bluuebunny/crossref_metadata_2025 huggingface dataset:
An Experimental Study on Wax Removal in Pipes With Oil Flow
Authors: Wang Qian , Sarica Cem , Volk Michael
Published: May 2009

Pigging is recognized as one of the most used techniques for removing wax deposits in pipelines. In an earlier paper, the mechanics of the wax removal was studied using an experimental setup under dry conditions, i.e., no oil presence. In this study, the pigging experiments are conducted for both regular disc and by-pass disc pigs under flowing conditions. A new test facility was designed and constructed. The test section is 6.1 m (20-ft) long Schedule 40 steel pipe with an inner diameter of 0.0762 m (3-in.). A mixture of a commercial wax and a mineral oil is cast inside the spool pieces for different wax thicknesses and wax oil contents. The wax breaking and plug transportation forces are investigated separately. The results indicated that the wax breaking force increases as wax thickness increases, and the wax plug transportation force gradient is independent of the wax plug length. In comparison to previous test results, presence of oil reduced the wax plug transportation force. Experimental results also showed that the wax transport behavior of the by-pass pig is significantly different than that of the regular pig. The by-pass pig allows the oil to flow through the by-pass holes and mobilizes the removed wax in front of the pig resulting in no discernible wax accumulation in front of the pig. Therefore, no measurable transportation force was observed for the by-pass pig tests.
Local Buckling Assessments for the Medgaz Pipeline
Authors: DeGeer D. , Timms C. , Wolodko J. , Yarmuch M. , Preston R. , MacKinnon D.
Published: May 2009

Medgaz is a consortium of leading international energy companies, with the aim of designing, building and operating an Algerian-European gas pipeline via Spain. The offshore section of this pipeline will be 210 km long, traversing the Mediterranean Sea floor at a maximum depth of 2160 metres. The 24-inch diameter, grade X70 line will provide up to 8 billion cubic metres of natural gas per year, with first gas flow expected in 2009. To support the technical issues surrounding such an ultra-deepwater pipelay, a number of full scale local buckling tests and detailed finite element analyses were undertaken at the C-FER facility in Edmonton, Canada. Local buckling conditions of concern included buckling of the pipe section at the pipe-buckle arrestor interface and collapse of the plain pipe under high external pressure. These conditions may arise during various phases of pipeline installation and operation, but the primary focus was to evaluate the local buckling integrity of the pipe during installation using the S-lay method. These conditions were assessed for both as-fabricated pipe and pipe that was heat treated to simulate a pipe coating process. This paper describes the Medgaz pipeline, its current state of development, the installation challenges that necessitated the buckling assessments, and some of the work performed throughout the study, including full scale tests, finite element analyses, and regression analyses. Collapse and critical bending strain predictive equations were developed and are also presented, and are compared to other well known collapse and critical bending strain equations. The results of these assessments have suggested that, for the local buckling conditions presented herein, the S-lay method can be successfully employed for ultra-deep water pipelay. The results demonstrated that the proposed pipe-buckle arrestor connection design will not cause premature buckling as the pipe traverses along the stinger during installation. In addition, potentially high bending strains in the overbend will not significantly influence the collapse strength of the pipe. The regression equations presented in this paper have also been shown to provide an accurate means of predicting pipe local buckling and collapse.
Strain Capacity of X80 High-Strain Line Pipes
Authors: Suzuki Nobuhisa , Kondo Joe , Ishikawa Nobuyuki , Okatsu Mitsuru , Shimamura Junji
Published: May 2009

Two compression tests and two bending tests of X80 high-strain line pipes were conducted to investigate the compression capacity and the bending capacity. The high-strain line pipes had the outside diameter of 762 mm (30”) and the D/t ratio of 49. The compression tests revealed that the pipes had the critical compressive strains of 0.90 and 0.78%. The bending tests of the pipes clarified that the 2D average critical compressive strains were 2.40 and 2.15% and the 1D average were 2.67 and 2.28%. The analytical solutions gave very fine predictions about the critical compressive stress and strain of the pipes subjected to axial compression. Based on the FEA results, while almost no effects of the geometric imperfections on the compression capacity were recognized, the effects of the geometric imperfections on the bending capacity were significant.
A PMM Experimental Research on Ship Maneuverability in Waves
Authors: Xu Yongze , Bao Weiguang , Kinoshita Takeshi , Itakura Hiroshi
Published: May 2009

Recently, much attention has been paid on the maneuvering characters of ships in waves. Apparently, ships steering in waves are subjected to linear wave loads at the encountering frequency. Nevertheless, the maneuvering characters of ships are more affected by the nonlinear wave drift forces at low frequencies. In the present work, to examine the wave effects on the ship maneuverability, a serial of towing tests with a Planer Motion Mechanism system (PMM) is carried out in waves. Forces and horizontal displacements of the model are measured throughout the tests. The measured data are analyzed by the Fourier analysis to obtain components at different frequencies. Discussion on the importance of the force components is made.
Using Transient Inflow Performance Relationships to Model the Dynamic Interaction Between Reservoir and Wellbore During Pressure Testing
Authors: Costantini Aldo , Falcone Gioia , Hewitt Geoffrey F. , Alimonti Claudio
Published: May 2009

The fundamental understanding of the dynamic interactions between multiphase flow in the reservoir and that in the wellbore remains surprisingly weak. The classical way of dealing with these interactions is via inflow performance relationships (IPR’s), where the inflow from the reservoir is related to the pressure at the bottom of the well, which is a function of the multiphase flow behaviour in the well. Steady-state IPR’s are normally adopted, but their use may be erroneous when transient multiphase flow conditions occur. Transient multiphase flow in the wellbore causes problems in well test interpretation when the well is shut-in at surface and the bottomhole pressure is measured. Pressure build-up (PBU) data recorded during a test can be dominated by transient wellbore effects (e.g. phase change, flow reversal and re-entry of the denser phase into the producing zone), making it difficult to distinguish between true reservoir features and transient wellbore artefacts. This paper introduces a method to derive the transient IPR’s at bottomhole conditions in order to link the wellbore to the reservoir during PBU. A commercial numerical simulator was used to build a simplified reservoir model (single well, radial co-ordinates, homogeneous rock properties) using published data from a gas condensate field in the North Sea. In order to exclude wellbore effects from the investigation of the transient inflow from the reservoir, the simulation of the wellbore was omitted from the model. Rather than the traditional flow rate at surface conditions, bottomhole pressure was imposed to constrain the simulation. This procedure allowed the flow rate at the sand face to be different from zero during the early times of the PBU, even if the surface flow rate is equal to zero. As a result, a transient IPR at bottomhole conditions was obtained for the given field case and for a specific set of time intervals, time steps and bottomhole pressure. In order to validate the above simulation approach, a preliminary evaluation of the required experimental set-up was carried out. The set-up would allow the investigation of the dynamic interaction between the reservoir, the near-wellbore region and the well, represented by a pressured vessel, a cylindrical porous medium and a vertical pipe, respectively.
Investigation of a Cup Anemometer in Oblique Flow for Measurement of Wind Velocity in Offshore Installations in the Field of Aquaculture
Authors: Winkel Hans-Joachim , Paschen Mathias , Jensch Mario
Published: May 2009

In Germany the combination of environmentally friendly changed wind energy with aquaculture is seen as an opportunity for multiple use of natural marine resources. Loads and stresses of “open ocean” offshore installations depend on wind velocity. For measurements of wind velocity within the field of wind energy conversion the only cost effective device is the cup anemometer. It has a linear relation between number of revolutions and velocity. Questionable is the measuring result if the wind vector is not within the ‘measuring plane’. Results of CFD calculations of the flow around an anemometer cup are given. With the force coefficients (lift and drag coefficients) an anemometer in inclined flow is calculated. It will be shown that the measured velocity within the measuring plane is higher than the cosine component of velocity.
Calculation of In-Line Vortex Induced Vibrations of Free Spanning Pipelines
Authors: Larsen Carl M. , Yttervik Rune , Aronsen Kristoffer
Published: May 2009

Pure in-line (IL) vibrations will in many cases contribute significantly to fatigue damage for free spanning pipelines. This might be the case even if IL amplitudes are smaller than cross-flow (CF). While CF response has been subjected to research for a long time, little attention has so far been given to the pure IL VIV case. The hydrodynamic coefficients needed for response calculation have in fact not been available until recently, but results from forced IL oscillations have improved this situation. Data for added mass and force in IL direction has been used to establish a general response model along the same lines as for traditional CF response analysis. This has made it possible to calculate stresses from IL VIV in free spanning pipelines, and include the influence from interaction with the seafloor at the span shoulders. A brief presentation of the analysis method is given, but the main part of the paper gives results from a case study that illustrates important effects and the significance of IL response as compared to CF.
Some Effects of Non-Linearity for High Wave Groups in Front of a Vertical Wall
Authors: Romolo Alessandra , Arena Felice
Published: May 2009

The upright fully reflective wall-types breakwaters have been widely adopted for development of harbours. The design of these structures depends on the characteristic wave field resulting from the interaction of the sea waves with the reflective structure, which is strongly irregular and nonlinear. This paper deals with the mechanics of long-crested wave groups in front of a vertical wall. For this purpose the ‘Quasi Determinism’ theory, particularized for the reflection of sea wave groups, is extended up to the second-order. The nonlinear processes free surface displacement and velocity potential, when a large wave crest occurs on the vertical wall or in front of it, are obtained. In the application some properties of nonlinear wave groups in reflection, when a high wave crest occurs on the wall or in front of it, are investigated.
Sloshing Effects Accounting for Dynamic Coupling Between Vessel and Tank Liquid Motion
Authors: Zalar Mirela , Diebold Louis , Baudin Eric , Henry Jacqueline , Chen Xiao-Bo
Published: May 2009

Sloshing, a violent behaviour of liquid contents in tanks submitted to the forced vessels’ motion on the sea represents one of the major considerations in LNG vessels design over several past decades. State of the art of sloshing analysis relies on small-scale sloshing model tests supported by extensive developments of CFD computation techniques, commonly studying one isolated tank submitted to the forced motion without their mutual interaction. In reality, wave-induced response of the vessel carrying liquid cargo is affected by internal liquid motion, and consequently, tank liquid flow is altered by the vessel motion in return. An efficient numerical model for dynamic coupling between motions exerted by tank liquid (sloshing) and rigid body motions of the vessel (seakeeping) was developed in Bureau Veritas, formulated under the assumptions of linear potential theory in frequency domain. As already experienced with anti-rolling tanks, strong coupling effect is perceived on the first order transverse motions. However, consequences of coupled motions on sloshing loads have not been explored yet. This paper presents comparative analysis of sloshing effects induced by coupled and non-coupled vessel motion, introduced as the excitation to 6 d.o.f. small-scale model test rig. Possible risk of coupled effects is demonstrated on the example of standard size of LNG carrier operating with partly filled cargo tanks.
A Discussion of Long Term Response Versus Mean Maximum Response of the Selected Design Sea State
Authors: Haver S.
Published: May 2009

The overall aim of the design process is to ensure that a structure can withstand with sufficient margin all foreseen load events. Offshore rules and regulations will typically define a set of rules to be followed by the designer. By fulfilling these it is tacitly assumed that the aimed safety level is achieved. The governing regulations and rules and, not the least, their interpretation may vary between various offshore regions. This may be the case both for the load side of the problem and the strength side of the problem. In connection with design of offshore structures, the largest variability and uncertainty is typically associated with the load side of the problem. The purpose of this paper is to discuss the consequences of two rather common definitions regarding the characteristic environmental load to be used for design purposes: i) The characteristic response is taken as the load for which the annual probability of exceeding it is q (here referred to as the q-probability load), ii) The characteristic response is taken as the mean extreme response of a sea state for which the annual probability of exceeding it is q (q-probability sea state). Results are shown for some generic response cases varying from a linear response case being rather insensitive to wave period to a quadratic response problem associated with a critical wave period band.
Simulation of Sloshing in LNG Tanks
Authors: Peric Milovan , Zorn Tobias , el Moctar Ould , Schellin Thomas E. , Kim Yong-Soo
Published: May 2009

The aim of this paper is to demonstrate the prediction of internal loads on liquified natural gas (LNG) tanker ships and on offshore platforms. We use the moving grid approach and a finite volume solution method designed to allow for arbitrary ship motion. The motion of liquid is computed using an interface-capturing scheme which allows overturning and breaking waves. By performing a coupled simulation of the flow and vessel motion, it is possible to obtain a realistic response of the liquid in a tank to external excitation, e.g. by sea waves. Results are first presented for an LNG tanks whose motion is prescribed in accordance with planned laboratory experiments. Both two-dimensional (2D) and three-dimensional (3D) simulations are performed. The aim is to demonstrate that 1) realistic loads can be predicted using grids of moderate fineness, 2) the numerical method is able to accurately resolve the free surface even when severe fragmentation occurs, and 3) long-term simulations over many oscillation periods are possible without numerical mixing of liquid and gas. The plausibility of a coupled simulation of both vessel motion and the flow inside tanks and outside the vessel is then demonstrated for a full-size ship with partially-filled tanks exposed to head waves. In this simulation the forces and moments exerted by the sea cause the vessel to move, exciting the sloshing of liquid in tanks. For the computation of vessel motion, both sea-induced forces and forces due to sloshing in tanks are taken into account when determining the resultant forces and moments. While there are no experimental data for comparison at this time, the results look plausible and encourage further validation and application studies.
Probabilistic Fatigue Reliability of Large Diameter Steel Catenary Risers (SCR) for Ultra-Deepwater Operations
Authors: Akpan U. O. , Koko T. S. , Rushton P. A. , Tavassoli A. , Else M.
Published: May 2009

For deepwater development in the Gulf of Mexico, steel catenary risers (SCRs) supported from both SPAR and semi-submersible platforms have proven to be successful solutions for in-field flowlines, tie-backs, and export systems. It is envisaged that this will continue to be a promising solution in ultra deep-water applications, up to and beyond 10,000 ft. The study, commissioned by the Mineral Management Service (MMS), investigated the reliability of large-diameter SCRs in ultra-deepwater operations. The primary damage mode considered is fatigue failure. A probabilistic methodology for fatigue reliability is developed, which utilizes deterministic cumulative fatigue damage indicators, namely the stress levels and cycles associated with the various sea states and the fatigue strength of the members. Uncertainties in structural load and material properties are accounted for by assigning probability distributions and standard deviations to the deterministic stress levels. Furthermore, fatigue strength parameters, Miner’s indices, and capacities are modeled as random variables. First order reliability method (FORM) is employed for estimating fatigue reliability. The methodology is applied to three deterministic case studies presented by Intec Engineering (2006a, 2006b). The case studies involved either a SPAR or a semi-submersible platform. For the sake of brevity, a case study involving only a SPAR platform is presented in this paper. The effect of uncertainties in parameters on fatigue reliabilities is investigated. It is observed that the fatigue reliability estimates followed similar trends as the deterministic cumulative damage results, and hence can be used to complement deterministic estimates. Additional benefit and insight gained from the probabilistic study, which can be used for design decisions, include information regarding probabilistic importance and probabilistic sensitivity analysis. For case study presented here, it is seen that in general, uncertainty in the fatigue strength exponent (m) has the highest impact on fatigue reliability of SCRs. The second most important random variable is the stress range (S), which captures uncertainties in parameters such as loads and material properties. Parametric sensitivity studies on the fatigue strength parameters indicate that SCR reliability is sensitive to both the standard deviation and probability distribution of the parameters, thus highlighting the need for accurate probabilistic calibration of the random variables.
Tidal Power and Ocean Ice in the Bay of Fundy, Canada: 1968-2007
Authors: Sanders Richard , Baddour Emile
Published: May 2009

Ocean ice capable of impacting marine operations occurs periodically in the most favorable sites for energy harvest from the tidal currents of the Bay of Fundy in Nova Scotia and New Brunswick. In-stream tidal current harvesting devices deployed at these sites should to be engineered to tolerate at least 30% ice cover with 15 cm (6 inch) thick floes at least 100 metres in length. Propelled by tidal currents and prevailing winds, ice floes may achieve velocities in excess of 8 knots in some locations. In very severe winters, in-stream tidal current harvesting devices may be subjected to periods of 90% cover of rapidly moving or packed ice thicker than 30 cm (12 inches). Markets for ice-tolerant tidal current harvesting devices developed under the moderate ice conditions in the headwaters of the Bay of Fundy may exist in other jurisdictions with energetic tidal flows which experience more severe conditions of ocean ice.
Utilization of Nonlinear Wave Motions in Magnetic Fields
Authors: Frandsen Jannette B.
Published: May 2009

In this contribution, the energy from steep waves is extracted from liquid sloshing in tanks. The aim is to develop a damping device to control structural vibrations in for example free standing towers, offshore platforms and other engineering structures. The focus of the work is to add further energy to the liquid sloshing motions through a magnetic field. Without the magnetic field, the damping device may be too heavy and thus impractical. We seek to develop light weight environmental friendly dampers. Liquid sloshing in tanks exhibit complicated free surface behavior, especially when the waves becomes steep and break. As a result, mode interactions occur which make it difficult for practitioners to know the performance of the tuned liquid dampers. It is also known that wave breaking in tanks is the source which provide maximum energy. It is this situation, we wish to exploit further in a magnetic field. Herein, small scale physical tests are undertaken in a square tank. The parametric study includes a variation of liquid, liquid depth, magnetic strength, forcing frequency and forcing direction. The preliminary results show that a magnetic field can suppress the free surface significantly, and thus provide additional energy, potentially resulting in smaller damping devices.
An Integrity Management Strategy Based on Flexible Risers Conceived to Be Self-Monitored
Authors: Brack Marcelo , Alves Mendes Sergio Roberto , Lancelotti Campos Rodrigo Cesar , Lobianco e Souza Luiz Antonio
Published: May 2009

Many of the current practices applied for offshore production managing emphasize those aspects related to the efficiency of oil & gas production and exportation. Nowadays, due not only to the inherent technical challenges associated to deep-water applications but also to the increasing importance of the HSE aspects and requirements faced by the industry, a crescent demand to implement a philosophy which focalizes safety, reliability and integrity of their subsea flowing systems is taking place. The handling of monitored data in order to help the pipe operator to control fluid transportation throughout flexible pipes is an old practice performed by Petrobras and other oil companies in the world. However, the idea of acquiring a product which has been conceived, designed and manufactured with both intrinsic monitoring and expert systems is a recent idea. The tendency of the main flexible pipe manufactures is normally to face the problem from the traditional point of view: those systems are considered dissociated one from the other and as appendices to be installed, in the field, after system connection and start-up. Experience demonstrates that the installation of those systems during the operation phase has a number of limitations, restrictions, and associated problems. The main objective of this paper is to present an integrity management strategy based on the concept of the self-monitored flexible riser. Self-monitoring is understood as the own capacity of a product (flexible pipe and components with built-in devices) to acquire data about itself and make use of data from the internal and external environments, during its service life. Monitored data is then processed in order to generate relevant information for the fluid transportation business. The basic premise is to select, during product conception phase, some key-parameters to be further monitored, during the operation phase. The selection is determined by technical criteria depending on the potential failure mechanisms and modes related to the particularities of each different application.
Design and Installation of a Tension Moored Wind Turbine
Authors: Zambrano Thomas , MacCready Tyler , Roddier Dominique , Cermelli Christian
Published: May 2009

This paper describes the design of the floater and the mooring system for a small wind turbine. The engineering basis and the hydrodynamics calculations are described, as well as the installation and commissioning sequences.
Wave Climate Analysis for a Wave Energy Conversion Application in Brazil
Authors: Beserra Eliab R. , Mendes Andre´ L. T. , Estefen Segen F. , Parente Carlos E.
Published: May 2009

A variety of ocean wave energy conversion devices have been proposed worldwide considering different technology and energy extraction methods. In order to support full-scale prototype design and performance assessments of a conversion scheme to be deployed on the northern coast of Brazil, a long-term wave climate analysis is under development. A 5-year pitch-roll buoy data series has been investigated through an adaptive technique to enhance spatial resolution and allow for accurate wave directionality evaluation. Device design most influential variables such as extreme significant wave height, peak period and directionality were considered. Temporal variability in wave energy levels was particularly investigated for energy resource assessment. The major findings of this work include the narrow directional amplitude of the incident wave and higher significant wave heights of locally generated waves. The estimated energy resource levels agreed well with literature, also showing little annual fluctuation. The wave climate demonstrated to be in full agreement with the large-scale Equatorial Atlantic atmospheric variability, dominated by either local wind waves or by distant storm swells.
A More Efficient Form of the Method of Moments for Derivation of Morison’s Force Coefficients
Authors: Najafian G.
Published: May 2009

Morison’s equation is the most widely used method of predicting wave forces on slim cylindrical members of offshore structures. The equation assumes that the total force of the fluid is made up of a drag force and an inertial force, where the drag component is due to water particle velocity and the inertial component is due to water particle acceleration. In practice, the force uses two empirical coefficients, which are usually referred to as the drag and inertia coefficients. The values of these empirical coefficients are determined from laboratory and/or field experiments. In a typical wave load investigation, the wave force together with corresponding water particle velocity and acceleration are measured. The measured data is then analysed to calculate constant values for drag and inertia coefficients. One of the methods used in derivation of these coefficients is the method of moments. However, the coefficients obtained from this method show considerable scatter due to large sampling variability. The purpose of this paper is to introduce a more efficient form of the method of moments, which will lead to more accurate estimates of Morison’s coefficients by reducing their sampling variability. Simulated data has been used to compare the new method of moments with the conventional one. The results indicate that the new method is superior to the conventional one. This is particularly the case for drag-dominated forces.
Numerical Prediction of MODUs Drift During Hurricane Katrina
Authors: Tahchiev Galin , Zhang Jun
Published: May 2009

During severe hurricanes, such as Katrina, the mooring system of a number of Mobile Offshore Drilling Units (MODUs) in the Gulf of Mexico failed. Drifting MODUs may potentially damage other critical elements of the offshore oil and gas infrastructure by colliding with floating or fixed production systems and transportation hubs, or by rupturing pipelines owing to their dragging anchors over the seabed. To avoid or mitigate the damage caused by a drifted MODU, it is desirable to understand the mechanics of the drift of a MODU under the impact of severe wind, wave and current and have the capability of predicting the trajectory of the drift. To explore the feasibility and accuracy of predicting the trajectory of a drifting MODU based on real-time or hindcast met-ocean conditions and limited knowledge of the condition of the drifting MODU, this study employed a simplified equation describing only the horizontal (surge, sway and yaw) motions of a MODU under the impact of steady wind, current and wave forces. The simplified hydrodynamic model neglects the first- and second-order oscillatory wave forces, unsteady wind forces (owing to wind gustiness), wave drift damping, and the effects of the body oscillation on the steady wind and current forces. It was assumed that the net effects of the oscillatory forces on the steady motion are insignificant. To verify the accuracy and feasibility of our simplified approach, the predicted drifts of two MODUs were compared with the corresponding measured trajectories recorded by the Global Positioning System (GPS).
New Buoy Designs for Ocean Observatories
Authors: Steen Atle , Halkyard John , Berger Jon
Published: May 2009

The Ocean Research Interactive Observatory Networks (ORION - www.orionprogram.org) is a federally funded program that focuses the science, technology, education and outreach of an emerging network of science driven ocean observing systems. The objective of this initiative is to provide the scientific community with ocean observatories around the globe that will provide the infrastructure necessary to collect, among other things, time series data of oceanographic and geophysical phenomena which can’t be collected from vessel expeditions alone. The program calls for placing buoys in all water depths and a full range of metocean domains, from calm equatorial sites to high latitude deepwater southern ocean locations. Conventional oceanographic buoys will not meet all the requirements of this program, so new buoy designs are being developed. The authors have developed a variation of the spar buoy design for the more robust requirements, and are currently looking into other options. This paper will review the more demanding criteria for the Orion buoys, and the proposed solutions for new buoy designs.
PA 12 for Flexible Flowlines and Risers
Authors: Buchner Stefan , Baron Christian , Sheldrake Terry , Tu¨llmann Ralf , Dowe Andreas , Bulmer Graeme
Published: May 2009

In their supply chain strategy in 2003 Wellstream decided to expand the application of Nylon (PA) grades for off-shore use in order to move away from a single source supply. Degussa have developed a new grade of PA 12 for use in offshore applications, such as flexible pipes. In a joint effort Wellstream International Limited and Degussa AG have performed a qualification test programme on this new grade. On the basis of these tests Wellstream has obtained a Lloyds “Certificate of Material Qualification for the Use of Polyamide 12 Grade - VESTAMID® LX9020 in non-Bonded Flexible Pipes”. Since this certificate was obtained in June 2006 additional test results were obtained which confirm the outstanding material properties and long term performance of the new material. This presentation focuses on results from long term ageing and compatibility experiments of PA 12 in air, in water with different CO2 levels, in crude oil with different TAN, in methanol and other oilfield additives. Some of these tests have been performed over a period of almost three years. The presented data is relevant to predict the long term performance of PA 12 in flexible pipe service.
The Effect of Structural Compliance on Fatigue Crack Growth in Jacket Structures
Authors: Schneider Ronald , Sanderson David J. , Thurlbeck Simon D.
Published: May 2009

Quantifying the fatigue crack growth and remaining life in joints making up jacket structures forms one of the basic requirements of a sub sea structural integrity assurance scheme. The accurate prediction of the likely failure time of welds allows a realistic estimate of the risk of structural collapse at any stage in a structure’s life. It is current practice to consider the welds making up the member as individual components rather than looking at the whole compliant system of welds, joints, members, and structural framing arrangement. In this approach, the nominal loads in any one member are determined from an analysis of the undamaged structure and are then applied to a crack growth solution using handbook stress intensity factor solutions to determine the fatigue life of that component. This method assumes that the applied load is purely load-controlled whereas in reality it is a combination of both load and displacement controlled. A study was performed to investigate the influence of the surrounding structure on crack growth in tubular members located in jacket structures. The aim of the study was to verify whether the traditional approach, which uses stresses from undamaged structures to evaluate crack growth in individual components, is appropriate. The findings of the study showed that structural compliance has only a beneficial effect on fatigue growth in the latter stage of the crack growth process with crack lengths greater than 40% circumference. It was shown that the beneficial effect of structural compliance on fatigue crack growth in the later stage of the growth process does not significantly increase the overall fatigue life. It was concluded, that the current approach, which uses stresses from undamaged structures to evaluate crack growth in individual components, is valid and safe to use.
Simplified Wave In Deck Analysis In Irregular Seas
Authors: Johannessen Thomas B.
Published: May 2009

Using a simple von Karman approach, Baarholm (2005) obtained good agreement between measured and predicted impact force underneath the deck of a GBS. This method rested, however, on a very careful second order modelling of the diffracted free surface elevation underneath the deck. In particular, the method was found to be very sensitive to the maximum wave elevation (i.e. the magnitude of the negative airgap). These requirements limit the applicability of the method since an accurate representation of the surface elevation is not usually available. In the design of an offshore structure, however, much effort is spent on determining the airgap underneath the deck. Zones with negative airgap are found by approximate calculations or by extrapolating on model test results. The method described in the present paper simplifies the problem by using Baarholm’s method subject to a known negative airgap height above the deck rather than attempting to solve the whole problem. With a known negative airgap height and using the well known expected linear shape of the wave around this maximum, a robust and conservative estimate of the deck loading can be obtained. The nonconservative property of linear theory that it underestimates the crest height is thus removed and replaced with the conservative property that too much water is present above the deck level. The paper describes this method, provides a practical design example and compares the results with model test results. The agreement with model test results is good for the single event which has been compared, but there is a clear scope for further comparison.
Studies on a Submerged Flexible Sheet Type Wave Energy Device With Distributed Point-Wise Secondary Power Conversion
Authors: Korde Umesh A.
Published: May 2009

This paper reports ongoing work on a horizontally submerged tension-moored flexible sheet serving as a wave energy absorber. The device represents a “smart structure” designed for efficient wave energy conversion, and would have such advantages as low primary converter costs, multiple resonant peaks, and ready tunability with reactive loading. The natural response of the sheet is studied analytically, and both flexural and membrane effects are modeled. Natural frequency dependence on device length and in-plane applied tension is investigated. Secondary energy conversion is via hydraulic actuators applying point loads, and desirable locations for the actuators are identified. Finally, the effect of reactive loads on the dynamic response is explored through calculations on the lowest oscillation mode.
Performance of Steel Jacket Platforms in Recent Gulf of Mexico Hurricanes
Authors: Puskar Frank J. , Verret Sean M. , Hariharan Aditya
Published: May 2009

In the past few years there have been several large hurricanes in the Gulf of Mexico that have destroyed or damaged over 200 fixed offshore platforms. These include hurricanes lvan (2004), Katrina (2005) and Rita (2005). Prior to these, the most recent hurricane to cause this level of damage was hurricane Andrew in 1992. Below water damage consisted of separated underwater braces, buckled braces, broken legs, and cracked connections. Above water damage consisted of wind and wave damage to decks and topsides equipment. Interestingly there has been little if any pile damage in any of these hurricanes. Although some newer platforms suffered damage and even destruction, most of the destroyed and damaged platforms were of older vintage and designed to American Petroleum Institute (API) Recommended Practice 2A-WSD (RP2A) practices that have since been improved. This paper summarizes the types of damage and destruction that has been found, including likely causes. It describes how some of this damage correlates to API design procedures. The work is based upon a series of projects funded by the Minerals Management Service (MMS) to study the effects of hurricanes on these types of offshore facilities.
Contributions to Second Order Directional Sea Simulation and Wave Forces
Authors: Sharma Jagat N. , Dean Robert G.
Published: May 2009

We briefly describe a method for simulating second order directional seas and associated wave forces. We present wave force calculations for simplified forms of real offshore structures. Compared to unidirectional wave force calculations, this method reduces total design wave force. Complex subharmonic motion of large floating structures can be easily understood within the framework of this simulation method. A real sea is first represented by discrete linear waves of many frequencies traveling in many directions. Then, second order effects are calculated using equations derived for this purpose. The sum of linear and nonlinear waves is used to calculate wave forces on example offshore structures. The simulation method has been applied to calculate total wave forces on a single pile and a 4-pile group. Simulation method calculated wave forces on a single pile and a 4-pile group on a 60 ft square array are only 61% of wave forces using unidirectional wave methods. These results are the same as those obtained by a so-called hybrid method (Dean 1977) for the drag dominant case. When the pile separation is increased to 300 ft (similar to a TLP or a semisubmersible) the corresponding reduction factor varies from 0.79 for a unidirectional random sea to 0.61 for an omnidirectional random sea. Large offshore structures are relatively insensitive to the linear frequencies but could have very large response to the subharmonic frequencies in the simulation method. This is consistent with the field and laboratory observations.
Dynamic Interaction Between SCR and the Seabed
Authors: Nakhaee Ali , Zhang Jun
Published: May 2009

The interaction between a Steel Catenary Riser (SCR) and seabed soil is extremely complicated, in which the supporting force of soil to the riser is nonlinear and time dependent, and soil absorbs the energy of the motion of a SCR near its Touch Down Zone (TDZ). The magnitude of soil stiffness is crucial to the fatigue life of a SCR and the damping of soil may play an important role in reducing the motion of a SCR near its TDZ. Therefore, it is important to allow for a realistic behavior of seabed soil in simulating the interactions between SCRs and the seabed and between a floating structure and its SCRs. Recent studies on the behavior of seabed soil in interacting with risers have resulted in more realistic P-y curves of the seabed soil, where P stands for the supporting force of soil and y for the vertical penetration of the riser with respect to the seabed surface. This study extends an existing finite element numerical code for dynamic analysis of mooring lines and risers, known as CABLE3D, to allow for the low boundary condition of a SCR at the seabed to be described by a P-y curve or an approximation of the P-y curve instead of simplified linear springs. The results of the extended CABLE3D can lead to a more accurate estimate of the fatigue life of a SCR.
Lessons From Failure of the Flood Protection System for the Greater New Orleans Area During Hurricane Katrina
Authors: Bea Robert
Published: May 2009

This paper summarizes the key lessons that have been learned from the failure of the flood defense system for the greater New Orleans area during hurricane Katrina. This was a manmade disaster whose roots were firmly embedded in failure of the Technology Delivery System that had been charged with providing adequate flood protection for the public. Two basic approaches for going forward are explored. Recommendations for improvements in engineering criteria and guidelines and in the Technology Delivery System charged with providing acceptable flood protection are advanced.
Influence of the Geometric Imperfection on the Buckling Behavior of Floating Platform Column Under Axial Load
Authors: Estefen Tiago P. , Werneck Daniel S. , Estefen Segen F.
Published: May 2009

The present work focuses on the design of the new generation of semi-submersible platforms for oil&gas production offshore that is based on column square cross-sectional area. The platform column is based on an arrangement of stiffened flat panels having their ultimate strength characterized by buckling under in-plane compressive loading. Distortions induced by fabrication have considerable influence on the buckling behavior and are discussed in order to provide design recommendations. The aim of the paper is to study a segment of the column structural arrangement between robust transverse frames to analyze the failure behavior of the stiffened panels. Previous research demonstrated the strong influence of both mode and magnitude of the geometric imperfection distribution, as well as the boundary conditions. Numerical and experimental simulations for small scale isolated panels are carried out in order to perform a correlation study to adjust the numerical model for further use in more complex numerical simulations of the structural failure of the column arrangement. The stiffened panels are analyzed to identify the buckling initiation on a particular panel and then the interaction between plates, longitudinal stiffeners and the robust transverse frames during the progressive failure of the whole column.
Experimental Research on Methods of Preventing Stray Children
Authors: Miyazaki Wataru , Kondo Takeo , Yamamoto Kazukiyo
Published: May 2009

An experiment aimed at reducing a lost child by a solution for the lost child early stage when I used a prevention of lost child policy and the personal information that I posted position information on because a lot of lost children were protected conducted an investigation at a Japanese beach. As a result, it became clear for a prevention of lost child policy to lead to decrease of a lost child. In addition, it was a solution, and stable solution was enabled in comparison with conventional correspondence in a short time for the lost child early stage.
Experimental Study on Interaction Between Waves and Netting: Damping Effects and Forces
Authors: Lader Pa˚l F. , Jensen Atle , Sveen Johan Kristian , Fredheim Arne , Olsen Anna , Enerhaug Birger
Published: May 2009

One of the possibilities to expand sea-based fish farming is to move the aquaculture installations away from the conflicts of the coastal zone, and into more open ocean locations. However, open ocean aquaculture puts other demands on the structures than aquaculture in sheltered locations, and in this context it is necessary to understand the behaviour of the aquaculture structures as they are exposed to large sea-loads from waves and current. Flexible netting is a main part of most sea-based aquaculture structures, and in this paper the interaction between waves and netting is studied. Experiments were conducted at the narrow wave flume facility at the University of Oslo, Norway, where several different regular wave cases were run through netting with different solidity. The wave energy was measured after the wave had passed through the net and compared with the energy of an undisturbed wave to assess the wave damping properties of the net. The vertical and horizontal forces were also measured. The findings show that the damping effects of the netting are not necessarily correlated with the wave forces, indicated complex nonlinear processes contributing to the fluid-net interaction. The amount of nonlinear energy in the wave and force waveforms is also investigated, and it is shown that the nonlinear energy in the incoming wave results in an even higher level of nonlinear components in the forces experienced by the net.
Quantification of Uncertainty in Reserves Estimation From Decline Curve Analysis of Production Data for Unconventional Reservoirs
Authors: Cheng Yueming , Lee W. John , McVay Duane A.
Published: May 2009

Decline curve analysis is the most commonly used technique to estimate reserves from historical production data for evaluation of unconventional resources. Quantifying uncertainty of reserve estimates is an important issue in decline curve analysis, particularly for unconventional resources since forecasting future performance is particularly difficult in analysis of unconventional oil or gas wells. Probabilistic approaches are sometimes used to provide a distribution of reserve estimates with three confidence levels (P10, P50 and P90) and a corresponding 80% confidence interval to quantify uncertainties. Our investigation indicates that uncertainty is commonly underestimated in practice when using traditional statistical analyses. The challenge in probabilistic reserves estimation is not only how to appropriately characterize probabilistic properties of complex production data sets, but also how to determine and then improve the reliability of the uncertainty quantifications. In this paper, we present an advanced technique for probabilistic quantification of reserve estimates using decline curve analysis. We examine the reliability of uncertainty quantification of reserve estimates by analyzing actual oil and gas wells that have produced to near-abandonment conditions, and also show how uncertainty in reserves estimates changes with time as more data become available. We demonstrate that our method provides more reliable probabilistic reserves estimation than other methods proposed in the literature. These results have important impacts on economic risk analysis and on reservoir management.