bluuebunny/crossref_metadata_2025 huggingface dataset:
Hamiltonian Formulation and Long Wave Models for Internal Waves
Authors: Craig Walter , Guyenne Philippe , Kalisch Henrik
Published: May 2009

We derive a Hamiltonian formulation of the problem of a dynamic free interface (with rigid lid upper boundary conditions), and of a free interface coupled with a free surface, this latter situation occurring more commonly in experiment and in nature. Based on the linearized equations, we highlight the discrepancies between the cases of rigid lid and free surface upper boundary conditions, which in some circumstances can be significant. We also derive systems of nonlinear dispersive long wave equations in the large amplitude regime, and compute solitary wave solutions of these equations.
Turbulence Measurements in a Gulf of Mexico Warm-Core Ring
Authors: Mitchell Thomas M. , Lueck Rolf G. , Vogel Michael J. , Raye Robert E. , Forristall George Z.
Published: May 2009

Oceanographic measurements were made in a Loop Current Eddy in the Gulf of Mexico to characterize the turbulence associated with these eddies. Measurements were made within the eddy, and across the strong frontal boundary delineating the eddy from the surrounding waters. The survey was conducted August 23–30, 2003, from the R/V Pelican. The towed vehicle, the TOMI, was equipped with a special 300 kHz acoustic Doppler current profiler (Medusa ADCP) that had its four beams directed fore, port, starboard and down. The along-beam velocities resolved structures with wavelengths of 4 to 60 m. The vehicle also carried shear probes for measuring velocity fluctuations in the dissipation range (0.5 to 100 cycles per meter), and other environmental sensors for measuring temperature, salinity, depth and vehicle orientation. Ship equipment included a 75 and 300 kHz hull-mounted ADCP, CTD, and meteorological sensors. Tows were conducted at 25, 50, 100 and 150 m depths around the northern edge of the Loop Eddy in currents of up to 1.7 m s−1. Turbulence was detected with the shear probes, but mostly in the 130–150 m depth range around the local salinity maxima. The level of turbulence is weak and it is distributed intermittently in both space and time. The most energetic events of turbulence have eddy scales of at most 4 meters and velocity scales of only 1 cm s−1. The typical and average values are more than 10 times smaller. The concurrent measurements of velocity with the Medusa ADCP did not reveal any signals significantly larger than the noise level of this instrumentation, namely 2 cm s−1. Overlap averaging of the forward directed beam reduced the noise level to 0.5 cm s−1 but still failed to reveal real environmental signals. This “null-result” is consistent with the simultaneous measurements taken with the shear probe. These low levels of turbulence are also consistent with reports of measurements in the Gulf Stream, the Florida Current, and a Gulf Stream Warm-Core Ring. Funding was provided by the DeepStar oil industry research consortium. Complete details of the program are provided in Reference [6].
Development and Qualification of Alternative Solutions for Improved Fatigue Performance of Deepwater Steel Catenary Risers
Authors: Aggarwal Rajiv K. , Mourelle Marcio M. , Kristoffersen Steinar , Godinot Henri , Vargas Pedro , Else Michael , Schutz Ronald W. , Portesan Giovanni , Izquierdo Alfonso , Quintanilla He´ctor , Schamuhn Stefan , Sches Celine
Published: May 2009

Several initiatives have been undertaken by the operators, engineering companies, product manufacturers, and regulatory bodies to enable increased use of steel catenary riser (SCR) design in development of deepwater and ultra-deepwater fields. Some of these efforts focus on improvement in understanding of soil-structure interaction at SCR touch down zone (TDZ) and its impact on fatigue damage estimates through analytical studies, laboratory testing, or in-field monitoring of SCR behavior. Through recent studies and laboratory testing work for floating platforms with SCR, the need for significant enhancement of SCR design at TDZ through implementation of alternate solutions has been identified. This paper presents a summary of the work undertaken in a Joint Industry Project (JIP) during 2004 to 2007 [1, 2] to develop solutions and undertake qualification tasks for four alternatives with potential to improve fatigue performance at TDZ by factor of up to 10 or more. The solutions considered at SCR TDZ include: thick light-weight coating over steel riser sections; steel riser sections with upset ends; high strength steel riser sections with integral connectors; and a titanium segment. The major qualification tasks undertaken for each solution will be identified and discussed. The qualification program undertaken for each solution varied and in some cases, it also included manufacturing of samples, laboratory and full-scale fatigue testing, and post-failure evaluation. Through significant qualification activities undertaken in this JIP, progress has been made to bring these solutions to project ready state for their consideration at the frond end engineering design (FEED) stage. Such design enhancements would enable increase in selection of SCR design for production and export riser applications under severe operating conditions, harsh environment, and floating systems with high motions.
Numerical Seakeeping Simulation of Model Test Condition for Two-Ship Interaction in Waves
Authors: Li Lin
Published: May 2009

In order to validate the numerical prediction of two-ship interactions in waves, the model test calibration has to be conducted. However, motion constraints are usually applied when model tests are carried out for the measurement of wave excitation forces and moments in certain selected motion modes for both ships. Therefore, in current paper, a numerical seakeeping simulation of the model test conditions has been studied. Restraints in surge, sway and yaw, as well as the free motion modes in heave, roll and pitch have been all applied to both the ships, experimentally and numerically. The restraint forces and the unrestrained motions have been compared with the model test results and numerical simulation results. Fairly good agreements have been found.
Probabilistic Fracture Mechanics Methodology Applied to Pipes Subjected to Multiple Reeling Cycles
Authors: Ernst Hugo A. , Bravo Richard E. , Schifini Ricardo , Passarella Diego N.
Published: May 2009

Reeling process is one of the more used methods for installations of linepipes in recent years. Pipes are welded onshore and subsequently reeled onto a drum. During installation, the line is unreeled, straightened, and then laid into the sea. The pipe is subjected to severe cyclic plastic deformation. Due to the characteristics of the process, it is necessary to guarantee the integrity of the components during and after the process. For this reason, structural reliability analyses are essential requirements. In a previous work [1], a fracture mechanics based methodology was developed to obtain a method to assess the structural reliability of reeled pipes. The problem of several reeling cycles was considered. In addition to a fracture mechanics methodology, a formulation considering fatigue crack growth (FCG) controlled by ΔJ parameter was developed. This formulation accounts for the crack growth produced during subsequent reeling cycles. In another work [2], a probabilistic fracture mechanics assessment approach to perform the structural reliability analysis of tubes subjected to a reeling process was developed. This procedure takes into account the statistical distributions of the material properties and pipe geometry, using a fracture mechanics approach and the Monte Carlo method. In this work, the probabilistic fracture mechanics approach was applied for the case of multiple reeling cycles that includes ΔJ-based fatigue crack growth and reliability analysis. A particular case of interest was studied and tolerable defect sizes were determined for different number of reeling cycles taking into account the parameters variability.
Study the Effect of Wave Directionality on Dynamic Nonlinear Behavior of Jack-Up Subjected to Wave and Earthquake Loading
Authors: Abbasi M. , Gharabaghi A. R. M.
Published: May 2009

Sea waves are random in nature as they propagate with different frequencies and in different directions. In literature, there are several studies about dynamic behavior of marine structures under random waves in frequency domain, but little or nothing has been done about wave directionality. In this paper, the behavior of a typical jack-up platform operating in Caspian Sea is studied. In order to model the interaction between spudcan and surrounding soil, it was modeled separately by PLAXIS software. The applied soil properties are based on the field measurements. The results from plastic analyses show that they can differ up to 18 percent compared with those obtained based on the recommendations of API. A 3D model of Iran-Khazar Jack-up was studied using ANSYS software. All elements of triangular legs were modeled by PIPE59 elements and the results obtained from previous step were used to model nonlinear interaction of spudcan footings. The wave and current characteristics are based on the field data. The time-history records of earthquake used in this research are based on Manjil earthquake related to the same area. Nonlinear dynamic analyses including large deflection and material non-linearity was performed. The temporal variation of displacement at deck level was compared under solely wave or earthquake loading and simultaneously acting wave and earthquake loading assuming to be in the same direction and in different directions. It is found that when earthquake loading is applied simultaneously with wave loading in the same direction, displacements are less than wave loading alone. However, when they are applied in different directions, especially when the direction of applied earthquake differs about 30 or 90 degrees with respect to wave direction, displacements become larger. In addition, the effect of wave directionality on the maximum displacement of structure was considered.
Low Cycle Fatigue of T-Tubular Joints With Out-of-Plane Bending Loading
Authors: Boge Fro̸ydis , Helland Torgeir K. , Berge Stig
Published: May 2009

Tubular T-joints were fatigue tested with out-of-plane bending loading. Six models were tested, three models with R-ratio of 0 and three with R = −1. Hot spot stress was measured for the brace and for the chord using the ECSC linear extrapolation procedure. Fatigue loading was applied in load control, to obtain through thickness cracking at a number of cycles in the range 4 000–200 000 cycles. The data were analysed and compared with published data and with current fatigue design criteria for tubular joints.
Assessment of the Horn Mountain Spar in Hurricane Ivan
Authors: Xu Qi , Perryman Stephen , Prislin Igor
Published: May 2009

The Horn Mountain Spar experienced the full strength of Hurricane Ivan, a virtually 2500-yr wave and 600-yr wind in the Gulf of Mexico [1], in 2004. The measured and hindcast sea state reached a 53ft significant wave height and 88 knot wind speed (1-hr average at 10m elevation), which is significantly higher than the spar’s maximum design criteria of 41.7 ft wave height and 78.2 knot wind speed. Both the environment and spar responses in the hurricane were measured. However, 6 hours prior to the peak of the storm, the generator which powered the instrumentation stopped, preventing further measurements. In order to estimate the spar’s actual responses at the peak of the storm and to evaluate the accuracy of the existing spar motion analysis tools, a blind analysis test was performed using the measured and hindcast environmental conditions as input. The analysis results were then compared to the last available measurements of the spar responses in the hurricane. Since the analysis from the design phase was known to be conservative, a fully coupled spar motion analysis model was used in this blind analysis test in order to achieve a higher level of accuracy. The environmental condition was also modeled to a higher level of detail than in the design phase, including wave spreading, dual-peak wave spectrum, etc. Some minor adjustments to the fully coupled model were necessary to achieve the desired accuracy. From the blind test and the subsequent motion analysis for the peak of the storm, the following conclusions can be drawn: 1) The result from the fully coupled analysis model generally agrees very well with the measured spar motion responses early in the storm (when measurements were still available). 2) The spar’s pitch, roll and heave responses were estimated to be within the design limit even though the environment measured at a nearby platform significantly exceeded the maximum design condition at the peak of the storm. There was no drilling rig on the spar at the time of the storm. The findings confirm both the robustness of the current spar design practice and the accuracy of the spar analysis tools.
Corrosion Fatigue Behaviour of Welded Risers and Pipelines
Authors: Baxter D. P. , Maddox S. J. , Pargeter R. J.
Published: May 2009

The fatigue design of pipelines or risers in deepwater oil and gas developments, is often critically dependent on quantifying the extent to which aggressive service environments affect performance. Girth welds in these structures are often exposed to seawater on the external surface, and sweet or sour production fluids on the internal surface. All of these environments can lead to higher rates of fatigue crack growth and lower overall life compared to performance in air. The seawater environment has been studied in some depth, and design codes provide advice on how steel structures are likely to behave under conditions of either free corrosion or cathodic protection. However, it is important to note that there are limits to how widely these guidelines can be applied, and for more complex environments, such as production fluids which are inevitably project specific, design guidance is rarely available. Laboratory testing provides a means of quantifying material behaviour in a simulated service environment, and allows the impact of various environmental variables to be explored. This is important as parameters such as temperature, H2S concentration or loading frequency can have a significant effect on the extent to which performance is affected. This paper provides a review of published information and recent research data, and highlights particular areas where existing data are limited and design challenges remain.
Acoustics and Hydrodynamics of Circle and Square Moonpool: An Experimental Research
Authors: Kang Zhuang , Salman Sadiq , Yao Xiong-Liang
Published: May 2009

In the present study, hydrodynamic forces, resonance ranges due to flow induced oscillations and shear layer self sustaining oscillations for circular and square shaped moonpool; beneath a low speed, laminar boundary layer were studied through model test experiments in a towing tank. The research concentrated upon determining the effects upon resonance and comparison of hydrodynamic forces by rotating the leading edge of the cavity away from normal to the flow direction. Cavity resonance was identified through spectra of flow fluctuations sensed with hydrophone installed inside model. Pressure fluctuations were monitored using pressure sensors installed inside circular and square moonpool separately. Square moonpool was rotated for different attack angles varying from 0–45 degrees with an interval of 15°. Data analysis was conducted using ‘Empirical Mode Decomposition Method’. The natural frequency of water governs the water oscillations inside the moonpool which remains the same (0.7 Hz) from Froude number 0.26 to 1.3 in calm water with forward model speed condition for circular shape, while the Froude number for square moonpool was from 0.23 to 1.17. ‘Piston Phenomenon’ is the condition of resonance when free surface water oscillation frequency and shear layer frequency are equating. Piston phenomenon was observed between Fr = 0.26–0.39 for circular shaped; while for square shaped this phenomenon was different for changing attack angles; ranges were; 0–30 degrees Fr = 0.23–0.47; and 45 degree Fr = 0.23–0.58. The resonance frequency was found to decrease gradually as the cavity leading edge was rotated up to 45° from normal to the flow. Hydrodynamic forces increased at angles to the flow direction. Moreover, hydrodynamic noise is distinguished from structural noise and relationship between St and Fr number is presented. Noise generated is highest (154.14 dB) at beginning of resonance in circular moonpool, while it is lowest between Fr = 0.52–0.63. This paper gives complete understanding for the physics in moonpool related to the underwater noise and hydrodynamic forces. The drag on surface can be altered due to the location of moonpool, and forces exerted by fluid on the cavity walls can be strong enough to cause structural failure, if the location of moonpool is not appropriate and optimized.
Cost Effective Minimum Platform Designs Considering Damage Tolerance and Potential Consequences of Failure
Authors: Ronalds Beverley F. , Pinna Rodney , Cole Geoffrey K.
Published: May 2009

With their simplicity and light structural weight, minimum platforms are a popular solution — even though they may not have the same robustness levels as conventional jackets to withstand extreme or accidental events. These competing attributes are investigated by comparing the cost and structural reliability characteristics of five structures designed using API RP2A. The braced monopod and tripod structures have considerably lower ultimate strengths and residual strengths, and damage scenarios play a much greater role in the overall probability of failure, than for the four-legged jacket. In determining the relative economic merits of the different configurations, the magnitude of the potential consequences of failure becomes an important parameter. Bands of conditional costs of failure for which each structure is the most cost-effective option are derived, with the sturdy monopods and tripod shown to be the optimal solutions for moderate consequence levels.
Non-Linear Finite Element Static Analysis of Flexible Risers With a Touch Down Boundary Condition
Authors: Hosseini Kordkheili S. A. , Bahai H.
Published: May 2009

In this paper a non-linear finite element formulation is presented for static analyses of the flexible risers in present of a touch down boundary condition. A four-nodded with twenty four degrees of freedoms pipe elbow element is used to derive the governing finite element formulation. A pipe-soil interaction model is used to identify seabed boundary condition. The effects of buoyancy force as well as steady-state current loading are considered to obtain useful finite element formulation for riser structures. The results of some sample solutions are given to illustrate the capability of the formulation.
Nonstationary Extreme-Value Predictions in the Gulf of Mexico
Authors: Stefanakos Christos N.
Published: May 2009

In the present work, return periods of various level values of significant wave height in the Gulf of Mexico are given. The predictions are based on a new method for nonstationary extreme-value calculations that have recently been published. This enhanced method exploits efficiently the nonstationary modeling of wind or wave time series and a new definition of return period using the MEan Number of Upcrossings of the level value x* (MENU method). The whole procedure is applied to long-term measurements of wave height in the Gulf of Mexico. Two kinds of data have been used: long-term time series of buoy measurements, and satellite altimeter data. Measured time series are incomplete and a novel procedure for filling in of missing values is applied before proceeding with the extreme-value calculations. Results are compared with several variants of traditional methods, giving more realistic estimates than the traditional predictions. This is in accordance with the results of other methods that take also into account the dependence structure of the examined time series.
Design and Performance of Slug Damper
Authors: Reinoso Antonio , Gomez Luis E. , Wang Shoubo , Mohan Ram S. , Shoham Ovadia , Kouba Gene
Published: May 2009

A novel flow conditioning device, namely, the slug damper, which can be used upstream of compact separation systems, is investigated theoretically and experimentally. In the experimental part, a 3” ID slug damper facility has been installed in an existing 2” diameter two-phase flow loop. This flow loop includes an upstream slug generator, a Gas-Liquid Cylindrical Cyclone (GLCC©) attached to the slug damper downstream, and a set of conductance probes for measuring the propagation of the dissipated slug along the damper. Over 200 experimental runs were conducted with artificially generated inlet slugs of 50 ft length (Ls/d = 300) that were dumped into the loop upstream of the slug damper, varying the superficial liquid velocity from 0.5 to 2.5 ft/s and superficial gas velocity between 10 to 40 ft/ (in the 2” inlet pipe) and utilizing segmented orifice opening heights of 1”, 1.5”, 2” and 3”. For each experimental run, the measured data included: propagation of the liquid slug front in the damper, differential pressure across the segmented orifice, GLCC liquid level, GLCC outlet liquid flow and static pressure in the GLCC. The data prove that the slug damper is capable of dissipating long slugs, ensuring fairly constant liquid flow rate into the downstream GLCC. Also, the damper capacity to process large slugs is a strong function of the superficial gas velocity (and mixture velocity). The theoretical part includes the development of a mechanistic model for the prediction of the hydrodynamic flow behavior in the slug damper. The model enables the predictions of the outlet liquid flow rate and the available damping time, and in turn the prediction of the slug damper capacity. Comparison between the model predictions and the acquired data reveals an accuracy of 30% with respect to the available damping time and outlet liquid flow rate. The developed model can be used for design of slug damper units in the field.
The Hydrodynamic Theory, Ecological Effect and Risk Assessment of the Artificial Fish-Reef
Authors: Liu Shunbin , Liu Defu , Liu Weiwei , Zhang Shouyu , Zhao Lin
Published: May 2009

Based on the urgent requirement for protection inshore fishing resources and rehabilitation of ecosystem, the theoretical study and practical use of artificial fish-reef (AFR) is necessary for fishery. In this paper the physical experiments and numerical simulation are carried out for hydrodynamics and ecological effects of the AFR. The most important nature hazards in China Sea—the typhoon induced extreme sea combinations (extreme wave height with the lowest sea level) are involved to risk assessment of AFR.
The Concomitant Model Testing Approach for the Development of the Pendulous Installation Method of Heavy Devices In Deep Water
Authors: Fernandes Antonio Carlos , Neves Cassiano Rodrigues , Sales Junior Joel Sena
Published: May 2009

This paper synthesizes a method for model testing of non-conventional devices and operations. The basic idea is to perform numerical analysis in advance (or concomitantly) of the model test itself. This has been very effective over the years and its application to the development Pendulous Installation Method (PIM) of deepwater heavy devices has been effective as shown in the paper. Specifically, the PIM does not require boats with compensation systems for the usual vertical launching. The device to be installed is released from the free surface. It is hold by an auxiliary cable from a boat and connected by the installation cable to another boat some distance away. The auxiliary cable is then released and after an almost vertical trajectory of the manifold, the installation cable traction increases and the device rotates (pendulous motion) about the turning point on the other support boat. During the development of novel systems or operations, the industry is learning the importance of model testing. During the model tests, it has been understood that a two-scale problem had to be faced by the model test design. There are large bodies (the device) together with slender bodies (cables) in the same hydrodynamic situation. These two bodies have different scaling laws. How cope with it depends on the several aspects (inertia, damping, etc). However, with the help of a modern numerical model computer code, these problems may be anticipated by creating a framework of results that help in the interpretation of the model testing result itself.
Condition Based Integrity Assessment of Pipelines
Authors: Kumar Amrita , Hannum Robert , Beard Shawn J. , Salama Mamdouh M. , Durnie Will
Published: May 2009

The integrity of pipelines transporting hydrocarbon is critical to economy, safety and environment. One of the leading cause of pipeline failures is 3rd party damage during excavation activities, followed by corrosion, which is becoming increasingly significant as the pipeline infrastructure ages. Current inspection techniques for corrosion monitoring may require the pipeline to be shutdown during inspection reducing overall availability and a potential loss of revenue. Structural Health Monitoring (SHM) offers the promise of a paradigm shift from schedule-driven maintenance to condition-based maintenance (CBM) of pipeline structures. Built-in sensor networks integrated with the pipeline can provide crucial information regarding the condition and damage state of the structure. Diagnostic information from sensor data can be used for prognosis of the health of the structure and facilitate informed decision processes with respect to inspection and repair, e.g., repair vs. no repair or replacement. Asset management can be performed based on the actual health and usage of structures, thereby minimizing in-service failures and maintenance costs, while maximizing reliability and readiness. This paper provides an overview on the design of a SHM system for in-situ real-time, rapid assessment of pipeline integrity using a built-in sensor network. Results of a cost-benefit study conducted for the system usage on pipeline structures will also be presented.
Significance of Biaxial Stress on the Strain Concentration and Crack Driving Force in Pipeline Girth Welds With Softened HAZ
Authors: Liu Ming , Wang Yong-Yi
Published: May 2009

The effect of the biaxial stress and HAZ softening on the crack driving force of girth weld defects was investigated using finite element analyses (FEA). The defects were of elliptic shape and located on the inner surface of the pipe. The crack driving force is represented by the crack tip opening displacement (CTOD) normal to the cracked plane (Mode I). The effect of hoop stress on a homogeneous pipe was revisited at first. It was found that the application of hoop stress tends to increase the crack driving force. However, in the practical range of longitudinal strains (≤4.0%), the effects of hoop stress is not monotonic. For example, at a constant longitudinal strain, as the pre-existing hoop stress increases, the driving force may firstly increase then decrease. The combined effect of HAZ softening and biaxial stress was then studied. With the application of hoop stress, the increase of the crack drive force due to HAZ softening was amplified. It was found that the crack driving force can be closely correlated with the surface strain measured over a structurally significant scale right above the defect. In addition, the effects of loading sequence and material anisotropy on the crack driving force were also briefly examined. The increase of the crack driving force from the hoop stress is more pronounced when it is applied prior to the application of longitudinal strains than the reverse loading sequence. The material anisotropy was found to further increase the crack driving force and therefore representative material models are necessary to analyze the anisotropy effects.
S-Lay Installation of Riser Fairings for the Independence Hub
Authors: West Christopher , Armstrong Stephen , McMillan Damon , Allen Don , McMillan David , Henning Dean , Mekha Basim , Blockus D. J. , Doiron Dwayne , Lemker Mike
Published: May 2009

One of the challenges of insuring adequate fatigue life of marine risers due to vortex-induced vibrations (“VIV”) is the installation of VIV suppression devices in a cost effective manner. In the past, short fairings for production risers have been restricted to J-lay or Reel-Lay installations, where the fairings are installed either before or during pipelay, or retrofit installations where the fairings are installed underwater, using specially designed tooling and Remotely-Operated Vehicles, after the riser has already been installed. Both of these options can be expensive by either restricting the vessels that can install the risers (in the case of J-lay installation) or through expensive rental of ROV and associated equipment for retrofit installation. S-lay installation of fairings is particularly challenging due to the shape of the fairings and the need to install thrust collars to keep the fairings in their axial position once they are installed. Additional complexity is added by the requirement that the fairings be able to withstand mud forces from the temporary placement of the riser on the seabed. These challenges were addressed for the Independence Hub project, which required fairings for both adequate fatigue life due to VIV and for thermal heat transfer considerations. An S-lay testing machine was developed for testing fairings experiencing roller loads up to 115 kips. In addition, wave tank tests were performed to insure that the fairings would stay in proper position during S-lay while experiencing wave forces. Upon successful test results, the S-lay fairings were installed on the Spiderman East, Spiderman West, Jubilee East, Jubilee West, and Merganser flowline risers for the Independence Hub project. These flowline installations are the deepest so far in the world. This required close collaboration by all personnel involved to meet the challenges of insuring success for this type of installation. This paper discusses the methodology used to produce successful S-lay fairing installations for the Independence Hub risers. Presented are test results from the roller and wave tank tests, and a discussion on how the final design was derived. Finally, field experiences from the offshore installations are discussed. This paper should be of interest to engineers involved in the design and installation of deepwater risers.
Wave Sector Dependent Contour Lines
Authors: Baarholm Gro Sagli , Haver Sverre , Larsen Carl M.
Published: May 2009

This paper is concerned with estimating the response value corresponding to given annual exceedance probability. In principle, this requires that a full long term analysis is executed. For a linear response this can easily be done. For a non-linear response quantity however, where time domain simulations are required in order to obtain the short term stochastic structure a full long term analysis will be time consuming. An approximate method to determine the long-term extremes by considering only a few short term sea states is outlined. All sea states corresponding to a certain probability of occurrence and are given by a contour line of Hs, Tp for each wave direction. The advantage of the method is that a proper estimate of the long term extreme can be obtained by considering the most unfavourable sea state along the contour line. This will make possible practical estimation of the extreme loads the structure is exposed to. The purpose of the present paper is to illustrate how to apply directional contour lines in order to obtain a characteristic design value according to requirements regarding the marginal exceedance probability.
Deterministic-Spectral Fatigue Analysis of a Typical Jacket Platform (SPD1) Using Directional Wave Spectrum
Authors: Azimirad M. , Gharabaghi A. R. M. , Chenaghlou M. R.
Published: May 2009

Fixed offshore platforms or Jacket type platforms are the most common offshore structures used for oil & gas Exploration & Production industry in Persian Gulf, because water depth is such that the shallow water condition is dominant. Sea waves as dominant environmental loading are cyclic and have random nature. The applied cyclic sea wave forces will lead to fatigue damages in jacket’s joints. There are different methods to investigate the fatigue life of jackets such as deterministic method, simplified method, spectral method and transient method. Spectral method is a suitable method, which can consider the random nature of sea waves in fatigue analysis. Deterministic-spectral method developed by Bishop et al. is used to estimate the fatigue life of shallow water jacket platforms. However, in this method the frequency spectrum of waves is used in the analysis, but generally sea waves are propagating in different directions with different frequencies, so directional wave spectrum can consider wave randomness more properly. In this paper, frequency domain spectral method using Deterministic-Spectral approach has been used to estimate the fatigue life of a typical jacket platform (SPD1 at South Pars Field - Persian Gulf). Base wave cases were chosen from joint histogram of height & period that is calculated based on scatter diagrams of South Pars Field. First the jacket was modeled by ANSYS software, then by applying base wave cases to it and analyzing the critical TT joint under internal cyclic forces, hot spot stress transfer functions at 8 nodes around the intersection of joint were obtained. Using JONSWAP standard spectrum and the spreading function proposed by Goda, sea state’s Power Spectral Densities (PSD) and directional spectrums are multiplied to obtain stress spectra. The fatigue damage and fatigue life then are calculated. Results indicate that the fatigue life based on frequency spectrum is less than the fatigue life based on directional spectrum.
Integrated Structural Analysis Methodology for Truss Spars
Authors: Luo Michael Y. H. , Zhang Bob L. X. , Tallavajhula Sudhakar , Srinivasan Sanjay
Published: May 2009

Eleven truss spars have been successfully installed in the deep water fields since late 2001. Compared with other floating systems, Truss spars offer significant advantages in motions, stability, and project schedule. One of the unique aspects of a truss spar is that it exhibits both high-frequency and low-frequency motion responses. The high-frequency motions, or wave frequency motions, are peaked around the wave spectral energy, while the low-frequency motions correspond to the natural periods of the spar’s rigid-body motions. Accurate structural design should include loads due to both wave and low frequency motions. The wave-frequency motions can be accurately estimated with potential/diffraction theory, but the low-frequency motions cannot be accounted for using the traditional spectral method. The traditional spectral method may be acceptable to other types of platforms such as Semi-submersibles and TLPs, but can become non-conservative for a spar structure. In the past, this challenge was overcome by performing time domain analysis to design the truss and a combined time and frequency domain analysis to design the other structural components. The procedure proved to be time consuming and inefficient, requiring extensive engineering hours. The hull design process was enhanced by developing an integrated structural analysis methodology. The methodology significantly reduces engineering hours and maintains accuracy in the estimation of loads by a combination of the wave frequency and low frequency motion responses. Efficient use of personnel for the labor-intensive structural modeling tasks was also achieved. Use of this methodology in two spar projects has proved to add significant value. The procedure is also applicable to a range of floating platforms such as Technip’s extended draft platform (EDP) and other deep draft floating platforms. Salient features of the integrated structural analysis methodology for both strength and fatigue analysis of the truss spar are discussed in the paper. Structural loads determined from the integrated methodology are compared with those from a complete time-domain analysis of the truss spar.
Seakeeping of a Non-Wall Sided Vessel in Shallow Water: Lessons Learnt From Experiments and Numerical Analyses
Authors: Naciri Mamoun , Southall Neil
Published: May 2009

Extensive model tests of a soft moored LNG carrier in shallow water have been performed in 2004. The focus of this test campaign was on the hydrodynamics of non wall-sided, low drag vessels and the characterization of the model basin performance. The motivations behind this test campaign have been discussed in ref [1] and [2]. Numerical calculations have been performed using industry state of the art tools and compared to experiments. Having uncovered differences, further numerical investigations have been performed with a nonlinear potential time domain solver. Comparison between experiments and numerical predictions will be presented for monochromatic waves in 30m depth. Lessons learnt in the process will be highlighted.
Sensitivity Study on Nonlinear Seismic Response of Offshore Jackets Due to Different Earthquake Events
Authors: Kimiaei Mehrdad
Published: May 2009

Earthquake design of offshore platforms is one of the main parts in offshore platforms design on which seismic soil pile structure interaction could be the main concern. In seismically active areas, it is often necessary to perform a dynamic analysis that accounts for nonlinear pile soil structure interaction effects. Based on a previously developed numerical BNWF-FE model for nonlinear seismic response of offshore platforms, in this paper, sensitivity of seismic response of an existing sample offshore platform to different earthquake events (with different duration and different frequency contents) together with the sensitivity of the results to the model main parameters are investigated.
Interaction of Waves With Steady Intake/Discharge Flow Emanating From a 3-D Body
Authors: Padmanabhan Bala , Ertekin R. Cengiz
Published: May 2009

It has been proposed that the warm surface-water intake pipes distributed around an OTEC plant can generate adequate momentum to globally position a platform to overcome the second-order drift forces, thereby eliminating the need for additional power for thrusters or for mooring lines. It is evident that if the intake rate of the flow is high, there will be interaction among the locally created steady flow due to the intake, the incoming wave and the ensuing platform motions. In this work, we address such concerns by developing a linear theory for obtaining the motions (in the presence of incoming waves) of arbitrary 3-D bodies from which there is a steady intake/discharge. The boundary-value problem is formulated within the assumption of linear potential theory by decomposing the total potential into oscillatory and steady components. The steady potential is further decomposed into double-model and perturbation potentials. The time harmonic potential is coupled with the steady potential through the free-surface condition. The potentials are obtained using the quadratic boundary-element method. The effect of the steady flow on hydrodynamic force coefficients and RAOs are studied.
Drag Reduction of Deepwater Risers by the Use of Helical Grooves
Authors: Huang Shan , Clelland David , Day Sandy , James Richard
Published: May 2009

Model tests were carried out in a towing tank to investigate the effects of helical grooves on the drag loading of stationary circular cylinders in uniform and steady currents. A series of models were made and tested, including smooth and rough cylinders with and without grooves. The maximum Reynolds number achieved in the tests was about 4×105. The comparative results between the smooth and grooved cylinders show that the helical grooves reduce the drag loading by between 18 and 25% depending upon the cylinder surface roughness and the Reynolds number. This is a continuation of our work published in OMAE06 in Hamburg on VIV suppression by the use of helical grooves.
Offshore Pipeline and Riser Geotechnical Model Testing: Practice and Interpretation
Authors: Langford Thomas , Dyvik Rune , Cleave Ra
Published: May 2009

Offshore developments may typically feature a number of subsea structures for which pipelines and risers play an integral role. Especially for deepwater projects, very soft clays will be encountered at the seabed; these can be difficult to characterize by standard in-situ and laboratory testing. This means that geotechnical model tests are increasingly used to investigate the complex interaction between the seabed soils and the risers or pipelines. The prototypes for these model tests are typically at or close to the actual dimensions used by industry and often include specific coatings for the pipeline or riser section being investigated. The models may be loaded or displaced statically or cyclically in different directions to evaluate the different mechanisms involved. Although it is a challenge to recreate the undrained shear strength found at the seabed, experience and theoretical knowledge may be combined to give good agreement between the shear strength level in the test tank and on site. This may then be verified by in situ testing in the test tank and compared with high quality data from the field itself. The model test results themselves serve as input to pipeline or riser design which can incorporate geotechnical, structural and hydrodynamic effects. This paper describes the typical procedures involved in model testing and investigates the interpretation of the data using theoretical and empirical methods. The impact of results on pipeline and riser design within a project is also considered, where the focus is on very soft clays which are often encountered in offshore projects, especially in deepwater.
The Future of Environmental Riserless Drilling: Dope Free Drill Pipe Connections Using Thread Saver Technology
Authors: Teodoriu Catalin , Ohla Klaus , Nielsen William
Published: May 2009

Threaded connections which employ a metallic “thread saver” for OCTG in lieu of a thread compound are potentially valuable for the drilling industry because they offer a fast make-up process and an environmentally friendly method of making secure threaded connections without damaging threads. Using a thread saver to protect the tool joint thread is not a new idea, but new developments in materials technology combined with a better understanding of the makeup process appear to offer a new solution for compound-free drill pipe connections. The paper presents the latest tests conducted on NC38 drill pipe connections using thread savers fabricated from a CuBe alloy. Different tempers of the alloy and different thread saver surface coatings were tested. The results reported are the data from the first series of more than 60 tests that were carried out at the Technical University of Clausthal. The tests were designed to provide a characterization of the mechanical behavior of the connection process using the thread saver. Data from the tests is compared to similar data previously developed for the more-familiar processes which use thread compound. The results show the new technology is viable and may be used for heavy duty applications such as drill pipe and drill collar connections. Additionally, a short review of the thread saver history for the drilling industry is presented. Results suggest some important applications for the thread saver technology may be in riserless drilling or in scientific drilling such as the Integrated Ocean Drilling Program. A reliable “dope-less” connection technology is particularly valuable for operations where the adverse impact of environmental contamination by stray thread compounds is potentially higher than in some other operations.
Hydrodynamic Modeling of Three-Phase Flow in Production and Gathering Pipelines
Authors: Zaghloul Jose , Adewumi Michael , Ityokumbul M. Thaddeus
Published: May 2009

The transport of unprocessed gas streams in production and gathering pipelines is becoming more attractive for new developments, particularly those is less friendly enviroments such as deep offshore locations. Transporting gas, oil, and water together from wells in satellite fields to existing processing facilities reduces the investments required for expanding production. However, engineers often face several problems when designing these systems. These problems include reduced flow capacity, corrosion, emulsion, asphaltene or wax deposition, and hydrate formation. Engineers need a tool to understand how the fluids travel together, quantify the flow reduction in the pipe, and determine where, how much, and the type of liquid that would from in a pipe. The present work provides a fundamental understanding of the thermodynamics and hydrodynamic mechanisms of this type of flow. We present a model that couples complex hydrodynamic and thermodynamic models for describing the behavior of fluids traveling in near-horizontal pipes. The model incorporates: • A hydrodynamic formulation for three-phase flow in pipes. • A thermodynamic model capable of performing two-phase and three-phase flow calculations in an accurate, fast and reliable manner. • A new theoretical approach for determining flow pattern transitions in three-phase (gas-oil-water) flow, and closure models that effectively handle different three-phase flow patterns and their transitions. The unified two-fluid model developed herein is demonstrated to be capable of handling systems exhibiting two-phase (gas-water and gas-oil) and three-phase (gas-oil-water) flow. Model predictions were compared against field and experimental data with excellent matches. The hydrodynamic model allows: 1) the determination of flow reduction due to the condensation of liquid(s) in the pipe, 2) assessment of the potential for forming substances that might affect the integrity of the pipe, and 3) evaluation of the possible measures for improving the deliverability of the pipeline.
Behaviors of Drag and Inertia Coefficients of Circular Cylinders Under Vortex-Induced Vibrations With Forced Oscillation Tests in Still Water
Authors: Ikoma Tomoki , Masuda Koichi , Maeda Hisaaki , Hanazawa Sayaka
Published: May 2009

This paper describes the drag coefficient and the inertia coefficient on rigid circular cylinders under oscillating flows. And also VIV behavior of tubing is investigated by using horizontal orbit patterns of test models including effect of VIV. Rigid circular cylinders are used for the model experiment. The test models are suspended into still water through a flat spring, and hence VIV is observed found with hydroelasticity in spite of a rigid circular cylinder. This is a forced oscillation test in still water. The circular cylinders for the model test have drafts of 30 cm, 60 cm and 80 cm, and diameter of 5 cm and 8 cm. The models are oscillated with periods of 0.5 s to 4.6 s and with about 7 cm amplitude of the oscillation. Model tests of forced oscillation are carried out under transverse motion (VIV) fixed conditions because VIV effects are investigated to the drag and the inertia coefficients. It is shown that the drag coefficient increases and the inertia coefficient decreases when larger VIV response occurs under forced oscillation in an inline direction.