Efficient Computation of Irregular Wave Wire Stresses in Flexible Risers

2018 ◽  
Author(s):  
Gabriel Rombado ◽  
Nathan Cooke ◽  
Dharma Pasala ◽  
Xianglei Ni ◽  
Andrew Low ◽  
...  
Keyword(s):  
Fatigue Life ◽  
Dynamic Simulation ◽  
Irregular Waves ◽  
Local Model ◽  
Regular Wave ◽  
Flexible Riser ◽  
Irregular Wave ◽  
Wave Approach ◽  
Dynamic Substructuring ◽  
Time Histories

Accurate computation of tensile armor wire stresses remains a major challenge in flexible riser fatigue life predictions and integrity management. Accuracy of the results relies heavily on capturing the kinematics of the flexible’s helically contra-wound tensile armor layers and their interaction with the other metallic and thermo-plastic layers in a dynamic simulation. The standard industry practice to assess the fatigue life of flexibles is to use high fidelity 3D Finite Element Models (FEMs) to capture the complex kinematics and produce accurate stresses. However, direct simulation of flexible riser detailed FEMs is limited to regular wave analyses and computation of wire stress time-histories subjected to irregular waves have been computationally infeasible. This is due to the complexity of the nonlinear FEM and the long simulation time of the irregular wave environment coupled with large number of fatigue sea states. As a result, simplified approaches which do not directly simulate the local model and instead assume that wire stresses can be interpolated based on static stress versus curvature material curves within a pre-defined tension /pressure envelope have been utilized. This paper utilizes Nonlinear Dynamic Substructuring (NDS), a simulation-based approach that that extends the framework of dynamic substructuring to nonlinear problems. NDS enables the efficient nonlinear dynamic simulation of multiple pitch lengths of detailed flexible riser FEM subjected to irregular wave inputs and the computation of wire stress time-histories at any location on the local model. In this paper, a 14-inch diameter flexible riser under consideration by ExxonMobil is subjected to vessel motion and wave load in irregular wave environments and is modeled using a detailed 3D FEM and simulated via NDS. The flexible riser design features four tensile armor layers to mitigate localized lateral buckling of the wires near the touch down point. Tension and curvature time-histories of the riser near the hang-off, calculated from a conventional beam model global analysis, is used to drive a 5.1m long local model. Irregular wave wire stress time-histories extracted at the corners of the tensile armor wires are used to compute the fatigue life of the flexible. To demonstrate the inaccuracies associated with the regular wave approach, fatigue life is computed via the regular wave approach and compared against the irregular wave approach. It is shown that the NDS capability to efficiently compute irregular waves mitigates over- and under-predictions due to environment idealizations leading to a more accurate and reliable flexible riser life prediction and structural integrity assessment.

Methodology for Calculating Irregular Wave Stress Time Histories of Tensile Wires in Flexible Risers

2007 ◽  
Author(s):  
Krassimir Doynov ◽  
Christoffer Nilsen-Aas ◽  
Rune Haakonsen ◽  
Wan Kan ◽  
Robert Bjærum
Keyword(s):  
Time Domain ◽  
Transfer Functions ◽  
Stress Transfer ◽  
Regular Wave ◽  
Flexible Riser ◽  
Irregular Wave ◽  
Wave Approach ◽  
Stress Components ◽  
Time Histories ◽  
Flexible Risers

Flexible risers are being deployed in more and more demanding applications in terms of water depth, remote locations, temperature, pressure and corrosive fluids. Focus has been put on long term riser integrity in general, and on fatigue performance in particular, as knowledge of pipe behavior and properties has been advanced over the last decade. In this context, accurate and consistent estimation of riser global and local response to external loading is essential. A methodology has been developed to efficiently calculate irregular wave stress time histories of tensile armour wires for flexible risers. The stress time histories are calculated directly from the global loads which are usually generated by using commercially available well proven global analysis tools. The methodology elevates the dynamic analysis of flexible risers from the conventional regular-wave approach to irregular-wave time-domain approach. This in turn allows a better assessment of the fatigue performance and provides a better fit-for-service assessment or an opportunity to reduce design conservatism. This methodology also allows for consistent stochastic fatigue evaluations to be performed in time domain simulations using the well established stochastic analysis approach. All flexible riser non-linear hysteretic effects are included and phase shift between tension and curvature is also fully accounted for. The key ingredient lies in the generation of transfer functions of all stress components using a validated local analysis (LA) tool based on finite element method. This is done because direct use of the LA tool for long time domain simulations is very computationally intensive and impractical. The stress transfer functions allow direct mapping of the tension and curvature readings to individual stress components, which are combined in a phase consistent manner to obtain the total stress-time histories. This methodology should also work well for other systems having complicated cross sections such as dynamic umbilicals and integrated production bundle, etc. Accuracy of the proposed methodology should be equivalent to that of using the LA tool directly provided that the stress transfer functions are constructed appropriately. In comparison with the traditional regular-wave methodology, this irregular wave approach has been shown to provide a significant fatigue-life improvement for the flexible riser tensile-wire in a deep water West Africa application.

Irregular Wave Simulation and Fatigue Damage Evaluation of a Flexible Riser Subjected to Bi-Modal Sea States

2012 ◽  
Author(s):  
Zhimin Tan ◽  
Yucheng Hou ◽  
John Zhang ◽  
Terry Sheldrake
Keyword(s):  
Fatigue Damage ◽  
Wave Train ◽  
Time History ◽  
Regular Wave ◽  
Flexible Riser ◽  
Single Wave ◽  
Irregular Wave ◽  
Wave Approach ◽  
Wave Simulation ◽  
Wind Sea

This paper presents the fatigue evaluation of a flexible riser subjected to bi-modal sea states, where the local wind and swell conditions act simultaneously, and is observed in many offshore regions including Brazil and West Africa. Due to the irregularity of the riser responses, the traditional, regular wave approach for assessing the fatigue damage of a flexible pipe cannot be applied without significant simplifications. A typical deviation would be to treat the combined swell and wind conditions at sea as two sets of separate cases. The regular wave approach can then be applied and the summation of the damage of both cases defined as the final damage of the pipe. As an alternative, this paper presents a more theoretically accurate irregular wave approach. The entire irregular wave simulation was first performed using the commercial software, OrcaFlex™, together with a tensile wire stress model developed in-house. The model implements the pipe bending hysteresis behavior during dynamic excitation, producing corresponding time history stress results, which are used to assess the fatigue damage using a rain-flow counting method. Two case studies are presented, the first being a dynamic simulation performed with two wave trains generated based respectively on the given swell and wind sea spectrums. In the second case study, a single wave train is generated based on the combined spectrum of the swell and wind sea states. Both results are compared with those obtained by the traditional regular wave approach and a preferred analysis method is recommended based on the conservatism and time efficiency.

A Study for Statistical Characteristics of Riser Response in Global Dynamic Analysis With Irregular Wave

2014 ◽  
Author(s):  
Yanqiu Zhang ◽  
Zhimin Tan ◽  
Yucheng Hou ◽  
Jiabei Yuan
Keyword(s):  
Dynamic Simulation ◽  
Wave Train ◽  
Time History ◽  
Irregular Waves ◽  
Statistical Characteristics ◽  
Normal Process ◽  
Autocorrelation Functions ◽  
Irregular Wave ◽  
Global Dynamic ◽  
Time Histories

A study was conducted to have a deeper understanding to the statistical characteristics of response of flexible riser in global dynamic simulation with irregular wave. If consider the numerical simulation model as a system and the input wave train as an excitation to it, the time histories of riser load should be the response of the system to the excitation. In order to look the effect of riser configuration and water depth, the study was conducted with three kinds of configuration: Free-Hanging, Lazy-S and Tethered-Wave, which were in different water depths. In order to examine the stationarity and ergodicity of riser response, 100 simulations were performed. Each simulation was performed with a 3-hours-long storm. Except the seeds used to generate the random phases to the wave components, the 100 irregular wave processes for each riser are completely the same. When the number of wave components is enough large, the input irregular wave train should be a stationary normal process. Since the software used for the dynamic simulation is high nonlinear, however, the time history of riser response may not be perfectly stationary normal process. Then different probability distribution theories were applied to fit these time histories and the most fitting one was found out for different riser responses and for different riser configurations. The ensemble autocorrelation functions and the time autocorrelation functions were also examined for both irregular waves and the riser responses. Then the study indicated that both irregular waves and riser responses as random processes should be ergodic stationary. Finally the cross correlations between the irregular waves and riser responses were also examined and it was found that the irregular wave for each riser should be jointly stationary with each response of the riser.

Improving Operating Window for Disconnect Operations of CWO Risers

2010 ◽  
Author(s):  
Guttorm Gryto̸yr
Keyword(s):  
Rigid Body ◽  
Wave Height ◽  
Irregular Waves ◽  
Regular Wave ◽  
Irregular Wave ◽  
Wave Approach ◽  
Tension Level ◽  
Rigid Body Model ◽  
Industry Practice ◽  
Operating Window

The term ‘riser recoil’ refers to the situation when the lower end of a top tensioned riser is released, and the riser is lifted up by the riser tensioner and/or top motion compensator system on the supporting vessel. The elastic energy stored in the riser is then released, and the riser ‘recoils’. This paper focuses on the case of planned disconnect, and builds on ref. [1] which was based on a simplified riser analysis using a rigid body to represent the riser. In the present paper, the methodology has been applied to an elastic riser model in the riser analysis software RIFLEX, from MARINTEK in Trondheim, Norway, which includes axial damping elements required for modeling of the tensioner systems. Completion and Work Over (CWO) risers are unique in the sense that they may be simultaneously connected to both the riser tensioner system and the top motion compensator system of a drilling vessel. A Marine Drilling riser, on the other hand, is only connected to the riser tensioner system. Typically the riser tensioner system has a stroke of ± 8–9 m, whereas the top motion compensator system has only ± 3.5–4 m. It is imperative that the connector is lifted clear of the subsea structure in order to avoid damage to the equipment after the riser has been disconnected. The operating window for planned disconnect of CWO risers is severely limited by the available stroke of the top motion compensator. One of the purposes of the disconnect analysis is to establish the maximum wave height at which there is still sufficient clearance between the connector and the subsea structure after disconnect. Previous experience has shown that this may be the governing limitation for workover operations. The analysis may also establish a maximum tension level, and seastate, to avoid hard stroke-out of the top motion compensator cylinders. This requires an elastic riser model, since a rigid body will yield unphysically large impulse loads in case of stroke-out. The current industry practice is to use a regular wave approach in the analysis. In accordance with ref. [1], the present analysis is performed with irregular wave analyses. The results are documented through a case study of a typical CWO riser system connected to a semi-submersible in typical North Sea environmental conditions. The semi-submersible and the CWO riser system are exposed to irregular waves. Comparison of the resulting allowable wave height shows that using the approach presented here with an elastic riser model yields less conservative results than the previous methodology with a rigid body model. This should be coupled to the findings with the rigid riser model, ref. [1], that irregular waves yield a considerable increase in the operating window, and the resulting operability, compared to a regular wave analysis. Hence, using a regular wave approach combined with a simplified riser model that neglects the flexibility of the riser is expected to yield overly conservative results for the EQDP elevation after disconnect.

Methodology for Disconnect Analysis of CWO Risers in Random Seas

2009 ◽  
Author(s):  
Guttorm Grytoyr ◽  
Anne Marthine Rustad ◽  
Nils Sodahl ◽  
Per Christian Bunaes
Keyword(s):  
Wave Height ◽  
Irregular Waves ◽  
Regular Wave ◽  
Irregular Wave ◽  
Wave Approach ◽  
Random Seas ◽  
Extreme Response ◽  
Industry Practice ◽  
Operating Window

The term ‘riser recoil’ refers to the situation when the lower end of a top tensioned riser is released, and the riser is lifted up by the riser tensioner and/or top motion compensator system on the supporting vessel. The elastic energy stored in the riser is then released, and the riser ‘recoils’. This paper focuses on the case of planned disconnect. Recoil of Marine Drilling Risers has been the subject of several research papers over the past two decades. Some examples are listed in references [2] through [7]. Completion and Work Over (CWO) risers are unique in the sense that they may be simultaneously connected to both the riser tensioner system and the top motion compensator system of a drilling vessel. A Marine Drilling riser, on the other hand, is only connected to the riser tensioner system. Typically the riser tensioner system has a stroke of ± 8–9 m, whereas the top motion compensator system has only ± 3.5–4 m. It is imperative that the connector is lifted clear of the subsea structure in order to avoid damage to the equipment after the riser has been disconnected. The operating window for planned disconnect of CWO risers is severely limited by the available stroke of the top motion compensator. One of the purposes of the disconnect analysis is to establish the maximum wave height at which there is still sufficient clearance between the connector and the subsea structure after disconnect. Previous experience has shown that this may be the governing limitation for workover operations. The current industry practice is to use a regular wave approach in the analysis. The wave frequency is varied in order to find the maximum response, and hence one is actually searching for the extreme response, without paying attention to the probability that this will occur. In this paper a new method is presented, where the analysis is based on an irregular wave approach and the Monte Carlo technique, using time-domain simulations. Acceptance criteria are established based on a stochastic analysis, and are based on target levels of probability of exceedance. The results are documented through a case study of a typical CWO riser system connected to a semi-submersible in typical North Sea environmental conditions. The semi-submersible and the CWO riser system are exposed to both regular and irregular waves. Comparison of the resulting allowable wave height indicates that using the approach presented here with irregular waves will give a considerable increase in the operating window, and the resulting operability, compared to a regular wave analysis.

Alternative Methodologies for Subsea Flexible Strength Analysis

2018 ◽  
Author(s):  
Anskey A. Miranda ◽  
Fred P. Turner ◽  
Nigel Barltrop
Keyword(s):  
Real World ◽  
Wave Train ◽  
Irregular Waves ◽  
Wave Crest ◽  
Strength Analysis ◽  
New Wave ◽  
Regular Wave ◽  
Wave Analysis ◽  
Sea State ◽  
Irregular Wave

This paper presents a study of the analysis methodologies used to predict the most likely response of flexibles in a subsea environment, with the aim of determining an efficient and reliable prediction methodology. The most accurate method involves simulating multiple wave realisations of a real world sea state, i.e. irregular waves, and post-processing the results to determine the most probable maximum (MPM). Due to the computationally intensive nature of this approach, however, regular wave analysis is typically used to determine flexible response. This approach considers the maximum wave within a design storm at a desired period; the choice of periods may leave room for uncertainty in the conservatism of the approach. With proper screening, regular wave analysis can be a valid yet overly conservative approach resulting in over design and additional cost. However, if screened incorrectly, there is a possibility that the choice of periods could give results that are under conservative. In addition to regular wave analysis, the paper presents two alternative methodologies to determine the most likely response, with the focus on reducing the computational resources required. The first alternative is an ‘Irregular Wave Screen’ approach in which the wave train is screened at areas of interest for waves within a user defined threshold of the maximum wave height, in addition to other user defined parameters. Only waves within these parameters are simulated to determine responses. The second alternative is the ‘New Wave’ approach, which models the most probable wave elevation around the maximum wave crest. The calculated new wave is then placed at the desired location to determine responses. The responses of the Regular, Irregular Wave Screen and New Wave methodologies are compared with the Irregular MPM approach to determine their feasibility to predict the response of flexibles in a real world irregular sea state with lower computational requirements.

Effect of Local Model Dynamics on Flexible Riser Tensile Armor Wire Stress Predictions

2019 ◽  
Author(s):  
Gabriel Rombado ◽  
Krassimir Doynov ◽  
Nathan Cooke ◽  
Arya Majed
Keyword(s):  
Local Model ◽  
Local Analysis ◽  
High Fidelity ◽  
Reduced Order Models ◽  
Flexible Riser ◽  
3D Fem ◽  
Reduced Order ◽  
Irregular Wave ◽  
The Impact ◽  
Stress Hysteresis

Abstract Accurate time-consistent computation of tensile armor wire stresses remains a major challenge in flexible riser fatigue life predictions and integrity management. Accuracy requires capturing the kinematics of the flexible’s helically contra-wound tensile armor layers and their interaction with the other metallic and thermo-plastic layers in a dynamic simulation. It is generally accepted that high fidelity 3D Finite Element Models (FEMs) can best capture the complex kinematics and produce accurate stresses. The local model is typically constructed of few “pitch lengths” of the 3D FEM. Local analysis involves enforcing tension and nodal rotation time-histories on the local model and extracting wire stresses at critical fatigue locations along risers. While local analysis involving a few bending cycles can be executed on modern multi-core computers, static simulations typically require computation times of 24–48 hours for a single cycle and do not capture the effect of dynamics of the local model. With this computational constraint, 1-hr long irregular wave fatigue simulations with 3D FEM local model become computationally infeasible. The nonlinear dynamic substructuring (NDS) approach has been utilized in the past to overcome this computation challenge. Reduced order models are numerical methods for efficiently solving high fidelity FEM. NDS utilizes reduced-order models and numerical algorithms to significantly decrease the computation time associated with the irregular wave fatigue simulations of the high fidelity flexible FEM. Because NDS is a simulation-based approach, effects such as local model tension stiffening and inertial resistance to the global rotation inputs are fully captured and the impact on wire stresses can be discerned. A 14” inner diameter (ID) flexible riser with a four-tensile armor layer configuration is modeled and simulated using the NDS approach. The 5m long local model is first driven at different “speeds” of harmonic (regular wave) rotation inputs to illustrate inertial effects. For the faster input, the impact of local model inertia on wire stresses is immediately apparent by the increase in wire stresses and change in the shape of the wire stress hysteresis curve. Next, the local model is simulated to irregular wave inputs. It is again shown that the inclusion of local model inertia increases wire stresses and modifies the shape of the wire stress hysteresis.

Time Domain Simulation of the 3D Bending Hysteresis Behavior of an Unbonded Flexible Riser

2009 ◽  
Vol 131 (3) ◽  
Author(s):  
Zhimin Tan ◽  
Peter Quiggin ◽  
Terry Sheldrake
Keyword(s):  
Dynamic Simulation ◽  
Time Domain ◽  
Bending Moment ◽  
Dynamic Responses ◽  
Life Assessment ◽  
Flexible Riser ◽  
Detailed Model ◽  
Hysteresis Behavior ◽  
Wave Approach ◽  
The Difference

This paper presents a “state-of-the-art” development in time domain dynamic simulation of 3D bending hysteresis behavior of a flexible riser under offshore environment loading. The main technical challenge is to understand and model the riser tensile armor behavior under continuous changes in both the magnitude and direction of bending, and its subsequent impact on the riser’s bending hysteresis characteristics. On account of this technical obstacle, the current industry practice is to model the riser as a linear structure, with certain conservatism enforced, and then to extract the global dynamic loads to a detailed local model for stress and life assessment. Two 3D flexible riser bending hysteresis models developed by Wellstream and Orcina are introduced in this paper, with their calibrations against the bending hysteresis loops measured in full scale tests. Both models are implemented using the analysis program ORCAFLEX. The Wellstream model is a detailed model that calculates both the total bending moment and the stresses in the tensile armor, whereas the Orcina model is a simpler model that only calculates the total bending moment. The study presented illustrates the difference in riser dynamic responses with and without consideration of the bending hysteresis behavior and assesses the difference in dynamic responses between the Wellstream and Orcina 3D bending hysteresis models. This development permits the modeling of more realistic riser structural properties in the dynamic simulation and reports detailed time history stress or strain results for strength components of the riser, and so expands the current practice of riser fatigue analysis, which uses the regular wave approach only, to using an irregular wave approach employing the rainflow counting method.

Flexible Riser Fatigue Re-Evaluation to Extend the Service Life

2008 ◽  
Author(s):  
Carlos A. D. Lemos ◽  
Fernando J. M. Sousa ◽  
Jose´ R. M. Sousa
Keyword(s):  
Fatigue Life ◽  
Service Life ◽  
Phase Difference ◽  
Coupled Analysis ◽  
Wave Analysis ◽  
Flexible Riser ◽  
Irregular Wave ◽  
Deep Waters ◽  
Special Procedure ◽  
Flexible Risers

Some PETROBRAS fields are near mature now, around 15 years of production, and their production still important to the company portfolio, the possibility of extending the service life of these flexible risers becomes extremely attractive. This work addresses the re-evaluation of the fatigue life of old flexible risers aiming to extend their fatigue life at the same environment conditions or at new and less challenging ones. To fulfill this condition a special procedure is being applied to stretch the service life of the installed flexible risers, considering irregular wave analysis conditions, distributions of damage around the circumference and along the bend stiffener area and phase difference between tension and bending and in some cases a coupled analysis of the ship, mooring and risers systems. This kind of new fatigue procedure could also become of paramount importance to Petrobras to allow the design of conventional flexible risers for ultra deep waters.

Statistical Assessment and Validation of Experimental and Computational Ship Response in Irregular Waves

2018 ◽  
Vol 3 (2) ◽  
Author(s):  
Matteo Diez ◽  
Riccardo Broglia ◽  
Danilo Durante ◽  
Angelo Olivieri ◽  
Emilio F. Campana ◽  
...  
Keyword(s):  
Fluid Dynamics ◽  
Degrees Of Freedom ◽  
Bootstrap Method ◽  
Irregular Waves ◽  
Vertical Acceleration ◽  
Block Bootstrap ◽  
Regular Wave ◽  
Bootstrap Methods ◽  
Irregular Wave ◽  
Head Waves

The objective of this work is to provide and use both experimental fluid dynamics (EFD) data and computational fluid dynamics (CFD) results to validate a regular-wave uncertainty quantification (UQ) model of ship response in irregular waves, based on a set of stochastic regular waves with variable frequency. As a secondary objective, preliminary statistical studies are required to assess EFD and CFD irregular wave errors and uncertainties versus theoretical values and evaluate EFD and CFD resistance and motions uncertainties and, in the latter case, errors versus EFD values. UQ methods include analysis of the autocovariance matrix and block-bootstrap of time series values (primary variable). Additionally, the height (secondary variable) associated with the mean-crossing period is assessed by the bootstrap method. Errors and confidence intervals of statistical estimators are used to define validation criteria. The application is a two-degrees-of-freedom (heave and pitch) towed Delft catamaran with a length between perpendiculars equal to 3 m (scale factor equal to 33), sailing at Froude number equal to 0.425 in head waves at scaled sea state 5. Validation variables are x-force, heave and pitch motions, vertical acceleration of bridge, and vertical velocity of flight deck. Autocovariance and block-bootstrap methods for primary variables provide consistent and complementary results; the autocovariance is used to assess the uncertainty associated with expected values and standard deviations and is able to identify undesired self-repetition in the irregular wave signal; block-bootstrap methods are used to assess additional statistical estimators such as mode and quantiles. Secondary variables are used for an additional assessment of the quality of experimental and simulation data as they are generally more difficult to model and predict than primary variables. Finally, the regular wave UQ model provides a good approximation of the desired irregular wave statistics, with average errors smaller than 5% and validation uncertainties close to 10%.

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