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API REPORT 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:
Mar 20, 1980
Scope:
Fatigue has been judged to be an important problem with regard to integrity of fixed offshore
structures of the "jacket" or "template" type. The situation is particularly significant in deep
water or rough sea locations, where it is anticipated that these structures may experience 10 7
- 10 9 stress cycles of relatively low amplitude over the design life. Fatigue cracking and
failure is most likely to occur at the welded joints of tubular members, since at such sites stress
is concentrated due to both metallurgical and structural (geometrical) irregularities.
A recent literature study conducted under API sponsorship has comprehensively reviewed the state of
knowledge regarding fatigue of welded structural steel in sea water, and a research plan for future
studies was recommended. Evaluations such as this and others have placed emphasis upon the fact
that most existing data which is applicable to fatigue of welded steel structures in sea water has
addressed the low cycle (relatively few cycles-to-failure) regime. The limited high cycle fatigue
data which is available has been obtained primarily from tests performed at stressing rates in
excess of those experienced by tubular joints of offshore structures. Since it is generally
considered that damage due to cyclic stressing in a corrosive environment is more accelerated the
lower the frequency, such low stress range tests could result in an incorrect and overly optimistic
assessment of fatigue resistance. Figure 1, which is reproduced from Figure 12 of reference 5 ,
presents S-N curves from various investigations, thus summarizing the extent of existing data for
relatively large welded steel specimens in sea water or a similar test electrolyte at the time that
report was prepared. Included also are the AWS-X and AWS-X Modified design curves. For these the
strain axis designates the "hot spot" value on the outside surface of the main member weld toe for
simple T, Y and K connections. While the AWS-X curve considers that cycles-to-failure becomes more
sensitive to strain range beyond 2 × 10 6 cycles, the AWS-X Modified curve represents a
more conservative stance where log fatigue strength is assumed to decrease at a constant rate with
decreasing log strain range, presumably as a consequence of severe notches and corrosion. The
degree of conservatism, if indeed there is conservatism, associated with either of these design
curves in the high cycle range cannot at this time be projected based upon presently available test
data which properly models the stress-environment state of welded joints in offshore structures.
The U.K. Offshore Steels Research Project has for the past several years been comprehensively
addressing the problem of sea water corrosion fatigue. This program is investigating the numerous
facets of this problem, including not only development of S-N and crack growth rate data, but also
welding techniques and material properties, experimental and finite element evaluation of the
mechanics of welded connections and fracture mechanics and fracture toughness testing. While the
U.K. investigations have developed additional, relevant S-N data in the 10 6 - 10 7
cycles-to-failure range, still no test results extending beyond this (cycles-to-failure >
10 7) have been reported. Also, most of this data has been for freely corroding specimens and
little new information is available regarding the cathodically protected case.
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