Pipeline Welding Technology

Line pipes can be connected by mechanical connectors or welding. Threaded and coupling (T&C) or pin and box connectors are used for drilling riser and top tensioned riser connections. However, welding is more commonly used for offshore pipelines due to its proven technology and lower cost than mechanical connectors. Advantages of connectors are: use of high grade pipes (up to 125 ksi SMYS), fast make-up, no welding (no heat-affected zone, no welding inspection), no field joint coating, etc. Disadvantages of connectors are: high material cost, leak test for each connection, weak for torsion and fatigue, etc. Integral connectors, without requiring twist the pipe or connector, have
been developed. The available integral connectors are Jetair PSC, Hydil 2000, OSI Merlin, etc.

The maximum pipe grade which can be welded offshore is X-70. Pipe grade higher than X-70 requires induction heat treatment which is impossible for continuous long pipeline welding. The induction heat treatment is normally done in an oven so it is limited by the welded products’ size and length.

There are diversity of welding processes such as solid state welding (resistance, cold, friction, ultrasonic, etc.), soldering/brazing, and fusion welding. Soldering/brazing melts only filler materials not base materials. However, the fusion welding involves partial melting of base material (called heat affected zone). Electrical energy (electrode) is commonly used for the fusion welding.
 

Heat affected zone

Source: Introduction to Offshore Pipelines & Risers - Jaeyoung Lee

• SMAW or Stick Welding

Shielded Metal Arc Welding (SMAW) is frequently referred to as stick welding. The flux covering the electrode melts during welding and this forms the gas and slag to shield the arc and molten weld pool. The slag must be chipped off the weld bead after welding.

• GMAW or MIG Welding

Gas metal arc welding (GMAW) uses an arc between a consumable constant filler metal electrode and the weld pool. Shielding is provided by an externally supplied shielding gas. This method is also known as metal inert gas (MIG) welding or metal active gas (MAG, i.e. carbon dioxide or oxygen) welding.

GMAW consists of a DC arc burning between a thin bare metal wire electrode and the work piece. The arc and weld area are encased in a protective gas shield. The wire electrode is fed from a spool, through a welding torch which is connected to the positive terminal. The technique is easy to use and fast (high productivity) and there is no need for slag-cleaning since no flux is used. The MAG process is suitable for steel, low-alloy, and high-alloy based materials. The MIG process, on the other hand, is used for aluminum and copper materials.

• GTAW or TIG Welding

Gas tungsten arc welding (GTAW) is more commonly known as tungsten inert gas (TIG) welding. It is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The electrode used in GTAW is made of tungsten, because tungsten has the highest melting temperature among metals. As a result, the electrode is not consumed during welding, though some erosion (called burn-off) may occur.

GTAW is most commonly used to weld thin sections of stainless steel and light metals such as aluminum, magnesium, and copper alloys. The process is known for creating stronger and higher quality welds than SMAW and GMAW. However, GTAW is comparatively more complex and difficult to master. It is also significantly slower than most other welding techniques.

Welding types

Source: Introduction to Offshore Pipelines & Risers - Jaeyoung Lee

Each welding should be examined for its completeness and quality by non-destructive
test (NDT). Generally four (4) NDT methods are widely used in welding inspection as
shown in table below.

Non-destructive test

Source: Introduction to Offshore Pipelines & Risers - Jaeyoung Lee

Source: Introduction to Offshore Pipelines & Risers - Jaeyoung Lee

Source:
Introduction to Offshore Pipelines & Risers - Jaeyoung Lee