Page 405 - Pipelines and Risers

312 Chapter 19

Onshore
The SMAW process is the standard welding methods for onshore pipelines. Low hydrogen
SMAW has been used on pipelines up to AFT 5L X80 grade. Cellulosic SMAW is not
generally used on linepipe above API 5L X70 strength due to problems with hydrogen
cracking. For onshore pipelines above API 5L X70 grade, low hydrogen processes such as
GMAW or FCAW are required. Refer to Section 19.3.2 below for details of project welding
experience.

Offshore
The semi-automatic GMAW process is used extensively on laybarges for offshore pipelay.
GMAW is sensitive to changes in carbon equivalent of the material. Generally, the carbon
equivalent of linepipe increases as the grade is increased. GMAW is also sensitive to Boron
alloying in the linepipe, however control of Boron in Japanese and European mills is very
good and hence this is not considered to be an issue provided high quality linepipe is used. It
is possible that development of GMAW procedures will take significantly longer for linepipe
above API 5L X70 strength.

SAW is used on third generation lay barges for double jointing. SAW is a high heat input,
high dilution process. Therefore, the chemistry of the linepipe being welded has a large
influence on the properties of the final weld.

Welding API 5L X70 and X80 linepipe with SAW will require careful control of alloying
elements to ensure that the final properties of the weld will be satisfactory. There have been
problems with poor root toughness of SAW welds due to pick up of elements such as
aluminium from the linepipe. The construction contractor should be given the opportunity to
review chemistry requirements prior to linepipe manufacture in order to ensure compatibility
with proposed SAW procedures.

FCAW is currently used for structural welding and for performing certain types of repairs on
pipeline welds. Properties of FCAW welds are generally good, however, there have been
historical problems in obtaining consistent weld toughness.

FCAW consumables have been developed for welding linepipe up to API 5L X80 grade.

GTAW produces very high quality welds with excellent properties. However, the process is
slow and is not generally used offshore (with the exception of hyperbaric welding and
welding of Corrosion Resistant Materials).

In principle all the standard pipeline welding methods (with the exception of cellulosic
SMAW) should be suitable for welding API 5L X70 and X80 linepipe provided additional
time is allocated for weld procedure and consumable development.