Page 404 - Pipelines and Risers

Use of High Strength Steel 37 1

Repair by hyperbaric welding, whether at diverable or diverless water depths, for material
grade of X70 or above has not been undertaken and therefore there is currently no information
regarding its behavior under hyperbaric conditions. Research programs should be monitored
and initiated to develop understanding in this area.

An alternative repair method is to use the hot tap technique to bypass the area of pipeline
damage. However, for offshore use this experience is limited and certainly unproven in high
strength material pipelines.

Hot tap repairs are regularly performed onshore for API 5L X65 pipe grades and lower. BS
6990 states that hot tap welding of material above X65 yield strength should not be performed
without welding trials being performed. The inferior weldability of high grade linepipe
combined with the high cooling rates experienced during welding onto a live pipeline increase
the safety risks associated with hot-tapping operations. For linepipe grades above API 5L
X70, it is recommended that hot tapping is not performed unless extensive weld testing can be
conducted.

Additionally, the subsea hot tap technique is limited to a maximum size of 24/36-inch (i.e. 24-
inch bypass into 36-inch pipeline) at a limited water depth of 100 m for relatively low
pressure lines (1,000 psi). This technique needs to be further evaluated.

19.3 Welding of High Strength Linepipe

19.3.1 Applicability of Standard Welding Techniques
The range of welding techniques used for pipeline construction includes Shielded Metal Arc
Welding (SMAW), Gas Metal Arc Welding (GMAW), Submerged Arc Welding (SAW), Flux
Cored Arc Welding (FCAW) and Gas Tungsten Arc Welding (GTAW). All of these
techniques have been applied successfully to API 5L X65 linepipe and lower in accordance
with internationally recognized pipeline construction codes and standards.

When welding higher strength grades of linepipe (X70 and above), special techniques are
generally specified to avoid defects in high strength welds. Some of the additional measures
that are necessary include:

control of joint preparation and line-up;
using adequate preheat;
additional inter-run griding;
careful selection of electrical characteristics;
no movement of the pipe until completion of the root pass.

The specific application of standard welding technology to onshore and offshore pipeline
construction is discussed in the following sections.