Effect of seawater environment on the fracture toughness of mooring chain link material under cathodic protection


High strength C-Mn mooring chains are used to keep floating offshore oil and gas platforms in position and as structural components in oil and gas subsea production systems. Cathodic protection (CP) can effectively prevent general corrosion of the chains from the surrounding seawater. However, hydrogen atoms in the water can be liberated at the surface of the links because of the cathodic reaction and diffuse into the steel, causing hydrogen embrittlement of the material. As a result, the fracture toughness of the chain link material drops. There is a dearth of knowledge about the fracture toughness of chain link material when subjected to the operating environment. This paper presents and discusses results from full-scale fracture toughness testing of a studless mooring chain link grade R5 in NaCl solution under CP.

The chain link was subjected to tensile step loading while being completely submerged in the environment. There was evidence of crack extension beyond the fatigue pre-crack that was imposed on the chain link. However, there was no definitive quantitative evidence at which step load the crack extension began. The post-test metallographic analysis showed that the crack extension due to hydrogen embrittlement has taken place in two steps. Taking into account the compressive residual stress caused by manufacturing at the crack tip, the maximum stress intensity factor (K) value that came from the second step load of 7035kN was 2735N/mm1.5. The average fracture toughness of the material in the same environment measured from small scale testing was 2372N/mm1.5. This value is acceptably near the full-scale value considering the difference in the notch depth ratio, and the design of small-scale specimens having higher constraint, expected to give lower toughness. The results at this stage suggest that small-scale specimens can give conservative predictions of full-scale behaviour of mooring chain links.

Countries: Brazil ; United Kingdom

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