200
250
300
SImdwrwrr
Rlllo
(Sf.)
Figure
616
The
column
slenderness
ratio,
S,,
is
given
by
FQuation
6.11
:
(6.11)
Where:
E
=
Modulus
of
elasticity,
psi
302
Advanced
Blowout
and
Well
Control
Fy
=
Yield strength, psi
The effective slenderness ratio,
Sre,
is given by the larger result of
Equations 6.12 and 6.13:
(6.12)
(6.13)
Where:
UL
=
Unsupported length, inches
t
=
Wall
thickness, inches
Dp
=
Outside diameter of the tubular, inches
Di
=
Inside diameter
of
the tubular, inches
Inelastic column buckling can occur if the effective slenderness
ratio,
Sre,
is less than the column slenderness ratio,
S,,
and is equal to or
less than 250
(Sre
<
S,).
Inelastic column buckling can occur as either
local or intermediate. Whether inelastic buckling is local or intermediate
is determined by
a
comparison of the effective slenderness ratios
determined
from
Equations 6.12 and 6.13. If Equation 6.12 results in an
effective slenderness ratio less than that obtained
from
Equation 6.13,
local buckling occurs. If Equation 6.13 results in an effective slenderness
ratio less than Equation 6.12 (but less than
Src) (S,
>
S,,,
>
S,,,)
,
intermcdiate-column buckling occurs. In either situation, a compressive
load, which will cause a
work
string
to
buckle, is
known
as
the buckling
load,
Pbkl
,
and is defined by Equation 6.14:
(6,14)
'I
pbpl=
F,(D,2
-42)
[
0.7854Srci;13927S,
for:
S,
<
Src
-
Inelastic buckling
S,,,
<
S,,,
-
Local buckling
Special Services in Well Control
303
S,
>
S,,,
>
S,,,
-
Intermediate buckling
Fy
=
Yield strength, psi
Di
=
Inside diameter of the tubular, inches
Dp
=
Outside diameter of the tubular, inches
sre
=
Effective slenderness ratio, dimensionless
L
=
Column slenderness ratio, dimensionless
Where:
In
inelastic buckling, the buckling load,
Pw,
can be increased by
increasing the yield strength, size and weight of the work string or
decreasing the unsupported section length.
Elastic (long-column) buckling is critical if the effective
slenderness ratio,
S,,,
is greater than the
column
slenderness ratio,
S,,
and the effective slenderness ratio is equal
to
or less
than
250
(S,
I
250).
When these conditions exist, the buckling load,
PMl,
is defined by
Equation
6.1
5
:
225(
1
06)(
0,’
-
01.2)
8kl
=
Sre’
(6.15)
for:
Sre
>
S,
and
Sre
I250
-
Long-column buckling
Under
this
condition, the buckling load,
Pw,
can
be increased by
decreasing the unsupported section length or increasing the size and
weight
of
the work
string.
Consider the following examples:
Example
6.7
Given:
Work string:
=
2
318
inch
=
5.95
Iblft
=
P-105
304
Advanced Blowout
and
Well
Control
Unsupported length,
U,
=
23.5 inches
Modulus elasticity,
E
=
29X106
psi
Yield strength,
=
105000psi
Outside diameter,
Dp
=
2.375 inch
Inside diameter,
Dj
=
1.867inch
Wall thickness,
t
=
0.254inch
Required:
The buckling load
Solution:
The column slenderness
ratio
is given
by
Quation 6.1
1:
1
-
s,
=4.44[$
2
1
-
s,=4.44(
29(106)
)
105,000
s,
=
73.79
The
effective slenderness ratio,
S,,,
will be the greater value
as
calculated
from
Equations 6.12
and
6.13.
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Well
Control
305
Equation 6.12:
1
s,
=
4UL
(0,’
+
D,’):
4(
23.5)
(2.3752 +1.867’)?
s,
=
1
Sre
=
31.12
Equation 6.1
3
:
1
-
1.867+0.254 1.867 +0.254
450(0.254)
][
2(0.254)
)
sre
=
9.85
Therefore, the correct effective slenderness ratio is the greater and
is given by Equation 6.12
as
3
1.12.
Since
S,
(31.12) is
<
S,
(73.79)
and
S,
is
5
250,
failure
will
be
in
the intermediate (inelastic) mode and the buckling
load
is
given by Equation 6.14: