Fatigue S-N design curves for steels in seawater with cathodic protection (CP) - such as those found in DNVGL-RP-C203 and BS 7608 - are between 2 and 3 times lower than in air. This is based on tests conducted at typical wave loading frequencies. The SAFEBUCK JIP recommended a higher knockdown factor (KDF) of nine for pipelines designed to buckle laterally, and this factor should be confirmed through more validation tests.
This project will include a targeted program of low-cycle fatigue testing on C-Mn steel pipe girth welds in seawater + CP. This will establish if a higher KDF is necessary for the design of subsea pipelines subject to lateral buckling and other low-cycle fatigue applications compared with published design curves. If the results indicate a higher KDF is required, additional tests will help to establish a suitable design curve.
The results of this project will clarify the low-cycle fatigue performance of pipe girth welds exposed to seawater + CP. It will produce updated guidance for inclusion in fatigue design standards.
An accurate and industry accepted fatigue curve for safe low-cycle fatigue design of offshore pipelines exposed to seawater + CP.