One of the flow assurance challenges in subsea production systems is the occurrence of erosion damage due to the existence of sand particles and high production Gas Oil Ratio (GOR) as such erosion mostly occurs in highly gas dominated operating conditions in the annular flow regime. The erosion rate for an elbow with a constant flow velocity and with all other factors equal is higher in gas systems than liquid systems as more particles will impact on the inner wall of the outer curvature of the elbow. The maximum wear location and the penetration rate for multiphase flows are often an intermediary of gas and liquid systems occurring at 55 degrees from the inlet of the elbow, however this depends heavily on the multiphase flow regime. A challenge facing industry is availability of erosion prediction models; the majority of available models are based on singlephase liquid or gas as the carrying medium. This can result in large discrepancies in erosion rates and potentially increased wall thickness, fabrication and subsequent intervention costs.To predict the flow regime in greater clarity requires the use of computational power and / or instrumentation that can accurately characterize the flow within the pipes. Since experimental work is costly and unlikely to be representative of a large integrated production system, Computational Fluid Dynamics (CFD) is used to perform erosion assessments and can also aide in corrosion prediction and inhibitor selection. Only erosion assessments by CFD methods are discussed in detail within this paper. CFD has been extensively applied for erosion analyses; it is commonly used for identifying potential failure locations, improving understanding of failure mechanisms and only qualitatively used for erosion rates.CFD erosion modelling capability in this paper has been enhanced by simulating flow regime characteristics, in particular the liquid film for annular flow. This benefits the simulation to obtain greater accuracy for sand particle impact angles, area, speed and thus the erosion rate is significantly enhanced. In addition, the local volume fraction of sand has been considered in order to accurately evaluate the impact force. The research to date shows that a promising agreement is obtained between predicted erosion rate and the empirical predictions (Salama, Salama & Venkatesh and DNV RP-501 methods).Further comparisons to empirical model predictions are carried out to address the importance of flow regime on the results as current empirical models lack this consideration. The influence of the flow orientation (upwards and downwards flow), has also been investigated in this work due to current lack of publically available data. The paper presented hereafter illustrates that considerable difference in flow orientation is revealed and the prediction can be improved by considering the flow characteristics. An example is provided highlighting the use of liquid film and droplet velocity to replace the mixture velocity implemented in empirical models for annular flow. All of the fi...
Offshore pipe-in-pipe systems require high performance thermal insulation to maintain high fluid temperature at arrival and to avoid hydrate formation during the cool-down process that follows a pipeline shut-down. At field joints, it might be difficult to achieve the design insulation performance due to installation challenges. In these cases, the insulation layer partially fills the gap between the inner and outer pipes and thus “cold spots” could potentially arise at field joints during the pipeline operation and cool-down. In this paper the impact on the thermal performance of partially insulated pipe-in-pipe field joints is evaluated through Computational Fluid Dynamics (CFD). Thermal convection is included in the fluid model for the pipe content and the air gap between the inner and outer pipes. Comparison is also made between the numerical analysis and simplified lumped-parameter models. Results from numerical simulations show that for the case considered no cold spot arises due to a lack of field joint insulation and length-averaged Overall Heat Transfer Coefficient (OHTC) can be used to predict the pipeline cool-down time. Numerical predictions have been compared to simulated service test results, which confirm the length-averaging effect on the OHTC. Further studies are recommended to assess potential cost savings that could be achieved for uninsulated field joints.
A method for accurate prediction of flexible riser behavior and component stress computations under irregular wave inputs is a subject of major interest to offshore oil and gas operators. The initial full field procurement for flexible risers can be upwards of 500M USD, with replacement costs of a single flexible riser often approaching 40M USD. The reliability of fatigue life estimates is therefore critical to the successful long term operation of flexible risers. Computational constraints, however, continue to prevent realistic simulations and meaningful fatigue life predictions. To address this challenge, a next generation computational capability for nonlinear dynamic simulations of flexible risers has been developed. The FLEXAS solver overcomes computational constraints which limit conventional flexible riser analysis methods by implementing Nonlinear Dynamic Substructuring (NDS). This advanced multibody framework enables the incorporation of detailed finite element models into global nonlinear dynamic simulations under realistic environmental and system loads. Prior to this development, simulating these complex models for spans greater than a few pitch lengths was computationally not feasible even in static cases. The purpose of this DeepStar project is to validate the FLEXAS solver for nonlinear dynamic simulations of flexible risers against numerical and experimental references, and includes extensive local and global benchmarking. The local benchmarking scope involves comparing FLEXAS simulations against numerical and experimental references of pitch-length and bench test configurations. These benchmarks comprise a wide array of tensile armor stress and strain comparisons made against both a commercial solver and strain gauge measurements. The global benchmarking scope involves the comparison of FLEXAS simulations of a full length flexible riser configuration against the industry accepted numerical benchmarks, which includes large displacement nonlinear statics, vessel motion nonlinear dynamics and regular wave motion nonlinear dynamics. Results from all nonlinear simulations were in excellent agreement with their respective benchmarks. This work was initiated, technically guided and funded by DeepStar Global Deepwater Technology Development Program as part of Phase XII projects in the DeepStar X400 Floating Systems Committee. The successful completion of the project, established herein, is to build confidence within the industry, which will benefit from the validated FLEXAS simulation technology by improving decision making associated with flexible riser integrity management and continued service, with major cost savings realized across the entire flexible riser life-cycle.
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