bluuebunny/crossref_metadata_2025 huggingface dataset:
An Investigation of Fatigue Damage due to VIV in Flexible Riser With Different Methodologies
Authors: Zhang Yanqiu , Tan Zhimin
Published: November 2011

An investigation was carried out for the fatigue damages due to VIV in a flexible riser evaluated with three different VIV simulation methodologies. One methodology is provided in commercial software Shear7 which is established based on mode superposition. The other two are provided in commercial software OrcaFlex which are established based on global dynamic simulation in time domain. One of the two time-domain methods is referred to as Iwan and Blevings model formed with a wake oscillator, and the other is referred to as Tracking (1) model made with vortex tracking method. Fatigue damages in tensile armoring wires of the riser were evaluated with RMS curvature ranges resulting from the three methodologies separately. A detailed comparison between the damages and between RMS curvature ranges is made and then some discussions and recommendations are presented.
Reynolds Number Effect on Vortex-Induced Vibration on a Two-Dimensional Circular Cylinder With Low Mass-Damping Parameter
Authors: Wanderley Juan B. V. , Soares Luiz F. , Vitola Marcelo , Sphaier Sergio H. , Levi Carlos
Published: November 2011

The vortex induced vibration (VIV) on a circular cylinder with low mass-damping parameter and low Reynolds number is investigated numerically as basis for applications on dynamics of risers used in the offshore oil and gas industry and as a first step before tackling the harder high Reynolds number problem. The cylinder is supported by a spring and a damper and free to vibrate in the transverse direction. The numerical solution of the Reynolds average Navier-Stokes equations written in curvilinear coordinates is obtained using an upwind and Total Variation Diminishing conservative scheme and the k-ε turbulence model is used to simulate the turbulent flow in the wake of the body. Results were obtained for the phase angle, response amplitude, frequency, and lift coefficient for a variation of reduced velocity from 2 to 12 and three different proportional variations of Reynolds number, 2000–6000, 2000–12000, and 2000–24000. The numerical results indicate the strong effect of the Reynolds number range on the response amplitude, lift coefficient, and frequency of oscillation for a low mass-damping parameter.
A Gust Factor Model for Storm Winds
Authors: Fournier N.
Published: November 2011

This paper describes the development of a gust factor model for use in the design of offshore, onshore and coastal structures. The gust factor is an important parameter for design with the gust speed being routinely used to evaluate the maximum wind pressure acting on structures. This is of special interest when dealing with extreme wind risk assessment at locations where only scarce measurements are available. In the oil and gas industry, the ISO 19901-1 (2005) code is the standard approach to estimate the gust factor for offshore structural design. The methodology is based on the analysis of extensive marine data from the Island of Fro̸ya on the Norwegian coast by Andersen and Lo̸vseth (1993). However, this approach exhibit significant limitations at coastal and onshore locations. Therefore, in this study, another methodology is proposed. This is based on the latest developments of the Engineering Sciences Data Unit gust factor model (ESDU, 2003; Vickery and Skerlj, 2005). This model has been further validated by making use of additional storm wind records at various offshore, onshore and coastal locations. The relationship is function of latitude, height and surface roughness. The latter is prescribed for onshore locations and calculated offshore as a function of the mean wind speed. The model compares well with the observed gust factors and outperforms, especially over land, the ISO code currently used in the oil and gas industry.
Caisson Capacity in Clay: VHM Resistance Envelope: Part 3—Extension to Shallow Foundations
Authors: Kay S. , Palix E.
Published: November 2011

Suction embedded caissons are efficient and economic solutions to anchor floating structures. A more recent caisson application is to support seafloor structures such as manifolds, PLEMs, pumps, etc. For a deepwater hydrocarbon field, many types of seafloor structures are required, each with their own characteristics and slightly different design. Caisson designs increasingly use resistance envelope methodology. This eliminates non-linear 3D FE analyses (except for assessing responses or soil reactions), and facilitates probabilistic and optimisation analyses. In general, there is a requirement for a reliable method of assessing caisson capacity under general VHM load. Resistance envelope equations for “deep” circular caissons (1.5 < L/D < 6) have been presented by Kay and Palix (2010) for a wide range of soil undrained shear strength profiles. This paper extends the study to cover near-surface caissons (i.e. 0 ≤ L/D ≤ 1.5). As in previous studies, a quasi 3D non-linear finite element program (HARMONY) was the primary numerical analysis tool. Three soil shear strength profiles were investigated for 13 caisson embedment ratios. In the range 0 ≤ L/D ≤ 1.5, VHM envelope shapes transform from a “scallop” at L/D ≈ 0 into a “tongue” above a critical caisson embedment ratio (L/D)crit of about 0.5 The equations originally developed for the rotated ellipse/ellipsoid (i.e. “tongue”-shaped envelope) in Kay and Palix (2010) for L/D ≤ 1.5 have been extended for (L/D)crit ≥ L/D. All parameters are simple functions of L/D and soil shear strength profile. Major limitations and assumptions made were (a) foundation-soil tension was permitted and (b) no internal scoop failure (i.e. within the soil plug inside the caisson) was possible. These are important for low L/D: both may adversely affect VHM resistance.
Dynamic Response Analysis of the Offshore Wind Turbine Support Structure Under Extreme Conditions
Authors: Chen Qian , Fu Shixiao , Zou Zaojian
Published: November 2011

Based on modal analysis, the environmental loads of an offshore wind turbine under extreme conditions are analyzed, including the static wind load on the support structure, the static current load and wave load on the foundation. The vortex-induced vibrations caused by wind and current have also been studied. The dynamic displacement and bending moment characteristics of the offshore wind turbine support structure under the extreme conditions have been analyzed.
Determination of Critical Factors for the Strength Assessment of Damaged Steel Ship Structures
Authors: Underwood J. M. , Sobey A. J. , Blake J. I. R. , Shenoi R. A. , Cuckson B. R.
Published: November 2011

A significant number of damage events continue to occur to ocean going vessels, many of which remain afloat in need of assistance to evaluate remedial actions to minimise the risk of further damage and conditions for onward transit for a repair facility. Therefore, it is vital that the post damage situation is rapidly understood to reduce damage propagation occurring, reducing the cost of repair and allowing more vessels to be recovered safely, ensuring the safety of personnel onboard. After an incident, it is often difficult to determine and model the precise damage scenario due to the inability to survey the area. Each scenario will have variability in geometrical and material properties, which will affect the residual strength of the structure. Variations in these aspects are not accounted for in methods currently utilised in damage response scenarios. Therefore, to be able to more accurately analyse the damage and provide better guidance to the crew in real time, it is important that this variability can be analysed, allowing an understanding of worst and best case scenarios, the probabilities of these occurring and their affect on the structural strength. Finite element analysis has been used to model damage scenarios due to the high level of accuracy that can be achieved. This paper demonstrates the implications of damage aperture on the limit state of stiffened steel panels, investigating the residual strength of the damaged structures and their sensitivity to variations in damage event, geometric and material properties. The results are then compared with the Smith and Dow progressive collapse method, [1,2], with conclusions being drawn about the use of this method in damage situations. Further to this, the effects of variability in the ship are investigated for damage scenarios, showing that these influence the ultimate strength of the structure to a larger extent in the failure of damaged plates than intact scenarios; however, lack of knowledge relating to the area of the damage could overshadow other potential variability within these scenarios.
A Novel Method for Generating Continuously Surfable Waves: Comparison of Predictions With Experimental Results
Authors: Schmied Steven A. , Binns Jonathan R. , Renilson Martin R. , Thomas Giles A. , Macfarlane Gregor J. , Huijsmans Rene
Published: November 2011

In this paper, a novel idea to produce continuous breaking waves is discussed, whereby a pressure source is rotated within an annular wave pool. The concept is that the inner ring of the annulus has a sloping bathymetry to induce wave breaking from the wake of the pressure source. In order to refine the technique, work is being conducted to better understand the mechanics of surfable waves generated by moving pressure sources in restricted water. This paper reports on the first stage of an experimental investigation of a novel method for generating continuously surfable waves utilising a moving pressure source. The aim was to measure and assess the waves generated by two parabolic pressure sources and a wavedozer [1] for their suitability for future development of continuous breaking surfable waves. The tests were conducted at the Australian Maritime College (AMC), University of Tasmania (UTas) 100 metre long towing tank. The experimental results as variations in wave height (H) divided by water depth (h) as functions of depth Froude number (Frh) and h, together with predictions from both methods, are presented in this paper. Finally, measures of the wave making energy efficiency of each pressure source, and the surfable quality of the waves generated by it, were developed and are presented.
Tangguh LNG Project Statistical Young’s Modulus and Thermal Expansion Coefficient Testing of Clad Pipe Specimens
Authors: Griffiths Terry , Hadley Isabel , Johnson Richard , Micari Fabio
Published: November 2011

Material testing was undertaken on samples taken from clad pipe manufactured by JSW for the Tangguh LNG project. The test programme involved testing Young’s Modulus (E) and Coefficient of Linear Thermal Expansion (α) from room temperature to above 110° on each layer. This paper summarises testing and analysis of results which enabled mean and variance on each material property to be found. Checks were also undertaken for any correlations in properties between clad and parent layers, and between Young’s Modulus and Coefficient of Thermal Expansion. Analysis results are compared to existing industry norms and their implications for the Tangguh project UHB (Upheaval Buckling) SRA (Structural Reliability Analysis) are summarised.
Wave Drift Forces Affecting on Maneuverability Derivatives
Authors: Xu Yongze , Kinoshita Takeshi , Bao Weiguang , Itakura Hiroshi
Published: November 2011

Maneuverability in waves is important for the safety of ships and other marine vehicles. Conventionally, the maneuvering experiments are performed in still water at a constant forward velocity. In order to investigate the wave effects on ships steering, a series of towing tests with a Planer Motion Mechanism System (PMM) are carried out in waves in the present work. The ship model is forced to oscillate in horizontal plane at a low frequency when being towed forward at a small constant speed. As an extension of the previous work, the low-frequency oscillation of yaw motion is also considered. Hydrodynamic forces and the displacements of the ship model are measured throughout the tests. Then, the measured data are analyzed by the Fourier analysis to obtain components at different frequencies. By comparing the hydrodynamic forces measured in still water and that in waves, the wave effects on the maneuverability of ships are estimated. The variation of these nonlinear hydrodynamic coefficients against the wave amplitude and wavelength is examined.
A Framework for the Management of Ageing of Safety Critical Elements Offshore
Authors: Sharp John V. , Terry Edmund G. , Wintle John
Published: November 2011

Many offshore installations in the North Sea have now exceeded their original design life and are in a life extension phase. A Framework of six processes has been developed for the management of ageing of Safety Critical Elements (SCEs) in offshore installations. The processes include an analysis of the effect of ageing modes on SCE performance. Examples of performance indicators for typical SCEs are proposed based on how their condition and performance as may be affected by physical deterioration and other effects of ageing. Indicators for calibrating the maturity and effectiveness of the management processes are also suggested.
Experimental Investigation and Numerical Modeling of Hydrodynamic Force Characteristics and Deformation of an Elastic Net
Authors: Ito Sho , Kinoshita Takeshi , Kitazawa Daisuke , Bao Weiguang , Itakura Hiroshi
Published: November 2011

With the increasing worldwide demand of seafood and environmental problems in coastal aquaculture, offshore area has been increasingly expected to be utilized for aquaculture. An offshore aquaculture system has fewer effects on the surrounding marine environment through the rapid diffusion of organic wastes from the cultured fish than a coastal one. The offshore area then provides clean waters for cultured fish. On the other hand, the offshore aquaculture system is subject to the severe natural condition such as typhoon attack. Actually, in the current aquaculture system, the offshore sea cages are always submerged around 10m below the sea surface to avoid the effects of high waves and strong currents. However, the safety of the sea cage against the incident wave has seldom been examined, while that against the water current has been analyzed by model tests in tank and numerical simulation. We investigated the hydrodynamic force properties of a heaving sea cage in the previous work and its results proved that the deformation of net much affects on sea cage’s motion characteristics. To study the most fundamental problem, we investigated the hydrodynamic force characteristics of a rigid net as a first step and explained that these relate to its porosity and wave conditions. As a second step, in the present study, we investigate hydrodynamic force properties of an elastic net. Six square-shaped textile net models which are made of elastic strings and metallic frame are arranged. Three different porosities (0.53, 0.64 and 0.74) and two different net’s tension (10 and 50 N/m) on each porosity are employed; that is, we arranged 6 elastic net models. Forced heaving tests and diffraction tests are carried out. In addition to measuring heave force, the deformation of net surface is measured by a video image analysis system. To compare with the experimental results, we calculate hydrodynamic forces and the deformation of net using a boundary element method. The boundary condition on elastic net is that the normal velocity to the surface is not zero but proportional to the pressure difference. The deformation is also taken into consideration.
Hyperbaric Evacuation in the North
Authors: Serck-Hanssen Bjo̸rn , Gudmestad Ove T.
Published: November 2011

In this paper we will look into Hyperbaric Evacuation in the Arctic and Cold Climate regions and particularly in the Barents Sea. By hyperbaric evacuation is meant all means for safe transportation of the divers in saturation until decompressed onshore. Further the hyperbaric life boat on Subsea 7’s Diving Support Vessel (DSV) Seven Havila is investigated for use in these areas and the potential limitations the existing design has on diving operations in the area will be discussed.
Numerical and Experimental Investigation on Hydrodynamic Characteristics of FLNG With an Account of the Inner-Tank Sloshing
Authors: Zhao Wenhua , Yang Jianmin , Hu Zhiqiang , Peng Tao
Published: November 2011

FLNG is a new type of floating LNG platform which is ship-shaped FPSO hull equipped with LNG storage tanks and liquefaction plants and has been developed as an alternative to long pipelines to LNG liquefaction plant on land for stranded offshore fields. Different to the conventional FPSO, the hydrodynamic performance of the being-proposed FLNG can be significantly affected by the inner-tank liquid sloshing, due to the fact that LNG performs better liquidity than crude oil. In order to investigate the hydrodynamic characteristics of FLNG with large hull and high center of gravity, a series of numerical simulations on an FLNG designed by CNOOC and MARIC were conducted with the help of the well-known code WADAM. In addition, corresponding model tests on the same vessel with solid ballast were completed, serving as a verification of the numerical simulations. In order to inspect the sloshing effects of inner tanks on the global motion responses of the FLNG vessel, model tests with liquid ballast were also conducted. Comparisons were made between numerical and experimental results with methods of Fast Fourier Transformation technology and spectral analysis. The numerical simulation results were in good accordance with those of the model tests, which proves the feasibility of the numerical simulations. Furthermore, the effects of the inner tank sloshing on the FLNG hydrodynamic characteristics were summed up. Conclusions obtained from the comparison results would be of significant importance for the design of the FLNG system.
Cancellation of Non-Harmonic Waves Using a Cycloidal Turbine
Authors: Imamura John T. , Siegel Stefan G. , Fagley Casey , McLaughlin Tom
Published: November 2011

We computationally investigate the ability of a cycloidal turbine to cancel two-dimensional non-harmonic waves in deep water. A cycloidal turbine employs the same geometry as the well established Cycloidal or Voith-Schneider Propeller. It consists of a shaft and one or more hydrofoils that are attached eccentrically to the main shaft and can be independently adjusted in pitch angle as the cycloidal turbine rotates. We simulate the cycloidal turbine interaction with incoming waves by viewing the turbine as a wave generator superimposed with the incoming flow. The generated waves ideally are 180° out of phase and cancel the incoming wave downstream of the turbine. The upstream wave is very small as generation of single-sided waves is a characteristic of the cycloidal turbine as has been shown in prior work. The superposition of the incoming wave and generated wave is investigated in the far-field and we model the hydrofoil as a point vortex. This model has previously been used to successfully terminate regular deep water waves as well as intermediate depth water waves. We explore the ability of this model to cancel non-harmonic waves. Near complete cancellation is possible for a non-harmonic wave with components designed to match those generated by the cycloidal turbine for specified parameters. Cancellation of a specific wave component of a multi-component system is also shown. Also, step changes in the device operating parameters of circulation strength, rotation rate, and submergence depth are explored to give insight to the cycloidal turbine response characteristics and adaptability to changes in incoming waves. Based on these studies a linear, time-invarient (LTI) model is developed which captures the steady state wave frequency response. Such a model can be used for control development in future efforts to efficiently cancel more complex incoming waves.
Integrity of Mechanically Lined Pipes Subjected to Multi-Cycle Plastic Bending
Authors: Tkaczyk Tomasz , Pe´pin Aure´lien , Denniel Sylvain
Published: November 2011

There is an increasing demand for submarine pipelines which can transport highly corrosive products. Due to the significant cost of solid stainless steel pipes, the interest in more economical mechanically lined pipes (i.e. carbon steel pipes lined with a thin stainless steel layer) is growing. There is further incentive to combine this technology with the highly efficient reel-lay method, which is an alternative to the S-lay and J-lay methods for installation of small to medium size steel submarine pipelines. Reeled pipelines, however, are subjected to at least two symmetrical plastic strain cycles during installation. The plastic straining associated with reeled installation may trigger wrinkling of the corrosion resistant liner. Technip have undertaken a development programme in accordance with DNV-RP-A203 to design and qualify mechanically lined pipes for reeling without the use of internal pressure. The proposed novel design enables a safe and reliable reeled installation and a subsequent lifetime operation of mechanically lined pipes by controlling the liner thickness.
Infragravity Waves Induced by Short-Wave Groups in Coastal Regions Characterized by General Bottom Topography
Authors: Belibassakis K. A.
Published: November 2011

The free long-wave generation by short-wave groups over a sloping bottom is studied both experimentally and theoretically by various authors showing important results concerning the modelling of energy transfer from the short waves to subharmonics. In the present work, the coupled-mode model developed by Athanassoulis & Belibassakis (1999) for the propagation of water waves over variable bathymetry regions, as generalized to include dissipation due to bottom friction and breaking effects, is applied to calculate the spatial evolution of short-wave groups propagating over a shoaling area, characterized by general bottom topography. Following Scha¨ffer (1993), the present model is appropriately modified in the surf zone in order to destroy the short-wave modulation, keeping the wave height decay in proportion to the local water-depth, and is then used to calculate radiation stresses associated with shoaling and breaking of short-wave groups in the area of general bathymetry and in the surf zone. Subsequently, the system of long wave equations, corresponding to zero (set-down/set-up) and first few harmonics, forced by the radiation stresses, is numerically solved. Results are presented showing that the present model provides reasonable predictions, supporting the study of infragravity waves induced by shortwave groups and their effects on harbors and mooring systems of large vessel operating in nearshore/coastal regions.
A New Interpolation Method for FPSO Motion and Wave Drift Load Transfer Functions
Authors: Stafrach Ste´phanie , Naciri Mamoun
Published: November 2011

A new method of direction-wise interpolation is proposed. Its merits are first presented by considering the interpolation of 1st order motion Response Amplitude Operators (RAOs) and 3D (ω1, ω2, θ) wave drift load Quadratic Transfer Functions (QTFs). In a recent publication (see Ref. [1]), the importance of wave spreading on the spectral density of wave drift loads for a standard storage capacity LNG Carrier (135,000m3) in shallow water has been demonstrated. The computation of above-mentioned spectral densities requires the precalculation of a large number of 4D (ω1, ω2, θ1, θ2) wave drift load QTF and interpolations between the calculated directions. Application of the new method is investigated in this more challenging context. Examples are selected in the buoyant LNG floating system area with an LNG Carrier in shallow water and an FLNG in deep water.
Dynamic Positioning: Early Design, Capability and Offsets—A Novel Approach
Authors: van ’t Veer R. , Gachet M.
Published: November 2011

Many vessels use a Dynamic Positioning (DP) system that automatically controls vessel position and heading with its own propulsion system. The limiting environment in which a vessel can maintain heading and position under DP operation is typically shown in a capability plot. Capability is usually evaluated assuming constant environmental forces — the static DP approach. In this paper we discuss the methodology of how such a capability assessment can be made early in the vessel design stage. The insights obtained in the heading and position variability over time, and the effects of the environmental forces varying with time can be assessed through model tests and time domain simulations: the dynamic DP approach. This implies that the DP system characteristics are modeled. In a numerical approach, the time variable wind load and low-frequency drift forces are included in the assessment. This paper presents a novel approach on how static DP capability calculations can be corrected to account for the time variable wind and wave drift forces. This leads to a DP capability plot that provides a more realistic insight in the actual DP capability of the vessel in the dynamic environment.
Dynamic Response of SPAR in Internal Solitary Waves
Authors: Qu Yan , Song Zhijun , Teng Bin , You Yunxiang
Published: November 2011

Internal solitary wave is considered as a potential hazard environmental condition to the floating structures in South China Sea. This paper presents results of the dynamic response analysis of a SPAR in internal solitary waves (ISW). Mathematical model of the ISW is selected to simulate the current process induced by the ISW. The result shows that the Korteweg–de Vries (KdV) gives rational result compared with the Modified Korteweg–de Vries (MKdV) equation. Dynamic motion of SPAR were estimated by using the current profile derived from KDV theory, load determined by Morrison equation and the nonlinear model of the mooring system.
Limit State Design Based on Experimental Methods for High Pressure, High Temperature Riser and Pipeline Design
Authors: Shilling Roy , Alexander Chris , Livesay Ron
Published: November 2011

A full-scale test program was conducted for BP America, Inc. to evaluate the performance of pipe material selected for use in high pressure, high temperature (HPHT) riser applications. Full length ultrasonic (FLUT) wall mapping was then used to select samples, and burst tests were performed at pressures exceeding 40,000 psi. The tests’ results clearly demonstrated the accuracy of the capped end burst pressures predicted by API RP 1111 as demonstrated by the low standard deviation of experimental burst pressures. The test program validated the strain-based design methodology embodied in API RP 1111, especially the empirically-based design methodology presented in Appendix B of API RP 1111. This paper presents details on the completed program and how the industry can use the insights gained in completing this study to establish design pressures that more fully utilize material strengths for thick-wall riser pipe materials while maintaining conservative factors of safety. A performance and reliability-based design procedure based on FLUT wall mapping has been proposed and verified in this study; the use of this design procedure can improve true reliability by ensuring a better quality riser product.
A Double Birkhoff Wake Oscillator for the Modeling of Vortex-Induced Vibration
Authors: Ogink R. H. M.
Published: November 2011

A double Birkhoff wake oscillator for the modeling of vortex-induced vibration is presented in which the oscillating variables are assumed to be associated with the boundary layer/near wake and the far wake. The fluid forces are assumed to consist of a potential added mass force and a force due to vortex shedding. In the limit of vanishing incoming flow velocity, the model equations reduce to a form similar to the Morison equation. The results of the double wake oscillator have been compared with forced vibration measurements and free vibration measurements over a range of mass and damping ratios. The model is capable of describing the most important trends in both the forced and free vibration experiments. Specifically, the double wake oscillator is able to model both the upper and lower branch of free vibration.
Evaluation of an Extended Operational Boussinesq-Type Wave Model for Calculating Low-Frequency Waves in Intermediate Depths
Authors: de Jong Martijn P. C. , Borsboom Mart , de Bont Jan A. M. , van Vossen Bas
Published: November 2011

The motions of (LNG) vessels moored offshore at depths ranging from about 20 to 100 m may depend significantly on the presence of (bound) low-frequency waves with periods in the order of 100 s. This is because these moored vessels show a large motion response in this frequency range and because the energy contents of low-frequency waves at these ‘intermediate’ depths is relatively large. As part of the Joint Industry Project HawaI, the operational Boussinesq-type wave model of Deltares, TRITON, was used to investigate whether this type of wave models could predict bound low-frequency waves (setdown waves) at intermediate depths. Comparison to measured and theoretical data, however, showed an underestimation of the computed levels of bound low-frequency wave heights for this depth range by a factor 2 to 4. Recently, additional tests were made with TRITON in situations for which the model has been designed: coastal engineering applications in shallow water (depths up to at most 20 m). These also showed an underestimation of the bound low-frequency wave heights, albeit smaller, up to a factor 2. In view of the importance of the energy contained in the low-frequency range for certain nearshore and shoreline processes, such as morphological processes, this underestimation is also of concern in coastal engineering. This triggered the development of a higher-order extension of the TRITON model equations (Borsboom, 2008, Wellens, 2010), with the aim to improve the accuracy of the model for long waves while still keeping computational times within acceptable (operational) limits. This paper reports on the usefulness of the extended model for the field of application considered in JIP HawaI/II: providing wave data for calculating the motions of vessels moored in intermediate depths. The results show a significant improvement of the modeling of nonlinear wave dynamics, including the prediction of bound low-frequency waves. This means that the model extension is an important step towards an operational Boussinesq-type wave model with sufficient accuracy in both the wave-frequency (sea, swell) and the low-frequency range for applications in intermediate depths.
The Effect of Rotational Friction on the Stability of Short-Tailed Fairings Suppressing Vortex-Induced Vibrations
Authors: Assi Gustavo R. S. , Bearman Peter W. , Tognarelli Michael A. , Rodrigues Julia R. H.
Published: November 2011

Experiments have been carried out on a free-to-rotate short-tail fairing fitted to a rigid length of circular cylinder to investigate the effect of rotational friction on the stability of this type of VIV suppressor. Measurements of the dynamic response are presented for models with low mass and damping which are free to respond in the cross-flow and streamwise directions. It is shown how VIV can be reduced if the fairing presents a rotational friction above a critical limit. In this configuration the fairing finds a stable position deflected from the flow direction and a steady lift force appears towards the side the fairing has deflected. The fluid-dynamic mechanism is very similar to that observed for a free-to-rotate splitter plate of equivalent length.
A Low-Cost, High Rate Motion Software Sensor System Based on Novel Data Fusion for Unmanned Surface Vehicle Navigation and Oceanographic Instrumentation Motion Correction
Authors: Gelin Chrystel R. , Xiros Nikolaos I.
Published: November 2011

One of the major challenges in the navigation of underwater vehicles is obtaining precise and reliable positioning updates. Dead-Reckoning aided with Doppler velocity measurement has been, and remains, the most common method for underwater navigation for small vehicles. DR uses a set of navigation instruments to estimate the position of the vehicle by integrating the body-fixed velocity, accelerations, and angular rates with respect to time. Instrument error and bias lead to position error that increases exponentially with time. Thus, current DR systems require frequent position recalibrations. The Global Positioning System (GPS) provides measurements of geodetic coordinates for air and surface vehicles and it is often used to correct positioning error. However, underwater vehicles cannot use GPS for inflight navigation because GPS signals only penetrate a few centimeters past the air-sea interface. Thus, underwater vehicle navigation systems are limited to periodic position update from the GPS when they surface and extend an antenna through the air-sea interface. Standard GPS receivers are unable to provide the rate or precision required when used on a small vessel such as an Unmanned Surface Vehicle (USV). To overcome this, a low cost high rate motion measurement system for an USV with underwater and oceanographic purposes is proposed. The proposed onboard system for the USV consists of an Inertial Measurement Unit (IMU) with accelerometers and rate gyros, a GPS receiver, a flux-gate compass, a roll and tilt sensor and an ADCP. Interfacing all the sensors proved rather challenging because of their different characteristics. Some of the instruments have digital output (Compass/ADCP/GPS) while others have an analog output (IMU/tilt sensor). The proposed data fusion technique integrates the IMU, GPS receiver, flux-gate compass as well as tilt sensor and develops an embeddable software package, using real time data fusion methods, for a USV to aid in navigation and control as well as controlling an onboard Acoustic Doppler Current Profiler (ADCP). While ADCPs non-intrusively measure water flow, they suffer from the inability to distinguish between motions in the water column and self-motion. Thus, the vessel motion contamination needs to be removed to analyze the data and the system developed in this text provides the motion measurements and processing to accomplish this task.
Towards a Time-Domain Finite Element Analysis of Vortex Induced Vibrations
Authors: Mainc¸on Philippe , Larsen Carl M.
Published: November 2011

Slender structures immersed in a cross flow can experience vibrations induced by vortex shedding (VIV), which cause fatigue damage and other problems. Engineering VIV models tend to operate in the frequency domain. A time domain model would allow to capture effects beyond the scope of today’s frequency domain empirical codes: interaction between in-line and cross-flow vibrations, higher order frequency components, structural non-linearities, simultaneous actions from other loads like waves and forced motions at boundaries. There is also the potential to capture the chaotic nature of VIV. Such a model was formulated in the present work: for each cross section and at each time step, the recent velocity history is described as a combination of Laguerre polynomials. The coefficients of that combination are used to enter an interpolation function to predict the instantaneous force, allowing to step the dynamic analysis. An offshore riser was modeled in this way: Some analyses provided an unusually fine level of realism, while in other analyses, the riser fell into an unphysical pattern of vibration. It is concluded that the concept is very promising, yet that more work is needed to understand trajectory stability and related issues, in order to further progress towards an engineering tool.
Design Optimization of Top-Tensioned Risers for Deepwater HPHT Applications: Part II
Authors: Xu Lixin
Published: November 2011

Dry-tree solutions with top-tensioned risers (TTRs) have been successfully used with floating production systems (FPS), such as Spars and TLPs, in a wide range of deepwater applications. Both single-casing and dual-casing top-tensioned risers are field-proven in existing field developments. The top-tensioned risers can bring technical advantages and operational cost benefits. Moreover, recent oil and gas developments that have high pressure and high temperature (HPHT) in combination with severe environmental loads lead to more design challenges for steel risers in deepwater, pushing the design limits of conventional steel pipes in deepwater risers. High-strength steel pipes are therefore considered for both technical and economic reasons. The objective of the study that forms the basis for this paper is to provide top-tensioned riser system configurations that meet challenges of the extremely high operational pressure and environmental loads in deep and ultra-deep waters. Part I of the paper was published in OMAE 2010 [1], addressing strategies for top-tensioned riser sizing and weight management for HPHT applications in deep and ultra-deep waters, and also design considerations for TTR specialty joints. Part II here present spar top-tensioned risers and their support tensioning systems. The paper illustrates the HPHT riser global configuration on spar and the tensioning system performance optimization, as well as coupled motion compensation with the spar platform. The impact of riser loads on spar global performance is also discussed.
Development and Testing of a Point Absorber Wave Energy Conversion
Authors: Foster Jacob , Ghorbani Reza , Garambois Pierre
Published: November 2011

Wave energy conversion as a means for small scale energy production is approaching commercial viability. This paper presents the undergoing development of a wave energy conversion device at the University of Hawaii at Manoa. The device is a three part point absorber with two buoys, one floating and absorbing incoming waves; the other maintaining tension on the third mechanism, the submerged power-take-off unit. This design is discussed as three concept configurations for WEC construction. The analytical solution is developed, and the buoys response is computed due to a selected and analyzed sea-state.
The Dynamic Response of an Offshore Wind Turbine With Realistic Flexibility to Breaking Wave Impact
Authors: de Ridder Erik Jan , Aalberts Pieter , van den Berg Joris , Buchner Bas , Peeringa Johan
Published: November 2011

The effects of operational loads and wind loads on offshore monopile wind turbines are well understood. For most sites, however, the water depth is such that breaking or near-breaking waves will occur causing impulsive excitation of the monopile and consequently considerable stresses and displacements in the monopile, tower and turbine. To investigate this, pilot model tests were conducted with a special model of an offshore wind turbine with realistic flexibility tested in (extreme) waves. This flexibility was considered to be necessary for two reasons: the impulsive loading of extreme waves is very complex and there can be an interaction between this excitation and the dynamic response of the foundation and tower. The tests confirmed the importance of the topic of breaking waves: horizontal accelerations of more than 0.5g were recorded at nacelle level in extreme cases.
Qualification of Enhanced Collapse Capacity UOE Deepwater Linepipe
Authors: Slater Simon , Devine Robin , Aamlid Olav , Hernandez David , Swanek Doug
Published: November 2011

The local buckling of pipelines under external pressure is comprehensively addressed in section 5 of DNV-OS-F101 Rules for Submarine Pipeline Systems. The equations used, calculate the plastic and elastic components to give an overall collapse pressure. These equations include factors that are controlled by the pipe manufacturer. A key feature of the collapse design formula is that the compressive yield stress of UOE pipes is de-rated by 15 per cent through the use of a fabrication factor, αfab. This de-rating is used to account for the Bauschinger effect caused by the pipe forming process, in particular the final expansion. It is well documented that the cold forming (compression & expansion) and light heat treatment can have a beneficial effect on the compressive strength, leading to higher fabrication factors for UOE linepipe. DNV-OS-F101 states, “The fabrication factor may be improved through heat treatment or external cold sizing (compression), if documented”. The standard does not specify what documentation or quality control is required at the pipe mill to ensure every pipe length has the same collapse resistance to allow the increase in fabrication factor. Tata Steel Tubes Europe (Energy), together with Williams Field Services and Det Norske Veritas have recently concluded a technology qualification process, according to DNV-RP-A203 (Qualification Procedures for new Technology), with the specific aim of detailing the documentation and Quality Control needed to satisfy the requirements of DNV OS F101. This would then allow the use of increased fabrication factors in deepwater linepipe design. A key part of the technology qualification was the an extensive testing program that included small-scale compression tests, full-scale collapse tests and the newly developed ring collapse test procedure, which can be utilised as part of the mill quality control system for more representative assessment of the collapse resistance of linepipe material. This paper presents the systematic qualification process; including pipe manufacture, quality control and verification. It also presents some of the key mill capability requirements for producing deepwater UOE linepipe and additional factors that should be considered when optimising for local buckling resistance. Using this approach collapse pressures of above 585bar were achieved for a 457mm diameter × 31.75mm UOE pipe, equivalent to installation depths of over 5000m.
Coupled Analysis of Floating Structures With a New Mooring System
Authors: Yuan Zhiming , Ji Chunyan , Chen Minglu , Zhang Yun
Published: November 2011

As the exploitation of hydrocarbon reserves moves towards deeper waters, the floating structures are becoming more and more popular, and the catenary and taut mooring systems are two widespread mooring systems which are used for these floating structures. However, both of them have their inherent drawbacks. The aim of the present work is to develop and validate a new mooring system which will overcome these shortcomings. To this end, the motion performance of a semi-submersible platform is simulated by employing a full time domain coupled analysis method. It is shown that the new mooring system yields very good motion performance when benchmarked against the taut mooring system, and the reasons for this improved performance are discussed. Also, the new mooring system is compatible with the characteristic of catenary mooring system, which eliminates the requirement of anti-uplift capacity of the anchors. The second aim of this paper is to explore the proper water depth in employing this new mooring system. For this purpose, several typical water depths are simulated. It is found that the new mooring system works well both in deep water and ultra-deep water. But, as the water depth becomes deeper, the advantages of the new mooring system are reduced.
Free Standing Hybrid Riser Global Parametric Sensitivity Analysis and Optimum Design
Authors: Qin Wei , Kang Zhuang , Kang Youwei
Published: November 2011

With a rigid pipe for the vertical free standing portion and a flexible pipe for the near surface dynamic motion region, Free Standing Hybrid Riser (FSHR) has been widely accepted within the offshore oil and gas industry, especially in the field of West of Africa, Gulf of Mexico and field of Brazil. So the optimum design of FSHR system is becoming increasingly meaningful and necessary. This paper mainly presents the parametric sensitivity analysis and optimization of deepwater Free Standing Hybrid Riser (FSHR). First of all, a general description about FSHR system is given in the introduction. The components of FSHR and their function are presented in detail. Secondly, the procedure for parameters optimum design is discussed, and in order to make it looks more clearly and directly, a flowchart is illustrated to show the process. Considering the design requirements for a deepwater FSHR, the optimum design mathematic model is presented with the optimal objective - minimum of FSHR total weight which is directly proportional to project cost. Thirdly, a design instance is given to make clear the flow of FSHR optimum design with the best performance. The global strength analysis is carried out with the use of generic FEA tools. Then the parametric sensitivity analysis is performed through Single-Variable Control Method. Based on FSHR global strength analysis, four sensitivity parameters are carried out, and the minimum bending radius (MBR) of Flexible Jumper and the max von Misses stress are checked according to API RP 2RD. The relationship between sensitivity parameters and the response of the riser system is briefly discussed and the regress analysis can be used to quantify the relativity. Finally, the parameters optimum design is analyzed and the optimum results are checked to ensure its validity.
Coupled Eulerian Lagrangian Approach to Model Offshore Platform Movements in Strong Tidal Flows
Authors: Van den Abeele F. , Vande Voorde J.
Published: November 2011

Offshore platforms are subjected to wind, waves and tidal flows. Tidal flow will generate a steady current, which induces a lift force and a drag force on the platform legs. In addition, water particle velocities induced by waves give rise to an oscillatory flow. As a result, the structure will experience a lift, drag and inertia force when subjected to wave-induced flow patterns. On top of that, a turbulent Von Karman vortex street can appear in the wake of the platform legs for certain combinations of dimensions and flow velocities. Vortex shedding can lead to vortex induced vibrations, which may jeopardize the integrity of the entire offshore platform. Environmental loads can cause significant deformations of offshore structures, which can in turn influence the fluid flow. Multiphysics modelling is required to capture the mechanisms governing fluid-structure interaction. In this paper, a Coupled Eulerian Lagrangian (CEL) approach is pursued to simulate offshore platform movements in strong tidal flows. In a CEL analysis, the fluid flow is modelled in an Eulerian framework: the water is described by an equation of state, and can flow freely through a fixed mesh. The offshore platform is modelled as a compliant structure in a traditional Lagrangian formulation, where the nodes move with the underlying material. Interaction between the fluid domain and the offshore structure is enforced using general contact conditions. The strongly coupled problem is then tackled with an explicit solver. Here, the CEL approach is demonstrated to simulate the movement of an offshore jack-up barge. The response of the vessel is calculated for different flow conditions. The multiphysics model allows evaluating the added value of structural redundancy, e.g. in the number of platform legs required for a safe design. In addition, it provides a valuable tool to predict the tidal windows allowed for given operational conditions.