Page 407 - Pipelines and Risers
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374 Chapter 19
properties obtainable from the basic electrodes. The increased time was due to more ‘arc off
time for removal of the basic slag between passes.
The repair rate for manual field welding is reported to have been less than 3%. Maximum
hardness of 350 HVlO are reported in the cap HAZ.
Mechanized Welding
A general discussion of mechanized welding of X80 is provided in Price (1993). Experience
with the use of mechanized welding on three projects as identified in Table 19.4 is available
in Chaudhari et al. (1995), Laing et al. (1995). The CRC Evans GMAW mechanized system
was used in all three cases.
The Mitzihwin project achieved an average rate of 103 butts at 48-inch OD x 12.1 mm WT in
an 8 hour day though the repair rate was considered high at 6%, compared with 4% achieved
on the other two projects (Laing et al. 1995). It is stated that repair rates have been less than
1% in comparable subsequent projects.
Properties of Field Welds
A detailed review of the inter-relation of welding process and properties is beyond the scope
of this study. In the present context, the main point to be noted is that project specifications
for weld quality, strength and toughness were met in all cases for X80 with wall thickness in
the range 10.6 - 18.3 mm and that techniques have been developed sufficiently to allow
consideration of X80 for both land and offshore pipelines.
19.4 Cathodic Protection
Subsea pipelines require compatibility with CP in sea water. High hardness steels are at risk
of brittle failure caused by hydrogen embrittlement. Compatibility is conventionally satisfied
by hardness values below 350HV10. The limit applies to parent metal and all weld zones.
Chaudhari et al. (1995) and Laing et al. (1995) report maximum values of 350HV10 for
manual welding (Ruhrgas project) and 303HV for mechanized welding (three projects, test
load not given). The value of 350HV10 (10 for 10 g load in Vickers Hardness test) has been
shown to be an acceptable maximum hardness for avoiding hydrogen embrittlement of
structural steels and welds under CP in seawater (to minimum negative potential, maximum
polarization’s) of conventional sacrificial anodes. In all cases maxima were in the HAZ. This
data indicates that X80 can be welded within the conventional limit for compatibility with CP.