From measurement collection to remaining useful life estimation : defining a diagnostic-prognostic frame for optimal maintenance scheduling of choke valves undergoing erosion
Condition Based Maintenance (CBM) aims at regulating maintenance scheduling based on data analyses and system condition monitoring. Clear advantages of optimizing maintenance scheduling include relevant cost savings and improved safety and plant availability. A critical aspect is the integration of CBM strategies with condition monitoring technologies for handling a wide range of information sources and eventually making optimal decisions on when and what to repair. In this work, a practical case study concerning maintenance of choke valves in offshore oil platforms has been investigated. Choke valves used in offshore oil platforms undergo erosion caused by the sand grains transported by the oil-water-gas mixture extracted from the well. Erosion is a critical problem which can affect the correct functioning of the valves, result in revenue losses and cause environmental hazards. In this respect, this work proposes a diagnostic-prognostic scheme for assessing the actual health state of a choke va lv and eventually estimating its Remaining Useful Life (RUL). In particular, the focus has been on th e identification of those parameters which contribute to the actual erosion of the choke valve, the development of a model-based approach for calculating a reliable indicator of the choke valve health state, the actual estimation of the choke RU L based on that indicator using statistical approache s and, finally, the investigation of methods to reduc e the uncertainty of the RUL estimation by adding highly meaningful knowledge on the erosion state of the choke valve .
Technical Condition Assessment and Remaining Useful Life Estimation of Choke Valves subject to Erosion
Components that are part of an industrial process are normally degraded after some time in use. Efficient maintenance is of fundamental importance to the safety and economics of the plant operation. Predicting the future evolution of the degradation of components and estimating their remaining useful life are central elements for the implementation of optimal maintenance strategies. To achieve this, one must first identify the parameters providing a reliable indication of component performance or technical condition and then develop models for predicting their future trend. This paper presents an approach to tackle a practical problem concerning the estimation of remaining useful life of choke valves located topside at wells on the Norwegian Continental Shelf. Choke valves suffer from erosion due to sand production in mature fields and eventually need to be replaced. One indicator of the erosion process is the increase of the difference between the theoretical and estimated valve flow coefficients. To achieve a reliable estimate of this parameter an empirical model-based approach is used accounting also for the historical well test measures collected during the choke valve life. A gamma process is then devised to model the temporal variability trend of the erosion. Finally, the choke valve remaining useful file is estimated based on an erosion threshold established by expert judgment. * * This is an open-access article distributed under t h terms of the Creative Commons Attribution 3.0 United States License,
Effect of stress conditions on collapse deformation behavior of a rockfill material
A set of saturated and dry-saturated large-scale triaxial tests were conducted in order to explore the effects of stress conditions on the saturation-induced collapse deformation behavior of a rockfill material. Specimens were tested in different confining pressures and for dry-saturated tests the specimens were submerged in various shear stress levels. The results indicate that in all of the dry-saturated tests a sudden reduction in the volume of the specimens was observed during the submerging process. The ratio of the minimum axial strength of a submerged specimen (at the end of the saturation process) to the shear strength of the specimen before saturation is defined as the coefficient of stress recovery, Csr. Results of the dry-saturated tests showed that this ratio increases as the confining pressure increases and decreases by increasing the shear stress level. According to the results of dry-saturated tests the values of internal friction angle reduction caused by saturation (Δϕc), the ratio of the elasticity modulus of the material after saturation to its elasticity modulus in dry condition, i.e., Ewet/Edry, and also the saturation induced sudden volumetric strain (εvc) decrease as the confining pressure increases. However the shear stress level does not have any meaningful effect on the variation of Dfc, Ewet/Edry and (εvc).