Efficient Modal Decomposition and Reconstruction of Riser Response due to VIV

2011 ◽  
Author(s):  
S. I. McNeill ◽  
P. Agarwal
Keyword(s):  
Fatigue Damage ◽  
Traveling Wave ◽  
Angular Rate ◽  
Normal Modes ◽  
Structural Response ◽  
Measured Data ◽  
Vibration Monitoring ◽  
Wave Number ◽  
Modal Decomposition ◽  
Basis Vectors

Vortex-Induced-Vibrations (VIV) due to ocean currents can consume a sizable portion of the allotted fatigue life of marine risers. Vibration monitoring and concurrent estimation of fatigue damage due to VIV can significantly enhance the safe and reliable operation of risers. To this end, riser response can be characterized by using sensors (e.g. accelerometers and/or angular rate sensors) to measure the motion of the riser at a few locations. Fatigue damage can be predicted along the entire length of riser from measured data using the method of modal decomposition and reconstruction. In this method the structural response of interest, such as stress and fatigue damage, is expressed by modal superposition, where the modal weights are estimated using measured data and analytical modeshapes. However the accuracy of this method declines as the sensor density (number of sensors per unit riser length) decreases, especially when the riser vibrates in high-order modes and exhibits traveling wave behavior. In this paper, an efficient frequency-domain methodology allowing for accurate reconstruction of the riser response along the entire riser using a limited number of sensors is proposed. We first identify the excited VIV modes (natural frequency and modeshape) using principal vectors of the cross spectral density. Modal decomposition and reconstruction is performed separately for each VIV band surrounding each excited mode. This allows us to use several (as many as the number of sensors) participating modes in each band, and thus improve the accuracy. Since the stress distribution is sensitive to the chosen set of participating modes, we optimize over several candidate sets, selecting the set of modes that result in the lowest prediction error. In order to improve the reconstruction of complex modes, particularly traveling waves, the modeshapes can be augmented with additional basis vectors. The additional basis vectors are obtained by shifting the phase of the normal modes by 90 degrees at every wave number using the Hilbert transform. Though developed in the context of VIV, the method can be used to estimate fatigue damage due to vibrations regardless of the excitation mechanism. The methodology is demonstrated using the NDP (Norwegian Deepwater Program) test data on a 38 meter long slender riser, using data from eight accelerometers. Results show that the proposed algorithm can reconstruct stresses and fatigue damage accurately along the length of the riser in the presence of traveling wave behavior using relatively few sensors.

Spectral Formulations for Vortex Induced Vibration Modal Decomposition and Reconstruction

2012 ◽  
Vol 134 (4) ◽  
Author(s):  
S. I. McNeill
Keyword(s):  
Fatigue Damage ◽  
Angular Rate ◽  
Computational Cost ◽  
Time History ◽  
Computational Effort ◽  
Measured Data ◽  
Scale Model ◽  
Mode Shapes ◽  
Modal Decomposition ◽  
Vortex Induced Vibration

Modal decomposition and reconstruction (MDR) of marine riser vortex induced vibration (VIV) is a technique where vibration is measured using accelerometers and/or angular rate sensors, the modal displacements are solved for and the stress and fatigue damage is reconstructed along the riser. Recent developments have greatly increased the accuracy and reliability of the method. However the computational burden is onerous due to stress time history reconstruction and rainflow cycle counting at every desired location along the riser. In addition, fully synchronous data are required to reconstruct the stress histories. Dirlik’s method for obtaining rainflow damage for Gaussian random stress using only spectral information (four spectral automoments) has proven to be quite accurate with a significant reduction in computational effort. In this paper two spectral formulations of MDR are introduced. The first method is applicable when all the measured data are synchronous. In this method, spectral cross moments of the modal displacements are solved from the spectral cross moments of the measured data using basis vectors consisting of normal mode shapes. The spectral automoments of stress are obtained from the modal displacement cross moments and analytical stress mode shapes. Dirlik’s method is then applied to obtain rainflow damage. The second method is a generalization of the first, where the measured data cross moments are only partially known. This method is applicable when measured data are partially synchronous or asynchronous. A numerical root-finding technique is employed to solve for the modal response cross moments. The method then proceeds in the same manner as the first. The spectral methods are applied to simulated VIV data of a full-scale deepwater riser and to Norwegian Deepwater Program (NDP) scale-model test data on a 38 m long slender riser. Comparisons of reconstructed fatigue damage versus simulated or measured damage indicate that the method is capable of estimating fatigue damage accurately for Gaussian VIV even when data are not fully synchronous. It is also shown that computational cost is greatly reduced.

Real-Time Riser Fatigue Monitoring Routine: Architecture, Data and Results

2013 ◽  
Author(s):  
Scot McNeill ◽  
Puneet Agarwal ◽  
Dan Kluk ◽  
Kenneth Bhalla ◽  
Tomokazu Saruhashi ◽  
...  
Keyword(s):  
Real Time ◽  
Fatigue Damage ◽  
Water Depth ◽  
Angular Rate ◽  
Measured Data ◽  
Automated System ◽  
Time Data ◽  
Joint Rotation ◽  
Computational Environment ◽  
Fatigue Monitoring

Recently, the Modal Decomposition and Reconstruction (MDR) algorithm was developed to accurately estimate fatigue damage in marine risers based on measured acceleration and angular rates at several locations. The greatest benefit for drilling risers can be derived by incorporating the method in an online, fully automated system. In this way, fatigue damage estimates are available to the crew on the rig in real-time for risk quantification and mitigation. To this end, the MDR routine was implemented for online assessment of fatigue damage along the entire riser from acceleration and angular rate measurements at typically 5–10 elevations. This paper discusses the architecture, highlights some measured data and provides results for modes, stress and fatigue damage rate for the Chikyu drilling vessel during two scientific drilling campaigns. These campaigns occurred at the Shimokita site (1180-meter water depth) and the Nankai trough site (1939-meter water depth). To the authors’ knowledge, real-time fatigue monitoring of the entire riser has not been accomplished previously. Robust incorporation of the MDR algorithm into an online computational environment is detailed, including incorporation of top tension and mud weight data from the rig, detection and removal of data errors, and streamlined flow of the data through the computational modules. Subsequently, it is shown by example how the measured accelerations and angular rates are used to determine excited modes, participating modes, stress distribution and fatigue damage along the entire Chikyu drilling riser in an online setting. The technology highlighted advances riser integrity management two steps forward by first using measured data at 5–10 locations and the MDR algorithm to reconstruct stress and fatigue damage along the entire riser, and secondly integrating this approach into a fully automated, real-time computational environment. As a result, drilling engineers are empowered with a tool that provides real-time data on the integrity of the drilling riser, enabling informed decisions to be made in adverse current or wave conditions. Measured data also serves as a benchmark for analytical model calibration activities, reducing conservatism in stress and fatigue in future deployments. Furthermore, cumulative fatigue damage can be tracked in each riser joint, enabling more effective joint rotation and inspection programs.

Traveling Wave Response in Full-Scale Drilling Riser VIV Measurements

2011 ◽  
Author(s):  
Hayden Marcollo ◽  
Adrian Eassom ◽  
Emmanuel Fontaine ◽  
Michael Tognarelli ◽  
Pierre Beynet ◽  
...  
Keyword(s):  
Standing Wave ◽  
Traveling Wave ◽  
Angular Rate ◽  
Structural Response ◽  
Full Scale ◽  
Loop Current ◽  
The Novel ◽  
Novel Methods ◽  
High Level ◽  
First Time

The dominance of traveling wave VIV response is observed in full-scale measured drilling riser data for the first time. This paper presents the novel methods developed to identify the presence of traveling versus standing wave riser structural response in the full-scale data and the observations. This paper reports on some of the work conducted under the most recently completed phase of the DeepStar JIP (Phase 9). The paper uses four different novel methods to identify the presence of traveling versus standing wave structural response in data obtained from a full-scale Gulf of Mexico drilling riser during a loop current event. The techniques are: 1) Observation of RMS accelerations between synchronized accelerometers; 2) Observation of filtered displacements between synchronized accelerometers (using a new postprocessing synchronization technique); 3) Displacement versus angular rate phase diagrams; and 4) Derivation of upward/downward curvature components via an algorithm proposed by two of the co-authors. High level conclusions are drawn about the structural response types. Recommendations for future instrumentation campaigns are made.

Excitation of internal waves in a stably-stratified atmosphere with considerable wind-shear

1968 ◽  
Vol 32 (1) ◽  
pp. 145-171 ◽  
Author(s):  
A. A. Townsend
Keyword(s):  
Boundary Layer ◽  
Internal Waves ◽  
Wind Shear ◽  
Wave Packets ◽  
Normal Modes ◽  
Wave Number ◽  
Spectral Energy ◽  
Air Turbulence ◽  
Wave Numbers ◽  
Horizontal Wave

The rate of generation of internal waves by a thin turbulent boundary layer was calculated in a previous paper for a stably-stratified atmosphere with no significant wind-shear outside the boundary layer by considering the excitation of normal modes of wave propagation. By using the concept of wave-packets propagating upwards from the boundary layer, the effects of wind-shear can be included. Conditions for the validity of the approximation are given. In general, the spectral distribution of wave-energy at a particular height takes large values in two bands of horizontal wave-number, one band deriving from wave-packets undergoing internal reflexion near that height and the other from wave-packets of very small local frequency that accumulate there. The ‘reflexion’ wave-numbers are dominant if the wind increases with height and the ‘accumulation’ wave-numbers if the wind initially decreases with height. The spectral energy distributions and intensities of the wave-motion are discussed in more detail for an atmosphere of uniform stability and unidirectional wind-shear. The accumulation process may lead to instability or overturning of the waves, and estimates are made of the probable scale and intensity of the ‘clear-air’ turbulence produced. An interesting point is that the rate of energy loss from the boundary layer by radiation of internal waves turns out to be comparable with the rate of production in the outer nine-tenths of the layer, both for atmospheric boundary layers and for the surface layer of the ocean. It seems likely that radiation limits the layer thickness to some extent.

Real-time estimation of the structural response using limited measured data

2014 ◽  
Author(s):  
Hassan Sedarat ◽  
Iman Talebinejad ◽  
Abbas Emami-Naeini ◽  
David Falck ◽  
Gwendolyn van der Linden ◽  
...  
Keyword(s):  
Real Time ◽  
Structural Response ◽  
Time Estimation ◽  
Measured Data ◽  
Real Time Estimation

Mooring Chain Fatigue Assessment Methodology and Software of Williams Gulfstar 1 Spar Platform Using Comparison Between Field Measured Data and the Design Basis Information

2015 ◽  
Author(s):  
Young-Chan (Y. C.) Park ◽  
Arun Antony ◽  
Hisham Moideen ◽  
Arada Jamnongpipatkul ◽  
Jinsang Hwang ◽  
...  
Keyword(s):  
Fatigue Damage ◽  
Environmental Conditions ◽  
Measured Data ◽  
Design Parameters ◽  
Mooring Line ◽  
Graphic User Interface ◽  
Mooring System ◽  
Operating Life ◽  
Design Data ◽  
Mooring Chain

In 2014 Williams Companies delivered a Spar-based FPS to be used by the Hess Corporation for developing their “Tubular Bells” field as well as for future tiebacks for other developments. Tubular Bells is located in the Mississippi Canyon region of the Gulf of Mexico. The Spar hull is the ‘Classic hull form’, 584 ft (178 m) long, 85 ft (26 m) in diameter with 60 ft (18 m) hull freeboard. The mooring system consists of 9 mooring lines in 3 groups composed of a chain-polyester-chain configuration. This paper describes the tools and methodology Williams plans to use for assessing the fatigue damage of the mooring chain over the operating life of the platform. The basic plan is to collect field-measured data for parameters that impact mooring fatigue damage, such as: environmental conditions, Spar motions (particularly Vortex Induced Motions data) and mooring line tensions, then process that data with specialized software so it can be effectively compared to the applicable data that was used in the design of the mooring system for fatigue. A key element in the plan to compare the field data with the design data is new computer software developed specifically to process the field measured data and generate the field measured, fatigue design parameters on a regular basis and in a form that is readily comparable to the design data. Additionally, a graphic user interface in this software will provide a clear visualization of the Spar motions that is linked with the associated environmental conditions for the same time periods.

scholarly journals Dispersion relation analysis of turbulent magnetic field fluctuations in fast solar wind

2013 ◽  
Vol 31 (11) ◽  
pp. 1949-1955 ◽  
Author(s):  
C. Perschke ◽  
Y. Narita ◽  
S. P. Gary ◽  
U. Motschmann ◽  
K.-H. Glassmeier
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
High Speed ◽  
Energy Transport ◽  
Normal Modes ◽  
Frequency Dispersion ◽  
Dispersion Relations ◽  
Wave Number ◽  
Magnetic Fluctuations ◽  
Speed Solar Wind

Abstract. Physical processes of the energy transport in solar wind turbulence are a subject of intense studies, and different ideas exist to explain them. This manuscript describes the investigation of dispersion properties in short-wavelength magnetic turbulence during a rare high-speed solar wind event with a flow velocity of about 700 km s−1 using magnetic field and ion data from the Cluster spacecraft. Using the multi-point resonator technique, the dispersion relations (i.e., frequency versus wave-number values in the solar wind frame) of turbulent magnetic fluctuations with wave numbers near the inverse ion inertial length are determined. Three major results are shown: (1) the wave vectors are uniformly quasi-perpendicular to the mean magnetic field; (2) the fluctuations show a broad range of frequencies at wavelengths around the ion inertial length; and (3) the direction of propagation at the observed wavelengths is predominantly in the sunward direction. These results suggest the existence of high-frequency dispersion relations partly associated with normal modes on small scales. Therefore nonlinear energy cascade processes seem to be acting that are not described by wave–wave interactions.

scholarly journals Thermal-structural response and low-cycle fatigue damage of channel wall nozzle

2013 ◽  
Vol 26 (6) ◽  
pp. 1449-1458 ◽  
Author(s):  
Cheng Cheng ◽  
Yibai Wang ◽  
Yu Liu ◽  
Dawei Liu ◽  
Xingyu Lu
Keyword(s):  
Fatigue Damage ◽  
Channel Wall ◽  
Low Cycle Fatigue ◽  
Structural Response ◽  
Cycle Fatigue

Effects of Hull Flexibility on the Structural Dynamics of a Tension Leg Platform Floating Wind Turbine

2019 ◽  
Vol 142 (1) ◽  
Author(s):  
Carlos Eduardo Silva de Souza ◽  
Erin E. Bachynski
Keyword(s):  
Wind Turbine ◽  
Fatigue Damage ◽  
Structural Response ◽  
Wave Excitation ◽  
Short Term ◽  
Tension Leg Platform ◽  
Flexible Beams ◽  
Fatigue Damage Accumulation ◽  
Vertical Deformations ◽  
Motion Amplitude

Abstract Dynamic analysis of floating wind turbines often considers the hull as a rigid body. This paper explores the consequences of modeling the pontoons of a tension leg platform (TLP) wind turbine as flexible beams. The analysis is based on numerical simulations of free decays, structural response to wave excitation, and short-term fatigue damage accumulation at tower base and tendons. In addition, the importance of hydroelastic effects due to the pontoons’ vertical deformations is evaluated. Pontoon flexibility changed the platform natural periods and motion amplitude significantly, and the adoption of flexible pontoons reduced the predicted fatigue damage in the tower base and tendons. On the other hand, hydroelasticity had negligible consequences for motion and load responses considered here.

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