Cathodic protection of tubular joints: a reappraisal of the evidence

Abstract

Fatigue life data for plate to plate and tubular joints tested in seawater with cathodic protection (CP) derived in an Italian collaborative program is reviewed along with literature data. The applicability of a single linear log-log relationship to model the relationship between fatigue life and stress range is assessed. The derived curves are compared with standard curves for air and so called 'penalty factors', the ratio of life under CP to that in air, are derived (values range from 0.4 to 0.7). A fracture mechanics based model is used to assess the scatter in the fatigue data that may be associated with variations in weld geometry. Comparison of this predicted scatter with the observed experimental scatter indicates that they are of the same magnitude. The variation in the experimental data can be explained if it is assumed that the efficiency of CP remains relatively constant, with variations in life arising primarily due to variations in weld geometry. The fracture mechanics model is also used to assess the fatigue in both air and CP and thus to derive the penalty factors at low stress ranges. It is found that such factors remain relatively constant at values not greater than 0.6. The relevance of the data to fatigue design curves is assessed.