Investigation of Multi-degradation Mechanisms in Different Stainless Steels in Direct Riser Tensioning Systems for Offshore Oil&Gas Industry
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Many components utilized in offshore industry are complex tribological systems, for instance Direct Riser Tensioner (DRT) cylinders which include the use of seals, guide bands, hydraulic fluids/lubricants and materials in relative movement. However, those components are often load bearing elements and thus fail due to the combined effect of multi-degradation processes (wear, corrosion and mechanical stresses). Moreover, the requirements for material selection and pre-qualification testing do not consider the synergy of the degradation phenomena, which leads to inaccurate evaluations and results in much shorter component lifetime than expected. It is therefore of a great importance to explain what happens in a material during multi-degradation exposure and how particular factors influence such a system, with emphasis on bending conditions, to be able estimate lifetime of components more precisely, thus diminish cost of maintenance and increase safe operation. During experimental work two materials were tested: AISI 316L austenitic stainless steel (UNS S31603) and 25% Cr super duplex stainless steel (UNS S32750). Test were carried out at the multi-degradation test rig (LSMD) developed by NOV in cooperation with the Tribology research group at NTNU. Both environment related (normal and applied sustained/cyclic load, exposure time) and samples related (different materials and thus microstructure, grain size etc.) variables were changed and the effects were studied. Experiments were performed at OCP (with reference to Ag/AgCl reference electrode) in 3.4 wt% NaCl solution during reciprocating ball-on-plate sliding contact (4.76 mm alumina ball). Simultaneously either static or cyclic 4-point bending was applied and results compared to tribocorrosion exposure (also performed during tests). The tests showed that tensile stresses from 4-point bending applied to a tribocorrosion system affect its volume loss and subsurface microstructure transformations. The effect on subsurface microstructure: tensile stresses release some fraction of compressive stresses induced by sliding contact, enlarge energy dissipation zone, and thus provide less subsurface deformations. Volume loss is affected as well - tensile stresses influence oxide kinetics growth on metal surface. The thickness of such surface film determines the size of debris particles generated during rubbing, and thus the volume loss.