Szilas A.P. Production and transport of oil and gas, Gathering and Transportation
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170
7.
PIPELINE TRANSPOR7ATION
OF
OIL
describe an ultrasonic dividing detector. Here, the perforation of the pipeline is not
necessary. On the outside wall of the pipeline a transmitter, operating
with
piezo-
crystal, is attached and ultrasonic signals continuously are transmitted through the
pipe. The signals, arriving at the other side of the pipe, are sensed by a receiver.
At
the moment, when a mechanical dividing instrument moves
in
the section, measured
by the'instrument, the intensity of ultrasonic signal significantly decreases.
On
this
basis the dispatcher and/or the computer is informed about the passing of the fluid
slug.
7.3.
Leaks and ruptures in pipelines
Leaks, occurring in buried pipelines may be classified into two groups. The first
group includes the big holes, caused by the rupture
or
break of the pipeline. They are
mostly due to the too high internal pressure,
or
to external impact (earthquakes,
landslides, the damaging operation of the bulldozers, etc.).
It
may be also promoted
by ruptures,
or
fault
of the pipeline that remained undetected during hydrostatic
pressure test. Through ruptures and breaks
of
this type a relatively large oil volume
pours out from the pipeline
within
a short time. The second group includes small
leaks that develop generally due to corrosion
in
a longer period of time. Through
these leaks a relatively small oil rate seeps into the soil. Between the two groups,
holes and outflow ofmean size practically do not occur (Kreiss
1972).
Thus the main
tasks
of
the leak detection are:
(i)
the checking of the integrity of the pipeline before
starting its operation;
(ii)
the determination of the existence, and the locality of great
leakages within a very short period
of
time;
(iii)
the recognition and location of small
seepages, generally periodically. In the following section we discuss problems
(ii)
and
(iii).
For
the detection
of
the existence and the spot
of
the leaks in case of operating
pipelines several methods have been elaborated. The most ancient method is the
regular ranging over of the trace and its visual observation. Today, this operation is
partly,
or
totally performed from vehicles (helicopters, planes). Particularly in case
of great leaks and breaks the outpouring oil may cause considerable damages, that
is why the aim is that the existence and locality of the rupture should
be
found in the
shortest possible time after its induction. Beside the visual observation, systematic
instrumental tests, and observation methods are also applied. Small leaks, caused by
corrosion can
be
visually observed at the ranging only,
if
the seeping oil
contaminates already the surface. In order to recognize the phenomenon
in
the
shortest possible time the instrumental observation of this type
of
leakage should be
also introduced.
Table
7.3-1,
after Takatsu, gives a survey of the instruments
determining the existence, and sometimes also the place of the leaks. The symbols,
used for the qualification are as follow:
I
to be applied in the first place;
2
is to be
applied in the second place,
or
less suitable;
0
not suitable;
+
stands for instruments
recommended in Japan; while
-
means that the device is officially not recommend-
ed for application in Japan. In the following part some methods of major impor-
tance are discussed. The interpretation of the type marks is shown on the Table.
Table 7.3-
1.
Leak detecting methods
for
pipeline after Takatsu and Sugaya
(1977)
with small modification
Code
Detected
parameter
Flow
Pressure:
*dp
dx
'
dr
Noise
Gas
concentration
Air
pressure
Impedance
Essence
of
the method
a) Comparison
of
in- and outflowing crude rates
b) Comparison
of
differential value
of
flow
rate difference
between inlet and outlet point
a) Measuring the change
of
hydraulic
b) Negative pressure wave detection
c) Detection
of
differential pressure increase between block
d) pressure drop in the pressurized section during downtime
a) Detection
of
supersonic waves caused by leakage and
of
b) elastic waves caused by leakage from outside
of
the pipe
gradient
walves during downtime
of
pipeline and
of
by noise meter
Detection
of
inflammable gas vaporized
from
spilled oil
Detection
of
pressure drop
of
an air containing
oil
soluble
tube
dissolved by the spilled oil
Detection
of
change in impedance
of oil
soluble insulator
dissolved by spilled oil
Small
leak
detec-
tion
1
0
0
0
1
I
I
2
1
1
1
Time
for
detection
during
operation
1
I
1
I
0
0
1
1
1
1
1
during
downtime
0
0
0
0
I
1
0
2
I
I
1
Leak
loca-
tion
0
2
2
2
2
2
I
2
I
2
I
Japanese
stipula-
tion
code
Note:
I
means best method.
2
means good method.
0
means not adviwble
172
7.
PIPELINE TRANSPORTATION
OF
OIL
7.3.1. The
detection
of
larger leaks
Type
T/I.
A
rapid, and relatively simple detection is rendered by the so-called
ERM
system (Kreiss 1972). The essence of this method is that at the two end-points
of
the pipeline (or pipeline section) sensors are mounted which measure and
transmit signals, directly proportional to the actual pumping rate. The signal device,
e.g. may be two measuring probes, transmitting the pressure in the pipeline,
or
orifices, do not hindering line scraping,
or
an instrument, measuring the pressure
differential at two ends
of
a pipe bend
of
small radius. The signals, measured at the
ends
of
the pipeline are transmitted to a computer that continuously evaluates them.
The derivative
of
the differences, in case
of
leaking, significantly change. After Kreiss
(1972).
Fig.
7.3-I
represents two curves that can be defined by such a method.
Curve
I
shows the difference between the liquid rates, inflowing at the starting point,
and outflowing at the end point
of
the pipeline, respectively, i.e. the approximative
liquid rate passing through the suddenly emerged leak; while curve
2
represents the
total volume ratio of the ouflow liquid, as compared to the storage capacity
of
the
pipeline, as a function of time. From the shape
of
curve
I
it
can be read that this
phenomenon includes three periods. In the first period,
a,
the outflowing liquid rate
increases
to
a maximum value, and then
it
decreases to a certain value. This end
value is determined by the parameters
of
the still operating pumps, and by those of
the stationary out-flow through the leak. In period
b,
these parameters practically
remain constant.
At
the beginning
of
period
c
pumping is stopped, and then the
liquid pours out of the pipeline, which is supposed to be horizontal and closed at
C;"
7
2-
250
200
2
0
U
100
0
n
100
.
-
a
>
.
s
Y
50
7.3.
LEAKS
AND
RUPTURES
IN
PIPELINES
I73
both ends. The characteristic shape of curve
I
indicates the leak
in
1-2
minutes
after its occurrence, in spite
of
the fact that basic data used are not the exact
volumetric rates but only signals proportional to them. In the method described by
Cridner
(1974),
and Campbell
(1975)
turbineflow meters are usedfor the measurement
of
in- and outflowing liquid rates,
the results obtained are comparatively exact, and
the differences
of
measured data are recorded. Even
in
case of proper operation of
the pipeline small fluctuation can be inspected
(Fig.
7.3-2).
Differences may be
-
-
- -
-
-
-
-
-
-
-
- -
- -
-
-
-
- -
- - -
-
-
3
t
Fig.
7.3-2.
The change
of
the fluid
rate
dilTerence
v.
time existing between the pipeline inflow and
outflow, after Campbell
(1975)
attributed to the changes in temperature, pressure, and quality of the oil flowing
in
the pipeline, since due to the alteration of the above parameters the volume
of
the
liquid,stored in the pipelinechanges too.
By
experiments the upper, and lower limits
of
Aq
can be determined (lines
I
and
If),
i.e. the range within which the indicated
fluctuation does not mean a damage in the pipeline. The midline of the recorded
diagram is parallel to the abscissa, while its ordinate height corresponds to the point
Aq=O.
If a large leak, or break occurs in the pipeline, the midline of the curve
Aq
undergoes a one-directional change. Both the slope of the midline and the
surpassing of the limit lines indicates leakage.
The pipe-break can also
be
indicated by
periodical mass balance
of the liquid
volume flowing in the pipeline (Young
1975).
Its essence is that the inflowing and
outflowing liquid volume together with the temperature, pressure and density of the
given pipeline, is measured for a predetermined short time, and, by strain gauges the
tension in pipe is also determined. On the basis of these parameters, from time to
time, it is automatically determined,
if
the difference in inflowing, and outflowing
volumes at the pipe-ends is corresponding to the change in the oil volume contained
in the pipeline. The procedure is accurate enough
if
the evaluation is carried
out
on
the basis of a sufficiently great number
of
data. The speed of leakage sensing is the
function
of
the frequency of measurement and evaluation. For this procedure a
computer is needed and, as a whole, it is rather expensive.
Type
T/2.
By
T/1
types, we obtain information only for the existence of leakages in
the pipeline, but we do not receive information about the place
of
leakage, fracture
or break. The location
of
the leakage, by observing, and evaluating of the pressure
174
7.
PIPELINE TRANSPORTATION
OF
OIL
waves, is possible (Thielen 1972a). Being aware of the parameters
of
the sound wave
propagation
in
the pipeline, the time v. length curve of the negative pressure wave
front which develops by the impact of outflowing liquid through an abruptly
emerged leakage is previously determined and graphically represented (see also
Section 7.1). In
Fig.
7.3
-
3(a)
two superpositioned diagrams of this kind are shown.
In the function of pipeline-length, both of them show, between the
i
and
k
pressure
testing points the arrival time
of
the pressure front, propagating
in
the pipeline,
t
+
1
Fig.
7.3-3.
The location of the leakage
by
pressure
wave lines. after Thielen
(1972b)
supposing that the pressure wave starts from a leak occurred at the other point of
observation. The mirror image of the two curves is slightly different, since the
propagation velocity towards the liquid flow direction equals
(w
+
u),
while in
opposite direction
it
is
(w
-u).
As
a result
of
continuous telemetering we know the
exact time of the arrival of the pressure wave to the
i
and
k
testing points, i.e. values
ti
and
t,.
It can
be,
obviously, seen that
if
the above two curves, plotted on tracing
paper, are placed upon each other
so,
that the testing points
i
and
k
should cover
each other, and the two diagrams are vertically moved by the value of
(tk
-
ti),
then
the abscissa value of the intersection of the two lines determines the point of the
break. The accuracy
of
this process hardly decreases, if, in the analysis of the bi-
lateral pressure wave propagation, the flow velocity,
u,
is neglected. Excessive
accuracy is often unnecessary, since the leakage, detected from the centre, is
in
most
7.3
LEAKS
AND
KUPTUKCS
IN
PIPELINES
I75
cases seen by the service team sent to the spot, from distances of several hundred
meters.
In
Fig.
(h)
is shown that,
in
practice, the location of the leakage, on the basis
of the simplified curve, is performed by the help of the dashed line mirror curve.
Fig.
ic
)
shows the possibility of further simplification. Here, the point of the break is the
abscissa value of that curve-point whose ordinate value is
O.S[t,,/(t,
-
t,)].
It
can often be found that at certain sections of the pipeline oils of different quality
are transported. Thus,
it
may occur that even between two pressure testing stations
of the damaged pipe section, oil slugs of two different qualities flow.
In
such a case,
the time
v.
length diagrams of the pressure wave, for both oil qualities, must be
separately determined. By these curves and applying the above process two
hypothetical break points are determined. The real break point will fall between the
two calculated values. Being aware of the approximate place of the slug boundary at
the moment of the break, the curves, time v. length may be made more accurate and
thus, the location,
if
it
is necessary, can be improved.
In
spite ofthe fact that by applying the pressure wave method not only the place of
leakage, but, obviously its occurrence may be also determined, the
711
processes are
useful as well. The simultaneous application of different methods significantly
increases the reliability of leak detection. In the Federal Republic of Germany, e.g.,
the simultaneous application of two independent methods is prescribed.
7.3.2.
The
detection
of
small leaks
Type
773.
The
oil
flowing out of the pipeline through a small leak and seeping into
the soil generates sound. The flow being turbulent, a high frequency ultrasound
develops that may be detected by instruments. Depending on the fact whether the
measurement takes place outside or inside of the pipe wall, two types of solutions are
known. In the first case, the detection and simultaneously the location of the leak is
carried out by the help of a
contact microphone mounted on the outer surjiuce ofthe
pipe wall
sensing ultrasounds. Before the measurement, at certain points the soil
is
dug up and removed from the outside surface of the pipeline.
If
the leakage point is
bracketed, then the next measurement is carried out at the midpoint between the
two former test places. The first measurements were carried out by MAPCO in
distances ranging from
1.2
-
9.6
km, and the accuracy of location was about
180
m
(Pipe Line.
.
.
1973).
In the other type of detection, pipe-pigs,
pistons, moiling along
together with the liquid stream
are inserted into the line, that are suitable for
observing and recording ultrasounds. The signals are recorded on magnetic tape.
After the bringing out of the piston, the location may be carried out on the basis of
recorded velocity data of the pipe-pi& and the recorded signals of outside mounted
locators. In Karlsruhe, a significant amount of work was devoted to the
determination
of
the conditions, among which, at the outflow of the oil through a
small leak, ultrasonic signals of proper strengths can be obtained.
It
has been
established that the intensity
of
the sound significantly depends on the cavitation
occurring at the outflow, and on the stream being turbulent or not. The applicability
range of this process was also determined (Naudascher and Martin
1975).
176
7.
PIPELINE TRANSPORTATION
OF
OIL
Type
T/6.
The “heart”
of
this process is an electric cable conducting alternating
current. It consists
of
two metal wires insulated by a special material, soluble in oil.
If
this insulator is wetted by oil, or by any
oil
product, then
it
gets solved, the two metal
wires get into contact, and by changing the resistance
of
the cable, in the monitor
of
the control station a warning signal appears.
On
the basis
of
the modified resistance,
the leakage detector determines the distance where the short-circuit in the cable has
taken place. The “Leak
X”
system, operating on the very same principle, is also
suitable for indicating the damages caused by bulldozers
(Hydrocarbon.
. .
1973).
Other process-types.
Location by pipe-balls (Gagey
1975).
A
rubber ball
of
a
diameter exceeding the pipeline diameter by
1.5
percent is placed in the pipe, and
this ball is transported by the flow
for
a distance
of
A.
Then, the pumping is stopped,
and the pressure
of
pipe-section
A,
and that
of
the remaining section
B
is
measured
by a sensitive gauge.
If
leakage occurs in section
B,
then at an ideal case, supposing
11
t,
min
Fig.
7.3
-4.
Ideal pressure lines
of
leak location by pipe ball in isothermal
flow,
after Gagey
(1975)
+8......T7,.
1-
11.111,31111....
-
30
40 50
€id
t,
rnin
Fig.
7.3
-
5.
Real pressure lines
of
leak location
by
pipe
ball in isothermal
flow,
after Gagey
(1975)
7.4.
ISOTHERMAL OIL TRANSPORT I77
isothermal flow, pressure profiles, shown
in
Fig.
7.3-4
are obtained. Pumping
stopped at moment
t,.
In section
A
the pressure remains unchanged, while
in
the
section
B
it decreases continuously to the point, at which across the two sides
of
the
ball
a pressure differential of
0.1
-0.5
bar, required for the moving the ball develops.
Then the ball starts to move, and after covering a short distance
it
stops again. Due
to this ball displacement, pressure in section
A
slightly decreases, and then remains
bar
P
1%
1
D
1,
min
Fig.
7.3
-6.
Ideal pressure lines of leak location
by
pipe
ball. after non isothermal
flow
(Gagey
1975)
constant. The pressure in section
B,
after a sudden, slight rise, decreases further
continuously. In reality, due to the peculiar motion of the ball, getting stuck from
time to time, the ideally straight pressure lines are modified.
In
Fig.
7.3-5
real,
measured pressure lines are shown. If, before the stopping, flow was not isothermal,
then the theoretical pressure lines are similar to those, shown
in
Fig.
7.3
-6.
Because
due to the cooling of the oil, the pressure
in
section
A
is also reduced. The rate of
decrease in the pressure, however, is smaller here than in section
B.
If, after the first
experiment, the ball is transported at
a
farther point of the pipeline, then other
experiments, similar to the one described above, may be carried out, and
so
the spot
of the leakage may be determined with an ever increasing accuracy. By this method
even relatively small leaks can be well detected.
7.4.
Isothermal
oil
transport
The temperature of the oil, flowing in a pipeline, generally varies along the length
of the line. If, however, the viscosity
of
the crude is low, and the starting temperature
does not significantly differ from soil temperature, then, for practical reasons, the
flow may
be
considered isothermal.
7.4.1.
Oil
transportation with or without applying tanks
Main parts of the transporting system are the pipeline and the pump stations.
If
the oil is to be transported to a place located at a relatively small distance, then, one
pump station at the head end point may prove to
be
sufficient.
If,
however, the
pipeline is long then the building
of
several, so-called booster pump stations is
I2
I78
7.
PIPELINE
TRANSPORTATION
OF
OIL
required. Formerly, at the booster stations, also cylindrical, vertical storage tanks of
atmospheric pressure were used. The pump station in the head end
k
of a pipeline
section transported crude to the tanks of the tail end point
i,
and from these tanks
the pumps of station
i
sucked oil and transmitted
it
into the next section. The
schedule
of
this solution is shown in
Fig.
7.4-1fa).
The main advantage of this
I
1
(C)
LEGEND
w
got.
safely
YOLY.
valve
Fig.
7.4-
1.
Oil pipeline
transport
systems.
after Hathaway
(1972)
system lies in its simplicity and reliability. The tank farms of the booster stations
ensure the elasticity of the transport rate, that is proportional to their storage
capacity. The pipeline is the aggregate
of
hydraulically independent pipe sections.
From this system the present method
of
transportation, operating with reservoirless
booster stations had developed. The main stages of the development, after
Hathaway
(1972)
are as follows.
For
different reasons (e.g. because of the installing
scraper traps) mounting of gate valves on the pipeline became necessary. Already at
this time, there existed the possibility that due to carelessness the gate valve remains
closed at pump starting, and overpressure develops in the pipeline. In order to avoid
harmful effects, in case
of
solution (b) shown in
Fig.
7.4-1,
at the pumps by-pass
lines equipped with safety valves were mounted.
If
the pressure, after the pump,
exceeded the allowed value, the valve opened, and a part of the oil flowed back
7.4.
ISOTHERMAL OIL TRANSPORT
179
through the valve into one
of
the tanks of the station. The sketch of the first type of
transport system with the booster stations (TBS) operating without storage tanks is
shown in
Fig.
(c).
Since the suction pressure may also exceed the allowed value,
another branch was installed upstream
of
the pump, through the safety valve of
which the high-pressure oil could be led to a safety tank of relatively small storage
capacity. One of the main conditions of the successful operation of TBS without
storage tanks is that each pump station, within a very short period of time, should
attain the same transport capacity, otherwise heavy positive
or
negative suction
pressure changes may occur. In case of the up-to-date system, shown
in
Fig.
Id)
the
protection against the harmful pressure surges is automatically provided.
No
safety
tank is required. (The required safety equipments and control devices
will
be
discussed, more in detail, in Section 7.5.)
The main advantages of the tankless TBS are the following: the vapour
loss
during the transport is smaller; the system is less incendiary; because of the lack of
the tank farm and its equipment the investment costs are significantly lower;
it
can
work without personnel, and especially, in case of unpopulated areas considerable
additional costs may be spared; the greater leaks in the line are easier to detect
because of the higher line pressure.
The disadvantages are, that the system requires a technically more advanced
control system, and better trained personnel for periodical maintenance and
checking; the transport capacity may decrease due to the fact that the oil must arrive
at the pump with a higher than the bubble point pressure; any change
in
the
parameters of the transporting process has an effect on the total length of the
pipeline, and this, on the one hand may lead to dangerous pressure surges, on the
other hand
it
requires the permanent co-ordination of the pump-station operation.
The pressure changes may be
of
two types. In the first group the relatively slow
changes can be classified. These may be caused e.g. by the parafin deposits on the
inside pipe wall, or by the vapour caps developing at the top points
of
the pipeline.
The change in these cases generally does not exceed the value of
0.1
bar/day. Rapid
changes in pressure may originate, from the pressure surges due to the changes
in
transport parameters (see Section
7.l.l),
or by batch transport. Flowing a slug
interface through the pump, pressure change of even
10
bar/min order may occur.
Against the damaging impacts
of
such changes in pressure only an automated
control system may provide protection.
7.4.2.
Design of fundamental transport operations
(a)
Pressure traverse
and
maximum capacity
of
pipelines
Let the specific energy content
of
the flowing oil be
W,
(J/N)
at the head end, and
W,(J/N)
at a distance of
I,
m from the head end. The decrease
in
specific energy
content is equivalent to friction
loss,
i.e.
it
is
A
P,x
P9
29
PY
+z,
-z,=
~
PI-Px
v:-4
w,
-
w,
=
___
i
-
7.4-
1