how to design casing strings
TRANSCRIPT
8/10/2019 How to Design Casing Strings
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Reprinted from
etmleum
~~NBGGPHTERNATWALecember 989
How
o Design
Casing
Strings For Horizontal
Wells
by
John
F.
Greenip
Jr.,
H y d ~ i l
ous ton
Tes.
C
wing loads in
an
extended reach well or a horizontal
tended
reach
s trings . Unders tanding these re la tion-
well
are
no different than those developed in
a
conven-
ships may simplify the design process as well as allow
tionaloilorgaswell.Thereis,however,adistinctdif- t h e e n g i n e e r t o b e t t e r t a i l o r t h e d e s i g n t o t h e
ference in
the
magnitude
of
those loads.
eciiuse
of thi s
application.
difference,
casing stri ng design for thes e applications
In e i the r
an
extende d reach well or a horizontal well,
frequently rey
uires
structural considerations such as
t h e casing s tring can be divided into
segments
for
bending
and
torque.
These
factors
may
overshadow7
analysis. F o r illustration, th ree s egm ents will be used
more traditional design
parameters of
internal yield,
(Fig. 1):
h e reach
interval, eit her horizontal or sloping;
collapse,
and
tension.
the build interval;
and t he
vertical interval. For t h e
When developing a casing design for a horizontal
exam ple well calculations, t h e build is taken to
be
con-
well. here are several points to
consider:
s tan t th roughout i t s in te rva l
and
the reach interval is
The
axial
and
torque
loads
can
be
es t imated fo r taken to have
a
constant inclination angle.
A s
with
coil
extended reach and horizorltal wells
by
analyzing the
ventional wells , the s trin g then is analyzed from t he
seaarat
e intervals. bottom up.
~ L l a t i u n s h i ~ san be deve loped for the var ious
Reach Interval
parameters
which
facilitate casing
string
design
and
unders tanding.
For the reach in te rva l , the forces on t he pipe include
The
pick
set down,
and neutral
states
a
frict ion component tha t act s
in the
opposite direction
suffiriently ifferent
load
magni tudes
to
u an.ant
to
the pipe movement.
The
incremental tension,
dF,
is
analysis
of
each separately.
dF (W)(BF){Cos8 (p )(WXBF)(Sin l
where
As
with
casing
s tring design and analysis for the
w nomin l
pipe
weiRht,ft
conventional well , re la tionships can be d e v e lo p e d
RF
buoyancy k tor
among the various param eters for horizontal and ex-
inclination angle
Point 3 surface
Vertical interval
Po~nt -
OP
Build ~nte rva l
Point
\
Reach angle
TD
coefficient of friction
Tahng t h e reach in terval to he at a constant angle,
t h e tension at t h e t o p of the interval.
F,,
will
be
F, = dFi L1
wh e re
L reach i n t e r ~ a length
If the string is being set down , the friction com-
ponent
reduces
the o~erallension:
F,
L [IW)(BF ) ICo sb - (kjIW)(BF)(Sin
d l ]
F o r t h e esamyle shown in
Fig.
KOP 4.000 f t MD
build angle 2W1100 ft
reach
angle 80
reach
i n t e n a1 length 2,000 ft
pipe
size
5%
n.
17
lblft
coefficient. of fr ictio n 0.35
mud
weight
=9.0 lblgal (buoyancy factor, B F =0.86)
F, =
2,000[(17)(0.86)(Cos 80)
0.35) 17) Oh86) Sin 0)]
-
5,000
b
Thus 5,000 lb compression must
be
applied at the top
of
t h e
reach interv al, Point 1, to push t he pipe into final
position. This force would be 0 as the pipe enters the
reach interval and increases to t he 5,000 lb compression
as the s t r ing approaches TD.
Because it is generally desirable to
pick
up t h e string
after having
reached TD, the fric t ion, again acting
Fig .
1
When
anaIyzing
a r ~ i n g
esign for
n extended
opposite to the direction of pipe movement, will in-
r e ad
or horizontal
we l l
t h e
casing
str ing
should
be
crease the
tension
load
at
Point
divided
nto three
segments:
the re rh interval either Ror-
izontal or sloping; the build
inkrcol
nnd the
uertical
F, 2,000[(17)(0.86)(Cos 80)
interual.
+
(0.35)(17)(0.86)(Sin80)]
15,200
lb
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compression,
1,000
tb
Tension, 1,000b
-10 5 0 5 10 15 20 25 30
.-
B
rT
F 0.35
Build
angle
=
20 tlOO
t
Reach length 2,000
t
Reach angle 80
Fig. 2
xial
loud
versus
measured
depth.
The
maximum
Fig.
4.
Neutral
state
torque
r tio
Venus
build ang le.
The
cornpre~sionoad does
not
occur
at
P o i ~ t or 2 bu t accurrr
torque
ratio
for
each
reach
Islogth
approaches
the
aume
within the build in te rmi .
value about 1 20
EU
. .
6,000
5,000
P
4 000
3,000-
,.,*,
1,000-
0 5 10 15 20 25
30
35 40
Build
angle 1100
R
= 0.35
Reach
length
= 2,000 t
Reach angle 80
100-
Fig 3.
Bending load and
torque
versus
build
angle.
IrOrque
decreaeee an the build angle increaees while load in-
creme8
l inearly
with increasing build
angle
Keeping
the
build angle amall to reduce the loud due
to
bending
will
affect
mtation
t o q u e adversely at low
build
anghs .
4,000f i
Thus he swing between
pick
up and set
down for
t h e
reach section of
the
string
is
20 200
lb.
The
pipe will
remain stationary for any load applied to the top of
the
Teach
interval between 15,200
lb
tension and 5,000 lb
compression.
For
the neutral condition
where
there is
no
tendency toward
pipe movement,
the
friction com-
ponent is
0
and
the
tension at Point
1
is
F, 2,000[ 17) 0.86) Cos
80 ]
= 5,100
lb
Rotating during cementing
or wash-down
operations
requires that the torque
to
rotate
be estimated. This
torque
is a
function
of
the normal
force
between pipe
and
hole,
coefficient
of friction,
and
the pipe
radius.
With pipe body OD, the incremental torque, dM, is
dM (p)(W)[BF)(Sin0XOD)IZ
For the
reach
interval. the rate of
torque
increase
'
10 15 20 25 30 35 40
Build
angle, 1100
t
1,000 2,000 3,000
Aeach length,
Fig . 5.
xial load
at
KOP while
w t t i n g down caaing
string
uersu reach length. I f frequently become8 necessary to
applu
con~pres8ion t the top of
the
reach
inter~al
o ad
vance
the
pipe farther in the hok .
with length is
constant and
the
torque at the top
of
the
interval. M becomes
M
(k)(W)IBF)(Sin
0)(L)(O D)/24 ft-lb
For
the
example well
M (0.35)(17)(0.86)(Sin
80) 2,000) 5.5)/24
2,310
ft lb
This torque
must be applied at Point
1
to rotate
th t
pipe
in the reach interval. A
lower
torque could rotate
the string if the pipe were in a pick up
or
set down state
of
axial load.
However, because the axial load sta te will
cycle
during any
combined rotation-pick
up-set down
operation,
the neutral
state torque
is
used for the
reach
interval.
Build
nterval
While the
loads in
the
reach
interval change linearly
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with measured dep th, th e loads in the build interval do
not, and ar e thu s more difficult to estimate. The incre-
menta l tens ion, dF t , i s a functio n of
the
normal force
which i s itself a function of the tension,
the
inclination
angle, and t he build angle:
dYt
= (W)(BF)(Cos0 ) -c
(k)(Fn)
and the normal force,
Fn, is given by
F n = {[(Ft)(da)(Sin +
[ (F t ) (dS)
-
W)(BF) (S in
8 11 0.5
with da and
do
the incrementa l
changes
in azimuth an d
inclination angles, respectively.
As
with the tension
within
the
reach in terval ,
the drag due
to friction with-
in the build inte rval
increases t h e
tension du ring pick
u p , decre ases the tension during
set
down, and has no
axial effect for
the
neutra l s tate. (While not affecting
asia l load. friction rem ains a factor for th e torq ue calcu-
lation i n the neu t ra l s t a t e . ) Tak ing the change in
azimuth angle as
0, F n reduces to
F n = [ (Ft)(d0)- W)(BF)(Sin811
The calculation of the load increase between the top
of
the
reach interval, Point
1,
and th e KOP, Point
2,
can
be determined us ing an HP-41 program. The program
considers three sta tes of pipe movem ent: pick up, set
down,
attd
the neutra l s ta te .
F o r the
example,
t h e tension at Point 2 F,,
can
be
calculated. For F,
=
15,200 lb, t h e pick
up
load,
F: is
28,100 lb.
Thus
the
build interv al, which
spans
400 ft, increases
fro m a tension load of 15,200 lb at the lower end up to
28,100 Ib at
the
K O P when
picking up
on the s tring.
This 32
lblft average
rate
of increase is considerably
higher than the
14.6
Iblft
buoyed pipe
weight
and
pro-
vides insight into t h e influence of
drag
upon t he total
load. This ra te
of
tension increase is not constant but
increases nearer
t he
KOP Fig. 2 , appro aching 50 lblft
for th is
example.
For
F,
= - ,000 Ib, the set down
load
is
F, = -4,800
lb.
In Fig. 2, th e maximum compression load does not
occur
a t
eith er Point 1o r 2 bu t occurs within th e build
interval . Fo r the example , th is maximum compression
of abo ut 5,800 lb is not high, b ut d oes identify t h e need
to
make
intermediate calculations within t h e build
interval.
For
F, =
5,100Ib,
the
neutral state load
is F2
= 9.200 lb.
Because the torque a t Point 2 also is
a
funtticm o ft he
changing
normal
force
within th e build interva l, i ts
cal-
cula t ion w a s inc luded in
the
HP-41 p rogram. The
torque value
of
primary in teres t i s tha t
when
there
is
no axial pipe movement. Th is would correspond t o
the
condition
of
rota t ing during cementing. For the neutr l
s ta te of
t h e
example, using M
=
2,310 ft-lb.
hl =
2,800 ft-lb.
Another situation
w h w e
rotation may
be
desired
is
wash-down
operations.
The torque fo r the set
down s ta te
is
M-
3.200
ft-lb.
Because the
hole
above P oint 2 is taken to be vertical,
the torque to rotate at
the
surface,
M,
= M .
A
phenom-
enon
not
addressed in the program is the mutual e ffec t
rotation
and
axial
movement
have
on
each
other. Rota-
tion will reduce the axial
drag
on the pipe and thereb y
ease
movemel~t
p or down the hole. . Th e torque calcu-
lated fo r th e pick up and se t down s ta te s is thus some-
what conservat ive , having assumed axia l drag and
rotational dr ag t o act independently of each other.
One of
the
more recognizable differences between a
highly deviated well and
a
vertical well is the potential
for significant bending load within t he build in terval.
The build angle can produce sub stantial bending loads
t o a n e x t e n t t h a t they may become
the
overr id ing
design criteria. The
bending
s t ress ,
Sb,
in the
pipe
body can
be calculated for a
given
build angle,
BA:
S b = * ZII) OD) BA)pd
This
bending stress occurs
only
a t locations within
the s t r ing where the re is a change in the
angie
of the
hole. The
reach
interva l which has been taken to
be
a t a
constant inclination has
no
associated bending load. I t
is important to remember, however, that pipe which
ends u p in the reach interval
mus t pass
th rough the
build interval thus being subjected t o the associated
bending loads.
The
bending load produces an axial tensile stress on
t h e pipe s outside
curvature
(convex surface), and
an
equal magnitude but compressive s tre ss on the inside
curva ture (concave
surface). Thus, depending on the
axial
load
state
of
the pipe,
the
added tension
and
rom-
pression created by ben ding
may
aggravate the exis t -
ing tens ion
o r
compress ion s ta te . When des igning
casing string s, i t is easier to relate loads in force rather
than
stress
because pipe body yield and joint s tre ng ths
are in force. The bending stress can be converted in to a
corresponding tension and compression load, Fb. This
is
done
by dete rmin ing the axial force which creates the
same maximum s t re s s at the pipe body
OD as
the bend-
ing load
creates
using Ap
as
th e pipe body
area:
F b
= {Sb) Ap)
The bending s t ress for the ex mple well
Sb = z
211) 5.5) 20)
= 23,200 psi
and
Fb
= ? 23,200) 4.962) = ?11.5,100 1b
Thus the
axial load
state
of
the casing
at any
location
within
the
build interval h s an additional simultaneous
115.1
kips tension and compression.
A t t h e KOP, the
net
tension load,
F,b,
is the sum of
the axial force at
Point
2 , Fi , plu the force due to
bending, Fb.
For s t r ing
pick
up
F,h
=
28,100 + 115,100
=
143,200 1b and - 7,000 Ib
or
s t r ing
set down
F,h =
-
,800 - 115,100 = 110,300 lb and
-
19,900 lb
For the neu t ra l s t a t e
F,b = 9,200 ? 115,100 = 124,300 lb a nd - 105,900
Ib
The pipe
at t h e KOP could th us ex perienc e net axial
loads ranging from 120 kips compression to 143
kips
tension. F o r this medium radius build hole, th e load
d u e
to bending contr ibutes over 75 t o t h e to ta l load.
Vertical
Interval
Once the load at Point 2 has bee11 de te rmined , the
tension
at the
surface, F,, can
be
calculated.
F or pick u p
F,
= F, + KOP) W) BF)
=
28,100
+
(4,000)(17)(0.86)
= 86 600 b
For set down
F 4 800
t 58,500
=
53,700 Ib
For
t he u e u t ~ a l t a t e
F , = 9,200 + 58,500 = 67,700 lb
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Connection
Using an API L-80grade steel for the 5 +-in., 17-lb /ft
pipe and taking
the casing
connection t o
have
a tensile
efficiency of 758 , the jo in t s t ren gth , Pj, would be
Pj 10.75W4.96'7)(95,000) 354 000 b
The tension loads are highest when t he s t r i n g is
picked
up.
For t he joint
at the
surface, the tension
safety factor, SF t, is
S F t 354,000186,600 4.09
For
the joint just below the
KOP,
the tension safety
factor is
SFt 354,0001143,200 2.47
Taking the compression efficiency of
the
connection
to
be
85 , i ts compressive yield str en gth , PJC,s
Pjc (0.85)(4.9G2)(80,000)
337,000
lb
The highest compression loads are produced
when
set t ing down on the s t r ing . E'or the joint just below the
KOP, the compression safety factor, SFc, for the set
down sta te is
SFc
337,0001119,900
2.81
Fo r compression, the load a t the bottom of th e build
i n t e r v a l , F , b , a l s o s h o u l d
be
e v a l u a t e d . I n t h i s
e x a m p l e , it i s a p p r o x im a t e l y e q u a l t o t h e lo ad a t
t he KOP:
F,b -5 , 000
115,100 -120,100 Ib
and
SFc
337,000/120,100 2.81
I f the string is to be rotated, the connection must
have a yield torq ue ra t ing in excess of t he neutral s ta te
torque, 2,800 ft-lb for this example. In addit ion to i ts
basic torq ue capabil i ty , tha t pipe within th e build inter-
val
will
be subjected t o combined torqu e and bending
loads, a
dynamic
load environment general ly reserv ed
for
drill
pipe
tool joints rath ei* ha n
casing
connections.
f
the
pipe is ro t a t ed d u ri n g c e m ~ n t i n g .h e to rque
re-
quired will increase a s
the
cement fills the pipe w ithin
the reach inter\ ,al : ' This
h i g h e r
t o r q u e m i g h t b e
est imate d by using
a
heavier equivalent p i ~ ~ eeight for
the torq ue calculations.
Relationships
The example chosen used
a
build a ngle
of
20 JlOU
ft. A
diffe rent build angle
will
~)ruduce n interest ing change
in the t o rque required to ro tate .
The
torque, relatively
high for t h e very low build angles, decreases
as
t he
build angle itlcreases (Fig 3
.
At about 10"1100
f t
build,
the
torque
becon~es
early co nsta nt, increas ing slightly
with increasin g build angle. Also s h o wn in Fig. 3 i s the
ne t axial load created by the bending. This load increas -
es linearly with increasing build
angle.
A s
can be
seen
from
the two graphs, keeping
the
build angle small to
reduce the load due to bending wi l l affect rotation
to rque adversely a t t h e low build ang1c.s.
The ratio of the to rque a t Po int
2
to
tha t
at Point 1,
M,:M,, can
be
plotted against the build
angle
{Fig . 4) .
This plot is independent of coefficient of friction, pipe
OD, pipe weight, and buoyancy, assuming each s t a y s
constant throughou t t he build and reach intervals.
Ano ther relat ionship which
can
be seen from Fig.
4
is
that th e to rque rat io for
each reach
length approaches
t h e same value, about 1.20. This ra tio is depend ent on
reach angle only. Although
the
to rque rat ios are the
same, the actual torq ue values
vary
by th e rat ios of the
reach length.
F o r
a
90
reach interval , the torque rat io does not
become constan t bu t approaches
1.0.
However,
the
ratio at small build angles is higher than for th e 80"
reach angle hole and decreases more rapidly with in-
crea sing build an gle.
When running
the
string, it will
be
necessary to ap-
ply con lp re ~s ion r equent ly
at
th e to p of the reach in-
terval to advance the pipe far the r in to the
hole.
A s the
string advances, t h e load at Point
1 ,
F, , will increase
proportionallg (Fig.
5). For the example , t h e compres-
sion at Point 1 wi th 2.000 f t into t h e
reach
intervai was
calculated
t o
be 5,VOO Ib.
I f the
reach interval was
3,000h h e
compression
load upon
eaching
T D
would
be 7,500 Ib.
With
these
loads
and
using
the
set
down s t a t e
of the
HP-41 rogram, the
loads
a t the
KOP:
Point
2, are
cal-
culated. For t he example well,
t he
load at Point 2 is
about
3,000
Ib tension ivhen the bottom of the s t r i ng
enters t he reach
interval.
The load upon having3,WO ft
into the reach interval is about 8 800 Ib compression.
hus
or
the example well, th e load at Point
2
is increas-
ing faster than
the
load at Point 1. This ratio of load
change at Point
2
to load change
at
Point 1 becomes
eonsta ut beyond initial ntry into
the
re ch interval .
Also shiliru in Fig. is the load at Point 2
for
different
build angles. Each produces a separa te curve,
but
each
approaches
the
same
s lope a s th e p ipe
advances
within
the reach interval . The
ratio
of change of load, the s lope
of F1 o th e slope of F,, s independent of pipe OD and
weight
as
well as build angle. This
same
ratio also ap-
plies
t o
the s t r ing in the pick up s ta te . The ratio of
change of load is depe nd en t on th e coefficient of friction
and the reach angle.
Editor's note:
A copy of the HP-41 program can be
obtained
by
sending a sel f -addressed , s tamped en-
velope
to :
HP-41 Program, PETROLEUMN G I N E E R
IKTEHNATIONAL.
PO. 1589,
Dallas, Tex.
75221-1589.
References
1
Tolle. Glen anti Delli~igel:Thomas: "Mobil Identifies
Extenrlerl-
Reach-D~illing
dvantages,
Possibilities in N orth
Sea.
Holizon-
tal
D~ illin g echnology,Oil
LF
Gnu J . .adapted
from a
l ~ r e c e n t a t
on
to the Offshore Northern Seas Conference, Stavanger ,
Norway
( N u v e r n b ~ ~
985).
2. Johancsik. C . A . , k riesen. U . B . , Dawaon. R a p i e r To~que
nd
h a g n Ilil-ectionalM7el15-Prediction
and
Measu~ernent,
. Prt
Ttwli (June 1983)
987-99'2.
8.
Sheikholeslami, B.A.,
Schlottman,
B.W.,
Sietlel. E.4 . . Button.
D .M :
D1.illingand
P1.oduction
Aspects
of
Hol iz~)ntl
N'ella
in
t h e
Austin
Chalk, SPE papey 19X25.
3.
Gust , D.A.
and BlacDonald, R.K.:
"Rotation
of a
Long Llnrt-
111
a
Shallo\r Long-Reach
Well,
I
Pet
T ~ r l i
April lW9i 401-Wl.
N nghts reserved.No
part
o th~s
ubhcalron
may be eproduced wflhoutexpress psrrnfssfor t rhe publisher: