Page 402 - Pipelines and Risers

Use of High Strength Steel 369

By increasing the material grade, it is possible to lay pipeline in deeper waters. A thinner wall
thickness has a direct impact on this installation method since the requirements for lay barge
tensioners is related to the water depth and weight of pipe. For the Oman-India Gas Pipeline
project, the question is how this pipeline can be laid with a massive top tension of 10,600 kN,
during normal laying operation, necessary for controlling the catenary. It is recommended that
a laybarge that has a tension capability of at least 26,700 kN is used. This requirement is
dictated by a wet buckle abandonmentlrecovery scenario, that is, a buckle together with
rupture leading to pipeline flooding. J-lay techniques similar to drilling technology, may be
used but lay-rate can be low.

Weldability
Thick wall thickness creates additional problems related to weldability. As the wall thickness
of the linepipe increases, the cooling rate of the weld increases leading to possible problems
with hardness, fracture toughness and cold cracking (when non-hydrogen controlled welding
processes are used). A thinner wall thickness due to increase in material strength means that
the cooling rate of the weld will also decrease.

Pigging Requirements
The thicker walled sections of the pipeline in deeper waters may restrict the full capabilities of
intelligent pigging. There is a limitation on the wall thickness depending on the type of
pigging tool used.

19.2.2 Potential Disadvantages of High Strength Steels
Increase in Material Costs per Volume
Generally an increase in material grade will equate to an increase in cost of material. Refer to
Figure 19.4. However, it is also interesting to note that for a given design case, an increase in
the material grade equates to a slight decrease in cost per meter.