Szilas A.P. Production and transport of oil and gas, Gathering and Transportation
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210
7.
PIPELINE TRANSPORTATION
OF
OIL
respectively.
If
the gas pressure is smaller than the pressure prevailing
in
line
8,
then
the plastic pipe expands, and the oil from the pipeline, behind the core, through its
slots
10
and
11,
flows into the safety tank installed following the
f2
overflow line.
The task of throttling valve
2
is to delay the impact of line pressure exerted on the
outside wall of pipe
7.
On
rapid pressure increase a pressure difference develops for a
short time between the two sides
of
the pipe wall, resulting in the opening of the
valve.
If
the pressure rise is of long duration the pressure difference between the two
pipe surfaces is equalized, and the valve closes.
Fig.
7.5-7
shows how the line
1
12
10
9
li
Fig.
7.5
~
6.
Expansible tube type safety valve, after Cohn and Nalley
(1980)
t
Fig.
7.5-7.
Pressure lines
with
and without control, after Cohn and Nalley
(1980)
pressure at the receiving pump stations, without safety valve, would change (curve
I);
and, within what maximum (curve
2),
and minimum (curve
3)
values, the pressure
will
rise due to the safety equipment.
In the prevention of pressures, smaller, or greater than the allowed values, the
operation control equipment of the pump stations, has an important
role.
They
7.5
ISOTHERMAL
OIL
TRANSPORT
SYSTEM
21
1
receive the signals from the gauges measuring the pressures at the suction, and
discharge sides. If, at any pump station the section pressure is smaller than the
allowed value,
or
the discharge pressure is greater, than the set value, the control
unit stops first one
of
the pumps. Due to this, the intake pressure generally rises, and
the discharge pressure is reduced.
If
the rate in the change is not sufficient, the next
pump,
or
pumps, respectively are stopped by the control equipment.
If
with
the first
pump shut-down, pressure change in the desired direction does not occur at all, the
Fig.
7.5
-
8.
Pressure control system
of
a one-pump station, after
Cho
e/
al.
(1977)
alarm shut-down switch after a delay of
10-
30
seconds stops every pump.
A
case of
this sort may occur, e.g., when the pumps of the head station stop,
or,
the gate valve
after the station gets closed.
In
Fig.
7.5
-8
the configuration
of
a pressure control system, applied
in
the
USA
can
be
seen (Cho
et
al.
1977). The operating device, controlling the actions, is the
electrohydraulic throttling
value
2,
installed in the downstream of pump
I.
Check
valve
10
is operated independently. The suction pressure is measured by gauge
3,
and the corresponding signals are transmitted to the direct acting controller
4.
Gauge
5
measures the discharge pressure of the pump, and transmits the
corresponding signals to the reverse acting controller
6.
If
the signals, observed by
transmitter
3,
are smaller than the set point value,
or
the values measured by
transmitter
5
are higher, signal selector
7
is alerted. This is connected to the two
controllers and transmits the one with lower signals to the valve actuator.
If
the
controllers fail valve
2
also closes. By unit
9
this valve can be manually also
controlled. Motor overload could be similarly implemented.
A
reverse acting
controller could be used, with a setpoint equal to the highest allowable current. The
output
of
this would pass through signal selector
7
and would throttle the valve
2
if
the motor current rose above the designated value.
Figure
7.5
-
9
represents (after Cho
et
ul.
1977), two series connected pumps at the
booster station.
At
smaller rates one, while at higher rates two pumps transport.
If,
while operating the first pump, the second pump unit is
also
started, then the
212
7.
PIPELINE
TRANSPORTATION
OF
OIL
developing
of
a pressure wave must be considered. To avoid this,
in
case of manual
regulation, valve
2
is generally throttled before starting the pump, to reduce the
pressure rise, due to the pressure surge. After starting the pump the valve is
gradually opened. This solution, however, is characterized by a significant
loss
in
energy. In case
of
programmable set point generator this
loss
in energy may be
reduced. The scheme
of
thiscontrol unit is represented in
Fig.
7.5-
10.
At
stationary
transport, with one pump, the parameters
of
A
moment are valid.
In
that case the
I
r--
--
--------
Fig. 7.5-9. Pressure control system
of
a two-pump station, with programmable starting, after
Cho
e?
a/.
(1977)
s
P
q
Fig. 7.5-
10.
The change
of
characteristics
of
series connected pumps with programmable starting
v.
time, after Cho
er
al.
(1977)
adjusted pressure set point
pa
(dashed line) is slightly higher than the measured
po
discharge pressure (solid line). Before starting the second pump, the set point
generator is placed in track mode, and then, the set point decreases to the measured
pressure
(B
moment). At
C
moment the set point generator is commanded to hold its
present value and at
D
the second pump is started. In a short time the pump pressure
7.5.
ISOTHERMAL OIL TRANSPORT SYSTEM
Turn
on
pump
No
3
shut down
output
1
L
213
Check
oll
pump
running
I
Turn on pump
NO1
shut down
Yes
pump
No
1
output'
Wait
30s
No
2
shut
down
output'
1
-
N+
pump
NO3
No
valves
Fig.
7.5-
11.
Flow chart
of
centrifugal pumps' shut down
process,
after Muma and Henry
(1976)
214
7.
PIPELINE
TRANSPORTATION
OF
OIL
-
Recorder
for
Olfference
Account Alarm
Printer Prlnter
Teletype
-
t
Display
for
Pressure
Line Paper
5
DISCS
Card
Reading/
Punching
I
-Punch
Printer
1
Analogue Central Process
I/O
unit
Digital Memory Control Unlt
I
/o
E
xpander
lor
Display
I/
begins to increase above the set point value, and the
off
set causes the controller
6
(Fig.
7.5-9)
to throttle valve
2.
The throttling minimizes the amplitude
of
the
pressure wave.
At
moment
E,
after the pumping speed is great enough, the set point
generator is placed in the ramp-up mode. The controller gradually opens the valve,
and the rise at moment
F,
reaches the value, prescribed for the operation of the two
pumps.
In
case of remote control the input signal to controller
4
may
be
also
modified. The control valve is a floating seat full-bore ball valve with electrohydrau-
lic actuator. The up-to-date devices entirely shut down within
3
s.
Their parameters
are of “equal percentage” (see Section
6.2.1
-
(c)). Opening-closing hysteresis,
proportional to pressure drop across the valve, must be here also considered.
The Continental Pipeline Co. improved the transport system of an existing
30
in.
pipeline of
800
km length (Muma and Henry
1976).
Under the control of the central,
supervisory master computer, in a hierarchical system, a decentralized system was
established, controlled by separate PLCs from pump to pump. The task included
the starting-up of the motors, the starting, and shut down
of
the pumps in a given
sequence, the control
of
the gate valves’ operation, and alarm shut-down switches of
the station. There are four programs elaborated for the control.
As
an example,
Fig.
7.5
-
I1
shows the flow chart of the shut-down process of the p’imps, including that
of the gate valves of the stations.
Ope
ra
to
r
Desk
Process Peripherie Remote Control
/Units
for
Dota
-
Centre ond Port
-
Communication
I
Tank Station
Interface
cc
MODEM
Telecommunication Plant
lSu bstationl
IProcessl
4
7
6.
NON-ISOTHERMAL OIL TRANSPORT
215
There are several solutions for an up-to-date transport system (Szilas and Olih
1978). Their common feature is that they are controlled by computers.
A
good
example for a modern project is the north-west European pipeline (Nord West
Olleitung, i.e.
NWO),
that transports oil from the Wilhelmshafen port to six
refineries (Hoppmann 1976). The length of the pipeline is 721 km, and
it
consists of
pipe sections
of
28" and 40' nominal diameters. The informations, flowing through
the four wire communication channel (bandwidth ranges between 3000
-
3400
Hz),
is received by computers of two systems. The IBM-1800 that belongs to the first has
a core memory of 38
K,
and 5 peripherical disc memories of 51 2
K.
It
works partly
in
on-line, and partly in off-line mode. The main off-line duties are quantity
accounting, making statistics, disposition, planning of pump operation, deter-
mination of the load of the tanks, etc.
It
is used for solving on-line problems
requiring relatively high computer capacity. The stored data are at the disposal of
the off-line computing too. Such tasks are, among others, calculating the oil
quantity balance
on
the basis of the tank content data,
or,
the visualization of the
existent piezometric line of the pipeline on the computer display at 20 second cycles.
The upstream and downstream pressures of the pumps and control valves, the
curves, representing the maximum allowable pressures, and the geodetic profile are
also visible.
The task
of
the two Siemens PR 320 computers belonging to the second system is
the leak detection in on-line mode. Among the two units having 24
K
core memory
only one is working permanently. The other, stand-by computer is switched on by
the dispatcher only in case of a failure, occurring in the operating unit. The
computer, recognizing the leak on the basis of the telecommunicated pressure and
rate data, is also able to locate the leak.
Figure
7.5-
12
shows the scheme of the
system, connected to an
IBM
machine, and also the prepared report types.
7.6.
Non-isothermal oil transport
If
oil viscosity is comparatively high and the temperature of the flowing oil differs
appreciably from that of the line's environment, flow can no longer be regarded as
isothermal. Pipelines transporting oil are most often buried in the ground. The
thermal behaviour of the soil will accordingly affect the temperature of the flowing
oil.
7.6.1.
Thermal
properties
of
soils
The temperature of soil of a given nature, undisturbed by human hand, is
determined on the one hand by insolation, and on the other, by heat flowing from
the interior of the Earth towards its surface. Insolation has a double period, a daily
and a yearly one. The internal heat flow, due primarily to radioactive decay, is
constant in a fair approximation at any geographical location. Its magnitude is less
by about two orders of magnitude, in the depth range of 1-2 metres we are
216
7.
PIPELINE TRANSPORTATION
OF
OIL
concerned with, than the effects of insolation, and we shall accordingly disregard
it
in the sequel. Assuming the soil to be thermally homogeneous, temperature change
per unit time is described by Fourier's differential equation
dT
-
at
=a,V2T
7.6-
1
where
a,
is
the temperature-distribution
or
diffusion factor defined by the
relationship
7.6
-
2
Assuming the surface to be level in an approximation sufficient for
our
purposes,
the above equation describing temperature variations may be used satisfactorily
also in its one-dimensional form,
7.6
-
3
Let
us
characterize the temperature change
on
the ground surface, that is, at
h
=
0
by the relationship
A
To
=
A
Ton sin
(ot
where
w
=
2n/tp.
The other symbols are identified
in
Fig.
7.6
-
I.
Under physically
meaningful initial conditions, Eq. 7.6-
3
has the solution:
AT,=
AToa
exp
[
-
h
F]
sin
[F
-h
p].
astp
astp
7.6
-
4
The extreme temperature fluctuation
AT,,,
at depth
h
arises when the sine term
equals unity, that is,
7.6
-
5
This equation and
Fig.
7.6
-
1
reveal how the amplitude decreases with depth. Daily
fluctuation is almost imperceptible at
1
m depth, being less than
01
"C
in most cases.
The yearly wave will cause measurable temperature changes to depths of
25
-
30
m,
depending on the thermal properties
of
the ground. Beneath a depth of
30
m,
temperature will be controlled solely by the terrestrial heat flow.
Figure
7.6
-
I
further reveals that the temperature wave
I1
at depth
h
is displaced in phase against
the ground-surface wave. Phase shift is described by
7.6
-
6
7.6.
NON-ISOTHERMAL OIL TRANSPORT
217
Fig.
7.6-
1.
Fluctuation in time
of
soil
temperature
Example
7.6-
1.
Find the extremes of the daily temperature fluctuation and the
phase shift at 0-3 and
1.0
m
depth
if
a,=4.9 x m2/s, and surface temperature
varies from
+
22
to
+
2
"C.
-
In the notation of
Fig.
7.6
-
1,
surface amplitude is
By
Eq.
7.6-5,
the amplitude at depth 0.3 m
is
AT,,=
lOexp[ 49
x
-$""--I=
x 86400
-
+075
"C
whereas at depth
1.0
m it is
AT,,= +O.O018
"C.
The mean temperature is, in a fair approximation,
T-T
h-
o-
-
Tomi,+dToa=2+ lo= 12
"c.
Temperature extremes at 0.3 m depth are
T,,+
ATh=
12+075= 12.75
"C
T,min=T'h-AT,,=12-0~75=11~25"C
at
1.0
m
depth, they are
Kmax=
12.002
"C
and
qmin=
11.998
"C
that is, the fluctuation range at
1.0
m
depth is evanescent. Phase shift at
0.3
m depth
is stated by Eq.
7.6-6
to
be
218
7.
PIPELINE TRANSPORTATION
OF
OIL
Thermal conductivity ofthe soil depends on the conductivity of the soil matrix, on
grain size distribution, on the bulk density of the dry soil, and on humidity.
Makowski and Mochlinski, using an equation of Gemant, have developed a
relationship describing the thermal conductivity of humid soil (Davenport and
Conti
1971).
They considered the dry soil substance to be made up of two grain size
fractions: sand
(0.002 -2
mm particle size) and clay (less than
0.002
mm particle
Fig.
7.6-
u
E
--.
3
20
16
1.2
08
04
0
2.
Influence
of
water content upon thermal conductivity ofsand and clay,
Conti
(1971)
after Davenport
and
size). Let the weight percent ofclay be S,and the weight percent ofhumidity referred
to the total dry substance be
S,
.
The thermal conductivity of the humid soil is, then,
).,=(A
IgS,+B)
10'
7.6
-
7
where
A=0.1424-0.000465
S,;
B=0.0419-0400313
S,;
C=6.24
x
10
-4()sd.
the bulk density and specific heat of the soil. In humid soil,
The determination of the temperature distribution factor requires knowledge of
7.6
-
8
7.6
-
9
The simplification is based
on
the fact that the specific heat of water,
c,
=
1
kcal/
(kg
K)=4187
J/kg
K.
-
Introducing the factorsdefined by the aboveequations into
Eq.
7.6-2,
we get for humid soil
7.6-
10
Figure
7.6-2
is the diagram of Makowski and Mochlinski (in Davenport and
Conti
1971)
based on Eq.
7.6
-
7,
and transposed into the
SI
system
of
units.
It
gives
the thermal conductivity
of
humid soil
v.
percent humidity for sands and clays
of
7.6.
NON-ISOTHERMAL
OIL
TRANSPORT
219
different bulk density.
Figure
7.6
-3
(from the same source with modification),
shows heat diffusivites v. percent humidity, likewise for sand and clays of different
bulk density.
Various methods have been devised for determining the in situ thermal
conductivity
of
undisturbed soil.
Of
these, we shall outline the transient needle-
probe method after Makowski and Mochlinski
(1956).
The transient needle probe is
an electrically heated device which can in effect be regarded as a linear source of
1.
1
0.9
n
0.7
-1
E
E
0.5
0
0.3
0.1
0
I
I
I
1
0
20
40
60
80
Fig.
7.6-3.
Influence of water content
upon
temperature distribution factors
of
sand and clay, after
Davenport
and
Conti
(1971)
S,,
"la
heat. It is inserted into
a
hole drilled in the soil at the place where the measurement
is
to be performed, and heated electrically
so
as to generate a heat flow of constant
magnitude. Two thermocouples are attached to the outer, metallic sheath of the
probe. They are used to establish its surface temperature at frequent intervals.
It
can
be shown that if heat flow
is
constant, the temperature of the soil (on the outside of
the metal sheath) at a distance equalling probe diameter
r
from the linear heat
source
is
given after the elapse of time
t
by
T=
-[-
@*
EI(L)]
4nA,r2 4a,t
If
r2/4a,t
is low enough, that is,
if
t
is large as referred to
r2/us,
then
1
47t
T,
-
T,
7.6
-
1 1
that is,
if
heat flow
@*
is constant, temperature varies as the logarithm of time.
Plotting this relationship on log paper one obtains a straight line of slope
@*/47t1,,