Localized corrosion damage may be detrimental to the fatigue strength of offshore steel structures. An investigation was carried out in order to assess the effect of corrosion pits on the fatigue strength of planar and tubular welded joints. Pitting corrosion was selected in accordance with the geometry of pits observed in offshore structures. Tests on planar specimens were carried out in air and in natural sea water with and without cathodic protection. Tubular joints were tested in air. Fatigue notch factors were derived on the basis of nominal and net section stresses. The overall effect of pitting corrosion on the fatigue strength of tubular welded structures was assessed. Parts of the structures in which pitting corrosion would be of particular importance with respect to structural integrity were assessed.
The paper concludes with a set of criteria for inspection of offshore structures with respect to detrimental effect of corrosion pits.
Localized corrosion damage, or pitting corrosion on offshore structures has been reported from various areas - Gulf of Mexico, Cook Inlet and the North Sea [1-4]. Published data on pit depths are given in Table 1.
Localized corrosion may affect structural performance; the fatigue strength may in particular be impaired. In order to establish an inspection and maintenance strategy with regard to localized corrosion, SINTEF has carried out a joint industry project with participation from oil companies which are operating on the Norwegian Continental Shelf.
The affect of pitting with regard to effects upon fatigue performance will depend on the location of the pits with respect to geometry and stress.
Stresses in tubular joints are highly concentrated to hot spot regions along the welds. Outside the hot spot regions stresses decay rapidly. Weldments give rise to local notch stresses on top of the hot spot stress. Corrosion pits act in the same way, with a notch factor of about the same magnitude as a normal weld. This implies that pitting corrosion outside the hot spot region may be innocuous. An important inspection parameter is therefore the distance between a given pit and the hot spot region of the joint.
The assessment of an inspection zone for corrosion pitting in tubular structures was based on a comparison of stress concentration factors at appropriate sites of crack initiation. Weldments typically represent a stress concentration factor of 2. A lower bound estimate is most probably a value of 1.5. The. geometric stress concentration factor depends on joint geometry and mode of loading. Axially loaded T joints have large stress concentration factors, typically above a value of 5. K joints have small stress concentration factors, with a value of 2 as most probably a lower bound estimate. Hence, a resonable lower bound estimate of the stress concentration factor at the weld toe of tubular joints may be a factor of 3. (Superposition of the geometric stress concentration factor and the weld notch stress concentration factor.)
