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Yiannis Constantinides - One of the best experts on this subject based on the ideXlab platform.
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CFD Modeling and Validation of Steel Lazy-Wave Riser VIV
Volume 2: CFD and VIV, 2016Co-Authors: Yiannis Constantinides, Maeanna Stover, Amanda Steele, Markku SantalaAbstract:The steel lazy wave Riser is an emerging solution for deepwater applications in harsh conditions. The addition of buoyancy to provide the unique “lazy wave” shape reduces the dynamic stresses at the touchdown zone due to vessel motions and waves and results in improved performance. However, as the buoyant region cannot be easily fitted with VIV suppression, VIV becomes a critical aspect of the design. The present study progresses the modeling effort presented in [2] to model and understand the global Response of a deepwater lazy wave Riser using computational fluid dynamics (CFD). An industry first CFD simulation of a steel lazy wave Riser under in-plane currents is presented and validated against experiments with two different configurations. Results show good agreement between CFD and experiments and provide an initial understanding of the Riser Response under in-plane currents. The CFD method developed has been validated and will be an important tool for the design of lazy wave Risers.
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Experimental and Numerical Study of Steel Lazy Wave Riser Response in Extreme Environment
Volume 5: Pipelines Risers and Subsea Systems, 2016Co-Authors: Jingyun Cheng, Peimin Cao, Yiannis ConstantinidesAbstract:The paper presents numerical simulations and analysis of experimental results obtained from a model scale test on steel lazy wave Riser conducted by the DeepStar® consortium. The study focuses on the hydrodynamic Response of a steel lazy wave Riser under forced oscillatory and random motions in a wave basin. The Riser pipe deformations were measured by Fiber Bragg Grating (FBG) strain sensors, as well as force transducers. Both in-plane and out-of-plane signals were recorded. Numerical simulations were performed, focusing on in-plane Riser Response, and compared with the experiments. The typical hydrodynamic coefficients (added mass, and drag) for the Riser design are used in analysis and the calculated Riser reponses are compared with measurements. In addition, some coefficients are adjusted to improve the comparison and the results are promising. The test data post-processing approach and modeling of steel lazy wave Riser are also discussed in the paper.
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steel catenary Riser Response identification based on field measurements
ASME 2011 30th International Conference on Ocean Offshore and Arctic Engineering, 2011Co-Authors: Yiannis Constantinides, Prahlad Enuganti, Lee Tran, Mike CampbellAbstract:The existing Riser design and analysis methodologies rely on empirically derived parameters to conservatively represent the complex dynamic behavior. With exploration moving to deeper water and the increasing need of existing asset support, there is a strong need to evaluate and refine these methodologies. This is especially true for Steel Catenary Risers (SCR) as they are the most widely used Riser type and due to their complex soil-pipe interaction at the touchdown point. Given the small amount of small scale experiments that have been performed in the past, there is a strong industry need for large scale field measurements. This paper presents valuable field data collected from a deepwater SCR under storm conditions. The presented data includes Riser accelerations and strains compared against vessel motions. The measured SCR Response is also analyzed and qualitatively compared against the current understanding of SCR Response that constitutes the industry analysis methodologies.Copyright © 2011 by ASME
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COMPARISON OF SCR FIELD Response WITH ANALYTICAL PREDICTIONS
Volume 4: Pipeline and Riser Technology, 2011Co-Authors: Yiannis Constantinides, Prahlad Enuganti, Lee Tran, Jen-hwa Chen, Mike CampbellAbstract:Design of deepwater Risers involves the use of multiple conservative design parameters to account for the uncertainty in the understanding of the behavior of complex structures. As the oil industry moves into deeper and harsher waters, the design tolerances are getting stretched. Chevron has been monitoring the structural Response of a deepwater Gulf of Mexico steel catenary Riser (SCR) to improve the understanding of Riser behavior and to evaluate the existing analysis and design methodologies against actual field measurements. The following paper presents a selected set of results from benchmark of SCR Response in storm conditions against analytical predictions, based on industry standard methodologies. The predictions are based on a finite element analysis (FEA) modeling of the Riser structure with empirically formulated models for hydrodynamics and soil-structure interaction. Predicted Riser Response in terms of accelerations and stresses along the length are compared against field measurements showing good overall agreement.Copyright © 2011 by ASME
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Deepwater SCR Benchmarking Methodology
Volume 4: Pipeline and Riser Technology, 2011Co-Authors: Lee Tran, Mike Campbell, Prahlad Enuganti, Yiannis ConstantinidesAbstract:One of the primary goals of Riser monitoring is to build a database of measured Riser behavior during different environmental conditions and compare against design predictions during each period. A comprehensive database of field measured Riser Response provides not only a dataset to benchmark Riser performance but enables the calibration of design parameters for future Risers. The calibrated set of design parameters would feedback to establish a more representative Riser design process and provide greater confidence during future Riser designs. The following paper establishes a methodology to benchmark Riser behavior against software predictions with applications specific to a steel catenary Riser (SCR) suspended from a spar platform. Aspects and challenges dealing with processing of inclined sensors to derive global motions and operational effects are discussed and addressed. A demonstration of the methodology is presented using field measurements from a Gulf of Mexico deepwater SCR under storm conditions. The Riser behavior of interest for this study is specifically the touchdown motions and stress but additional comparisons are made along the entire Riser length.Copyright © 2011 by ASME
Carl M. Larsen - One of the best experts on this subject based on the ideXlab platform.
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flexible Riser Response induced by combined slug flow and wave loads
ASME 2013 32nd International Conference on Ocean Offshore and Arctic Engineering, 2013Co-Authors: Arturo Ortega, Ausberto Rivera, Carl M. LarsenAbstract:Flexible Risers provide optimum solutions for deep water offshore fields. Reliable dynamic analysis of this kind of slender structure is crucial to ensure safety against long time fatigue failure. Beyond the effects from wave loads, the influence from transient internal slug flow on the slender structure dynamics should also be taken into account.In this study two coupled in-house codes were used in order to identify and quantify the effects of an internal slug flow and wave loads on the flexible Riser dynamics. One code carries out a global dynamic analysis of the slender structure displacements using a finite element formulation. The other program simulates the behaviour of the internal slug flow using a finite volume method. The slug flow is influenced by the dynamic shape of the Riser, while the time varying forces from internal slug flow plus external waves will influence the shape. Hence, a fully coupled analysis is needed in order to solve the coupled problem. By means of the distributed simulation these two programs run synchronously and exchange information during the time integration process.A test case using hydrodynamic forces according to the linear Airy wave theory, coupled with an internal unstable slug flow was analysed and the results show: amplification of the dynamic Response due to the interaction between the two load types, effects on the effective tension caused by the internal two-phase flow, and influence on the internal slug flow caused by the wave induced Response.Copyright © 2013 by ASME
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flexible Riser Response induced by combined slug flow and wave loads
ASME 2013 32nd International Conference on Ocean Offshore and Arctic Engineering, 2013Co-Authors: Arturo Ortega, Ausberto Rivera, Carl M. LarsenAbstract:Flexible Risers provide optimum solutions for deep water offshore fields. Reliable dynamic analysis of this kind of slender structure is crucial to ensure safety against long time fatigue failure. Beyond the effects from wave loads, the influence from transient internal slug flow on the slender structure dynamics should also be taken into account.In this study two coupled in-house codes were used in order to identify and quantify the effects of an internal slug flow and wave loads on the flexible Riser dynamics. One code carries out a global dynamic analysis of the slender structure displacements using a finite element formulation. The other program simulates the behaviour of the internal slug flow using a finite volume method. The slug flow is influenced by the dynamic shape of the Riser, while the time varying forces from internal slug flow plus external waves will influence the shape. Hence, a fully coupled analysis is needed in order to solve the coupled problem. By means of the distributed simulation these two programs run synchronously and exchange information during the time integration process.A test case using hydrodynamic forces according to the linear Airy wave theory, coupled with an internal unstable slug flow was analysed and the results show: amplification of the dynamic Response due to the interaction between the two load types, effects on the effective tension caused by the internal two-phase flow, and influence on the internal slug flow caused by the wave induced Response.Copyright © 2013 by ASME
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Modeling of Riser Response for DP Control
Journal of Offshore Mechanics and Arctic Engineering, 2002Co-Authors: Bernt J. Leira, Asgeir J Sorensen, Qiaofeng Chen, Carl M. LarsenAbstract:The present paper is concerned with modeling of Riser Response to static and dynamic loading. Focus is on top and bottom angles which are of crucial importance, e.g., during drilling and workover operations. Parametric studies are performed for a representative deep-water Riser configuration. The relationship between surface floater motion and angle Responses is investigated. In a wider context, the possibility of reducing the maximum Response levels by dynamic positioning of the floater is the main objective. It is illustrated by a numerical simulation of the system behavior how the Response relations obtained herein can be utilized for such a purpose. Generally, minimization of one of the Riser end angles by adjusting the vessel position can only take place at the cost of increasing the other one. Hence, an optimum position should be defined by considering both angles but with different weight functions. An attractive approach is to determine these weights as functions of the respective reliability indices for each of the two angles. The viability of this scheme is also explored by numerical simulation for the same example Riser configuration.
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Reliability-Based Schemes for Control of Riser Response and Dynamic Positioning of Floating Vessels
21st International Conference on Offshore Mechanics and Arctic Engineering Volume 2, 2002Co-Authors: Bernt J. Leira, Asgeir J Sorensen, Carl M. LarsenAbstract:Application of structural reliability methods in relation to online control of dynamic systems is considered. The particular case in focus is dynamic positioning of marine vehicles as related to mechanical limit states for the Riser system. The magnitudes of top and bottom angles of the marine Risers are considered. These angles are of crucial importance during e.g. drilling and workover operations. The relationship between surface floater motion and angle Responses is first considered. The possibility of reducing the maximum angular Response levels by dynamic positioning of the floater is then investigated. Typically, minimization of one of the Riser top and bottom angles by adjusting the vessel position can only take place at the cost of increasing the other one. Hence, an optimum position should be defined by applying different weights for the two angles. Three different alternatives are considered: (I) The relative weights are kept fixed (II) The relative weights are determined as functions of the respective reliability indices for each of the two angles. (III) A loss function which is purely expressed in terms of reliability indices is introduced. The viability of different schemes of this type is explored by numerical simulation for a specific Riser configuration.Copyright © 2002 by ASME
Mike Campbell - One of the best experts on this subject based on the ideXlab platform.
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Effect of Mud Shedding on Riser Anti-Recoil Control at Emergency Disconnect
Volume 3: Pipeline and Riser Technology, 2012Co-Authors: Mike Campbell, Hugh Howells, Simeon PowellAbstract:With drilling capability extends to water depths up to 3000m, significantly increased is the risk associated with a failed Riser recoil control in the event of an emergency Riser disconnect due to loss of vessel station keeping. In deeper waters the tensioner system undertakes higher top tension due to the accumulation of Riser length and mud weight. During emergency disconnect, the Riser is disconnected between the blow-out preventer (BOP) and the lower marine Riser package (LMRP), releasing the base tension and mud pressure. Impact between the top Riser system and the diverter housing system should be avoided, and the clearance between the LMRP and BOP should be secured. Efforts have been continuously made in the industry to achieve a more accurate predict of the Riser recoil Response. The relevance of mud discharge to recoil control has been widely discussed but little quantitative data has been revealed in the literature.In this paper, effect of mud shedding on Riser recoil Response is discussed. The Herschel-Bulkley rheology model is utilized for mud flow and is considered the latest advance in the drilling industry. Water hammer theories with column separation are modified to account for mud discharge in laminar, transitional and turbulent flow regimes.As a case study herein, recoil Response of a drilling Riser attached to a dynamically positioned semi-submersible drilling vessel is assessed to present the mud discharge effect on Riser anti-recoil control. At emergency events, the Riser is disconnected above the BOP, which is located at a water depth of about 2150m for this study. Mud is generally preferred to be freely discharged during an emergency disconnect for ease of anti-recoil control and Riser integrity. The density difference between drilling mud in the Riser annulus and sea water outside the Riser outer casing before disconnect induces a high pressure difference, which drives mud shedding at Riser disconnect. Mud flow rate plays an important role in the speed control of the Riser uplift. 3D finite element analysis is performed in time domain to simulate Riser Response before and after disconnect. 2HRECOIL software is integrated into ANSYS user programmable features to better model the Riser Response and mud discharge.Copyright © 2012 by ASME
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steel catenary Riser Response identification based on field measurements
ASME 2011 30th International Conference on Ocean Offshore and Arctic Engineering, 2011Co-Authors: Yiannis Constantinides, Prahlad Enuganti, Lee Tran, Mike CampbellAbstract:The existing Riser design and analysis methodologies rely on empirically derived parameters to conservatively represent the complex dynamic behavior. With exploration moving to deeper water and the increasing need of existing asset support, there is a strong need to evaluate and refine these methodologies. This is especially true for Steel Catenary Risers (SCR) as they are the most widely used Riser type and due to their complex soil-pipe interaction at the touchdown point. Given the small amount of small scale experiments that have been performed in the past, there is a strong industry need for large scale field measurements. This paper presents valuable field data collected from a deepwater SCR under storm conditions. The presented data includes Riser accelerations and strains compared against vessel motions. The measured SCR Response is also analyzed and qualitatively compared against the current understanding of SCR Response that constitutes the industry analysis methodologies.Copyright © 2011 by ASME
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COMPARISON OF SCR FIELD Response WITH ANALYTICAL PREDICTIONS
Volume 4: Pipeline and Riser Technology, 2011Co-Authors: Yiannis Constantinides, Prahlad Enuganti, Lee Tran, Jen-hwa Chen, Mike CampbellAbstract:Design of deepwater Risers involves the use of multiple conservative design parameters to account for the uncertainty in the understanding of the behavior of complex structures. As the oil industry moves into deeper and harsher waters, the design tolerances are getting stretched. Chevron has been monitoring the structural Response of a deepwater Gulf of Mexico steel catenary Riser (SCR) to improve the understanding of Riser behavior and to evaluate the existing analysis and design methodologies against actual field measurements. The following paper presents a selected set of results from benchmark of SCR Response in storm conditions against analytical predictions, based on industry standard methodologies. The predictions are based on a finite element analysis (FEA) modeling of the Riser structure with empirically formulated models for hydrodynamics and soil-structure interaction. Predicted Riser Response in terms of accelerations and stresses along the length are compared against field measurements showing good overall agreement.Copyright © 2011 by ASME
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Deepwater SCR Benchmarking Methodology
Volume 4: Pipeline and Riser Technology, 2011Co-Authors: Lee Tran, Mike Campbell, Prahlad Enuganti, Yiannis ConstantinidesAbstract:One of the primary goals of Riser monitoring is to build a database of measured Riser behavior during different environmental conditions and compare against design predictions during each period. A comprehensive database of field measured Riser Response provides not only a dataset to benchmark Riser performance but enables the calibration of design parameters for future Risers. The calibrated set of design parameters would feedback to establish a more representative Riser design process and provide greater confidence during future Riser designs. The following paper establishes a methodology to benchmark Riser behavior against software predictions with applications specific to a steel catenary Riser (SCR) suspended from a spar platform. Aspects and challenges dealing with processing of inclined sensors to derive global motions and operational effects are discussed and addressed. A demonstration of the methodology is presented using field measurements from a Gulf of Mexico deepwater SCR under storm conditions. The Riser behavior of interest for this study is specifically the touchdown motions and stress but additional comparisons are made along the entire Riser length.Copyright © 2011 by ASME
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STRAKED Riser DESIGN WITH VIVA
29th International Conference on Ocean Offshore and Arctic Engineering: Volume 6, 2010Co-Authors: Dhyanjyoti Deka, Paul R. Hays, Kamaldev Raghavan, Mike CampbellAbstract:VIVA is a vortex induced vibration (VIV) analysis software that to date has not been widely used as a design tool in the offshore oil and gas industry. VIVA employs a hydrodynamic database that has been benchmarked and calibrated against test data [1]. It offers relatively few input variables reducing the risk of user induced variability of results [2]. In addition to cross flow current induced standing wave vibration, VIVA has the capability of predicting traveling waves on a subsea Riser, or a combination of standing and traveling waves. Riser boundary conditions including fixed, pinned, flex joint or SCR seabed interaction can be modeled using springs and dashpots. VIVA calculates Riser natural frequencies and mode shapes and also has the flexibility to import external modal solutions. In this paper, the applicability of VIVA for the design of straked steel catenary Risers (SCR) and top tensioned Risers (TTR) is explored. The use of linear and rotational springs provided by VIVA to model SCR soil interaction and flex joint articulation is evaluated. Comparisons of the VIV fatigue damage output with internal and external modal solution is presented in this paper. This paper includes validation of the VIVA generated modal solution by comparing the modal frequencies and curvatures against a finite element (FE) model of the Risers. Fatigue life is calculated using long term Gulf of Mexico (GoM) currents and is compared against the industry standard software SHEAR7. Three different lift curve selections in SHEAR7 are used for this comparison. The differences in Riser Response prediction by the two software tools are discussed in detail. The sensitivity of the VIVA predicted Riser Response to the absence of VIV suppression devices is presented in this paper. The Riser VIV Response with and without external FE generated modal input is compared and the relative merits of the two modeling approaches are discussed. Finally, the recommended approach for VIVA usage for SCR and TTR design is given.Copyright © 2010 by ASME
Arturo Ortega - One of the best experts on this subject based on the ideXlab platform.
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flexible Riser Response induced by combined slug flow and wave loads
ASME 2013 32nd International Conference on Ocean Offshore and Arctic Engineering, 2013Co-Authors: Arturo Ortega, Ausberto Rivera, Carl M. LarsenAbstract:Flexible Risers provide optimum solutions for deep water offshore fields. Reliable dynamic analysis of this kind of slender structure is crucial to ensure safety against long time fatigue failure. Beyond the effects from wave loads, the influence from transient internal slug flow on the slender structure dynamics should also be taken into account.In this study two coupled in-house codes were used in order to identify and quantify the effects of an internal slug flow and wave loads on the flexible Riser dynamics. One code carries out a global dynamic analysis of the slender structure displacements using a finite element formulation. The other program simulates the behaviour of the internal slug flow using a finite volume method. The slug flow is influenced by the dynamic shape of the Riser, while the time varying forces from internal slug flow plus external waves will influence the shape. Hence, a fully coupled analysis is needed in order to solve the coupled problem. By means of the distributed simulation these two programs run synchronously and exchange information during the time integration process.A test case using hydrodynamic forces according to the linear Airy wave theory, coupled with an internal unstable slug flow was analysed and the results show: amplification of the dynamic Response due to the interaction between the two load types, effects on the effective tension caused by the internal two-phase flow, and influence on the internal slug flow caused by the wave induced Response.Copyright © 2013 by ASME
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flexible Riser Response induced by combined slug flow and wave loads
ASME 2013 32nd International Conference on Ocean Offshore and Arctic Engineering, 2013Co-Authors: Arturo Ortega, Ausberto Rivera, Carl M. LarsenAbstract:Flexible Risers provide optimum solutions for deep water offshore fields. Reliable dynamic analysis of this kind of slender structure is crucial to ensure safety against long time fatigue failure. Beyond the effects from wave loads, the influence from transient internal slug flow on the slender structure dynamics should also be taken into account.In this study two coupled in-house codes were used in order to identify and quantify the effects of an internal slug flow and wave loads on the flexible Riser dynamics. One code carries out a global dynamic analysis of the slender structure displacements using a finite element formulation. The other program simulates the behaviour of the internal slug flow using a finite volume method. The slug flow is influenced by the dynamic shape of the Riser, while the time varying forces from internal slug flow plus external waves will influence the shape. Hence, a fully coupled analysis is needed in order to solve the coupled problem. By means of the distributed simulation these two programs run synchronously and exchange information during the time integration process.A test case using hydrodynamic forces according to the linear Airy wave theory, coupled with an internal unstable slug flow was analysed and the results show: amplification of the dynamic Response due to the interaction between the two load types, effects on the effective tension caused by the internal two-phase flow, and influence on the internal slug flow caused by the wave induced Response.Copyright © 2013 by ASME
Lance Manuel - One of the best experts on this subject based on the ideXlab platform.
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non parametric prediction of the long term fatigue damage for an instrumented top tensioned Riser
Applied Ocean Research, 2019Co-Authors: Lance ManuelAbstract:Abstract Marine Risers are susceptible to sustained vortex-induced vibration (VIV) because of their slenderness and light damping. Commonly used tools for analyzing VIV and the associated fatigue damage are based on the finite element method and rely on simplifying assumptions on the Riser's physical model, the flow conditions, and characteristics of the Response. In order to assess the influence of VIV and to ensure the integrity of the Riser, field monitoring campaigns are often undertaken wherein data loggers such as strain sensors and/or accelerometers are installed on such Risers. Given the recorded Riser's dynamic Response, empirical techniques can be used in VIV-related fatigue estimation. These empirical techniques make direct use of the measurements and are intrinsically dependent on the actual current profiles. Damage estimation can be undertaken for the different current profiles encountered and can account explicitly even for complex Riser Response characteristics. With a significant amount of data, “short-term” fatigue damage probability distributions, conditional on current, can be established. If the relative frequency of different current types is known from a separate metocean study, the short-term fatigue damage distributions can be combined with the current distributions to yield an integrated “long-term” fatigue damage model, which then can be used to predict the long-term cumulative fatigue damage for the instrumented Riser. Non-parametric statistical techniques (that do not assume a specific function for the underlying distribution as parametric techniques do) are employed to describe the short-term fatigue damage data. In this study, data from the Norwegian Deepwater Programme (NDP) model Riser experiments are used to demonstrate the effectiveness of empirical procedures and non-parametric statistics applied to field measurements to predict long-term fatigue damage, life, and probability of fatigue failure.
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A Comparison of Empirical Procedures for Fatigue Damage Prediction in Instrumented Risers undergoing Vortex-Induced Vibration
2018Co-Authors: C. Shi, Lance Manuel, Michael TognarelliAbstract:To gain insight into Riser motions and associated fatigue damage due to vortex-induced vibration (VIV), data loggers such as strain sensors and/or accelerometers are sometimes deployed on Risers to monitor their motion in different current velocity conditions. Accurate reconstruction of the Riser Response and empirical estimation of fatigue damage rates over the entire Riser length using measurements from a limited number of sensors can help in efficient utilization of the costly measurements recorded. Several different empirical procedures are described here for analysis of the VIV Response of a long flexible cylinder subjected to uniform and sheared current profiles. The methods include weighted waveform analysis (WWA), proper orthogonal decomposition (POD), modal phase reconstruction (MPR), a modified WWA procedure, and a hybrid method which combines MPR and the modified WWA method. Fatigue damage rates estimated using these different empirical methods are compared and cross-validated against measurements. Detailed formulations for each method are presented and discussed with examples. Results suggest that all the empirical methods, despite different underlying assumptions in each of them, can be employed to estimate fatigue damage rates quite well from limited strain measurements.
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An Empirical Procedure for Fatigue Damage Estimation in Instrumented Risers
Journal of Offshore Mechanics and Arctic Engineering, 2017Co-Authors: C. Shi, Lance ManuelAbstract:In order to assess the effects of vortex-induced vibration (VIV) and to ensure Riser integrity, field monitoring campaigns are often conducted wherein the Riser Response is recorded by a few data sensors distributed along the length of the Riser. In this study, two empirical techniques–proper orthogonal decomposition (POD) and weighted waveform analysis (WWA)–are sequentially applied to the data; together, they offer a novel empirical procedure for fatigue damage estimation in an instrumented Riser. The procedures are briefly described as follows: first, POD is used to extract the most energetic spatial modes of the Riser Response from the measurements, which are defined only at the available sensor locations. Accordingly, a second step uses WWA to express each dominant POD mode as a series of Riser natural modes that are continuous spatial functions defined over the entire Riser length. Based on the above empirically identified modal information, the Riser Response over the entire length is reconstructed in reverse–i.e., compose identified natural modes into the POD modes and, then, assemble all these dominant POD modal Response components into the derived Riser Response. The POD procedure empirically extracts the energetic dynamic Response characteristics without any assumptions and effectively cleans the data of noisy or less important features; this fundamental application of WWA is used to identify dominant Riser natural modes–all this is possible using the limited number of available measurements from sensor locations. Application of the procedure is demonstrated using experimental data from the Norwegian Deepwater Programme (NDP) model Riser.
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An Empirical Procedure for Fatigue Damage Estimation in Instrumented Risers
Volume 2: CFD and VIV, 2016Co-Authors: C. Shi, Lance ManuelAbstract:Vortex-induced vibration (VIV) can lead to significant fatigue damage accumulation in deepwater marine Risers. In order to assess the effects of VIV and to ensure Riser integrity, field monitoring campaigns are often conducted wherein Riser Response is recorded by a few data sensors distributed along the length of the Riser. It is possible to empirically estimate the fatigue damage at “key” critical fatigue-sensitive locations, where sensors may not be available as part of the spatially distributed discrete measurements. In this study, two empirical techniques — Proper Orthogonal Decomposition (POD) and Weighted Waveform Analysis (WWA) — are sequentially applied to the data; together, they offer a novel empirical procedure for fatigue damage estimation in an instrumented Riser. The procedures are briefly described as follows: first, POD is used to extract the most energetic spatial modes of the Riser Response from the measurements. Often, only a few dominant POD modes preserve most of the Riser motion kinetic energy; other modes are less important. Identified POD mode shapes are discrete as they are defined only at the available sensor locations. Accordingly, a second step in the proposed procedure uses WWA to express each dominant POD mode as a series of Riser natural modes that are continuous spatial functions defined over the entire Riser length. Based on the above empirically identified modal information, the Riser Response over the entire length is reconstructed using backward procedures — i.e., compose identified natural modes into the POD modes and, then, assemble all these dominant POD modal Response components into the derived Riser Response. The POD procedure empirically extracts the energetic dynamic Response characteristics without any assumptions and effectively cleans the data of noisy or less important features, which makes it possible for WWA to identify dominant Riser natural modes — all this is possible using the limited number of available measurements from sensor locations. Application of the entire procedure is demonstrated using experimental data from the Norwegian Deepwater Programme (NDP) model Riser.
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A Data-Driven Mode Identification Algorithm for Riser Fatigue Damage Assessment
Journal of Offshore Mechanics and Arctic Engineering, 2014Co-Authors: C. Shi, Lance Manuel, Jinkyoo Park, Michael TognarelliAbstract:A well-established empirical procedure, which we refer to as weighted waveform analysis (WWA), is employed to reconstruct a model Riser's Response over its entire length using a limited number of strain measurements. The quality of the Response reconstruction is controlled largely by identification of the participating Riser Response modes (waveforms); hence, mode selection is vital in WWA application. Instead of selecting a set of consecutive Riser vibratory modes, we propose a procedure that automatically identifies a set of nonconsecutive Riser modes that can thus account for higher harmonics in the Riser Response (at multiplies of the Strouhal frequency). Using temporal data analysis of the discrete time-stamped samples, significant Response frequencies are identified on the basis of power spectrum peaks; similarly, using the spatial data analysis of the sparse nonuniformly sampled data, significant wavenumbers are identified using Lomb–Scargle periodograms. Knowing the Riser length, the most important wavenumber is related to a specific mode number; this dominant mode is, in turn, related to the dominant peak in power spectra based on the temporal data analysis. The Riser's fundamental frequency is estimated as the ratio of the empirically estimated dominant spectral frequency to the dominant mode number. Additional mode numbers are also identified as spectral peak frequencies divided by the fundamental frequency. This mode selection technique is an improvement over similar WWA procedures that rely on a priori knowledge of the Riser's fundamental frequency or on the knowledge of the physical properties and assumptions on added mass contributions. At selected target locations, we compare fatigue damage rates, estimated based on the Riser Response reconstructed using the WWA method with the proposed automated mode selection technique (we refer to this as the “improved” WWA) and those based on the “original” WWA method (that relies on a theoretically computed fundamental natural frequency of the Riser). In both cases, predicted fatigue damage rates based on the empirical methods and data at various locations (other than the target) are cross-validated against damage rates based directly on measurements at the target location. The results show that the improved WWA method, which empirically estimates the Riser's fundamental natural frequency and automatically selects significant modes of vibration, may be employed to estimate fatigue damage rates quite well from limited strain measurements.
