OBJECTIVE: Vortex-induced vibration (VIV) is an important issue in the design of deepwater riser systems, including drilling, production and export risers. The VIV can produce a high level of fatigue damage in a relatively short period of time for risers exposed to severe current environments. The wake interference between various risers in the same riser array may also lead to collisions between adjacent risers. Suppression devices, such as helical strakes or fairings may be needed to prevent unacceptable levels of fatigue damage. A computational fluid dynamics (CFD) code based on the Finite-Analytic Navier-Stokes (FANS) numerical method has been developed recently for the simulation of riser VIV responses at high Reynolds numbers under steady currents. The objective of this research is to complete the CFD studies of the responses of deepwater risers to VIV to provide practical advice on: (1) Riser response to VIV in various current profiles and water depths, and (2) Effectiveness of VIV mitigation devices and strategies
APPROACH: The riser VIV responses are affected by many parameters including the Reynolds number, surface roughness, strakes, fairings, 3D sheared currents and ambient turbulence. In order to provide accurate analyses of the VIV phenomena, the Finite-Analytic Navier-Stokes (FANS) numerical method will be employed in conjunction with a chimera domain decomposition approach to investigate the complex deepwater riser VIV induced by various current profiles. The FANS method has been successfully used for VIV analysis of smooth and roughened risers in uniform currents with translational and rotational motions. In the proposed research, the method will be further extended to predict the responses of deepwater risers to VIV under various flow conditions such as loop currents, jets, and hurricane. The effectiveness of VIV suppression devices such as helical strakes and fairings will also be evaluated. The simulation results will be compared with available experimental data to assess the accuracy of the CFD predictions.
DEPLOYMENT OF RESULTS: The CFD results will be synthesized for deepwater risers under various operating conditions. This will provide designers a better understanding of the complex VIV phenomenon, and provide insight and a basis for designing to minimize VIV fatigue damage.
PROJECT ORGANIZATION & TIMING: The project will be completed in two years. In Phase 1 (2006-2007) of the project, the PEGSUS program which provides interpolation information between chimera grid blocks will be improved to facilitate more efficient data communication on multiple processors. The parallelized 3D CFD code will be coupled with a six degree-of-freedom (DOF) motion program for the prediction of VIV and dynamic response of long flexible risers. The CFD model will be validated with available lab and field data. Time-domain simulation will also be performed for single and multiple risers to study the VIV responses to different current profiles and different water depths encountered in loop currents, jets, and hurricanes. In Phase 2 (2007-2008), the 3D CFD code will be extended to study the VIV suppression devices and configurations. The inference due to multiple closely spaced risers and impact loads due to riser clashing will also be investigated. The research findings and their implications on riser design will be summarized in the final project report.ANTICIPATED NUMBER OF PHASES: 2
PROJECT PLAN FOR PHASE 1 (2006 -2007):Scope and Plan: The primary focus of this phase is to generalize the 3D CFD code for time-domain simulation of riser VIV under various current profiles and water depths. The following tasks are planned.
1. Parallelize 3D CFD Code – The present CFD code solves the momentum equations using the Finite-Analytic Navier-Stokes (FANS) method, while the interpolation between different chimera grid blocks was obtained from the PEGSUS program. The FANS code is fully parallelized on multiple processors. However, the PEGSUS program was executed only on the master processor. In the proposed research, the PEGSUS program will be optimized to facilitate more efficient data transfer on large number of processors. This will allow us to simulate the VIV responses of long deepwater risers with reasonable computer run times.
2. Couple CFD code to six-DOF motion program – A six-DOF motion program will be developed for the prediction of riser VIV responses in 3D sheared currents. The forces and moments obtained from the 3D CFD code will be coupled with the six-DOF motion program to determine the riser motions.
3. Riser VIV simulation of a single riser in sheared currents – Riser VIV simulations will be performed for a long flexible riser under various current profiles encountered in loop currents, jets, and hurricanes.
4. Validation of CFD code – The 3D CFD model will be validated with available lab and field data. Potential sources include tank experiments, DeepStar model riser data from Lake Seneca & Florida Gulf Stream, and actual riser data from GOM FPSs.
5. Simulation of riser Inference - Time-domain simulations will also be performed for multiple deepwater risers to evaluate the interaction between closely spaced risers subjected to 3D sheared currents.
6. Prepare final report – a final report for Phase 1 study will be prepared to summarize the theoretical formulation, numerical method, and simulation results for VIV responses of single and multiple risers in 3D sheared currents.
Anticipated Results & Deliverable: It is anticipated that the 3D CFD simulation results will provide detailed flow field and motion history for single and multiple risers subjected to 3D sheared currents including loop currents, jets, and hurricanes. The numerical results and their design implications will be disseminated through interim progress reports and the end-of-phase final report.Proposed Project Term: September 1, 2006-November 30, 2007
PROJECT PLAN FOR PHASE 2 (2007 - 2008):Scope of Work: The primary focus of this phase is to use the 3D CFD model for numerical investigation of VIV suppression devices and strategies and the simulation of impact loads due to riser crashing. The following tasks are planned.
1. VIV responses of single riser with fairing – Time-domain simulation of a long flexible deepwater riser fitted with fairing will be investigated to determine the effectiveness of fairing on the suppression of VIV motions.
2. Inference of multiple risers with fairing – Time-domain simulations will be performed for multiple deepwater risers with vortex-suppressing fairings to investigate the interaction between closely spaced risers with fairings. Various riser configurations including tandem, side-by-side, square, or diamond arrangements will be investigated.
3. VIV responses of single riser with strakes – Time-domain simulation of a long flexible deepwater riser fitted with strakes will be investigated to determine the effectiveness of strakes on the suppression of VIV motions.
4. Inference of multiple risers with strakes – Time-domain simulations will also be performed for multiple deepwater risers with helical strakes to investigate the effectiveness of strakes for closely spaced risers. Various riser configurations such as tandem, side-by-side, square, or diamond arrangements will be investigated.
5. Impact loads due to riser crashing – The compression and repulsive forces associated with riser crashing can be modeled by small ring fenders (elastic springs) surrounding each riser.
6. Prepare final report – a final report for Phase 2 study will be prepared to summarize the theoretical formulation, numerical method, and simulation results for VIV suppression devices and configurations.
Anticipated Results & Deliverables: It is anticipated that the 3D CFD simulation results for VIV suppression devices will provide the designers better understanding on the effectiveness of various vortex-suppression devices and configurations. The numerical results and their design implications will be disseminated through interim progress reports and the end-of-phase final report.
Proposed Project Term for Phase 2: September 1, 2007 – November 30, 2008 (work on Phase 2 will begin while Phase 1 project report is under preparation).
PRINCIPAL INVESTIGATOR (S) & OTHERS INVOLVED IN PROJECT:
PI(s): Hamn-Ching Chen, Chia-Rong Chen and Richard S. Mercier
DATE: June, 2007Project Title: Deepwater Riser VIV/CFD Simulation of Riser VIV
MMS Project: 481 TO Number: 35983 & 39774
Project PI: H. C. Chen, C. R. Chen and Richard Mercier
COTR: M. Else
Estimated Completion Date: 11/30/2007
Project Description:
Vortex-induced vibration (VIV) is an important issue in the design of deepwater riser systems, including drilling, production and export risers. The VIV can produce a high level of fatigue damage in a relatively short period of time for risers exposed to severe current environments. The wake interference between various risers in the same riser array may also lead to collisions between adjacent risers. Suppression devices, such as helical strakes or fairings may be needed to prevent unacceptable levels of fatigue damage. A computational fluid dynamics (CFD) code based on the Finite-Analytic Navier-Stokes (FANS) numerical method has been developed recently for the simulation of riser VIV responses at high Reynolds numbers under steady currents. The objective of this research is to complete the CFD studies of the responses of deepwater risers to VIV to provide practical advice on: (1) riser response to VIV in various current profiles and water depths, and (2) effectiveness of VIV mitigation devices and strategies.The riser VIV responses are affected by many parameters including the Reynolds number, surface roughness, strakes, fairings, 3D sheared currents and ambient turbulence. In order to provide accurate analyses of the VIV phenomena, the Finite-Analytic Navier-Stokes (FANS) numerical method will be employed in conjunction with a chimera domain decomposition approach to investigate the complex deepwater riser VIV induced by various current profiles. The FANS method has been successfully used previously for VIV analysis of smooth and roughened risers in uniform currents with translational and rotational motions. In the proposed research, the method will be further extended to predict the responses of deepwater risers to VIV under both the uniform and sheared current profiles. The simulation results will be compared with available experimental data to assess the accuracy of the CFD predictions.
Progress:
During this reporting period, the FANS code was coupled with a finite-element tensioned beam model for riser motion predictions. The riser structural deformation was calculated using two different methods: (a) modal decomposition approach with pre-defined mode shapes, or (b) direct integration of the tensioned beam motion equation. CFD simulations were performed first for a 38-m long, horizontally-positioned riser with length-to-diameter ratio (L/D) = 1,400 under both the uniform and sheared current profiles. The simulation results were compared with available experimental data to illustrate the capability of the present CFD approach for accurate prediction of the motion responses of flexible long risers. Calculations were then performed for a practical single casing 10¾” top tensioned riser in 3,000 ft water depth (L/D = 3,350). It was found that the riser could experience multi-mode VIV under uniform current condition. The simulation results demonstrated that the present CFD approach is suitable for VIV analysis of deepwater risers with large L/D and complex current conditions.Reports and Publications:
1. Huang K., Chen, H.C., and Chen, C.R., “Deepwater Riser VIV Assessment by Using a Time Domain Simulation Approach,” OTC 18769, Offshore Technology Conference, 30 April – 3 May 2007, Houston, Texas, USA.2. Huang, K., Chen, H.C. and Chen, C.R., “Time-Domain Simulation of Riser VIV in Sheared Current,” OMAE 2007-29363, Proceedings of the 26th International Conference on Offshore Mechanics and Artic Engineering, June 10-15, 2007, San Diego, California, USA.
3. Pontaza, J.P., Menon, R.G. and Chen, H.C., “Three-Dimensional Numerical Simulations of Flow Past Smooth and Rough/Bare and Helically Straked Circular Cylinders, Allowed to Undergo Two Degree-of-Freedom Motions,” OMAE 2007-29366, Proceedings of 26th International Conference on Offshore Mechanics and Artic Engineering, June 10-15, 2007, San Diego, California, USA.
4. Huang, K., Chen, H.C. and Chen, C.R., “Riser VIV by a CFD Approach,” 17th International Offshore and Polar Engineering Conference, July 1-6, 2007, Lisbon, Portugal.
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Date: December, 2006Project Title: Deepwater Riser VIV/CFD Simulation of Riser VIV
MMS Project: 481 TO Number: 39774
Project PI: H. C. Chen, C. R. Chen and Richard Mercier
COTR: M. Else
Estimated Completion Date: 11/30/2007
Project Description:
Vortex-induced vibration (VIV) is an important issue in the design of deepwater riser systems, including drilling, production and export risers. The VIV can produce a high level of fatigue damage in a relatively short period of time for risers exposed to severe current environments. The wake interference between various risers in the same riser array may also lead to collisions between adjacent risers. Suppression devices, such as helical strakes or fairings may be needed to prevent unacceptable levels of fatigue damage. A computational fluid dynamics (CFD) code based on the Finite-Analytic Navier-Stokes (FANS) numerical method has been developed recently for the simulation of riser VIV responses at high Reynolds numbers under steady currents. The objective of this research is to complete CFD studies of the responses of deepwater risers to VIV to provide practical advice on: (1) riser response to VIV in various current profiles and water depths, and (2) effectiveness of VIV mitigation devices and strategies.The riser VIV responses are affected by many parameters including the Reynolds number, surface roughness, strakes, fairings, 3D sheared currents and ambient turbulence. In order to provide accurate analyses of the VIV phenomena, the Finite-Analytic Navier-Stokes (FANS) numerical method will be employed in conjunction with a chimera domain decomposition approach to investigate the complex deepwater riser VIV induced by various current profiles. The FANS method has been successfully used for VIV analysis of smooth and roughened risers in uniform currents with translational and rotational motions. In the proposed research, the method will be further extended to predict the responses of deepwater risers to VIV under various flow conditions such as loop currents, jets, and hurricanes. The effectiveness of VIV suppression devices such as helical strakes and fairings will also be evaluated. The simulation results will be compared with available experimental data to assess the accuracy of the CFD predictions.
Progress:
During this reporting period, we have investigated the performance of a generic helical strake with 3 protruding triangular shape strakes spaced 120 degrees from each other and having a spanwise pitch of 9 riser diameters. Numerical simulations were performed first for a fixed straked riser at a Reynolds number of Re = 105. The numerical results indicate that the helical strakes alter the flow separation along the span and prevent a single dominant shedding frequency from occurring, but the strake protrusions increase the average drag coefficient in comparison with the bare riser. Simulations were then performed for a low mass ratio and low damping straked riser under two degree-of-freedom VIV at Re = 105. The simulation results clearly demonstrated that the helical strakes are very effective in suppressing the vortex-induced vibrations with a dramatic reduction of VIV amplitude by more than 95%. The average drag force acting on the straked riser is also significantly lower than that for the bare riser due to the sharp reduction of relative velocity between the ocean current and the straked riser.For the simulation of flexible long risers, we have developed a finite-element model by solving a tensioned beam equation for the riser displacement and velocity. The riser motion solver was tested for both static and dynamic loading conditions and the results are in good agreement with those obtained from the commercial riser dynamics software Flexcom 3D. The riser motion program has been coupled with the FANS code for quasi-3D VIV simulations of a long riser with L/D = 100. Preliminary simulation results successfully demonstrated the feasibility of using the quasi-3D approach for time-domain simulation of VIV motions of long flexible risers.
Reports and Publications:
Pontaza, J.P. and Chen, H.C., “Three-Dimensional Numerical Simulations of Circular Cylinders Undergoing Two Degree-of-Freedom Vortex-Induced Vibrations,” ASME Journal of Offshore Mechanics and Arctic Engineering, to appear.Huang, K. and Chen, H.C., “Ultra Deepwater Riser Interference Analysis by Using a Time Domain Simulation Approach with VIV Effect,” Proceedings of D.O.T. XVIII (2006) Conference, Unlocking Deepwater Assets through Technology, November 28-30, 2006, Houston, Texas.
Progress Reports: June 2007 December 2006