API REPORT 79-12 Influence of Sea Water and Cathodic Protection upon Fatigue of Welded Steel Plates as Applicable to Offshore Structures
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API REPORT 79-12 Document Information:
Title
Influence of Sea Water and Cathodic Protection upon Fatigue of Welded Steel Plates as Applicable to Offshore Structures
American Petroleum Institute
Publication Date:
Apr 30, 1981
Scope:
SUMMARY
Fatigue at welded tubular connections has been judged to be critical
with regard to the integrity of offshore structures in deep water or
rough sea applications. Thus, numerous ocean structures are predicted
to experience 107-108 cycles of relatively low stress amplitude during
the design life. However, most experimental data relevant to fatigue
of welded tubular joints extends only to 106 cycles. This report
presents results of an API sponsored research project which has
addressed this high-cycle fatigue data gap. The ultimate objective of
the project has been to contribute to our understanding of this
fatigue problem and in so doing to insure that offshore structures are
designed as efficiently and safely as possible.
The experimental technique involved fatigue of modified tapered
cantilever beam, ABS DH32 steel specimens, measuring 25.4 by 152.5 mm.
(1.00 by 6.00 inches) in the cross section of the weld, by a reverse
bend constant deflection technique. The nominal stress range was 138
N/mm2 (20 ksi) and cathodic protection was maintained at −1.00v.
(Cu-CuSO4) to simulate overprotection relative to the potential
generally recognized as minimally necessary to prevent uniform
corrosion (−0.85v.). All other variables were selected so to
approximate as closely as possible the conditions which exist at
welded joints of offshore structures. Information regarding occurrence
and development of fatigue cracks was obtained by visual examination
with a low power microscope and by strain gage measurements.
For the particular experimental conditions investigated it was
determined that fatigue cracks initiated and grew to some small size
and then arrested. Because these cracks were non-propagating, no
specimens tested under the above conditions exhibited failure. This
suggests that a potential of −1.00v. is beneficial with regard
to sea water fatigue mitigation, since idential specimens tested as
part of an earlier program did fail under freely corroding and
−0.85v. conditions. Possible explanations for this enhanced
fatigue resistance at −1.00v. are presented, and the results are
discussed within the frame of existing design criteria.
Because of the significance of weld metal geometry to fatigue,
evaluation of this factor has been made for the specific case of the
specimens employed in the initial phase of this overall program. This
involved characterization of welds in terms of height, included angle
and toe radius; determination of the resultant stress concentration
factor by a finite element analysis and rationalization of the
significance of the above weld parameters and the stress concentration
factor with regard to fatigue life reduction. The conclusions are
discussed with regard to the specific case of offshore structures.
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