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7/27/2019 Premium Digest December 2010 Inspection of Gas Pipelines With Ultrasonic Measurement Techniques - Practical A

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5 PiPelines international digest | deCeMBer 2010

integritY

Inspection of gas pipelines withultrasonic measurement techniques:

practical aspectsBy Dr Jochen Stratmann, Open Grid Europe GmbH, Pipeline Engineering Section (TLE), Essen, Germany

Pipeline inspection tools based on ultrasonic measuring techniques are requently used or the assessment o oilpipeline integrity. Due to the requirement o a liquid being present between the detection device and the pipe wall,this technique is not readily applicable to gas pipelines. Recently, the inspection o gas pipelines with ultrasonicdevices has been conducted successully by completely flling the pipeline with a liquid (or example, water) beorethe inspection. As an alternative, a short section (batch) o liquid may be run through the pipe in order to avoidcompletely flling it. The liquid is separated by pigs rom the surrounding gas in the pipeline and the measurementdevice is kept within the liquid-flled section. Batches may, however, assume considerable velocities in the event o

large slopes along the path o the pipeline, and this may inuence the probability o detection and, more practically,it constitutes a risk o pipeline damage i velocities become too high. This article ocuses on the method o calculatingthe batch velocity, predicting water batch movements in pipes as well as other practical aspects.

Ultrasonic detection devices employed by in-line

inspections (ILI) tools are used for the detection of

pipeline defects such as corrosion, grooves, laminations,

or cracks. These tools enable the detection of defects which

cannot be achieved using other techniques such as magnetic-

ux leakage (MFL), especially with respect to cracks due to

fatigue or stress corrosion. However, the contact between the

ultrasonic detector and the pipe internal wall needs to be

established through an incompressible liquid, which rules outstraightforward use of these devices in gas pipelines.

Filling the pipeline with a liquid before or while conducting

the pigging provides the most intuitive solution to this

challenge. However, typical sections of large gas pipelines may

contain volumes of several tens of thousands of cubic metres.

Any liquid employed for this use will experience a certain

degree of contamination through the contents of the pipe,

which may be a result of compressor station oil losses or the

higher hydrocarbons contained in the gas itself. The disposal or

the decontamination of the liquid may be extremely expensive,

a condition which gives the pipeline operator reason to look for

an alternative.

Additionally, it presents a considerable challenge to obtain

public and/or private permits to obtain water from rivers

or lakes. The public water supply is generally insucient

regarding capacity. To transport the water using temporary

above-ground water lines to one of the sites where pig traps

are located may also constitute a handicap. Furthermore, thedisposal of the water after its use, even after it being cleaned

using activated carbon, may not always be allowed by local

authorities.

Symbol Property Unit

a speed of propagation m/s

di internal pipe diameter m

E Youngs modulus N/m2

k compressibility of liquid l/Pa

l batch length m

ow friction coecient

ppressure dierences

(e.g. from ow friction)Pa

liquid density kg/m3

s wall thickness m

v velocity dierence m/s

Figure 1: Example of a liquid batch set-up.

Figure 2: Example of general set-up and boundary conditions (top)

and photo of mobile air compressors (bottom).


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Set-up and goalsDue to the above-mentioned conditions, pigging of the gas

pipeline using short liquid batches, of only several hundred

metres length, is desirable. In this case, the batch consists of a

liquid-lled section within the pipe while the liquid is separated

from the gas by means of pigs at both ends. Figure 1 shows such

a set-up.

Depending on the pressure in front of and behind the batch

(i.e. the boundary conditions), the batch may move more or less

freely within the pipeline. In addition, the height prole of the

pipeline inuences this movement.

The set-up under consideration here is a short water batch

with a length of approximately 600 m. The gas volume behind

the batch is fed with air using multiple compressors in

continuous or intermittent operation. Therefore, the amount

of air in this section continuously increases; the pressure,

however, varies depending on the position of the batch. The

gas volume in front of the batch is vented to the atmosphere in

such a fashion that the gas pressure in this volume remains at a

constant level, while the volume continuously decreases as the

batch moves in forward direction. Figure 2 illustrates this set-

up, the questions associated with which may be subdivided into

the following aspects:

Operational

There is a need to calculate the pressure needed to drivethe batch through the pipeline considering the local height

dierences.

The time at which the batch will be expected at a certain

location within the pipeline needs to be known for general

planning and for positioning pig detection teams along the

pipeline if pig detection is done throughout the inspection at

given locations.

Measurement technique The ultrasonic device is operated at a certain detection

frequency at which the measurement is conducted. If the

velocity of the batch exceeds a certain value, for example1.5 m/s, the probability of detection for a specic size of crack

will become too low to be acceptable for the assessment of

pipeline integrity. Therefore, it becomes mandatory to nd a

means of estimating the expected velocity for each location

along the pipe. Exceeding the critical velocity with respect

to detection in a small fraction of the total pipeline may be

considered acceptable.

Pipeline integrity and safety aspects Due to the density of water, a considerable hydrostatic

pressure may act within the pipe. Naturally, the pressure in

the pipe must not exceed the design pressure. This aspect

is less relevant to short batches, but may become critical for

batches of lengths in excess of a thousand metres depending

on the height prole.

At certain conditions discussed below, the batch may

separate into two (or more) parts. This is entirely undesirable

and induces the risk of a water hammer which may occur

if two of these parts collide in the pipe at a considerable

velocity. To avoid the batch separation, a certain pressure

needs to be maintained within (and at the front) of the batch.

Also, the question arises which collision velocity leads to a

critical pressure shock in the pipeline.

At high velocities, the batch may cause a considerable

(centrifugal) force to act on bends of the pipe if the pipe is

not straight at locations characterized by high velocities,

i.e. downhill.

In summary, it appears vital to rst calculate the pressure

behind and in front of the batch as well as its velocity. From these

parameters, conclusions regarding the integrity/safety aspects

may consequently be drawn.

Pressure calculationIn the following, the batch is considered merely as an amount

of water within the pipe. The mass and volume of the individual

pigs is not taken into account, since it is considered to result in a

similar density as an equivalent section of water or less, and thus

to behave accordingly.

Three dierent characteristic pressures are discussed in this

section. These are:

The maximum pressure that occurs within the section in

which the batch is located: this pressure must not exceed the

maximum operating pressure/the design pressure of the pipe.

The minimum pressure to be maintained in front of the batch:

the reason why a certain pressure must be maintained infront of the batch is that the pressure at the highest point

within the batch (the top in Figure 3) must not become

lower than the vapour pressure of the liquid. If this is the

maximum pressure

600 m batch

minimum pressure in front of batch

pmax = 8.9 bar

pdrive,max = 23.6 bar

pmax = 23.6 bar

pfront = 10.0 bar

600 m batch

driving pressure

Figure 3: Illustration of relevant height dierences.

Figure 4: Result for the three dierent characteristic pressures for

a batch length of 600 m. The maximum pressure and the driving

pressure are calculated on the basis of a pressure in front of the

batch of 10 bar (abs). The height prole is given in each graph to

enable comparison. The location is dened as the position of the

front of the batch, see Figure 3.


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case, the liquid will become gaseous at this location and,

consequently, the batch will fall apart into two separate

parts. It is vital to avoid this, as it gives rise to the danger of

a so-called water hammer if the two parts later collide in the

pipeline. Even a low velocity may generate a high-pressureshock. The pressure caused by the collision is discussed in

one of the following sections.

The pressure needed to drive the batch through the pipeline

(driving pressure, pressure behind the batch): this pressure

must be generated by one or more compressors. It depends

on the location of the batch within the height prole. This

aspect may be critical as to the duration of the pigging

process if a high pressure is needed in long sections of the

pipeline. If lling this volume at high pressure takes too long,

the pig battery capacity may become critical.

The maximum pressure in the section of the pipeline filled

by the batch exists at the very bottom of this section. It is

influenced by the hydrostatic pressure resulting from the

height profile as well as the pressure in front of the batch.

Figure 3 shows an example of a height profile which includes

an indication of the section filled with water. At this batch

location, the maximum pressure occurs at the very front and

is therefore identical to the gas pressure in front of the batch

if pig friction is disregarded. It must be ensured that this

pressure does not exceed the pipe design pressure. It must

therefore be calculated for all possible batch locations. A result

in Figure 4 shows that, for the case in hand, the maximum

pressure does not constitute a critical issue. The maximum

pressure aspect is most critical for long batches and large

height differences.The minimum pressure in front of the batch is calculated on

the basis of the following scenario: At any time or location,

the gauge pressure in front of the batch must, as a minimum,

match the hydrostatic pressure difference between the top

and front locations as indicated in Figure 3. This ensures

that the pressure at the top location never becomes lower

than 1 bar and therefore not lower than the vapour pressure

of water. This avoids the vaporisation of water at this point

at low pressure. Naturally, a certain safety margin is added

on top of this pressure. Since the height difference between

the two locations varies significantly with the position of the

batch, it must be calculated for each batch location. To ensuresafe operation of the batch run, it is easiest to maintain a

constant pressure in front of the batch as indicated above in

the section Set-up and goals. However, in some instances it

may be appropriate to vary this pressure, for example to avoid

an unnecessary operation of the compressors to create a high

driving pressure or to avoid a critical level of pressure at the

lowest point in the batch with respect to the design pressure of

the pipe. Figure 4 shows the calculation results.The driving pressure may be estimated by adding the

hydrostatic pressure dierence between front and rear of the

batch as indicated in Figure 3 to the gas pressure in front of the

batch as well as an equivalent pressure which corresponds to

the pig friction. In the set-up discussed here, this pressure must

be generated by means of air compressors as shown in Figure

2. Figure 4 shows that, due to the necessary pressure in front of

the batch of 10 bar and the pig friction, the maximum driving

pressure gets close to 25 bar even for the short batch of 600 m

length and this height prole example.

An alternative to calculate the driving pressure is to use a mass

balance of gas (air) in the volume behind the batch. Here, the

mass ow rate of the compressors used to feed the gas/air to this

volume must be considered as well as the actual, time-dependent

location of the batch. The time-dependent location of the batch

within the pipe is known after solving the problem described in

the following section.

Batch movement predictionand velocity calculation

As stated above, the presence of pigs within the water batch

is disregarded and the batch is treated as one continuous

section of water. To calculate the velocity, a balance of forces

acting on this lumped mass of water is considered.

Primarily, the movement of the batch is inuenced by thepressure behind the batch (pressure 1) and in front of the

batch (pressure 2), see Figure 5. These pressures depend on the

location of the batch within the pipe and on time (for example,

a compressor continuously feeds gas to the gas volume behind

the batch). As an example, moving the batch to the right-hand

side in Figure 5 will increase pressure 2 and decrease pressure

1. Simultaneously, a compressor feeding gas into the section

behind the batch increases pressure 1; venting gas from the

volume in front of the batch (in the direction of movement) will

decrease pressure 2.

In addition, Figure 5 displays the height prole of a gas

pipeline as an example. Due to the slopes associated with thisprole, a gravitational force acts on the batch. Therefore, this

force depends on the batch location (intuitively, downhill

means fast).

Figure 5: Forces acting on the batch. Figure 6: Example of height prole and velocity.


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The force caused by friction between pig and pipe wall also

needs consideration. This force may be assumed to be constant

as it depends mainly on the geometry of pig and pipe, thematerials employed and other parameters, but not on the

location. A pig in a dual-diameter pipeline may, however, behave

dierently. Here, the friction force may be seen as constant for

each section with a constant diameter. When compared to a

pressure force, this force typically is the equivalent of 1 bar per

pig or less.

The force which has the largest inuence on the maximum

batch velocity is, however, the friction force caused by the ow

in the liquid-lled sections between the pigs. If the local ow

eld at the ends of these sections near the pigs where the ow

is reversed is disregarded (long batch), this ow has normal

turbulent pipe ow characteristics. This means that it causes a

pressure loss equivalent to such a ow. This pressure dierence

(loss) acts as a force which decelerates the batch. Equation 1

gives the relationship:

p = l/di p/2 v2 (1)

This pressure dierence is calculated using a constant ow

friction coecient of = 0.01. For other cases, this coecient is

readily obtained from textbooks on hydrodynamics.

As is typical with pipe ow, the force increases with the square

of the velocity. It is therefore most inuential at such conditions

where the velocity is high, i.e. downhill, and acts as a natural

limiter for the maximum velocity. If it is intended to reducebatch velocities, an appropriate means is to use longer batches.

This is counter-intuitive, as one would assume longer batches to

move downhill faster than short ones. Nevertheless, using this

as a remedy causes a higher hydrostatic pressure within the pipe

and creates the need for more water. The force balance results in

an ordinary dierential equation (second order) which is solved

using the numerical methods in MATLAB. The following section

shows the results.

Velocity prediction results and discussionIf applied to the given height prole example, the velocity

calculation using the method described above results in avelocity prole as shown in Figure 6. It becomes obvious that a

downhill scenario results in a high batch velocity. Due to this

height prole, the velocities predicted by the calculation are,

however, too high for the use of ultrasonic measuring devices.

Therefore, a pipeline with such a height prole must be subject

to a complete ll or a very long batch.Figure 7 shows the comparison between the calculation result

(blue line) and a measurement of the velocity of one of the pigs

from a real batch run, measured using a common odometer.

An acceptable degree of agreement between prediction and

measurement of the velocity is achieved.

At some of the locations, primarily those with high velocities,

the measurement shows higher velocity values than predicted.

These values are considered a measurement error due to the

behaviour of the odometer at high velocity for which it is not

designed. This assumption will be veried in the future by using

more advanced devices for measuring the pig velocity.

Additionally, the measurement result may be the consequence

of pig behaviour if gas is trapped within the batch causing an

oscillation with high peak velocities of the individual pigs. This

aspect will also be addressed in future simulations by solving

the coupled dierential equations.

Pipeline integrity and saety aspects

Forces in pipe bends at high velocitiesThe forces acting on pipe bends are the result of the change

of momentum of the liquid ow through the bend (centrifugal

force). The total force depends on the ow velocity and the pipe

cross-section area, but not on the radius of the bend. However,

at larger radii, the force spreads over a larger area. Here, thecentrifugal force acts in addition to the force resulting from

the pressure inside the curved pipe. Figure 8 shows the total

force from both pressure and change of momentum. It becomes

obvious that in the velocity range up to 20 m/s, pressure

forces dominate. Also, the centrifugal force at a velocity of

approximately 35 m/s has the same value (approximately

900 kN) as a pressure force resulting from a pressure of

10 bar. Due to this equivalence, centrifugal forces do not play a

signicant role, if a velocity prole according to Figure 6 with a

maximum velocity of 20 m/s is expected.

Batch collision and water hammerDiscussion of the water hammer problem is needed to assess

the consequences of a batch collision in the pipe as a what-

if scenario. At the location of the collision, the pressure is

Figure 7: Comparison between prediction and measurement.Figure 8: Total force on a pipe bend is dependent on pressure and

batch velocity.


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increased immediately to a value described by the Joukowsky

equation. This pressure may well exceed the pipe design

pressure:

p = p a v (2)

This pressure shock then propagates at the speed of wave

propagation along the pipe in the backward direction until theend of each of the two batches is reached. At the end, the pressure

dierence reverses its sign (reection at open end) and the shock

front begins to propagate in the forward direction. The pressure

level in the batch may become extremely low during this process

and may well cause the liquid to vaporise if the pressure falls

below the vapour pressure.

Both the pressure increase and the velocity of propagation

of the shock wave depend on the compressibility of the water

and the elasticity of the pipe. Figure 9 shows the solution of the

Joukowsky equation for three dierent cases depending on the

relative collision velocity. If the compressibility of water is taken

into account and the pipe is regarded as entirely rigid, even low

collision velocities lead to critical pressure levels since water

reacts very stiy. Taking into account the elasticity of the pipe,

a much higher velocity is needed to create pressure levels critical

for a pipeline. It must be stressed that this pressure must be

added to the initial pressure in the pipe.

SummaryPractical aspects of relevance to gas pipeline inspection

using pigs which employ ultrasonic measurement techniques

are discussed in this article. To warrant the coupling between

measurement device and pipe wall, it appears advantageous

to move short sections of liquid (batches) through the pipe in

which the measurement device is contained. Using this approach,

the velocity of the batch in the pipeline needs to be predicted

and certain characteristic pressures in the system have to be

calculated. Consequently, key safety aspects are discussed on the

basis of the results.

A simple approach for the prediction of the velocity of liquid

batches of any length in pipelines is developed. The comparison

with measurements indicates that the approach is suitable

for the prediction of the velocity as it occurs in the eld. If theheight prole of the pipeline exhibits large slopes, the velocity

is expected to exceed a critical limit up to which the detection

of pipe faults is possible. Hence, it is suitable for pipelines with

minor height dierences only. The approach also enables the

prediction of the location at which the batch is found in the

pipeline over time.

Another vital aspect is to ensure the integrity of the batch by

maintaining a certain location-dependent pressure in front of the

batch. The collision of two water batches in a pipeline results in a

pressure level in excess of the design pressure if a certain velocity

is exceeded. This critical velocity depends on the properties of the

pipe.

In addition, due to the high density of the liquid, it must be

ensured that the maximum (hydrostatic) pressure in the pipe

does not exceed the design pressure. This aspect is important for

longer batches of several thousand metres length.

The ow of water in sharp bends of the pipe at high velocity is

found to be of minor importance when compared to the eects of

internal pressure on the pipe.

Figure 9: Pressure increase vs relative (collision) velocity, where:

k = 4.8e-10 1/Pa (compressibility of liquid)p = 988 kg/m3 (liquid density)

di = 0.9 m (internal diameter)

s = 12.6 mm (wall thickness)

E = 206,000 MN/m2 (Youngs modulus)

This paper was presented at the Evaluation, Rehabilitation

and Repair of Pipelines Conference held in Berlin, Germany,

in October 2010, and organised by Tiratsoo Technical (a

division of Great Southern Press) and Clarion Technical

Conferences, Houston.

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