Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 296

5.4 FABRICATION
The main element in fabrication and erection is the welding and its quality con-
trol. Welding procedures should be well prepared, by defining the steel grades,
joint design, thickness range, welding process, welding consumables, welding
parameters, principal welding position, preheating at working temperature
and post-weld heat treatment.
The main factor in controlling the welding quality is selection of the welder.
Ideally, the welder will have been continuously working for 6 months before
the start of work. In addition, all welders should be qualified for the type of
work assigned to them and certified accordingly.
After fabrication, non-destructive testing (NDT) should be performed. In
most cases, NDT includes radiographic testing, such as gamma or x-ray, ultra-
sonic testing (UT) and magnetic particle (MP) testing. Both the weld itself and
the heat-affected zone should have notch toughness properties equal to those
specified for the members.
If you perform a macro-section cut through the weld, it should present a reg-
ular profile, with smooth transitions to the base material without significant
undercuts or excessive reinforcement. The quality-control supervisor should
not accept any cracks or cold lap as lack of fusion. The fracture mechanics
toughness of heavy welded joints should be verified by crack-opening displace-
ment tests.
The steel itself should be tested by mechanical testing, such as tensile tests,
bend tests, Charpy V-notch tests and hardness tests.
The construction supervisor should postpone any welding when surfaces are
humid or damp. In case of high wind speed, welding should be postponed
unless there is an arrangement for suitable protection. In cold weather, heating
of the enclosed space can be used to raise the temperature above the dew point,
so that the groove for welding is dry at the time of welding, since preheating
removes moisture.
Note that stress relief is normally not required for the range of wall thickness
used in the jackets and piles of offshore structures in moderate environments,
such as the Gulf of Mexico (GoM); on the other hand, it is frequently required
for the thicker members of large deck structures and for the joints of the thicker-
walled jackets of North Sea platforms.
Manual welding of all higher-strength steels and of normal-strength steel
having a carbon equivalent greater than 0.41 should be carried out with low-
hydrogen electrodes.
For special structural steels and for all repair welding, Det Norske Veritas
(DNV) requires the use of extra-low-hydrogen electrodes. It is recommended
( Gerwick, 2007 ) that all piling be welded with low-hydrogen electrodes in
order to prevent fracture under impact.
Welding consumables should be kept in sealed moisture-proof containers at

20 º C - 30 º C, but in any event at least 58 º C above ambient temperature. Opened