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Welding Technology In Offshore Structur

By nhazlee Nov 11, 2014 1394 Words
WELDING TECHNOLOGY IN OFFSHORE
STRUCTURE

NURUL HUSNA BINTI MAHAYUDDIN
SITI AISYAH BINTI MOHD YUSOF
NURFAEZAH BINTI MOHD HASSIM
AIDA BINTI ABD WAHIT
AQMAL ARIFFIN BIN ALIAS

2011127493
2011180123
2012248558
2011295216
2011255028










Recently, oil and natural gas are being more actively developed throughout the world to cope with the increasing demand. A rapidly growing number of offshore structures are being built, for example, in Southeast Asia, Korea and China. Offshore steel structures :welded structures made of homogeneous steel materials whose qualities remain stable on a lasting basis. However they require suitable measures to prevent corrosion of the steel members.

It is necessary to apply highly reliable corrosion-preventive treatment to offshore steel structures used in extremely corrosive marine environments such as splash zone and tidal zone. It has been common practice to apply corrosion-preventive organic material to them.

There are cases where corrosion-preventive metallic sheathing is applied to an offshore steel structure which is required to withstand use for an extremely prolonged periods, for example from 50 to 100 years. When this method is used, the steel structure seldom needs repair and hence, the cost of maintenance can be cut significantly.

The application of metallic sheathing has been limited because the cost of the material and fabrication are higher than those of the conventional corrosionpreventive methods. Minimizing the lifecycle cost of an off shore structure used in a highly corrosive environment, practical use metallic sheathing technology using a highly reliable and economic sea water-resistant austenitic stainless steel sheet.

1. Jacket
Function:
Stable platform for oil and gas
production facilities
 Structure:
Deck supported by a steel tubular
structure having its feet on the
sea bed


2. Gravity Based Structure (GBS)
 Function:
Base which supports several
vertical columns which supports a deck
carrying production facilities.

 Structure:
- Large reinforced concrete bottom
mounted structure which uses its
weight to resist environmental loads,
not attached to the bottom with piles.
- Easy to adapt for oil storage since the
base is large.

3. Spar
 Function:
Vertical floating oil platform
typically used in very deep water
which supports drilling and production
activities.

 Structure
 Consist of large-diameter, single
vertical cylinder supporting a deck
 The family of spar consists of
cylindrical, truss and cell spar.

1. Jack-up rig


Function:



Mainly used as exploratory
drilling platforms and offshore
and wind farm service platforms.



Designed to move from place to
place and anchor themselves by
deploying the legs to the bottom of
the ocean using a rack and pinion
gear system on each leg.

 Structure
- Consist of buoyant hull fitted with a
number of movable legs, capable of
raising its hull over the surface of the
sea.
- The hull is raised to required
elevation above sea surface on its
legs supported by sea bed.

2. Tension Leg Platform (TLP)


Function:
- Vertical moored floating structure
or offshore production of oil or gas
- Connected to the sea bed by
vertical tendons



Structure:
- Consist of tubular steel members
called tendons
- Group of tendons is called a
tension leg
- Fully buoyant and restricted
below floating line by mooring elements

3. Floating Production,
Storage and Offloading
Facility (FPSO)


Function:
- Processing of hydrocarbons and
for storage of oil
- Receive hydrocarbon produced
from nearby platforms, process
them and store oil until it can be
offloaded onto tanker /
transported through pipeline.



Structure:
- Can stand in critical
environmental conditions.
- Easy to install

4. Semi – Submergible Rig (SSR)


Function:
- offshore drilling rigs
- safety vessels
- oil production platforms
- heavy lift cranes



Structure:
- Consist of lower hulls, column, braces, decks and derricks. - Operating deck located high above sea level
- Structural column connect to pontoons and operating deck

Main structural components of SSR

Comparison of deepwater semi-submersible and drillship

5 . Drilling ship


Function
- Use in exploratory offshore drilling
of new oil and gas wells
- scientific drilling



Structure
- Have functional ability of SSR
- Kept stationary for long period
- Carry large pay load than SSR
- Greater mobility in contrast to SSR



Heat generated by an
electric arc to fuse metal
together in the joint area.



An arc is truck between
the tip of the electrode
and the workpiece and
the core wire begin to
melt.



The coating of the core
wire provides a
protecting gas to shield
the weld pool from the
surrounding air.



It is done semi-auto
by handled gun. It
uses shielding gas
such as argon,
carbon dioxide and
carbon dioxide
mixture, argon
mixture with oxygen
or helium. It uses
electrode.



Heat generated by a DC
electric arc to join
metals together.



The arc is struck
between a continuously
fed consumable filler
wire and the workpiece.



The filler wire and
workpiece are both
melted in the process.



Heat generated by an
electric arc.



The arc is struck between
a non-consumable
tungsten electrode and
the workpiece to join
them together.



The process may be
operated without filler
wire or the filler wire is
added by a consumable
wire rod to the weld pool.



Use arc struck between a
continuously fed
electrode and the
workpiece.



The metal is melted in
the process and an
additional filler metal is
provided by a granular
flux.



The arc is submerging
under the molten flux and
it provides protection to
molten metal against the
atmosphere.

ADVANTAGES

Allows easy access to
restricted areas which
cannot be accessed by dry
underwater welding.
 Welding support
equipment can be easily
mobilized to site.
 Repair operations are
easily planned and carried
out due to welder
accessibility.


DISADVANTAGES

Reduction in ductility,
impact strength and
increase in porosity and
hardness; surrounding
water causes rapid
quenching of the weld
metal.
 Poor visibility and the
welder cannot weld
properly.
 Presence of large amount
of hydrogen and causes
embrittlement.


ADVANTAGES





Welder/Diver Safety; Welding is
performed in a chamber.
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.

DISADVANTAGES








Requires large quantities of
complex equipment and much
support equipment on the
surface.
The chamber is extremely
complex.
Cost is extremely high.
Work depth has an effect; at
greater depths, the arc
constricts and corresponding
higher voltages are required.
Cannot use the same chamber
for another job, if it is a different
one.

 Process of welding at elevated pressures,

normally in underwater.
 Hyperbaric welding can either take place wet

in the water itself or dry inside a specially
constructed positive pressure enclosure and
hence a dry environment.

 The applications of hyperbaric welding are

diverse—it is often used to repair;
 ships
offshore oil platforms
pipelines
 Steel

is the most common material welded.

 Used a dry hyperbaric welding which is done

in the chamber by the well-trained welder.
 at a pressure equal to the hydrostatic head at
the depth of the weld.
 most hyperbaric welding is dine using
saturation diving techniques.The weld are
made in the dry chamber

DRY HYPERBARIC
 Exposed to the toxicity of the gas will caused
to the hard-breath
 The chamber can explode if no proper safety
guidance
WET HYPERBARIC
 Easy to get electric shock

Offshore steel structures are welded structures made of
homogeneous steel materials whose qualities remain stable on a lasting basis. They have a number of advantages over structures made of other materials, such as plenty of leeway in design and short construction period. On the other hand, they require

suitable measures to prevent corrosion of the steel members.  Corrosion-preventive coating must be repaired or renewed as required to maintain their long-term durability
 Structure builded must strong enough to overcome the
environment effects such wind, wave, ice and snow, water depth and variation of sea level, current, temperature and also marine growth
 The selection of steel type and its grade is important in designing the offshore structure


Material used should have high withstand to environment
conditions, should withstand force applied and should have
high weldability to obtain a longer structural life
 Structural steels for offshore application can be welded by the following welding processes, SMAW (Shielded Metal Arc
Welding), GMAW (Gas Metal Arc Welding), FCAW (Flux core
Arc Welding), GTAW (Gas Tungsten Arc Welding), and SAW
(Submerged Arc Welding)
 In producing the offshore structure, excellent skilled
workers are needed
 The welding processes are an important process to joint the metal permanently at a surface or in the water using various techniques


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