API RP2A hot spot X and X' fatigue design curves for welded tubular connections have been replaced by a basic SN curve with slope m=3 changing to m=5 at ten million cycles. In this paper, the new criteria are compared with test data from both API-sponsored and European research -- including inter-related corrections for seawater, thickness, and use of weld profile control. Corrections for grinding and peening are also introduced, as well as SN curves and fatigue design requirements for cast nodes and grouted joints.
The simple Alpha Kellogg SCF equations have been replaced by the more rigorous Efthymiou equations, with new adjustments for chord stress effects and weld toe position.
Impact of the new revision, and some alternatives, on design practice and structural weight are examined. The Simplified Fatigue Design Procedure has been maintained, with the allowable peak hot spot stresses rechecked for the new CP-seawater SN curve.
The API fatigue revisions have achieved better alignment with the ISO 19902 draft design procedures, without sacrificing the economy of existing practices. However, they are more complex. Designers will have a lot of new refinement to deal with, and having this background will help them understand it.
Fatigue provisions in API RP2A were last updated in 1992, taking into account API-sponsored research from 1976–89 and a larger body of European work done primarily in 1978–87. That research re-affirmed the hot spot stress approach which had been in wide use since 1972, but refined it to include a more rigorous definition of the relevant stress, and to include the effects of random loading, of sea water environments, size, and profile.
During the 1990s, API established the Offshore Tubular Joint Research Center jointly with the Edison Welding Institute and the Universities of Ohio and Illinois. OTJRC continued to review data on fatigue and static strength, and conducted new research in these areas. Unfortunately, their proposals for revision lay dormant because RP2A itself lay dormant for seven years, with most standardization efforts going into ISO.
Most recently, API contracted with MSL Services to draft new tubular joint provisions. Their fatigue proposals largely follow DEn (UK) and ISO 19902 (North Sea) practices. MSL's draft provided clear and concise guidance, with many worthwhile improvements over the old API provisions. Their proposal reflects the international consensus and an intent to adopt back ISO-like provisions into API.
The ISO 19902 draft and MSL proposal recognize no benefit for weld profiling. API Subcommittee 2 felt very strongly that some recognition of the benefit of weld profiling should be retained, keeping in step with AWS D1.1. The proposal also did not fully reflect the decade of corrosion-fatigue and weld profile research of Prof. Hartt at Florida Atlantic University, funded by API. With respect to the concept that both size and profile affect fatigue performance, there was a conflict between American practice and European orthodoxy.
There were other technical issues that related to whether RP2A should represent American, North Sea, or broad international practice.