Ce687 Lecture 19
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Transcript of Ce687 Lecture 19
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WELDING
AND
NDT
TECHNIQUES
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UNDERWATER WELDING
Underwater
welding
can
be
classified
as
follows:‐
Wet Welding
Dry Welding
In wet
welding
the
welding
is
performed
underwater,
directly exposed to the wet environment.
In dry welding, a dry chamber is created near the area
to
be
welded
and
the
welder
does
the
job
by
staying
inside the chamber
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Wet welding A special electrode is used and
welding
is
carried
out
manually just as one does in open air welding.
The increased freedom of
movement makes
wet
welding the most effective, efficient and economical method.
Welding power
supply
is
located on the surface with connection to the diver/welder via cables and hoses.
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The power
source
should
be
a direct
current
machine
rated
at
300 or 400 Amperes
The welding machine frame must be grounded to the ship
The welding circuit must include a positive type of switch, usually
a knife
switch
operated
on
the
surface
and
commanded
by the welder‐diver.
The knife switch in the electrode circuit must be capable of breaking the full welding current and is used for safety reasons
Special welding
electrode
holders
with
extra
insulation
against
the water are used. The underwater welding electrode holder utilizes a twist type head for gripping the electrode.
The electrode types used conform to AWS E6013 classification
The electrodes
must
be
waterproofed.
All
connections
must
be
thoroughly insulated so that the water cannot come in contact with the metal parts.
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Advantages of wet welding The versatility and low cost of wet welding makes this method
highly
desirable.
Other benefits include the speed. With which the operation is carried out.
It
is
less
costly
compared
to
dry
welding.
The welder can reach portions of offshore structures that could not be welded using other methods.
No enclosures are needed and no time is lost in designing
these
enclosures
(as
in
dry
welding).
Readily available standard welding machine and equipment are used. The equipment needed for mobilization of a wet welded job is minimal.
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DISADVANTAGES
OF
WET WELDING
There is rapid quenching of the weld metal by the surrounding
water.
Although
quenching
increases
the
tensile strength of the weld, it decreases the ductility and impact strength of the weldment and increases porosity and hardness.
Hydrogen Embrittlement
– Large
amount
of
hydrogen
is
present in the weld region, resulting from the dissociation of the water vapour in the arc region. The H2 dissolves in the Heat Affected Zone (HAZ) and the weld metal, which
causes
embrittlement,
cracks
and
microscopic
fissures.
Cracks can
grow
and
may
result
in
catastrophic
failure
of
the structure. Another disadvantage is poor visibility. The welder some times is not able to weld properly
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HYPERBARIC WELDING
(DRY
WELDING) Hyperbaric welding is carried out in chamber sealed around the
structure to
be
welded.
The chamber is filled with a gas (generally helium containing 0.5 bar of oxygen) at the prevailing pressure.
The habitat is sealed onto the pipeline and filled with a
breathable mixture
of
helium
and
oxygen,
at
or
slightly
above
the
ambient pressure at which the welding is to take place. This method produces high‐quality weld joints that meet X ‐ray and code requirements.
The TIG welding process is employed for this process. The
area
under
the
floor
of
the
Habitat
is
open
to
water.
Thus the welding is done in the dry but at the hydrostatic pressure of the sea water surrounding the Habitat.
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ADVANTAGES OF DRY WELDING
Welder/Diver Safety – Welding is performed in a chamber, immune
to
ocean
currents
and
marine
animals.
The
warm,
dry habitat is well illuminated and has its own environmental control system (ECS).
Good
Quality
Welds
–
This
method
has
ability
to
produce
welds of quality comparable to open air welds because water is no longer present to quench the weld and H2 level is much lower than wet welds
Surface Monitoring
– Joint
preparation,
pipe
alignment,
NDT inspection, etc. are monitored visually.
Non‐Destructive Testing (NDT) – NDT is also facilitated by the dry habitat environment.
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DISADVANTAGES OF DRY WELDING
The
habitat
welding
requires
large
quantities
of
complex equipment and much support equipment on the surface. The chamber is extremely complex.
Cost of habitat welding is extremely high and increases with depth. Work depth has an effect on habitat welding. At greater depths, the arc constricts and corresponding higher voltages are required. The
process is
costly
and
the
same
chamber
cannot
be
used
for another job, if it is a different one.
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Welding Parameters and its effects Type of current
Type of Arc voltage
Speed of welding
Diameter of electrode
Length of stickout
Electrode feed speed
Polarity (weld in DC supply)
Electrode Orientation
Shielding medium
Joint geometry
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NDT
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Non Destructive testing (NDT)
The
use
of
non‐
invasive
techniques
to
determine
the
integrity of a material, component or structure
To
quatitatively
measure
some
characteristics
of
an
object
Inspect
or
measure
without
doing
Harm
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NDE
(Non
Destructive
Evaluation)
methods
Flaw
detection
and
evaluation
Leak detection
Location determination
Dimensional
measurements
Structure and microstructure characterization
Estimation of mechanical and physical properties
Stress (Strain)
and
Dynamic
response
measurements
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When NDE applicable
To
assist
in
product
development
To screen or sort incoming materials
To monitor, improve or control manufacturing
processes
To verify proper processing such as heat treatment
To verify proper assembly/workmanship
To inspect
in
‐service
damage
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Methods of NDT
Visual
X ‐ray
Ultrasonic
Eddy
current
Liquid penetrant
Magnetic measurements
Thermography
Magnetic particle
Laser interferometry
Tap testing
Acoustic emission
Acoustic Microscopy
Flux leakage
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Visual Inspection
Most
common
and
basic
inspection
Tools Used
Fiberoscopes
Borescopes
Magnifying glasses
Mirrors
Robotic
crawlers
also
used
In hazardous areas like ducts, reactors, pipelines
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Liquid Penetrant Inspection A liquid with high surface wetting characteristics is applied
to
the
surface
or
to
the
part
of
the
surface
and
allowed to seep into surface breaking defects
Excess liquid is removed from the surface of the part
A developer
(powder)
is
applied
to
pull
the
trapped
penetrant out of the defect and spread it on the surface where it can be seen
Visual
inspection
is
the
final
step
in
the
process.
The
penetrant used is often loaded with a florescent dye and the inspection is done under UV light to increase test sensitivity
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Magnetic Particle inspection
The
part
is
magnetized
Finely milled iron particles coated with dye pigment are then applied to the specimen
These
particles
are
attracted
to
magnetic
flus
leakage
fields and will cluster to form an indication directly over the discontinuity
This indication can be visually detected under proper lighting conditions
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Magnetic particle crack indications
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Radiography
Radiation
used
in
radiography
is
a
higher
energy
(shorter wavelength) version of the electromagnetic waves that we see as visible light
The radiation can come from an X ‐ray generator or a radio
active
source.
The part is placed between the radiation source and a piece of film
The part
will
stop
some
of
the
radiation
Thicker and more dense area will stop more of the radiation
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Eddy Current Testing
Well
suitable
for
detecting surface cracks
Also can be used to measure
coating
thickness
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Ultrasonic Inspection (Pulse‐Echo) High frequency sound waves are introduced into a material and they are reflected back from surface or flaws
Reflected sound energy is displayed versus time
Inspector can visualize a cross section of the specimen showing the depth of features that reflect sound
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Secondary Processing Machining
Welding
Grinding
Heat treating
Plating etc.
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In‐service damages Cracking
Corrosion
Erosion/Wear
Heat damage
etc.
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Wire rope
inspection
Electromagnetic devices and visual inspections are used to find broken wires and other damage to the
wire rope
that
is
used
in
cranes, mooring tethers and other lifting devices
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Storage
tank
inspection
Robotic crawlers use
ultrasound technique
to inspect the walls of large overhead tanks for signs of corrosion
Cameras on long articulated arms are
used
to
inspectunderground storage tanks for damage
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Pressure vessel inspection Failure of pressure vessel can result in rapid release of large amount of energy.
To protect against this dangerous event, the tanks are inspected using radiography and ultrasonic testing
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Pipeline
Inspection
NDT is used to inspect
pipelines
to prevent leaks that could damage
the environment
Visual Inspection, radiography and electromagnetic testing
are
some
of
the common NDT methods used
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Underwater inspection Inspection of structures in ocean requires NDT to be carried
out
under
water
Materials mostly include steel, concrete and wood
Problems to be identified include
Cracks and other growing defects
Wall thinning due to corossion
Biological and chemical changes (damage caused by
insects or
wood
rot)
Damage caused by collision
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Objectives
of
underwater
inspection Detection of surface opening cracks in welded tubular joints
Detection of wall thickness in tubular members
Checking of corrosion systems
Mapping of marine growth, scour and debris
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Steel Platforms
Cracks and localized corrosion are most common damages
Cracks mostly occur at welded zones and due to fatigue
Tubular joints
located
near
the
splash
zone
and
near
the
sea floor are most susceptible
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Concrete structures Detection of surface opening cracks in areas of high bending
moment
Detection of erosion of concrete in splash zone
Corrosion of reinforcement, anchors for pre‐stressing tendons, and other steel structures
Checking for foundation on the sea bed
Mapping
of
marine
growth
and
debris
Detection of cracks in structural steel members
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NDT of pipeline risers Pipeline risers
Susceptible to damages below waterline
Thermals loads
High internal load
High internal
pressure
Environmental loads
Relative motion between the platform and the pipeline (in case of gravity platforms)
Corrosion growth in fixtures
Marine growth and debris
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Methods for underwater inspection Visual inspection
Magnetic particle testing
Ultrasonic measurements
Eddy
current
measurement
Radiographic photography
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Conventional NDT Visual inspection by divers and under water cameras are
most
common
Ultrasonic methods are being increasingly used to detect defects
Radiography are little applied because of radiation problem
Vibration analysis is also used
Advantage is that sometimes the defect anywhere in the structure can be detected
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Once the defect is identified, detailed examination to be
carried out
by
NDT
methods
Steel structure Magnetic particles Eddy current
Concrete and
wood
Rebound hammer Penetration techniques Core removal for detailed examination
For All NDT methods in underwater surface preparation is mandatory
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Visual inspection under water Mostly by Diver
Experienced persons are employed
Remotely underwater camera/ videos used
Limitations:
Poor visibility
Heavy fouling
Strong
currents
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Magnetic particle inspection Purpose: to detect fine surface cracks in ferromagnetic
materials
Method: Magnetise the area to be examined followed by application of liquid suspension of ferro magnetic particles
If
there
is
a
crack
on
surface,
these
particles
will
deposit
along
the crack due to leakage of magnetic flux at the discontinuity of the material
Difficulty: Sometimes the particles are mixed with
fluorescent agents
UV rays will produce a good contrast between the particle gathering along the crack and the dark surrounding
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Magnetic testing… For depths upto 100m DNV has fabricated a MPI (magnetic
particle Instruction
device)
unit
Weighs about 1.5 ton Necessary magnetic flux is obtained by passing a high amperage alternating current between two electrodes
which are
applied
to
the
inspection
site
Current is drawn from a transformer located in a cage which is lowered to the inspection site
Cage also contains the tank for magnetic particle
suspension
Tank is fitted with baffles driven by electric motor to prevent settlement of ferromagnetic particles during operation
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Ultrasonic testing underwater Uses high frequency mechanical stress waves, frequencies above
sound
Two methods One transducer – sender and reciever of pulse signals, commonly used for metals
Two
transducer
each
side,
through
transmission
measurement,
commonly used
for
concrete
and
wood
Used to measure thickness of ship hulls, using pulse echo In corroded plates, results not good Pitted surface, results shows multiple reflection and are easily misinterpreted
Pitted surface results only 50 % of the actual thickness, if a test is conducted
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Ultrasonic equipment DNV has developed a ultrasonic flaw detector for use
under water
The instrument is mounted in a pressure housing
Inspection unit is kept above water and cables and
transducer are
kept
underwater
One operator moves the transducer along the metal surface while another operator above water with the inspection unit interprets the readings and records the location
Communication between the two operators is by phone or radio
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Nano
layered
coatings
for
corrosion protection Nano structured coatings enhance resistance surfaces against
corrosion
Recent invention at Taribat Modares University in Tehran
Advantages of
this
method
are
its
simplicity,
homogeneity and high uniformity of the applied coating
In this
method,
alkoxide
tetra
n‐butyl
irto
‐titanate
(TBT) was used to prepare a soluble gel
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procedure Ethanol and ethyle acetoacetate are mixed together at room
temp
Add TBT to the solution and stir the solution well Add some drops of distilled water while it is being stirred For polymeric reactions, the prepared solution is left for 6
hrs
After surface preperation titanium oxide nano particle coating is applied to the steel surface by submerging method
It is
possible
to
reduce
probability
of
corrosion
in
petroleum, gas and petrochemical industries with this method