Elizabeth Passano

Efficient Analysis of a Catenary Riser

The paper deals with the challenge of predicting the extreme response of catenary risers, a topic of both industry and academic interest. Large heave motions introduced at the upper end of a catenary riser can lead to compression and large bending moments in the region immediately above the touch down area. In the worst case, dynamic beam buckling may occur. The focus of the paper will be on understanding the riser behaviour in extreme, low-tension response and in establishing suitable analysis strategies to predict the extreme response. Results from long nonlinear stochastic simulations of many sea states with varying environmental and operating conditions may be combined to describe the long-term response of a nonlinear structure such as a catenary riser. However, this theoretically straight-forward approach is very demanding computationally and ways to limit the extent of nonlinear stochastic simulations are therefore sought. The usefulness of simpler methods such as regular wave analysis to improve understanding of the physical behaviour and to aid in concentrating the nonlinear simulations to where they are most useful, will be demonstrated.Copyright

VIV of Free Spanning Pipelines: Comparison of Response From Semi-Empirical Code to Model Tests

The purpose of this paper is to compare predictions of vortex-induced vibrations (VIV) from a semi-empirical program to experimental data. The data is taken from a VIV model test program of a free span pipeline using a long elastic pipe model. Both in-line (IL) and cross-flow (CF) vibrations are compared. The Norwegian Ormen Lange field development included pipelines laid on very uneven seafloors, resulting in many free spans. As part of the preparations for this field development, VIV model tests of single- and multi-span pipelines were carried out at MARINTEK for Norsk Hydro, which later became a part of Statoil. The VIVANA program is a semi-empirical frequency domain program based on the finite element method. The program was originally developed by MARINTEK and the Norwegian University of Science and Technology (NTNU) to predict cross-flow response due to VIV. The fluid-structure interaction in VIVANA is described using added mass, excitation and damping coefficients. Default curves are available or the user may input other data. VIVANA originally included only cross-flow excitation but pure in-line excitation was later added. Recently, simultaneous cross-flow and in-line excitation has also been included. At present, the excitation in the cross-flow and in-line directions is not coupled. Coefficients for simultaneous cross-flow and in-line excitation have been proposed and are available in VIVANA. In this paper, response predicted by VIVANA has been compared to the Ormen Lange model tests for selected test series. The analyses with pure IL loading gave good estimates of IL response up to and beyond the start of CF response. The analyses with combined CF and IL loading gave good response estimates for the test series with a long span. The experiments with short spans tended to give CF and IL mode 1 response while the present version of the program gave IL response at higher modes. The present coefficient based approach is, however, promising. Further work should aim at establishing better coefficients and to understanding the interaction between CF and IL response.Copyright

Extreme Bending Moments on Long Catenary Risers Due to Heave Excitation

This paper deals with the dynamic behaviour of catenary shaped risers under imposed motions applied at the top. Particular attention is paid to the heave component of motion which is of substantial importance for practical applications as it results to the amplification of large bending moments in the touch down region. In fact, the bending moment obtains its maximum value at the vicinity of the touch down point and very close to the location of its maximum static counterpart. This singular behaviour is discussed using the results from the solution of the eigenvalue problem. To this end the eigenfrequencies and the corresponding mode shapes are calculated using the WKB approximation making no assumption regarding the variation of the static components. In addition, the feature of the correlation between the axial component of the velocity of the excitation and the extreme bending moments at the lower part (Passano and Larsen, 2006) is further investigated through comparative numerical calculations of the problem using both frequency and time domain techniques.Copyright

Time and Frequency Domain Analysis of Catenary Risers Subjected to Vortex Induced Vibrations

Catenary risers in deep waters will experience conditions with insignificant wave forces in combination with strong current. The response will in such cases be dominated by vortex induced vibrations (VIV). Dynamic bending stresses will vary along the riser, but a large peak will almost always be seen near the touch down point. This peak is caused by the restrictions on riser displacements from the presence of the seafloor, and the local bending stresses will be influenced by stiffness and damping propertoes of the bottom. Analysis models based on finite elements will represent the interaction between riser and seafloor by discrete springs, which for the linear case will remain constant independent of the displacements. This type of model may give a significant over-prediction of bending stresses at the touch down point since a linear spring will give tensile forces instead of being released and allowing the pipe to lift off from the bottom. A non-linear time domain model will, however, account for changes by releasing springs if tension occurs and adding in new springs if free nodes obtain temporary contact with the bottom. The results will hence become far more realistic. Traditional empirical models for VIV prediction are based on a frequency domain dynamic analysis with constant stiffness. There is hence an obvious need for improvements when dealing with catenary risers. This paper will describe a new approach that is based on combined use of an empirical linear frequency domain model for VIV, and a non-linear model for time domain analysis. The first step is to carry out the VIV analysis according to linear response theory, and next introduce the calculated hydrodynamic forces to the non-linear structural model. The benefit from using the non-linear model is that stresses in the touch down area are described more accurately. A case study is also reported. Bottom stiffness and friction are varied, and results are compared to a simple model with a hinge at the touch down point. The conclusion is that the interaction between riser and seafloor is crucial for accurate stress prediction, and that a non-linear time domain model will give the most accurate result.Copyright

Estimating Distributions for Extreme Response of a Catenary Riser

The paper deals with the challenge of predicting the extreme response of catenary risers, a topic of both industry and academic interest. Large heave motions introduced at the upper end of a catenary riser can lead to compression and large bending moments in the region immediately above the touch down area. In the worst case, dynamic beam buckling may occur. Results from long nonlinear stochastic simulations of many seastates with varying environmental and operating conditions may be combined to describe the long-term response statistics of a nonlinear structure such as a catenary riser (Sodahl, 1991). However, this theoretically straight-forward approach is very demanding computationally and ways to limit the extent of nonlinear stochastic simulations are therefore sought. Previous work by the authors (Passano and Larsen 2006) focussed on understanding the riser behaviour in extreme, low-tension response and on establishing suitable strategies for concentrating the nonlinear simulations to where they are most useful. The research presented in this paper is a continuation of this work. The clear trends found between the prescribed axial velocity at the upper end and response quantities in the touch down area (TDA) are used to estimate individual response values and complete samples of response minima / maxima. A method for estimating the underlying response distributions is also presented.Copyright

Comparison of Calculated In-Line Vortex Induced Vibrations to Model Tests

The purpose of the present paper is to compare vortex-induced vibrations (VIV) in both in-line and cross-flow directions calculated by a semi-empirical computer program to experimental data. The experiments used are the Bearman and Chaplin experiments in which a model of a tensioned riser is partly exposed to current and partly in still water.The VIVANA program is a semi-empirical frequency domain program based on the finite element method. The program was developed by MARINTEK and the Norwegian University of Science and Technology (NTNU) to predict cross-flow response due to VIV. The fluid-structure interaction in VIVANA is described using added mass, excitation and damping coefficients. Later, curves for excitation, added mass and damping for pure in-line VIV response were added. These curves are valid for low current levels, before the onset of cross-flow VIV response.Recently, calculation of response from simultaneous cross-flow and in-line excitation has been included in VIVANA. The in-line response frequency is fixed at twice the cross-flow response frequency and the in-line added mass is adjusted so that this frequency becomes an eigenfrequency. A set of curves based on forces measured during combined cross-flow and inline motions are used. At present, the in-line excitation curves are not dependent on the cross-flow response amplitude.In the paper, in-line and cross-flow response predicted by VIVANA will be compared to the Bearman and Chaplin model tests. The choice of added mass and excitation coefficients will be discussed.Copyright

FREQUENCY AND TIME DOMAIN ANALYSIS OF VORTEX INDUCED VIBRATIONS FOR FREE SPAN PIPELINES

The present paper will discuss various models for calculation of vortex induced vibrations (VIV) of free span pipelines, and present a new strategy for such analyses. Applications of traditional models are presented and their limitations discussed. The new approach is based on the combination of an empirical linear frequency domain model, and a non-linear time domain structural model. The first step is to carry out the VIV analysis according to linear response theory, and next introduce the calculated hydrodynamic forces to the non-linear structural model. The benefit from using the non-linear model is to describe stresses at the shoulders more accurately, which is important since fatigue damage in many cases will be largest in this area. The conclusion is that the interaction between pipe and seafloor is crucial for accurate stress prediction, and that a non-linear time domain model will give the most accurate result.

On the Influence From Hydrodynamic Forces on Speed and Footprint for a Falling Riser

This paper presents analysis method and key results from dynamic simulations of a drilling riser on 1900 metres water depth after release of upper end. Key results are the geometry of the collapsed riser on the seafloor (footprint) and the impact speed of the riser when hitting the seafloor. The purpose of the study has been to investigate the influence on the results from operational and model parameters such as vessel offset relative to the riser base, current speed, hydrodynamic load model, material model and interaction between the riser and the seafloor. The main conclusion from the study is that most trends from parameter variations are weak and often overshadowed by a more stochastic variation caused by the inherent complexity of the mechanical behaviour during collapse.Copyright

IMPROVED IN-LINE VIV PREDICTION FOR COMBINED IN-LINE AND CROSS-FLOW VIV RESPONSES

Slender marine structures are subjected to ocean currents, which can cause vortex-induced vibrations (VIV). Accumulated damage due to VIV can shorten the fatigue life of marine structures, so it needs to be considered in the design and operation phase. VIV prediction tools are based on hydrodynamic coefficients, which are obtained from forced motion experiments on a circular cylinder. Most of the forced motion experiments apply harmonic motions in either in-line (IL) or cross-flow (CF) direction. Combined IL and CF forced motion experiments are also reported. However, measured motions from flexible pipe VIV tests contain higher order harmonic components, which have not yet been extensively studied. This paper presents results from conventional forced motion VIV experiments, but using measured motions taken from a flexible pipe undergoing VIV. The IL excitation coefficients were used by semi-empirical VIV prediction software VIVANA to perform combined IL and CF VIV calculation. The key IL results are compared with NDP flexible pipe model test results. By using present IL excitation coefficients, the prediction of IL responses for combined IL and CF VIV responses is improved. INTRODUCTION State-of-the-art VIV prediction tools are semi-empirical, which means that they are based on hydrodynamic coefficients obtained from experiments. Pure CF forced VIV experiments were done by [1] on a rigid circular cylinder at RRRR = 111144 . These excitation coefficients have been widely used to predict pure CF VIV responses and, in some cases, also used to predict the CF part of combined IL and CF VIV responses. Pure IL VIV forced VIV experiments were carried out at RRRR = 22.44 × 111144 [2]. Pure IL excitation coefficients obtained from these experiments have been used to predict pure IL VIV responses, but are not valid for prediction of the IL part of combined IL and CF responses. Forced motion tests with two dimensional harmonic motions have been carried out by [13]. Realistic orbits measured from a flexible beam VIV model tests were applied in forced motion VIV model tests [9]. This experimental method was first applied by [4], and further used by [9] and [10]. Both non-periodic time history of the motions and representative periodic motions were used in the experiments, see Figure 2. The hydrodynamic coefficients from the periodic motion tests were calculated and presented in [6]. The sensitivity of the hydrodynamic force and vortex shedding modes on the different realistic orbits were investigated in [7]. It was found that harmonic orbits had larger uncertainties to predict VIV than realistic orbits, and that IL motions can result in large higher order force components. Results from non-periodic and periodic forced motion VIV tests were compared in [8]. Depending on the response types, for quasi-periodic VIV responses, periodic orbits are representative for non-periodic time histories; while when the responses are partly or fully chaotic, the hydrodynamic coefficients calculated from tests with selected periodic orbits have larger uncertainty or fail to represent the entire time history. NOMENCLATURE AACCCC Amplitude in CF direction AAIIII Amplitude in IL direction AAyy Amplitude in y direction (CF) AA DD ⁄ Amplitude to diameter ratio CCDD Drag coefficient CCDD0 Drag coefficient of a fixed cylinder CF Cross-flow DD Diameter DOF Degree of freedom ffnn Natural frequency ffoooooo Oscillation frequency ffvv Vortex-shedding frequency

Vortex Induced Vibrations of Deep Water Risers: Sensitivity to Current Profile, Shear and Directionality

Slender offshore structures such as risers experience vortex induced vibrations (VIV) when they are exposed to currents and accumulate significant fatigue damage through that process. VIV will depend on several structural properties of the riser and on the current profile that the structure is exposed to. In deep water regions, risers will be subject to intricate circulation systems that impose currents profiles which may vary in intensity, shear and direction throughout the water column. The increased complexity of currents will make the prediction of VIV more difficult and represents a clear challenge to the Oil and Gas Industry. The objective of this study is to investigate how selected properties of a current profile affect the development and excitation of VIV for a deep water tensioned riser. We employ a semi-empirical frequency-domain program to perform a series of numerical sensitivity analyses where the riser model is subject to current profiles that vary in complexity and include uniform profiles, linearly-sheared profiles and more realistic profiles that represent offshore boundary current regimes from SE Brazil. We address the sensitivity of the VIV response to current intensity, shear and directionality. Our results demonstrate that those properties of the current profile have significant influence on the range of VIV modes that are excited and on the VIV response. Overall, uniform profiles produced the largest responses and the linearly-sheared profiles demonstrated the large range of VIV modes that can be excited. The realistic profiles also excited a broad range of VIV modes and variations between the profiles produced changes in the VIV response. This study highlights the need to further understand how complex current profiles in the offshore region affect VIV development in comparison to simpler profiles that are recurrent in model test conditions.Copyright