| dc.description.abstract | In marine and offshore industry, components and systems are exposed to corrosive
media, wear and fatigue simultaneously (i.e. multi-degradation). These setups are
vastly operated in critical and demanding conditions and their failure can result in
irreversible environmental and health damages as well as introduce very high
maintenance costs and operation shut downs. Numerous research works have been
performed to study the effects of corrosion, wear and fatigue in offshore structures
however, only limited studies have recently been dedicated to investigate multidegradation.
In this PhD thesis, a fundamental approach has been taken to study the
multi-degradation phenomenon in practical conditions of hydraulic tension risers used
offshore. In house developed Lab-Scale Multi-Degradation (LSMD) test rig was used to
simulate corrosion, wear and fatigue (cyclic and static loading) of super duplex (SDSS)
and austenitic (ASS) stainless steels in offshore environment. Different mechanical and
chemical characterization methods were applied to investigate the effect of parameters
such as normal load, bending, electrochemical conditions, lubrication, etc. on multidegradation.
The effect of 4-point bending on tribocorrosion of stainless steels in 3.4 wt% NaCl
aqueous solution was investigated. It has been shown that formation of crack matrices
as a mixture of smeared folds and subsurface cracks followed by enhanced premature
detachment of these cracks matrices is the mechanism of debris formation. This
mechanism is enhanced by cyclic and static bending where passive film properties are
altered due to surface tension. Cyclic and static bending promoted the formation of a
thicker passive film. A distinct correlation between the passive film thickness and the
thickness of recrystallized zone beneath the wear track as well as wear was observed.
This suggests that a thicker passive film enhances material detachment and wear by
suppressing the annihilation of dislocations and promoting more brittle fracture.
In marine and offshore industry, components and systems are exposed to corrosive
media, wear and fatigue simultaneously (i.e. multi-degradation). These setups are
vastly operated in critical and demanding conditions and their failure can result in
irreversible environmental and health damages as well as introduce very high
maintenance costs and operation shut downs. Numerous research works have been
performed to study the effects of corrosion, wear and fatigue in offshore structures
however, only limited studies have recently been dedicated to investigate multidegradation.
In this PhD thesis, a fundamental approach has been taken to study the
multi-degradation phenomenon in practical conditions of hydraulic tension risers used
offshore. In house developed Lab-Scale Multi-Degradation (LSMD) test rig was used to
simulate corrosion, wear and fatigue (cyclic and static loading) of super duplex (SDSS)
and austenitic (ASS) stainless steels in offshore environment. Different mechanical and
chemical characterization methods were applied to investigate the effect of parameters
such as normal load, bending, electrochemical conditions, lubrication, etc. on multidegradation.
The effect of 4-point bending on tribocorrosion of stainless steels in 3.4 wt% NaCl
aqueous solution was investigated. It has been shown that formation of crack matrices
as a mixture of smeared folds and subsurface cracks followed by enhanced premature
detachment of these cracks matrices is the mechanism of debris formation. This
mechanism is enhanced by cyclic and static bending where passive film properties are
altered due to surface tension. Cyclic and static bending promoted the formation of a
thicker passive film. A distinct correlation between the passive film thickness and the
thickness of recrystallized zone beneath the wear track as well as wear was observed.
This suggests that a thicker passive film enhances material detachment and wear by
suppressing the annihilation of dislocations and promoting more brittle fracture. | nb_NO |
