In the early
19th century, when operational
personnel first started to realize that
contaminates were blocking
their
pipelines, they used bundles of rags tied
with baling wire to clear
them.
Over the years, pigging technology has
evolved somewhat and this
article
explains chronologically the evolution of
those simple pigs to the
technologically
advanced forms of modern pigging instruments
that are available today.
The term 'pig'
probably
derived from the squealing
noise these early bundles made during their
travels down the pipeline.
Another school of thought applies a bit more
technical derivation to
the
word, 'Pipeline internal Gauging'. Whatever the
derivation, the name
stuck
and is used throughout the industry. Other names
do crop up, such as
'Rabbit',
'Mole' or 'Go-devil', but the most recognized name
remains 'pig'. As
opposed
to the limited capabilities of make-do pigging
instruments of earlier
days,
a pig today is recognized more as a cylindrical
device inserted in a
pipeline
for the purpose of sweeping the line clean of
water, rust, or other
foreign
matter.
Pig functions
Pipeline pigs today perform diversified
operations. Applications include
line cleaning, line fill and de-watering,
commissioning, removal of
mill
scale, removal of contaminates and condensates,
product separation,
leak
detection, internal geometry corrosion surveys
and many others.
Pigs can be classified under the following
categories:
i. Mandrel
ii. Polly pigs
and
foam swabs
iii. Cast urethane
pigs
iv, Spherical pigs
v. Inspection pigs
As early as the late 1940s companies were becoming
extremely conscious
of internal problems associated with metal fatigue
and corrosion. Shell
Research successfully developed what is perhaps
the first intelligent
or
'smart' pig, which sought out areas of corrosion
and recorded the
defects
internally on film. The first and only run of this
pig was performed on
a 10-in. pipeline in Texas. The method utilized to
detect corrosive
areas
was eddy current technology, which was later
dropped in favor of
flux-leakage
technology, which in turn was the forerunner of
the many pigs currently
being run today.
Presently, a variety of pigs are available on
the
market. Pigs range
from steel mandrel types of many different
configurations, which
include
brushes and blades to foam Polly pigs, each with
its own attributes.
Mandrel pigs
Mandrel pigs are designed, utilizing a central
shaft as the main body
for pig sizes up to and including 12-in. in
diameter. Over this size,
it
is normal for the main body to take the form of
a spool, to which are
mounted
urethane or rubber-type cups or disks separated
by either metal
fabricated
or urethane spacers. Typical pigs of this design
are usually fitted
with
four urethane cups or bidirectional disks and
may be equipped with
wire
brushes or blades in order to remove contaminate
build up from the
inner
wall of the pipe.
Pigs range from 2-in. in diameter to 48-in.
with
some custom-built pigs
having been designed to run in lines of much
larger diameters. These,
however,
are rare cases for specialized operations.
Depending on the pipeline
configuration
and style of bends, mandrel pigs may be designed
and used as a single
unit
or as multi-section units with articulated
couplings. The latter is
usually
confined to inspection tools, which carry a
great deal of
electronics.
Mandrel pigs may be utilized for pipeline
cleaning, swabbing, gauging
(utilizing a sizing plate), commissioning, line
and de-watering during
hydro-test operations, batching, internal line
coating, leak detection
and inspection.
Polly pigs
These types of pigs are formed of a foam core
utilizing varying foam
densities. Normally two pounds per cubic foot to
ten pounds per cubic
foot,
on to which strips of urethane are bonded to
form a pattern, which
provides
strength sufficient to run in pipelines for the
purposes for which they
are intended. Polly pigs are manufactured in
many different
configurations,
which may include brushes, carbide abrasives,
disks and also cavities
for
housing various types of pig locators.
Their uses include establishing scraper
passage,
de-watering and drying,
line cleaning and scale removal from water
lines. Foam of a low density
(2lbs/cu.ft.) is used in the process for drying
pipelines usually with
a nitrogen propellant or heated air in order to
bring a gas pipeline
down
to its required dew point prior to
commissioning.
Cast urethane pigs
This type of pig is formed completely from
urethane, normally reaches
78-80 shore hardness although this may vary
depending on the operation
for which it is required. Whilst some pigs of
this type are a
'throw-away'
item after normal life, others are manufactured
utilizing a center
shaft
on to which cups or disks may be mounted. These
types can be used over
long periods. Spacing is achieved by the
utilization of an inverse cup
arrangement manufactured from the same
material.
These pigs are normally used for line fill and
de-watering, line cleaning
and batching.
Spherical pigs
These sphere-like pigs are normally formed from
either polyurethane or
neoprene and nitrile rubber compounds. They may be
formed from two
hemispherical
pieces, which are bonded together in order to
complete the final
product
or in some cases are manufactured as a single unit
without a joint. It
is normal to include fill valves during
manufacture in order for the
operator
to fill the sphere with a glycol and water mixture
prior to it being
run
in a pipeline. The reason for filling is to size
the sphere to the
correct
internal line diameter and to allow pressure
equalization. Although
most
spheres are of the type, which can be filled,
there are other
varieties,
which are manufactured, utilizing a foam core with
an outer skin of
polyurethane,
in which numerous small holes are drilled to allow
the equalization of
pressure. Other types utilize wire brushes molded
to the outer skin in
order to clean lines, which cannot utilize
conventional pigs.
Spheres are commonly used in hatching as a
method
of sealing one product
from another, or in systems such as natural gas
pipelines where it is
necessary
to run spheres against a timed program in order
to remove condensates
from
the system. Spheres are adaptable to varying
line sizes and do allow
for
automatic launching facilities to be utilized.
However, the main
problem
with spheres, apart from their inefficiency for
line cleaning, is the
fact
that they have only one sealing surface, which
can cause hang-ups in
tees
and valves of the non-conduit through
type.
Inspection pigs
Since the early development of the Shell Oil
Company corrosion detection
pig, vast changes have been made which provides
pipeline operators with
what one might describe as 'Eyes inside of a
pipeline'.
Inspection pigs may be placed into the
following
categories:
· Internal geometrical inspection
· Bend radius survey
· Corrosion and crack detection
· Leak detection
· Geographical survey
· Thermal survey
Internal geometry pigs
After the construction of new pipeline systems,
it
is advisable for
operators to be aware of any internal
geometrical defects that can be
corrected
prior to operations. Such a geometric survey of
the internal line may
be
carried out with a caliper-type of pig, which
normally utilizes sensors
or fingers. The sensors contact the inner wall
of the pipe covering a
360-degree
inner circumference in order to detect
deflections significant of
changes in the inner pipeline diameter.
 |
During the running of the
inspection tool, a continuous
record of the footage traveled is
maintained and allows an accurate
position
of an event or defect to be recorded.
Records of events are typically
stored,
either on magnetic tape or in electronic
memory systems. Bend radius
surveys
are achieved with a similar tool to the
caliper pig but with an
additional
sensor configuration, which allows the
pig's attitude through a bend to
be recorded thus allowing definition of
bend radius. |
Corrosion and metal loss
surveys
Material integrity usually suffers during
normal
pipeline life through
corrosion and stress, which can cause loss of
metal and cracks in the
material.
Companies such as Tuboscope Pipeline Services
and
Vetco Pipeline Services
originally championed the cause to allow
pipeline operators to be fully
aware of their pipeline's integrity.
Developments of their internal
inspection
tools determine areas of defects of both the
internal and external pipe
walls and provide recordings that indicate the
severity of the defect
together
with its location. Both companies’ tools are
designed using
'flux-leakage’
technology by which sections of the pipe are
saturated by a strong
magnetic
field and the flux path recorded. Leakage or
deviations in the flux
path,
as detected by magnetic sensors situated between
the magnetic poles,
can
be directly related to areas of metal loss.
Since the early '70s
several
other companies have entered into the field of
on-line inspection, each
with their own brand of technology but with the
purpose of providing
pipeline
operators with the insight to operate under safe
conditions. Presently,
there are at least five corrosion inspection
companies that include
British
Gas, Rosen, Pipetronics, Vetco and Tuboscope
Pipeline Services. These
companies
offer inspection surveys utilizing flux-leakage
technology and have
committed
themselves to highly expensive development
programs to enhance
accuracy.
The accuracy requirement for offshore
inspections is particularly
stringent
due to the fact that one slight error in the
determination of incorrect
information, either due to bad data collection
or incorrect location,
can
be extremely costly to the Pipeline Company
during final
examination.
Ultrasonic inspection
Other methods of metal loss detection are
achieved
by companies using
ultrasonic, and eddy current technologies.
Ultrasonic inspections of
pipe
have been applied for many years in pipe yards
for locating defects in
the metal prior to its acceptance for use in
pipeline construction. In
the early '80s development was started on a
pipeline inspection tool
which
would provide ultrasonic inspection from the
inside by means on pigging
methods.
The technology provides for a ring of
transducers
embedded and staggered
within a stainless steel housing or ring, which
is mounted directly to
the main body of a multi-section pig. The
multi-section pig has
couplings
between each section. The transducers fire a
pulse of ultrasonic sound.
The energy travels through the liquid coupling
to the wall of the pipe.
Reflections are received from both inner and
outer wall of the pipe
thus
providing a continuous measurement of the pipe's
wall thickness.
Due
to its electronic configuration, the pig carries
far less weight than
its
flux-leakage counterpart. The ultrasonic pig
utilizes a unique flexible
coupling between segments, which allows the pig
to traverse short
radius
bends.
There are drawbacks with this technology, which
can be overcome, but
do create additional problems to operational
staff especially on gas
pipeline
systems. Due to the fact that the ultrasonic
method requires a liquid
coupling
and a gas system does not, the pig has to be run
between a batch of
condensate
or methanol. This is normally achieved by
containing the liquid between
two pigs.
The batch must be sufficiently designed in
order
to handle the entire
journey without loosing station. As previously
stated, this operation
requires
good line cleaning, superior to that of the
requirements for
flux-leakage
tools.
Leak detection
Leaks normally show up during hydro-test
programs
when the pressure
exceeds the normal operating line pressure. Such
leaks can and do occur
on lines during normal service. In order to
provide some level of
insurance,
companies use dye and gas during line fill prior
to hydrostatic tests.
This system often fails due to the extent of
overburden and in the case
of offshore systems, both the extent of cover
and the sea
currents.
This therefore brings forth the utilization of
pigs, which can detect
the leak by either 'listening' for a sound as
caused by the leak, or
measuring
the differential pressure across the pig - Delta
'P'.
Due to the fact that leaks cause a loss in
pressure, the Delta 'P' system
can be employed especially when lines have been
removed from service,
or
during operations. The latter is normally
achieved by use of a StarTrak
'Pathfinder' magnetic pig, which is fully
detectable both during its
travel
and when it is stationary. Measurement of
pressure across the pig will
determine which side the leak is on in relation
to the pig's location.
The basic requirements in order for this system
to work correctly, are
good seals, which are mounted on the pig,
together with an accurate
means
of locating the pig both for land and marine
pipeline systems. Even
small
weeping joints have been detected by using this
method.
Different operational procedures are adopted
for
multiple leaks or in
lines which are operational but which do not
have facilities to reverse
flow. For these situations, two pigs are
utilized to isolate the
general
area of the leak by stopping at short intervals
and analyzing pressure
differential. The precise location of the leak
is determined by
analyzing
the pressure behind the second pig at frequent
intervals.
Other methods in which a pig is utilized to
locate
leaks, is by means
of an instrumentation pig. Through electronic
means, it detects the
sound,
which is made by a leak and records the event
against the distance
traveled.
A further method employed by H. Rosen
Engineering, meters the flow and
transmits both volume and direction of flow
through the pipe wall to
the
operator.
Geographical
survey
pigs
This pig was designed and is presently being
operated by Nowsco Pipeline
Services in order to determine information on
the location and profile
of the pipeline, geometry and curvature. The
information gathered is
related
to GPS (satellite positioning) which, by a
unique software package,
provides
a geographical survey of the pipeline to
identify areas of erosion and
subsidence on land and offshore.
Thermal survey pigs
These instrumented pigs are used to determine
the
integrity of the pipe's
wrapping or external coating. Poor thermal
insulation can cause heat
loss,
which in turn transfers a cooling effect to the
product. By use of a
pig,
which continuously monitors temperature against
distance traveled, it
is
possible to determine the areas of poor thermal
insulation and forecast
the places where wax build-up is likely to
occur.
In 1994, StarTrak designed 3-in. thermal survey
tools, spaced them in
an offshore facility at one-minute intervals
over a twenty four-hour
period
to evaluate the thermal protection. This type of
survey will become
commonplace
as lines go into deeper waters.
Pig monitoring and
locating
Over the past 25 years many pig tracking
systems
have been developed
in order to provide, either as an assurance,
that the pig is running as
planned or, in the event of a pig becoming
obstructed within the pipe,
so that it may be accurately located. This is an
area, which still lags
behind in general technology.
Radioactive tracers are still being used in
various countries but in
general this method has given way to less
hazardous systems. Recently,
it has been determined that low levels of
radioactivity can be traced
by
newly developed instrumentation.
Low frequency electromagnetic transmitters are
commonly utilized to
track pigs through sections and locate them in
the event that they
become
'lost'. These units obviously require power,
which is supplied by
lithium-type
batteries.
Offshore operators tend to rely on pingers,
which
give sonic, interrupted
pulses. It uses the liquid both inside the line
and the sea water
outside
as signal conductors. This method is often
extremely ideal for
monitoring
from a boat or even a helicopter, which is
capable of lowering a
transducer
to below the water's surface. It is not good for
gas lines, which are
dry
or lines which have a great deal of cover.
StarTrak Pigging Technologies has utilized its
'Pathfinder' magnetic
technology successfully for many years. The
system utilizes a permanent
magnetic circuit, which becomes a part of the
pig body for sizes above
6-in.
Advantages of this system is that it never
requires re-magnetization
or power and can be tracked through heavy cover
both on land or
offshore,
and with any types of pipeline product. For land
and offshore
operations
it lends itself to operate station magnetometers
with telemetry or can
be utilized, offshore, in conjunction with
pingers. The magnetic units
can be mounted in pigs or installed inside of
inflatable spheres during
manufacture. Due to the usage of
neodymium-iron-boron material, there
is
never a requirement for re-magnetizing of these
units which have an
added
bonus over past magnetic material as they are
lighter and have many
times
more energy - 45,000,000 Gs/Oe as opposed to
7,500,000 Gs/Oe energy
factor
for the Al-Ni-Co materials previously
used.
Recently, StarTrak has developed a
pig-signalling
unit known under its
trade name of 'Deep C'. It is utilized in
conjunction with deep-water
pipeline
applications tested at a depth of 10,000-ft.
Designed for a twenty-year
life span, it has twin sensing systems to allow
redundancy. The
system's
in-built CPU provides analysis of the
surrounding magnetic fields and
automatically
adjusts itself to the magnetic environment.
Speed ranges of these pigs
can be determined from less than six inches per
minute to over 100
miles
per hour.
Permanent
pig monitoring stations
For many years, the only method of signalling
the
passage of an outgoing
pig, its passage at strategic points in the
line, or its receipt into
the
receiving facility has been by mechanical means.
Recently, the industry
has become aware of the fact that it is not
necessary to utilize
mechanical
type signalling devices (flags).
The total strength of any pipeline system is
based
upon its weakest
point and in this case it must be recognized
that line intrusion by a
mechanical
device must cause weakness to the system. Apart
from this technicality,
maintenance is a further problem, which normally
requires the line to
be
shut in while either maintenance or change-out
of the unit is being
carried
out. Hence the introduction by several
manufacturers, including
StarTrak,
of magnetic signalling devices.
The magnetic pig signal does not require
intrusion
into the line. It
is normally positioned on the pipe by means of
either a strap or welded
saddle. As the pig passes the unit it becomes
activated, so providing a
voltage output, a contact closure or 4-20mA
indication. The pig must
carry
a magnetic source, which has sufficient strength
to provide a
signal through the pipe
wall.
Wax removal pigging system
Many problems associated with offshore
pipelines
and in some cases,
land pipeline systems, is the build-up of wax or
asphaltine in crude
oil
pipelines. This occurs as hot oil begins to cool
during its travels
through
the pipe. Wax adheres to the pipe wall and not
only builds upon itself
but continues to build over a larger area as the
initial deposits
provide
an insulator against the cooling effect.
Soft wax deposits provide little problem to the
operator as he can usually
remove these by pigging, or the use of a
combination of pigs and a
solvent
such as Xylene. However, many systems have
either been neglected or not
enough provision has been made for routine
pigging operations, the end
product of this is wax or, in many cases
hardened wax. In order to
remove
this form of contaminate, it requires a
combination of heat coupled
with
a mechanical action or turbulence.

StarTrak Pigging Technologies of Houston
together
with, their Associates,
CSI Inc. have developed a technology named
'Jet-Stream System'. This
technology
is designed around a pig which causes a jetting
action to take place
when
impeded by wax build-up and utilizes chemicals
which, when mixed,
provide
the necessary exothermic reaction to take place
in order to remove the
wax/
Asphaltine deposits, Operations consist of a
chemical train beginning
with a gel slug utilizing an HCI base that is
first inserted into the
line.
This is followed by a diluted form of HCI and
again followed by
anhydrous
ammonia separated by the 'Jet-Stream' pig.
Immediately after this batch
is a further separation pig and a slug of
Xylene. Action takes place
when
the pig becomes 'Jetstream' pig impeded by the
wax build-up which in
turn
causes an increase in the differential pressure
thus opening a pressure
relief valve into the pig's nose chamber. The
anhydrous ammonia will
then
surge into the chamber and onward through
jetting nozzles to the pipe
wall
immediately in front of the pig. The reaction of
the chemicals coupled
with the turbulence created causes the wax to
melt into the product
thus
returning line efficiency to normal. The batch
of Xylene following
prevents
crystallization which also aids the pacification
process. The
by-products
of the chemical reaction are salt water and
nitrogen both of that
create
little problem for disposal.
Batch separation
Over the years problems associated with the
separation of product have
caused much loss in revenue to energy companies,
due to the loss in
refined
product by downgrading or re-refining. In
multi-products pipelines,
batches
of products are transported often for hundreds
of miles. Such batches
may
not belong to one company as normally there are
several It
has been commonplace to separate batches with
spherical pigs due to the
ease of launching and retrieving of these pigs
however, it was found
that
spheres would 'hang' up at points along the
route, i.e., tees, check
valves,
etc., thus causing an undesired by-passing
effect, so creating a large
product interface.
In order to detect interface, the sphere was
used
to activate mechanical
pig signalling devices. Gravimeters do monitor
for a change in specific
gravity, but there are many cases where batches,
which are being
transported
for various companies, cause little or no change
to be recognized by a
gravimeter. To this very day, many companies
have a sampling room
whereby
an operator continues to sample product for
viscosity change and
directs
flow to the appropriate tanks.
Due to the ineffectiveness of the spherical
pigs
most companies accept
the interface and down grade this into batches
of a lesser grade. Due
to
the length of travel and the possible stoppages
along the route to
direct
batches into sub-stations, the interface spreads
and, appreciating that
product in the center of the line moves faster
than that nearest the
pipe
wall due to friction, it will continue to
spread. Many engineering
evaluations
of this problem have been made over the past
twenty-five years and most
have resulted in the recommendation of a more
effective method to
contain
interface.
StarTrak has also evaluated the problems
associated with product separation
and has arrived at a system, which allows a pig
to be launched
automatically,
and directly into the line at the interface.
Unlike spheres, which have
a single seal, the pig is equipped with a
multi-sealing surface. Due to
the pig's design, it has the strength of low
temperature carbon steel
yet
one third of its weight, thus allowing neutral
buoyancy during its
travel.
This factor alone eliminates much of the wear to
seals so allowing
station
in the interface to be maintained.
The pig is equipped with a permanent magnetic
circuit, which will not
only cause activation of external signalling
devices, but can be easily
located in the event of blockage or any other
contingency. Such
signalling
stations are equipped with instruments, which
monitor the passage of a
pig and its speed past that station. It further
allows forecasting of
ETA
at its next station, provides pipe/soil
potentials and also leak
detection/monitoring.
Other facilities for monitoring valve status,
line pressure and line
temperature
can be included in this monitoring process. Data
transmission can be
made
directly into SCADA microwave systems, directed
over cellular phone
lines
or by means of radio relay 'handshake'
stations.
Summary
As we now look back to the early 50s, much
technical progress has been
achieved for pipelines on land and offshore.
Pigs have brought with
them
a high quality assurance level that allows
operators to work within a
higher
level of safety. It is certain that inspection
tools will provide a
higher
degree of information and inspection companies
will undertake
preparation
work besides remediation.
All of these items will increase the safety
factor
of operational pipeline
systems. Communication between field and control
will reach higher
standards,
with many functions completely automated due to
company down sizing.
Methods
of communication using satellites have improved
dramatically and,
coupled
with GPS, will provide the required efficiency
to maintain the lowest
cost
of one of the safest forms of fluid
transportation known to the
world.
Ernest Casey
Back to Technical Papers
|