bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nonlinear Pitch Decay of a Floating Offshore Wind Turbine Structure
Authors: Thiagarajan K. P. , Urbina R. , Hsu W. 
Published: November 2013

Model tests were conducted on three generic floating wind turbine systems in 2011, and reported in a series of papers at OMAE 2012. These tests were conducted at the MARIN facility in the Netherlands, by a consortium of universities, government research organizations and industry. As part of the testing program, decay tests in platform pitch were conducted with and without wind. It was found that for spar and semi-submersible type structures, resonant pitch motion was damped due to wind in storm sea conditions.
The nonlinear decay motion of a floating wind turbine platform is modeled using a one degree-of-freedom nonlinear oscillation equation about a mean offset angle. Attention is paid to the turbine thrust coefficient and its variability with respect to oncoming flow speed, which in turn is affected by the structure pitch motion. The equation of motion reveals that the mean offset position has an important role in the stiffness, damping and consequently the natural period of pitch motion. Several important dimensionless parameters are introduced.
The paper discusses a simple thrust model for an offshore wind turbine based on rudiments of blade element theory. Using the simplified thrust coefficient formulation, the increase in platform pitch damping due to wind is formulated. Experimental data reported from prior tests described above show good agreement with the theoretical model.
A Mesh Dependency Study for the Mean Drift Forces by Pressure Integration
Authors: Pan Zhi Yuan , Vada Torgeir , Hanssen Finn-Christian Wickmann 
Published: November 2013

The convergence of drift force computation on a LNGC in shallow water is studied. It is found that the convergence of the result, especially for the pressure integration method, is largely dependent on the mesh quality. Furthermore, the irregular frequencies degenerate the results in the high frequency region. The LNGC analysis has numerical challenges related to the geometry and water depth. We also wanted to see if the convergence properties are due to this or if they are of more general nature.
To investigate this, an additional analysis was done for a Wigley ship in deep water. The convergence properties were found to be even poorer in this case. This shows that the convergence problem seems to be quite general. It is also found that the vertical components, which have to be evaluated by pressure integration, converge much better than the horizontal components. Hence all six components can be found using a moderately dense mesh by using the far field integration for the horizontal components and only the vertical components from the pressure integration.
Approach to Lateral Buckling and On-Bottom Stability Interaction Assessment
Authors: Anderson Benjamin , Shim Edwin , Zeitoun Hammam , Chin Eu Jeen 
Published: November 2013

Lateral buckling and on-bottom stability are often considered independently during pipeline design. However, it is expected that the interaction between lateral buckling and on-bottom stability will become an increasingly common and important pipeline design consideration; as projects move into higher temperature / higher pressure (HT/HP) design conditions in areas of cyclonic metocean environments. The interaction of lateral buckling and on-bottom stability occurs mainly at shallow water depths. This interaction is not expected in deeper water depths where the hydrodynamic loading from storms is less significant at the sea floor.
The main challenges faced in assessing lateral buckling and stability interaction include; bounding storm-induced horizontal out-of-straightness (HOOS), capturing the increased tendency to buckling due to lift and inline forces from hydrodynamic loading, and the post-buckle response at unplanned and planned initiation sites during a storm event.
In cases where pipeline absolute stability cannot be assured, storm-induced HOOS can be introduced into the pipeline. This HOOS can govern the buckling design; rather than HOOS due to pipelay, or vertical out-of-straightness (VOOS) due to bathymetric features. In addition, the lift and inline forces due to the storm loading increases the tendency for pipelines to buckle, and increases the likelihood of rogue buckle formation. Subsequently, storm loading may play a significant role in terms of initiation and response of uncontrolled lateral buckling, and therefore the extents of buckle control schemes (if required).
Furthermore, it is important to consider the response at planned initiation sites under storm loading. Severe storm loading can result in significant lateral displacement of planned buckles; affecting the strains and bending moments in the pipeline and influencing the buckling design.
This paper outlines the approach and knowledge gained from recent experiences in the design of pipelines subject to lateral buckling and severe cyclonic metocean conditions. The insight gained can be applied to other systems where the interaction of lateral buckling and on-bottom stability is of a concern. In order to perform appropriate assessments, Wood Group Kenny used a number of advanced finite element (FE) models which simulate buckle responses under various sea states for on-seabed and at snake bend location.
Time Domain Analysis Approach for Riser Vortex-Induced Vibration Based on Forced Vibration Test Data
Authors: Wang Kunpeng , Tang Wenyong , Xue Hongxiang 
Published: November 2013

As oil and gas exploration and production are pushed into deepwater area, the offshore industry is facing more challenges for riser vortex induces vibration (VIV). Although frequency domain approach has been widely used for the riser VIV prediction and fatigue design, several assumptions need to be made. In addition, frequency domain approach cannot account for the variable current and riser nonlinear boundary conditions, such as top boundary response, the interaction between riser and guides in the hull and soil-SCR interaction.
Considering above cases, several time domain codes have been developed for riser cross-flow (CF) VIV prediction. This paper presents a time domain approach based on forced algorithm. The exciting force is derived from the non-dimensional amplitude and frequency dependent lift coefficients from forced vibration test. The hydrodynamic damping model consists of empirical model and the extension of the lift curves. At each step, the displacement and velocity of each element would be obtained to calculate the response amplitude and frequency for the lift coefficient and damping. Expect for CF VIV, the mean drag force is also considered, which would be magnified by CF VIV.
The model test at Delta Flume of Delft Hydraulics is simulated using the proposed approach, and the CF VIV responses and the mean drag displacement are predicted. The results match well with the measured data.
CFD Analysis of a Unidirectional Axial Turbine for Twin Turbine Topology in OWC Plants
Authors: Pereiras Bruno , Valdez Pablo , Castro Francisco , Garrido Julio C. 
Published: November 2013

OWC devices are widely known among researchers in ocean energy. It is well-known that the efficiency of the device is closely related to the efficiency of the Power-Take-Off (PTO) which is usually a turbine. Traditionally, self-rectifying turbines are the most widely considered for working in an OWC because unidirectional turbines require a system of valves to rectify the flow. However, another option recently proposed is the use of the “twin turbine” configuration. This paper focuses on the performance of the turbines used in this configuration.
A numerical model has been developed and validated with data from the bibliography. This model has been used to analyze the flow field of the turbine when working in both performance modes: direct and reverse. Flow angles and loss distribution have been analyzed and interesting conclusions can be extracted.
Once the flow field has been analyzed, changes in the turbine geometry are proposed in order to improve the efficiency of the whole system by increasing the blockage made by the turbine in reverse mode. These changes, focused on the solidity of the rotor and guide vanes, were implemented and new simulations were carried out. The results obtained are the core of this work.
Study on Weld Fatigue Evaluation Under Sour Service Environment Using Battelle Structural Stress Method
Authors: Hong Jeong K. , Forte Thomas P. 
Published: November 2013

Risers, pipelines and flowlines for deep water applications are subject to corrosive environments. Especially, in the presence of hydrogen sulfide which makes the field sour, their fatigue performance becomes significantly degraded. In order to quantify the sour degradation effect, a knock-down factor has been introduced. This factor is defined as the fatigue life reduction relative to the in-air fatigue life.
Several sets of fatigue test results in sour service environments have been published. These include strip specimens of different sizes, e.g., diameters, wall thicknesses, and arc lengths. Naturally, the knock-down factor must be based upon a statistically valid number of fatigue test results obtained from the same specimen geometry and the same loading conditions tested in air and in sour conditions. Currently, the database available in the open literature is too limited to properly define a knock-down factor. Moreover, there is a great deal of scatter within the database and each test in a sour environment is costly and time consuming. Thus, it is difficult to establish a statistically valid database upon which to base the knock-down factor.
A mesh-insensitive structural stress method has been developed by Battelle researchers and has been proven to be highly effective in correlating the fatigue behavior of welded joints. In 2007, the Battelle structural stress based weld fatigue master S-N curve was included in ASME Section VIII Div. 2 because it successfully consolidated more than 800 fatigue test results for weld toe failures onto a single master S-N curve with very little scatter, regardless of specimen shape, size, loading type, and steel alloy [1–2].
A knock-down factor is derived by applying the Battelle structural stress method to the existing database for sour environment tests and by using the current in-air database as the reference condition. This approach will reduce the uncertainty in the knock-down factor because it allows a wider range of sour environment data from specimens of different sizes, types, and loading conditions to be combined, while simultaneously reducing scatter. As such, a unified knock-down factor can be determined with greater statistical validity and wider applicability for design recommendations in sour conditions.
State-Space Realization of the Wave-Radiation Force Within FAST
Authors: Duarte Tiago , Alves Marco , Jonkman Jason , Sarmento António 
Published: November 2013

Several methods have been proposed in the literature to find a state-space model for the wave-radiation forces. In this paper, we compared four methods, two in the frequency domain and two in the time domain. The frequency-response function and the impulse response of the resulting state-space models were compared against those derived from the numerical code WAMIT.
A new state-space module was implemented within FAST, an offshore wind turbine computer-aided engineering tool, and we compared the results against the previously implemented numerical convolution method. The results agreed between the two methods, with a significant reduction in required computational time when using the new state-space module.
Performance and Characteristics of Take-Off Power of a Vertical Axis Marine Turbine With Variable-Pitch Blades
Authors: Ikoma Tomoki , Masuda Koichi , Nakamura Yasunori , Rheem Chang-Kyu , Maeda Hisaaki 
Published: November 2013

This paper describes performances and characteristics of a vertical axis marine turbine (VAT) on which variable-pitch blades are equipped. Then, model experiments in a current channel and CFD calculations were carried out in this study. In the model testing, a variable-pitch type turbine model was used. Accuracy of the CFD computation is validated by comparing with experimental results of torque and number of rotation of the turbine model. The CFD calculation is three-dimensional as well as two-dimensional. The corresponding turbine model has three straight blades, which diameter is 0.3 m. Three kinds of tests were performed for model testing in the channel and CFD calculations as well. They were the starting-driving test, the braking test and the forced-rotation test in currents. The forced-rotation test was performed in still water as well in order to know a simple thrust and to estimate ideal efficiency of the turbine.
Form results of model experiments and numerical experiments, the CFD calculation was satisfactory to estimate performance of the turbine. The starting performance could be definitely improved by applying the variable-pitch system. For driving performance, it was very useful to control the pitch angle a little bit even in very fast velocity currents.
Protection of Subsea Pipelines Against Ice Ridge Gouging in Conditions of Substantial Surface Ice
Authors: Duplenskiy Stanislav , Gudmestad Ove Tobias 
Published: November 2013

The development of Arctic offshore hydrocarbon fields involves transportation systems for oil and gas, which comprise their shipping by tankers or pipeline systems. The later have sustained behavior with respect to hydrocarbons delivery and relatively non-sophisticated operational requirements.
However, some important challenges regarding Arctic conditions have to be assessed before the pipeline is constructed. Attention is given to the conditions of a specific hydrocarbon field off the Sakhalin Island, and the design of an offshore pipeline. The issue of the pipelines’ interaction with first year ice ridges is described, a design ridge is selected and two theoretical models for evaluation of the maximum scour depth produced by the ridge ploughing the seabed are introduced.
Thereafter a beam model of a pipeline exposed to bending and tension in terms of combined transverse and lateral loadings is established and analyzed. Based on the limit state design criteria the required trench features for different soil conditions for the safety of the pipeline are specified. Following the required pipeline failure probability, it is documented that the pipeline might be buried just below the probable scour depth if certain conditions of a “sandwich” backfilling with a weak soil layer on the bottom are met.
Conclusions summarizing the acquired findings provide reasonable recommendations for offshore pipeline design in the Arctic regions and suggest a scope for future works and studies.
Full Scale Thruster-Hull Interaction Improvement Revealed With CFD Analysis
Authors: Bulten Norbert , Stoltenkamp Petra 
Published: November 2013

The performance of two steerable thruster configurations has been compared: a more conventional straight unit and a tilted-shaft-line unit. Based on a theoretical approach, it has been shown that the full scale bollard pull performance of both units is close to the realistic optimum. The open water performance of the units revealed comparable performance at bollard pull for the straight and tilted unit. A more pronounced difference in favor of the straight unit is found in free sailing condition. When the thruster-hull interaction effects are taken into account, the trends change however. Some typical cases have been analyzed in detail with RANS-CFD numerical analysis methods: a drill ship in free sailing and bollard pull condition and a drill rig in bollard pull condition.
The thrust-deduction factors are reduced significantly for all analyzed cases when the tilted unit is compared to the straight unit. This leads to improved performance of the complete vessel. In order to capture the full potential of the tilted-shaft line thruster units, a detailed review of the currently used DP-capability calculation methods needs to be made. Besides the reduced thrust-deduction factors due to hull interaction, also the forbidden-zones due to thruster-thruster interaction need to be reviewed. The downward deflection of the jet in case of tilted units will reduce the zones of interaction with other thrusters.
Based on the currently presented research, it should be clear that the tilted thruster concept has a large potential for drill ships and drill rigs. A very good DP-capability will be achieved with the currently installed powers.
Vortex Induced Vibration of Circular Cylinder With Two Degrees of Freedom: Computational Fluid Dynamics vs. Reduced-Order Models
Authors: Wang Enhao , Xiao Qing , Srinil Narakorn , Zanganeh Hossein 
Published: November 2013

Computational fluid dynamics (CFD) studies capturing vortex-induced vibration (VIV) phenomena in a wide range of both the hydrodynamics and the structural parameters are important, because the analysis outcomes can be applied to numerical prediction codes, complement experimental measurement results and suggest a modification of some practical design guidelines. Nevertheless, in spite of many published studies on VIV, CFD studies for two dimensional coupled cross-flow/in-line VIV even with two degrees of freedom (2-DoF), are still quite limited. More CFD studies which can control the equivalence of system fluid-structure parameters in different directions with reduced uncertainty are needed to improve the numerical model empirical coefficients and capability to effectively match numerical predictions and experimental outcomes.
This paper presents a CFD study on the 2-DoF VIV of elastically mounted circular cylinder with a low mass ratio (m* = 2.55). The Reynolds number is fixed to be 150 and the reduced flow velocity parameter is varied by changing the cross-flow natural frequency. To model the problem, two-dimensional Navier-Stokes equations coupled with linear structural equations in the in-line and cross-flow directions are solved. Particular attention is paid to the determination of maximum attainable amplitudes and the associated instantaneous lift and drag forces and hydrodynamic coefficients. These results are compared with the obtained results from alternative numerical prediction outcomes using new reduced-order models with four nonlinearly coupled wake-structure oscillators (Srinil and Zanganeh, 2012). Some qualitative and quantitative aspects are discussed. Overall, the important VIV characteristics are captured including the dual-resonance and figure-of-eight trajectories. Through the flow visualization study, it is found that as the dual-resonance is excited, a P+S wake pattern appears.
Simulation of Hooking Event in Fish Trawling Operation
Authors: Wu Xiaopeng , Longva Vegard , Sævik Svein , Moan Torgeir 
Published: November 2013

Interference between trawl gears and subsea pipelines is an important issue. A special case called hooking, defined as the situation when the trawl gear gets “stuck” under the pipeline, should be a rarely occurring situation. In this case, however, the warp line tension could be high as its breaking strength. This may be detrimental with respect to both fishing vessel safety and pipeline integrity. This calls for a better understanding of the hooking phenomenon.
The goal of this study is to develop a proper numerical model to describe the hooking event. The proposed model is based on the finite element method. A special penalty-based contact element that includes the friction effect is utilized to deal with the trawl board and pipeline interaction. The trawl board and seabed (span shoulder) contact is also accounted for in order to simulate the hooking event. To validate the proposed model, numerical simulation results are compared with previous model test results. A rectangular type trawl board was selected as the target object. The pull-over cases with different span height were firstly tested and compared. Then, the hooking event set-ups were modeled.
Based on the model tests, there are two most likely scenarios for hooking: 1) a de-stabilized trawl board with small span height; 2) small crossing angle with large span height. The above two cases were both tested by the proposed model. In the first case, the trawl board is towed flat on the seabed. Permanent hooking was successfully obtained in the simulation. Then, two cases with small crossing angle were studied. Hooking event was reproduced in the case of a 20 degree crossing angle by introducing a disturbance on the trawl board. It shows that the proposed model could reproduce the hooking event, provided that the trawl board motion similar to the model test could be obtained. This gives a good basis for further studies.
Assessment of Effective Stiffness Formulation of Concrete Coated Rigid Pipeline
Authors: Chellakat Satyaraj Joe-Joe , John Manoj , Puthiyaparambath Vinod , Durairaj Senthilkumar 
Published: November 2013

Application of concrete coating on rigid pipelines alters the stiffness of the pipeline. Estimation of the effective stiffness of the concrete coated pipe is challenging as it depends on the diameter and thickness of the pipeline, thickness of the concrete coating, the corrosion coating and the cut back lengths.
Realistic estimation of the stiffness is essential in order to define the pipelay profile and for the estimation of pipeline stresses, which in turn governs the limiting weather window for the installation. This paper describes the effect of concrete stiffness of the rigid pipeline using three different methods of stiffness calculation, namely Mogbo, Lund and DNV methods.
Comparison of results from each of the method is presented with the aim of choosing the appropriate method for installation. The objective of this paper is to provide a better insight into the significant operational and economic influences these alternative approaches can have during the installation campaign.
Reassessment of Dynamically Stable Pipelines Accounting for Potential Clashing With New Assets
Authors: Carneiro Daniel , Powell Sabine , Timmins David , Bruzzo Pier 
Published: November 2013

Since mid-1980’s, guidelines permit the design of rigid subsea pipelines allowing significant lateral displacements under extreme environmental conditions. Such a design criterion might bring issues not foreseen in the original design when future assets are to be installed in close proximity — for example pipeline crossings, hot-tap tie-ins or parallel lines using the same corridor. Although not limited to any particular region of the globe, this issue has been showing increasing relevance in the Australian North West Shelf over the last few years. The combination of environmental and geotechnical conditions in that region has led to significant advances in engineering design techniques, and to audacious projects which made viable a number of important subsea pipelines which have been designed to be dynamically stable. As this oil and gas province matures, the installed infrastructure in the region forms part of an increasingly complex subsea network.
The design of new subsea assets close to, crossing, or tying into these dynamically stable pipelines require the reassessment of the existing infrastructure for potential clashing conditions. This paper discusses the techniques and integrity criteria available for reassessing dynamically stable pipelines whose originally expected displacements may be incompatible with new required infrastructure. Although the criteria for confirming the structural integrity is, and has to be, grounded on established design codes, the assessment often require advanced engineering and state-of-the-art analysis techniques.