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.
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