bluuebunny/crossref_metadata_2025 huggingface dataset:
Human and Organizational Factors in Reliability and Design of ‘Minimum’ Offshore Structures
Authors: Bea Robert G.
Published: March 2009

‘Minimum’ offshore structures represent a class of structures that have been optimized to deliver minimum Capital Expenditures. This paper summarizes a part of the work done for a Joint Industry –Government sponsored project to study the reliability characteristics of minimum offshore structures for the North Sea [1 ]. Three minimum structures were compared with a conventional 4-leg jacket structure. These structures all were designed for specific conditions at a site in the North Sea using current design and engineering criteria appropriate for these structures. The reliability characteristics of these structures were evaluated for in-place operating conditions considering strength — extreme storm conditions, fatigue conditions, and accidental – boat collision conditions. This paper addresses and summarizes the reliability characteristics of these structures evaluated for defect and damage states due to Human and Organizational Factors (HOF) that developed during design, construction, maintenance, and operation. The HOF based defect and damage states were based on information gathered from designers, builders, and owner – operators of similar structures and from databases that contain information on HOF based defects and damage in offshore structures. An advanced reliability formulation was developed and applied that allowed integration of the intrinsic (natural) and extrinsic (HOF) hazards. This paper summarizes the advanced reliability formulation that was applied, the processes used to define and evaluate the HOF defect and damage scenarios, the results from evaluations of the scenarios with conventional and advanced QA/QC (Quality Assurance/Quality Control) processes, and the results of platform structure system analyses with and without recognition of the HOF effects.
Changes in Tensile Properties Due to Cold Bending of Line Pipes
Authors: Fukuda Naoki , Yatabe Hiroshi , Masuda Tomoki , Toyoda Masao
Published: March 2009

The changes in the tensile properties of line pipes due to cold bending were experimentally and analytically investigated. Full-scale cold bending experiments were performed on API X60 and X80 grade line pipes. The reduction in the yield stress of the cold bends due to the Bauschinger effect was approximately 20% and 35% for X60 and X80, respectively. In order to evaluate the changes in the tensile properties of the pipes quantitatively, finite element (FE) analyses and small-scale experiments were conducted. The FE analytical model for simulating the strain distribution at various bending angles was verified with the results of the full-scale experiments. The tensile properties of the cold bends were in good agreement with those of the small-scale experiments using uni-axially prestrained specimens. Based on the present results, an estimation method was proposed for evaluating the distribution of the tensile properties after cold bending with the analytical model using the results of the tensile tests for prestrained specimens.
The Concept of Virtual Penetration and Its Application in Risers With Multiple Contacts
Authors: Li Yousun
Published: March 2009

Marine risers may involve multiple contacts, such as a marine riser consisting of centralized multi-tubes, and a riser with a trumpet bending restrainers. An algorithm has been developed specially for the time-domain simulation of the riser motions with a large number of intermittent contacts. In this algorithm, the contact constrains are lifted, and a pair of contact forces for each contact is added to the applied loads in the global equilibrium equations. Each pair of contact loads are either large enough to keep the wall penetration within a specified tolerance, or vanish when the contact pair is separated. These contact loads can be obtained by iterations, based on a concept called “viavirtual penetrationl”. Through a dual casing riser as an example, the time records of the riser motions are simulated by the algorithm of virtual penetration and by modeling contacts as nonlinear springs. These two results are in good agreement. However, the algorithm developed herein can handle much more intermittent contacts than other methods.
A Wave Maker With Active Reflected Wave Compensation System
Authors: Nascimento Fa´bio , Levi Carlos , Fernandes Antonio C. , de Tarso Esperanc¸a Paulo , Gomes Paulo Sergio
Published: March 2009

Important aspects in the studies to assess the dynamic behavior of ocean vessels or structures, like ships or offshore oil platforms is the capability of generating gravity waves under strict laboratory control. Model test techniques are continuously improving and are very much dependent on the good quality waves that could be generated in a basin. Since ocean basins have finite dimensions, the waves reflected by the models, walls and even to some extent by the beaches, may become a critical issue if you need to guarantee accuracy and reliability for the tests. Besides the undesirable pattern of reflected waves within the test area of the basin, these waves come back onto the wave maker, affecting the correct properties of the wave to be generated. Modern wave generator apparatuses are now being equipped with real time control systems that enable them to generate an irregular wave pattern. At the same time they correct their flap motions to compensate re-reflection of waves from the wave-boards. The quality of such a system depends very much on the efficiency of the algorithm to be implemented in it. This paper discusses the development of an effective mathematical model of a control system used in an irregular wave maker–hinged flap type, featuring active wave reflection compensation. An efficient real time algorithm has been selected to run the control system device. The system is able to generate first order irregular waves and detect reflected waves that approach the wave maker by means of wave probes mounted on the face of the flap. The probe registers the input data to be used by the actuator to compensate the incoming wave by controlling the flap motion. Computer simulations obtained for a wave-maker in a flume are used to demonstrate the efficiency of each step of the theory and the overall accuracy of the compensation system.
Wave Hindcast in the Eastern Mediterranean Sea
Authors: Eldeberky Y. , Metwally A. , Rakha K. , Cavaleri L.
Published: March 2009

Wave hindcast is widely used to assess the wave conditions in both offshore and nearshore regions. A third generation ocean wind-wave model is implemented to hindcast the wave climate in the eastern Mediterranean Sea offshore Egypt coastline. The effects of the spatial grid resolution, the model physics, and input wind fields are examined using results of wave hindcast. Model results of wave hindcast during two selected storms are compared against wave observations offshore the Egyptian and Italian coasts as well as remotely sensed wave height data recorded by TOPEX satellite in the eastern Mediterranean Sea. Sensitivity computations are performed to confirm that a spatial grid with 0.5° resolution is adequate for assessment of wave conditions in the open sea (deep water). In intermediate water near the coast, a spatial grid with 0.25° resolution is required to take into account the local bathymetry and coastal details. The model-data comparisons recommend the use of Tolman and Chalikov (1996) parameterizations, for the wave input and dissipation terms, and the NCEP wind fields for wave hindcast in the eastern Mediterranean Sea.
Application of Composites to Deepwater Top Tensioned Riser Systems
Authors: Saad Pascinthe , Salama Mamdouh M. , Jahnsen Ove
Published: March 2009

As offshore exploration and production activities head to deeper water, extensive efforts have been focused on mitigating the potential challenges associated with deep- and ultra deep-water riser systems. Such challenges include overcoming the hydrostatic stresses associated with the increased length of water column as well as the increase in overall riser system weight. The implementation of composites in offshore applications is perceived as a promising path forward with composite materials offering many advantages including high specific strength and stiffness, lighter weight, enhanced corrosion resistance, high thermal insulation, improved structural damping and favorable fatigue performance characteristics. This paper focuses on evaluating the potential of composite materials for applications to deepwater top tensioned riser systems from the standpoint of possible impact on overall system cost and reliability. Many deepwater development concepts utilize top tensioned production riser systems, mainly, for conveying production fluids from the wellhead to the surface processing facilities in a dry tree based field development solution. Top tensioned risers can be configured as dual or single barrier systems and can either be hydraulically or hydro-pneumatically supported as on a typical TLP system or pneumatically supported through positively buoyant air cans as on a Spar riser system. Many advantages can be derived from the use of composites on buoyancy can systems. Such advantages include a reduced system weight, higher net lift, smaller diameter cans, improved efficiency and a positive impact on required inspection frequency. Main advantages of using composites for an entire TLP riser system include a significant reduction in both tension requirement and tensioner weight resulting in substantial reduction in total deck loads. An overall assessment of the economic and structural impact of using composites as an alternative to steel will be presented for riser systems and riser system components of both deepwater development concepts.
Influence of Heave Plate Geometry on the Heave Response of Classic Spars
Authors: Thiagarajan Krish P. , Datta Indra , Ran Alex Z. , Tao Longbin , Halkyard John E.
Published: March 2009

A production spar designed for West African (WA) offshore conditions must consider possible resonance with long period swell, which might result in large amplitude heave oscillations. Preliminary study of a classic spar with diameter of 39 m (128 ft) and draft 198 m (650 ft) for a WA application led the authors to believe that excessive heave response of 5.2 m (17 ft) may occur at the natural period of 28 seconds. This led the team to investigate the possibility of adding a heave plate (circular disk) at the base of the spar to control the response to within 3.1 m (10 ft), which is the limit set by a typical compensation system. Important design issues arose with regards to the geometry of the plate, i.e. diameter and thickness. Numerical simulations and model testing were used to identify the influence of a heave plate on the heave response of the spar. Heave response for various diameters and thickness were investigated. Comparison of added mass and damping values were found to be in reasonable agreement. Issues such as effect of a centerwell and moorings, plate cutouts for ease of transportation were also investigated. Discussion of the experimental results and comparison with numerical simulations are presented in this paper, and some recommendations are made on optimum heave plate geometry.
Reliability Based Design Optimisation of a “No Burst” High Pressure Pipeline
Authors: Stewart Graham , Roberts Caroline , Matheson Ian , Carr Malcolm
Published: March 2009

The design philosophy of a pressure-protected subsea pipeline is intimately linked to the reliability of the Pressure Protection System (PPS) and to the probability of burst of the pipeline if it is exposed to full wellhead shut-in pressure. A reliability based design approach is presented that allows the pipeline wall thickness (and cost) to be reduced under the philosophy that the pipeline will “not burst” in the event of PPS failure. This paper describes how uncertainties in the pipeline design parameters may be initially modelled statistically to allow structural reliability techniques to be adopted at the design stage (before the pipe is manufactured). It further addresses how correlation of these parameters can be included and their extreme value distributions developed, which is particularly relevant as the length of the tieback increases. A method to incorporate inspection inaccuracy is also presented. The initial estimates of the design parameters necessarily err on the conservative side. These can be later updated when manufacturing data is available.
Intercomparison of Seastate and Zerocrossing Parameters From the WACSIS Field Experiment and Interpretation Using Video Evidence
Authors: Barstow Stephen F. , Krogstad Harald E. , Lo̸nseth Lasse , Mathisen Jan Petter , Mo̸rk Gunnar , Schjo̸lberg Peter
Published: March 2009

During the WACSIS field experiment, wave elevation time series data were collected over the period December 1997 to May 1998 on and near the Meetpost Nordwijk platform off the coast of the Netherlands from an EMI laser, a Saab radar, a Baylor Wave Staff, a Vlissingen step gauge, a Marex radar and a Directional Waverider. This paper reports and interprets, with the help of simultaneous dual video recordings of the ocean surface, an intercomparison of both single wave and sea state wave parameters.
Task-Related Rogue Waves Embedded in Extreme Seas
Authors: Clauss Gu¨nther F.
Published: March 2009

This paper describes a new technique for the generation of tailored design wave sequences in extreme seas which are simulated under laboratory conditions. The wave field is fitted to predetermined global and local target characteristics defined in terms of significant wave height, peak period as well as wave height, crest height, and period of individual waves. The generation procedure is based on two steps: Firstly, a linear approximation of the desired wave train is computed by a Sequential Quadratic Programming (SQP) method which optimises an initially random phase spectrum for a given variance spectrum. The wave board motion derived from this initial guess serves as starting point for directly fitting the physical wave train to the target parameters. The subplex method developed by Rowan (1990) is applied to improve systematically a certain time frame of the wave board motion which is responsible for the evolution of the design wave sequence. The discrete wavelet transformation is introduced to reduce significantly the number of free variables to be considered in the fitting problem. Wavelet analysis allows to localise efficiently the relevant information of the electrical control signal of the wave maker in time and frequency scale. Nonlinear free surface effects, even wave breaking are included in the fitting process since the simulation of the physical wave evolution under laboratory conditions is an integral part of the new technique. This feature is especially important for simulating experimentally wave/structure interactions in rogue waves and critical wave groups. As an illustration of this technique the Draupner “New Year Wave” is simulated and generated in a physical model wave tank. Also a “Three Sisters” wave sequence with succeeding wave heights Hs … 2Hs … Hs, embedded in an extreme sea, is synthesised.
On the Model Scaling of Polyester Mooring Lines for Offshore Applications
Authors: Fernandes Antonio Carlos , Rossi Ronaldo Rosa
Published: March 2009

With the introduction of the polyester ropes as mooring lines of large systems such as semi-submersibles, the need to simulate these lines in model tests became a necessity. Although the non-linear behavior is clear, depending on the type of cycling, the polyester rope responds in ways that may be considered linear as a steel wire rope. Because of that, the early model tests have been performed using a linear restoring capability, with different restoring coefficients. The use of equivalent springs seemed the proper way. However, with the help of fundamental investigation on the similarity laws, the present work shows that the use of very thin polyester lines in model scaling is feasible and will indeed allow a closer physical representation. By avoid using springs, but using the same material as in full scale, the same non-linear behavior is present during the tests and even the response to random excitation due to random waves is better represented. The paper closely describes the application of these ideas in a model test of a FPSO (Floating Production Storage and Offloading) comparing both the linear springs and new approach with the model scale equivalent polyester line.
Strength Reduction Factor for Tubular Joints Under Combined Loading
Authors: Saajedi M. A. , Wang J. , English R.
Published: March 2009

Tubular joints are used in a frame structure such as an offshore jacket and are under combined loading in normal service condition. In the design of such frame structures, selecting safety factor (FS) when determining dimensions for the joints generally depends on many factors such as previous experiences, material properties, fatigue performance analysis and history, etc. In this paper, the authors intend to show that understanding the interaction between the magnitude, direction, and combination of loading on the brace and chord would contribute to prediction of the strength of the tubular joint with a higher accuracy. Thus the ratio between the loading on the brace and chord could be used in determining the strength of the joint to a higher accuracy. The phenomenon of strength reduction for a tubular joint under combined loading in comparison with tubular joints under just single loading shall be explored and a definition for this phenomenon would be introduced. Hence, a more accurate FS for the design of tubular joints under known combined loading can be selected which in turn would potentially be smaller than the FS presented using the general FS selection method without compromising reliability and safety of the structure. By reducing the FS in this manner, many benefits would be potentially gained such as: 1. Reduced dimensions, hence reduced materials in the construction of the joints; hence easier fabrication. 2. Less potential deficiencies in the construction of members. 3. Lighter and more slender structures. 4. Potentially reduction in fabrication time and cost. This paper principally introduces and defines Strength Reduction Factor (QSRF) for Tubular Joints under Combined Loading Condition.
Predicting Ice Jams With Discriminant Function Analysis
Authors: White Kathleen D. , Daly Steven F.
Published: March 2009

Breakup ice jam prediction methods are desirable to provide early warning and allow rapid, effective ice jam mitigation due to the suddenness with which breakup jams and related flooding occur. However, prediction models are limited to empirical or stochastic models rather than deterministic models because of the difficulties in using deterministic models to forecast the formation of breakup ice jams. Existing ice jam prediction methods range from empirical single-variable threshold-type analyses to statistical methods such as logistic regression and discriminant function analysis. Empirical methods are highly site-specific and tend to over predict jam occurrence. In addition, existing models do not provide quantitative information regarding the risk of errors in prediction, which limits their usefulness in emergency situations. In this paper, existing methods are reviewed and a three-step process to predict breakup ice jams is proposed.
Application of Finite Element Modelling in the Qualification of Large Diameter Unbonded Flexible Risers
Authors: Zhang Wenchao , Tuohy Justin
Published: March 2009

Unbonded flexible pipe has a proven track record in the offshore oil and gas industry for more than 20 years. The product is synonymous with the use of floating production systems spanning the water column and connecting subsea structures to facilitate the retrieval of hydrocarbons, provision of water injection systems and the export of processed or semi-processed fluids to main trunk pipelines or onshore. Unbonded Flexible pipe is a technically complex multi-layer structure of helically wound metallic wires and tapes and extruded thermoplastics. In 1996 Wellstream was awarded a major contract for the supply of flexible risers and flowlines as part of the Norsk Hydro Troll Olje Gas Province Development located in 350m water depth 80km west of Bergen. The development consists of two main fields, Troll East (31/3 and 31/6) and Troll West (31/2) which together have an estimated production life in excess of 50 years, making it one of the worlds largest offshore developments. Norsk Hydro is responsible for the development and operation of the production facilities. The scope of supply included 15-inch internal diameter, 213 barg design pressure, dynamic risers for the export of oil and gas from the platform to shore. At contract award, Wellstream was finalising the location of their European Manufacturing site, a facility which would have the capability of manufacturing unbonded flexible pipe with external diameters up to 24-inches. The design, manufacture and qualification of a large diameter oil and gas export riser for service in the Norwegian sector of the North Sea, considered to be one of the most severe environments in the offshore industry, provided unique challenges and attributes. These risers have now been in service for over two year, following an extensive qualification programme. This paper provides an insight into the integrated approach adopted during qualification with the successful application of finite element technology to aid full-scale testing. During a full-scale test program a finite element simulation of a 15 metre long prototype pipe was performed with special emphasis on the evaluation of contact forces between the flexible pipe and a bend limiting structure. The finite element analysis program package ANSYS is chosen for this simulation due to its special feature of contact/target elements. The paper illustrates that the use of Finite Element Modelling is indeed capable of predicting the observed behaviour of prototype risers, which are subjected to a series of dynamic load cases, in a Dynamic Test Rig (DTR). Finally, the paper concludes that focus should now be given to the advantages of using finite element tools that are verified by full scale testing to reduce development costs and schedules.
Material Choice and System Design for Deep Water Single Pipe Thermal Insulation Applications
Authors: Jackson Adam , Simonsen Eirik
Published: March 2009

From the point of view of thermal design for fluid transport systems there are two operation regimes that dictate the requirements on performance of the system. There is the scenario of “steady state operation”, representing stable operation, and the transient scenario of the “shut-in”, where changes are occurring in operational regime. These two approaches to the design of a thermally insulated system often give differing results if looked at in isolation. The performance of the insulation materials over time, with a view to both thermal, mechanical and ageing performance are critical factors which must also be considered at the design stage in order to develop cost effective and robust solutions capable of meeting the industry’s demands. This paper deals with the choice and evaluation of materials, along with a conservative, robust design procedure suitable for use in the pipeline insulation design process.
Weld Repair of Shell Plates During Seagoing Operations
Authors: Lindstro¨m Per R. M. , Ulfvarson Anders
Published: March 2009

An algorithm to estimate the cooling rate of welding seams on the shell plating of a ship, below the waterline, while it is on voyage has been derived. The demand for this technique has arisen from the wish of ship operators to make it possible for the safe repair of ship structures without taking them out of operation. [1] The strength of the shell plating after welding is determined by its metallurgic structure, which is dependent on the cooling rate, its chemical composition and the original grain size of the base material. [2] The cooling rate for this type of welding seam depends on the velocity of the water flow, the distance from the bow, the thickness of the plate, and the heat from the heat input of the welding. The algorithm makes it possible to calculate the cooling rate for a base material affected by a forced flow of fluid by means of Rosenthal’s equation and thus enabling suitable welding parameters to be determined. As the welding parameters can be chosen to fit the specific repair to be made, it is now possible to determine the suitability of a welding procedure in advance. The algorithm is applicable when determining welding parameters at Hot-Tapping operations as well, where the base material is affected by a forced flow of fluid. A number of experiments have been performed and the results support the theoretical model. The research project continues with the aim of finding an algorithm to include the enhanced cooling rate due to the layer of boiling fluid on the back of the base material. A method to improve the measurements of the most important parameter in the algorithm has been developed and makes it possible to build up a quantitative database of typical values for various configurations.
Dual Process Welding of Steel Plate
Authors: Wildash Clint , Webster Steve
Published: March 2009

Large scale fabrication and welding industries, such as those involved in offshore construction, are continuously striving to improve productivity, while maintaining quality levels required by the applicable design codes and standards. To achieve this, new improved welding technologies are regularly being evaluated. One area of development is to combine different welding processes to produce a welded joint exhibiting properties and productivity benefits that neither process could achieve individually. One promising combination is the use of both arc and laser beam welding for products such as pipeline. The welding procedure development work described, was carried out in two stages. Stage 1 (discrete dual processing) investigated the production of a welded joint using both arc and laser welding at separate times. The welds produced for this work, demonstrate that significant increases in welding speed are achievable in comparison to using either process individually. Stage 2 (simultaneous dual processing) investigated arc and laser welding of the joint at the same time from opposing plate faces, with the laser weld pool trailing the arc weld pool so that the former was positioned in the area of highest preheat temperature. The use of arc preheat significantly reduced the hardness of the laser welds to more acceptable levels. The main disadvantage identified for both stages of work was that fit up of the laser welded part of the joint needed to be good to accommodate the autogenous laser weld. Both Stages were shown to be capable of producing full penetration welds at higher productivity than using either process individually. It has been demonstrated that for 19 mm thick plate, an overall doubling of welding speed could be achieved using dual process as opposed to submerged arc welding, which is currently widely used to weld fabricated products such as pipeline. Future work to be carried will include extensive destructive testing on the welds to assess the benefits of Stage 1 and 2, which will be reported in another paper.
On the Safety of Fixed Offshore Structures, Failure Paths and Barriers
Authors: Ersdal Gerhard
Published: March 2009

In order to ensure the safety of an offshore structure it is important to identify and maintain the barriers preventing hazardous events. Also, when monitoring the safety, the monitoring should be regarding how well these barriers are functioning, and utilise these to reassess the safety of the structure over time. The purpose of this paper is to apply a well-known method in risk assessment, Haddon’s energy and barrier model, to a new area; structural safety. The purposes of this exercise are to look at the structural safety from a risk assessment point of view, and to use this to identify and give an overview of the existing barriers. Furthermore, the purposes are to evaluate the efficiency and redundancy of these barriers, and to use this to evaluate the safety of offshore structures. This paper will analyse the safety of a fixed offshore structure through a qualitative approach. A possible event chart for a fixed offshore installation during operation in storms is established and analysed. Some of the root causes for potential structural failure are identified. These root-causes are kept on a general level, but considered in more detail than often seen in risk analysis. Hazards that are normally included in risk analysis, like boat collisions, fire, explosions, and dropped objects are not evaluated. Hazards that are evaluated are structural failure due to wave loading, fatigue damage, aging, and gross errors in design, fabrication, installation and operation. In order to identify the barriers (hazard reduction strategies, physical barriers and vulnerable target protection strategies), the different failure paths in the event chart are then analysed using Haddon’s ten preventive strategies for reducing damage from hazards. As an example a fixed offshore steel structure is used. A list of proposed barriers that influence the safety of such a fixed offshore installation are presented, and methods to measure these barriers are discussed.
Variable Amplitude Corrosion Fatigue Tests on a High Strength Jack-Up Steel
Authors: Dover W. D. , Stacey A. , Tantbirojn N. , Brennan F. P.
Published: March 2009

A series of fatigue tests were conducted on T-butt-welded specimens made from high strength (SE702) jack-up steel. Fatigue tests were conducted using a pseudo-random sequence (JOSH) designed to give typical jack-up loading at two levels of cathodic protection. The results are presented in terms of fatigue life and crack growth evolution. The results for the T-butt joints were compared to tubular joint fatigue test data obtained with conventional fixed platform steels (BS4360 Gr50D) and the same high strength steel (SE702). The study indicated that for high strength steel joints with relatively short fatigue lives the influence of the cathodic protection level is quite small and the fatigue resistance is slightly better than that of joints fabricated from conventional structural steels such as BS4360 50D.
Experimental Study of the Probability Distributions of Green Water on the Bow of Floating Production Platforms
Authors: Soares Carlos Guedes , Pascoal Ricardo
Published: March 2009

Results of an experimental program with a model of a moored Floating Production Storage and Offloading (FPSO) vessel are used to study the probability distributions associated with various phenomena related with green water loading. Separate analysis of wave height and crests are performed in order to assess the presence and significance of nonlinearities. Time series of pitch motion and relative motion are analysed to check for linearity of the response process. Probability distributions of the occurrence of water on deck and of the conditional distribution water height above deck are also studied.
Validation of the AeroDyn Subroutines Using NREL Unsteady Aerodynamics Experiment Data
Authors: Laino David J. , Hansen A. Craig , Minnema Jeff E.
Published: March 2009

Completion of the full-scale wind tunnel tests of the NREL Unsteady Aerodynamics Experiment (UAE) Phase VI allowed validation of the AeroDyn wind tuxbine aerodynamics software to commence. Detailed knowledge of the inflow to the UAE was the bane of prior attempts to accomplish any in-depth validation in the past. The wind tunnel tests permitted unprecedented control and measurement of inflow to the UAE rotor. The data collected from these UAE tests are currently under investigation as part of an effort to better understand wind turbine rotor aerodynamics in order to improve aero-elastic modeling techniques. Preliminary results from this study using the AeroDyn subroutines are presented, pointing to several avenues toward improvement. Test data indicate that rotational effects cause more static stall delay over a larger portion of the blades than predicted by current methods. Despite the relatively stiff properties of the UAE, vibration modes appear to influence the aerodynamic forces and system loads. AeroDyn adequately predicts dynamic stall hysteresis loops when appropriate steady, 2-D airfoil tables are used. Problems encountered include uncertainties in converting measured inflow angle to angle of attack for the UAE phase VI. Future work is proposed to address this angle of attack problem and to analyze a slightly more complex dynamics model that incorporates some of the structural vibration modes evident in the test data.
A Framework for Dynamic and Aeroelastic Analysis of Horizontal Axis Wind Turbines
Authors: Lee Donghoon , Hodges Dewey H.
Published: March 2009

This paper presents a framework for the dynamic and aeroelastic analysis of a horizontal axis wind turbine modeled as a multi-flexible-body system. The multi-rigid-body portions of the system, composed of the nacelle and hub, are modeled as a system of interconnected rigid bodies using Kane’s equations. The flexible portions, composed of the the blades and tower, are represented using nonlinear beam finite elements, taken from a mixed formulation for the dynamics of moving beams. Each analysis leads to a set of symbolic equations that can be coupled symbolically to represent the dynamic behavior of the wind turbine. A solution procedure is implemented to assess the dynamic stability of the system. Here the solution is divided into two parts: a set of nonlinear ordinary differential equations governing the periodic steady-state operating condition, and a set of equations that are linearized about the steady-state operating condition governing the transient dynamics. The harmonic balance method is used for the nonlinear periodic steady-state solution, and the finite element in time method is proposed as an alternative method. Linearization of the equations about the steady-state operating condition yields system equations with periodic coefficients which are solved by Floquet approach to extract the modal parameters. For the aeroelastic analysis, aerodynamic loads from an aerodynamic model to be selected in the future will be inserted into the present framework. Then, the framework can produce a symbolic system matrix, potentially useful for control design. Numerical results are presented for the dynamic characteristic of HAWT’s with flexible tower and blades.
Modal Response of 3-Bladed Wind Turbines
Authors: Malcolm David J.
Published: March 2009

A set of linear equations describing the motion of an operating 3-bladed HAWT are obtained from the dynamic characteristics of the stationary turbine by adding rotating frame effects. The approach makes use of the Coleman multi-blade transformation to present all results relative to the fixed frame. The formulation is in terms of a selected number of stationary, real, mode shapes. The formulation is applied to the expression of both the aerodynamic loading and the displacement response in terms of the operating mode shapes. This technique is applied to the conditions of vertical wind shear and off-yaw operation of a hypothetical 46-m wind turbine. The principal objective of the paper is to enable the characteristic of the inflow to be related to the nature of the response. A second objective is to illustrate a method of extracting linearized models from general aeroelastic codes such as ADAMS™.
Vibrations of a Three-Bladed Wind Turbine Rotor Due to Classical Flutter
Authors: Hansen M. H.
Published: March 2009

The aeroelastic stability of a three-bladed wind turbine is considered with respect to classical flutter. Previous studies have shown that the risk of stall-induced vibrations of turbine blades is related to the dynamics of the complete turbine, for example does the aerodynamic damping of a rotor whirling mode depend highly on the tower stiffness. The results of this paper indicate that the turbine dynamics also affect the risk of flutter. The study is based on an eigenvalue analysis of a linear aeroelastic turbine model. In an example of a MW sized turbine, the critical frequency of the first torsional blade mode is determined for which flutter can occur under normal operation conditions. It is shown that this critical torsional frequency is higher when the blades are interacting through the hub with the remaining turbine, than when all blades are rigidly clamped at the root. Thus, the dynamics of the turbine has increased the risk of flutter.
Effect of Turbulence Variation on Extreme Loads Prediction for Wind Turbines
Authors: Moriarty Patrick J. , Holley William E. , Butterfield Sandy
Published: March 2009

The effect of varying turbulence levels on long-term loads extrapolation techniques was examined using a joint probability density function of both mean wind speed and turbulence level for loads calculations. The turbulence level has a dramatic effect on the statistics of moment maxima extracted from aeroelastic simulations. Maxima from simulations at lower turbulence levels are more deterministic and become dominated by the stochastic component as turbulence level increases. Short-term probability distributions were calculated using four different moment-based fitting methods. Several hundred of these distributions were used to calculate a long-term probability function. From the long-term probability, 1- and 50-year extreme loads were estimated. As an alternative, using a normal distribution of turbulence level produced a long-term load comparable to that of a log-normal distribution and may be more straightforward to implement. A parametric model of the moments was also used to estimate the extreme loads. The parametric model predicted nearly identical loads to the empirical model and required less data. An input extrapolation technique was also examined. Extrapolating the turbulence level prior to input into the aeroelastic code simplifies the loads extrapolation procedure but, in this case, produces loads lower than the empirical model and may be non-conservative in general.
Investigation of four distinct glottal configurations in classical singing—A pilot study
Authors: Herbst Christian T. , Ternström Sten , Švec Jan G.
Published: March 2009

This study investigates four qualities of singing voice in a classically trained baritone: “naïve falsetto,” “countertenor falsetto,” “lyrical chest” and “full chest.” Laryngeal configuration and vocal fold behavior in these qualities were studied using laryngeal videostroboscopy, videokymography, electroglottography, and sound spectrography. The data suggest that the four voice qualities were produced by independently manipulating mainly two laryngeal parameters: (1) the adduction of the arytenoid cartilages and (2) the thickening of the vocal folds. An independent control of the posterior adductory muscles versus the vocalis muscle is considered to be the physiological basis for achieving these singing voice qualities.
Limits of temporal pitch in cochlear implants
Authors: Kong Ying-Yee , Deeks John M. , Axon Patrick R. , Carlyon Robert P.
Published: March 2009

A common finding in the cochlear implant literature is that the upper limit of rate discrimination on a single channel is about 300pps. The present study investigated rate discrimination using a procedure in which, in each block of two-interval trials, the standard could have one of the five baseline rates (100, 200, 300, 400, and 500pps) and the signal rate was a given percentage higher than the standard. Eight Med-El C40+ subjects took part. The pattern of results was different than those reported previously: six Med-El subjects performed better at medium rates (200–300pps) compared to both lower (100pps) and higher (400–500pps) rates. A similar pattern of results was obtained both with the method of constant stimuli and for 5000-pps pulse trains amplitude modulated at rates between 100 and 500Hz. Compared to an unmatched group of eight Nucleus CI24 listeners tested using a similar paradigm and stimuli, Med-El subjects performed significantly better at 300pps and higher but slightly worse at 100pps. These results are discussed in relation to evidence on the limits of temporal pitch at low and high rates in normal-hearing listeners.
Adaptation to frozen babble in spoken word recognition
Authors: Felty Robert Albert , Buchwald Adam , Pisoni David B.
Published: March 2009

Previous research has shown that listeners can adapt to particular samples of noise, a phenomenon known as “frozen noise” [Langhans and Kohlrausch, J. Acoust. Soc. Am. 91, 3456–3470 (1992)]. However, no studies have reported a similar effect for multi-talker babble. The results of this study comparing open-set word recognition in multi-talker babble showed that listeners are significantly more accurate when the babble is fixed than when the babble is random. This documents the effect the authors refer to as “frozen babble.”
Chirp excitation technique to enhance microbubble displacement induced by ultrasound radiation force
Authors: Hu Yi , Zhang Dong , Zheng Hairong , Gong Xiufen
Published: March 2009

Ultrasound radiation force has been proposed to increase the targeting efficiency in ultrasonic molecular imaging and drug delivery. A chirp excitation technique is proposed to increase the radiation force induced microbubble displacement and might potentially be used for enhancing the targeting efficiency of microbubble clouds. In this study, a modified Rayleigh–Plesset equation is used to estimate the radius-time behavior of insonified microbubbles, and the translation of insonified microbubbles is calculated by using the particle trajectory equation. Simulations demonstrate that the chirp excitation is superior to the sinusoidal one in displacing microbubbles with a wide-size distribution, and that the performance is dependent on the parameters of the chirp signal such as the center frequency and frequency range. For Gaussian size distributed microbubble clouds with mean diameter of 3.5 μm and variance of 1, a 2.25 MHz chirp with frequency range of 1.5 MHz induces about 59.59% more microbubbles over a distance of 10 μm during 200 μs insonification, compared to a 2.25 MHz sinusoidal excitation with equal acoustic pressure.
A multi-rate decay model to predict energy-based acoustic parameters in churches
Authors: Martellotta Francesco
Published: March 2009

Multi-rate decays are sometimes observed in room acoustics, appearing when markedly different volumes are coupled together and resulting in nonlinear decay curves. Such behavior appears in several churches at the very beginning of the decay process, although in conditions which cannot be explicitly referred to as coupling phenomena. Consequently, multi-rate exponential decays may be suitable to model energy distribution in this group of buildings, providing a more elegant and easily applicable set of equations in place of a previously defined “linear” model, used to adapt Barron’s revised theory. The paper shows that the multi-rate approach ensures ease of calculation, without significant loss in accuracy in predicting energy-based acoustic parameters.
Spectral integration under conditions of comodulation masking release
Authors: Buss Emily , Grose John H.
Published: March 2009

Detection of a pure tone signal in a narrowband noise masker can be improved by the introduction of coherently amplitude modulated masker bands in neighboring frequency regions, an effect called comodulation masking release (CMR). Experiment 1 tested the hypothesis that detection of a spectrally complex signal in a comodulated masker critically depends on the signal/masker interaction, with best sensitivity in conditions where the signal introduces across-frequency stimulus envelope differences. Consistent with this hypothesis, thresholds for a multi-frequency signal differed by approximately 10 dB depending on the relative patterns of signal/masker interaction across frequency. In comodulated maskers, there was no improvement in threshold relative to the single-frequency signal threshold even in cases where the multi-frequency signal introduced across-frequency envelope differences. Experiment 2 tested conditions that have previously been associated with large spectral integration in comodulated but not random maskers. Results depended on the masker configuration used as the reference condition, with comparable integration for random and comodulated noise in some cases. The results suggest that CMR obtained with a pure tone signal can differ greatly from that obtained with a complex signal, and that spectral integration is inversely related to the amount of CMR under some conditions.
Effect of electrical circuits on duration of an acoustic pulse radiated by a piezoplate
Authors: Konovalov S. I. , Kuz’menko A. G.
Published: March 2009

By means of numerical computation, pulse mode of a piezoelectric radiator in the form of a plate, loaded with a liquid and connected to R-L circuits in various combinations, is investigated. The computations were carried out having taken into account the internal resistance of the electric generator. Numerical results, based on the theory, are provided for examples of parallel and series circuits connected to a piezoelectric plate. The optimal values of parameters are determined, providing minimal duration of radiated acoustic pulses.
Contribution of very low amplitude-modulation rates to intelligibility in a competing-speech task
Authors: Füllgrabe Christian , Stone Michael A. , Moore Brian C. J.
Published: March 2009

It is generally agreed that the slow fluctuations in the envelope of speech in different spectral channels carry critical information for intelligibility. Previous studies in which amplitude modulation (AM) was selectively removed from the speech signal showed that modulation rates between 4 and 16Hz are most important, and that rates falling outside this range contribute little or not at all to speech intelligibility. The present study investigated the role of very low (<4Hz) AM rates in the ability to identify sentences in an interfering background talker. The mixture was processed through a noise vocoder. The depth of AM with rates below 4, 1.3, or 0.4Hz was reduced using a multi-channel envelope compressor with a high compression ratio. Data obtained using nine normal-hearing listeners demonstrate that low-rate AM, in the range 0.4–4Hz, contributes to the intelligibility of relatively long speech utterances, at least for adverse listening conditions in which background noise is present and listeners are forced to rely on envelope cues in a few spectral channels.
Two-dimensional wave propagation in an elastic half-space with quadratic nonlinearity: A numerical study
Authors: Küchler Sebastian , Meurer Thomas , Jacobs Laurence J. , Qu Jianmin
Published: March 2009

This study investigates two-dimensional wave propagation in an elastic half-space with quadratic nonlinearity. The problem is formulated as a hyperbolic system of conservation laws, which is solved numerically using a semi-discrete central scheme. These numerical results are then analyzed in the frequency domain to interpret the nonlinear effects, specifically the excitation of higher-order harmonics. To quantify and compare the nonlinearity of different materials, a new parameter is introduced, which is similar to the acoustic nonlinearity parameter β for one-dimensional longitudinal waves. By using this new parameter, it is found that the nonlinear effects of a material depend on the point of observation in the half-space, both the angle and the distance to the excitation source. Furthermore it is illustrated that the third-order elastic constants have a linear effect on the acoustic nonlinearity of a material.