bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison Between Numerical Prediction and Model Tests Measurements on the Centerwell Tank of a Radial Wellbay Spar
Authors: Muehlner Edmund , Murray John , Banumurthy Surya , Lakhotia Chandan
Published: August 2013

The radial wellbay (RAW) Spar is, for the most part, based on conventional Truss Spar technology. The primary difference between the RAW Spar and the conventional Truss Spar is the wellbay arrangement.

An integral structural component of the RAW Spar is the ABCD (Adjustable Buoyancy Centerwell Device) located in the lower portion of the centerwell. The ABCD is connected to the interior of the hard tank using a number of shear plates. The ABCD captures the buoyancy in the open centerwell and contributes to the total buoyancy of the hard tank. As an option, the ABCD can be used for in-hull storage. On the RAW drilling Spar, the ABCD can be used to support the setback and pipe racking system.

Design applications using the ABCD raise an additional requirement in the design of the hard tank, specifically, an accurate estimate of the load in the structure that connects the ABCD to the hard tank. Contributions to this load come from the inertia effect of the Spar motions, hydrostatic responses affected by ballast conditions in the device, and hydrodynamic forces affected by pressure in the riser slot gap. Connection loads are required to design the structure connecting the ABCD to the interior walls of the hard tank.

An ABAQUS™ based time domain semi-empirical model was developed to predict the local and global loads on the ABCD. A series of 1:50 scale model tests on the RAW Spar were carried out at the OTRC basin in College Station, Texas. The model’s ABCD, mounted on a dynamometer system inside the hard tank, was used to measure the six-degree-of-freedom forces between the hard tank and the ABCD. The paper focuses on a comparison of the predicted and measured loads.
Application of Fatigue Reliability Analysis to a Typical Upper Deck Penetration Structure of a Ship
Authors: Kim Bongjae , Radha P. , Cho Taemin , Suh Yongsuk , Eom Jaekwang
Published: August 2013

Fatigue crack is one of the most important failure modes for hull structures and so it is imperative to verify that the calculated fatigue life is beyond the vessel service life for fatigue prone structures in design stage. However, the amount of uncertainties lying in the analysis is quite large for such kind of analysis that the analysis often fails to explain the real phenomena correctly. To manage such a risk, design fatigue factor is usually introduced and for the determination of proper level of design fatigue factor, a quantitative assessment of the fatigue reliability is very useful. In this paper, a fatigue reliability analysis for typical upper deck penetration structures of a compact drillship is presented. Possible uncertainties in inputs and analysis models have been investigated and the variation of important factors has been quantitatively estimated. The estimated failure probability of the current practice design has shown that about 3.4% cases may have fatigue crack within 20-year service life. Proper design fatigue factors for current and higher level of load uncertainties have been investigated and suggested as 2.0 to 6.0 according to the desired safety level.
Experimental and Numerical Study of a Multi-Layer Flexible Pipe Depressurization
Authors: Lambert Anaïs , Felix-Henry Antoine , Gilbert Philippe , Gainville Martin
Published: August 2013

Unbonded flexible pipelines used for offshore fields developments usually rely upon a stainless steel carcass as innermost layer for collapse resistance and a polymeric pressure sheath for internal leakproofness. For some dynamic flexible risers, this pressure sheath is a multi-layer construction made of 2 or 3 layers. In this multi-layer configuration, during operation, the fluid transported in the bore of the pipe can penetrate the annular space between the pressure sheath layers by means of flow paths through the end-fitting. Then, when the pipe bore is depressurized, the pressure of the fluid trapped between the sacrificial sheath and the pressure sheath does not decrease as fast as in the bore. This is due to the small annular flow path between the sheaths. Depressurization of the pipe bore should thus be performed at a limited and controlled rate to avoid an excessive differential pressure between the pipe bore and the annulus that could potentially cause the collapse of the carcass. The allowable depressurization rate is a key parameter the field operators need to know to avoid such an issue.

A model was developed and implemented in a numerical software to calculate the evolution of the differential pressure during the depressurization. It is based on a one dimension gas/liquid transient transport model. More specifically, it is composed of the mass and momentum conservation laws. The thermodynamics properties of the fluid are computed with a simple model. In addition, the dynamic behavior of the annulus is coupled with the conservation equations and the pressure of both the bore and the external environment. A Finite Volume Method is chosen to both discretize and keep the conservative properties of the model on a discrete level.

In addition, a large test campaign was carried out on a full scale pipe to validate the model. A test consists in a pressure increase followed by a depressurization. The pressures in the bore and in the annulus were permanently recorded using a specific device which enables to know in real time the differential pressure between the pipe bore and the annulus. Various conditions were tested by varying the fluid viscosity, the initial bore pressure and the depressurization rate.

After explaining the physics of the depressurization for this specific flexible pipe construction, this paper will present the numerical model developed to calculate the maximum differential pressure and the test campaign performed to validate this model.
Effective-Stress Finite Element Analysis of Spudcan Penetration
Authors: Yi Jiang Tao , Lee Fook Hou , Goh Siang Huat , Li Yu Ping , Zhang Xi Ying
Published: August 2013

The numerical modeling of spudcan penetration involves technical challenges posed by large soil deformation coupled with significant material non-linearity. The Lagrangian approach commonly used for solid stress analysis often does not work well with large deformations, resulting in premature termination of the analysis. Recently, the Arbitrary Langrangian Eulerian (ALE) and the Eulerian methods have been used in spudcan analysis to overcome problems caused by the soil flow and large deformation. However, most of the reported studies are based on total stress analysis and therefore shed no light on the excess pore pressures generated during spudcan installation. As a result, much remains unknown about the long-term behaviour of spudcans in the ground, which is affected by the dissipation of excess pore pressures. This paper reports an effective-stress finite element analysis of spudcan installation in an over-consolidated (OC) soft clay. The Eulerian analysis was conducted using ABAQUS/ Explicit, with the effective stress constitutive models coded via the material subroutine VUMAT. The results demonstrated the feasibility of conducting effective-stress finite element analysis for undrained spudcan penetration in OC clays. The paper discusses the flow mechanism, stable cavity depths and bearing capacity factors when spudcan installation occurs in various OC soils. It was found that the pore pressure build-up concentrates in a bulb-shaped zone surrounding the spudcan. The size of the pore pressure bulb increases with increasing penetration. The maximum excess pore pressure, which is generated near the spudcan tip, is predominantly controlled by the undrained shear strength at the tip level.
Wellhead Fatigue Analysis Method: Benefits of a Structural Reliability Analysis Approach
Authors: Hørte Torfinn , Reinås Lorents , Mathisen Jan
Published: August 2013

Structural Reliability Analysis (SRA) methods have been applied to marine and offshore structures for decades. SRA has proven useful in life extension exercises and inspection planning of existing offshore structures. It is also a useful tool in code development, where the reliability level provided by the code is calibrated to a target failure probability obtained by SRA. This applies both to extreme load situations and also to a structural system under the influence of a time dependent degradation process such as fatigue.

The current analysis methods suggested for service life estimation of subsea wells are deterministic, and these analyses are associated with high sensitivity to variations in input parameters. Thus sensitivity screening is often recommended for certain input parameters, and the worst case is then typically used as a basis for the analysis. The associated level of conservatism embedded in results from a deterministic analysis is not quantified, and it is therefore difficult to know and to justify if unnecessary conservatism can be removed from the calculations.

By applying SRA to a wellhead fatigue analysis, the input parameters are accounted for with their associated uncertainty given by probability distributions. Analysis results can be generated by use of Monte-Carlo simulations or FORM/SORM (first/second order reliability methods), accounting for the full scatter of system relations and input variations. The level of conservatism can then be quantified and evaluated versus an acceptable probability of failure.

This article presents results from a SRA of a fictitious but still realistic well model, including the main assumptions that were made, and discusses how SRA can be applied to a wellhead fatigue analysis. Global load analyses and local stress calculations were carried out prior to the SRA, and a response surface technique was used to interpolate on these results. This analysis has been limited to two hotspots located in each of the two main load bearing members of the wellhead system.

The SRA provides a probability of failure estimate that may be used to give better decision support in the event of life extension of existing subsea wells. In addition, a relative uncertainty ranking of input variables provides insight into the problem and knowledge about where risk reducing efforts should be made to reduce the uncertainty.

It should be noted that most attention has been given to the method development, and that more comprehensive analysis work and assessment of specific input is needed in a real case.
Large Eddy Simulations of Three-Dimensional Flow Around a Pipeline in a Uniform Current
Authors: Prsic Mia , Ong Muk Chen , Pettersen Bjørnar , Myrhaug Dag
Published: August 2013

Three-dimensional (3D) modeling of the flow around a circular cylinder in a steady, uniform current subjected to low and intermediate Reynolds numbers (Re) is considered. Large eddy simulations (LES) are used to simulate the flow due to their ability to study fine structures in the turbulent wake of the cylinder. The turbulence is resolved by the Smagorinsky model. The open-source code OpenFOAM is used in the simulations. Influences of various numerical parameters, such as the grid resolution close to the cylinder, domain width and spanwise resolution are investigated through mean values of the drag and the lift coefficients, the Strouhal number, as well as through the details of the flow field in the near wake of the cylinder.

The main purpose of the present paper is to study the numerical influences on the 3D LES simulations of the flow around the cylinder in the free stream at Re = 13100. However, first the flow around a cylinder in the free stream at Re = 3900 is investigated. The latter case has previously been thoroughly investigated, both with LES and direct numerical simulations (DNS); hence these results will be used for verification of the OpenFoam solver.

At Re = 13100 the drag and lift coefficients and the Strouhal number are calculated for the comparable Re. Details of the flow in the intermediate wake of the cylinder are also investigated. So far, this has been investigated experimentally and by two-dimensional (2D) numerical simulations. Previous studies were conducted for moderate Re in the range 103–105.
Benefits of Laser Measurement Supported JCOC Process for Improvement of Clad Pipe Quality
Authors: Reichel Thilo , Pavlyk Vitaliy , Aretov Ivan , Beißel Jochem
Published: August 2013

Metallurgically bonded CLAD pipes consist of a thick carbon steel bulk and a thin layer of a corrosive resistant alloy (CRA). In order to perform a girth weld of such pipes, the clad layers of both parts must geometrically fit to each other. Thus, the field weldability of CLAD pipes are to a large extent determined by dimensional tolerances of pipe ends, which have to be smaller than CRA thickness, typically 3mm. To ensure such tight tolerances, EBK has developed and installed a new calibration machine called Impander and corresponding cold sizing technology, in which the pipes are compressed (impanded) from outside. Thus, the traditional JCO manufacturing process is supplemented by a Compression step, which leads to the abbreviation JCOC. In contrast to an expander, the Impader tools operate from the pipe outside and produce tight dimensional tolerances without contacting the sensitive CRA layer. Controlled adjustment and final inspection of pipe dimensions demand an accurate measurement of pipe shape. For this, a laser tracker which allows precise measurements of complete pipe circumference with an accuracy of 0.1 mm both from inside and outside was installed and a corresponding evaluation software was developed. The paper gives a comparison of different pipe production technologies pointing out the advantages of the JCOC process using the Impander for manufacturing of CLAD pipes. An analysis of the influence of dimensional tolerances on edge misalignment of two pipe ends during girth welding is given. The precise laser measurements allow to mark exactly the largest and smallest diameter on pipe ends, that helps to reduce time for pipe adjustment during field welding. In addition, using numerical and experimental investigations, the pipe impansion was shown to significantly reduce residual stresses in the pipe. Thus, when cutting the pipe, a change of pipe shape due to springback effect is considerably reduced. Additionally, the compressive strengthening in hoop direction due to cold work by impansion in combination with a low ovality along the whole pipe length considerably increases the external collapse pressure, which makes the pipes particularly suitable for deep water applications. So far, this has been experimentally proven with a full-scale-collapse-test on a conventional pipe, but is suggested to be applicable for the CLAD pipes too.
Fatigue Assessment of Subsea Wells for Future and Historical Operations Based on Measured Riser Loads
Authors: Russo Massimiliano , Holden Harald , Reinås Lorents , Sæther Morten
Published: August 2013

Operators in the North Sea have recently strengthened their efforts in documenting the integrity of subsea wellhead systems. As a part of this effort, fatigue damage estimation of subsea wells in service has been performed. Fatigue damage estimation on subsea wells due to drilling riser dynamic loads was carried out by the use of analytical model results. The applied analytical methodology is based on a decoupled approach, where global load analyses and local stress calculations are carried out prior to a SN based fatigue accumulation. Applying such methodology on safety critical systems the analytical philosophy should ensure conservative fatigue damage. For cases where the fatigue calculations returned unfavorable estimates, one corrective action has been to measure the actual riser response and to monitor the development of fatigue damage closely. For this purpose a methodology for fatigue estimation based on measured riser response was needed. In this approach of estimating the fatigue damage, the global load analysis results are replaced by measured dynamic load time series. By combining direct riser response measurements with local stress calculations, a revised SN based fatigue accumulation can be performed. The fatigue damage derived from measured riser response is compared to the fatigue damage based only on analytical results. From this comparison the conservatism in the analysis methodology for the global riser response is shown to be significant. As this method relays on measurements, it will only yield historical fatigue damage and at best it can return updated fatigue capacity usage on the fly. Forecasting fatigue damage still have to be established based on global riser analyses results, resulting in a conservative forecast. This paper suggests an updated methodology using actual measured response to both asses fatigue damages of historical operations and forecast fatigue damages based on historic operations. By cycle counts of measured response time series (one hour response) a link between this cycle count and the coexistent significant wave height and spectral peak period can be established. This relationship between observed weather and measured response is representative for the rig and riser system on which the measurements were performed. Then forecast and measurements of the weather conditions can be used to estimate the historical damage and the future fatigue damage respectively. The paper will present results from the suggested approach by use of examples from a real North Sea well in shallow water.
Modelling Details of Fenders in Float-Over Installation Experiments
Authors: Cole Jason C. , Islam Mohammed , Garvin Matthew , Herrington Paul
Published: August 2013

Fenders are a key element of float-over topsides installations protecting the vessel and jacket legs by absorbing impact energy. Fender capacity is defined in accordance with expected vessel motions, vessel hull strength, and anticipated impact energy. Translating those dependencies from full-scale float-over scenarios to model-scale experiments provides added challenges when devising experimental approaches.

This paper describes the design and development of model-scale fenders and their application in float-over topsides installation experiments. Current model fenders use multi-stage cantilevered leaf springs to develop the required non-linear stiffness curve which is calculated from full-scale fender data. Spring material and geometry, relative position, and loading location are key variables used to match the target fender stiffness as changes in these parameters directly affect the force-deflection characteristics of the model-scale fenders.

The force-deflection characteristics (stiffness) of the model-scale fenders were checked before, during, and after wave experiments, using both isolated bench tests and in situ verifications. Multiple iterations of stiffness checks were completed during the stiffness curve matching process prior to the experimentation. The achieved load-deflection characteristics of the fender units were within 2% of the target stiffness curve. The damping characteristics of the fenders were not specifically modeled, but were measured for each of the stiffness stages. The model fenders have successfully been used in multiple float-over experimental campaigns.
The Analytical Solution of Taut-Slack Mooring Line With Tanh-Method
Authors: Zhang Su-xia , Tang You-gang , Liu Xi-jun
Published: August 2013

In this paper, the continuous model is introduced and solitary solutions of the nonlinear partial differential equations of taut-slack mooring line are obtained by tanh method, and the plots of results are drawn. The results show that, with the continuous increasing of pretension, the tension curve convert from snap tension to continuous line, until to multi-peaks line. In the process of increasing the pretension, the mooring line transfer from slack to taut, until to breakage, accompanying with tension skip, which is reduced by the system parameters, and different combination of parameters may introduce to different tension in line, and the uncertainty may cause the breakage of mooring system.
Effects of Low Temperature on ASTM A131: An Experimental and Numerical Study
Authors: Park Dae Kyeom , Kim Do Kyun , Kim Bong Ju , Seo Jung Kwan , Paik Jeom Kee
Published: August 2013

The aim of this study was to investigate the effects of low temperatures on the material properties and crashworthiness of ASTM A131-type carbon steel. A series of tensile coupon tests and crushing tests on square tubes were performed in a liquid nitrogen cooled chamber to examine their material behavior and crashing behavior at a series of low temperatures under quasistatic conditions. The temperature conditions cover a cryogenic condition, the typical Arctic temperatures and room temperature.

In the previous study, ASTM A500-type carbon steel is considered [13] and in present study the different grades of ASTM A131-type carbon steel for ships and offshore structures are considered, including Grades A, B and D of mild steel and Grades AH and DH of high tensile steel.

The results of the tensile coupon tests showed an increase in the strength and in non-uniform fracture strain behavior in different grades and temperatures. Unstable crushing behavior and fractures were observed in the crushing tests. LS-DYNA nonlinear finite element method analyses were performed using reliable nonlinear finite element method modeling techniques to assess the structural crashworthiness of the materials at low temperatures.
One Optimization Method on Mechanics Performance Reliability of the High-Speed Monohull Ship
Authors: Yang Song-lin , Chen Peng , Wen Yi-yan , Cui Jian , Chen Shu-ling
Published: August 2013

The authors present an algorithm called P-C-GA (Parallel layered Complex method and Genetic Algorithm), which was based on delicate variables’ segments, parallel calculation thinking, genetic and complex algorithm, it had been applied on optimizing integrate the performance of navigational performance reliability and structure characteristic reliability by the computer programming language VC++. A large number of computation results in different solving methods show that compared with other methods such as chaos, GA and parallel GA algorithm, this algorithm is reliable and efficient.
Numerical Study of Hydrodynamic Forces on a Submarine Piggyback Pipeline Under Wave Action
Authors: Cheng Xiaofei , Wang Yongxue , Ren Bing , Wang Guoyu
Published: August 2013

In the paper, a 2D numerical model is established to simulate the hydrodynamic forces on a submarine piggyback pipeline under regular wave action. The two-dimensional Reynolds-averaged Navier-Stokes equations with a κ-ω turbulence model closure are solved by using a three-step Taylor-Galerkin finite element method (FEM). A Computational Lagrangian-Eulerian Advection Remap Volume of Fluid (CLEAR-VOF) method is employed to simulate free surface problems, which is inherently compatible with unstructured meshes and finite element method.

The numerical results of in-line force and lift (transverse) force on the piggyback pipeline for e/D = G/D = 0.25 and KC = 25.1 are compared with physical model test results, which are conducted in a marine environmental flume in the State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, China. It is indicated that the numerical results coincide with the experimental results and that the numerical model can be used to predict the hydrodynamic forces on the piggyback pipeline under wave action. Based on the numerical model, the surface pressure distribution and the motion of vortices around the piggyback pipeline for e/D = G/D = 0.25, KC = 25.1 are investigated, and a characteristic vortex pattern around the piggyback pipeline denoted “anti-phase-synchronized” pattern is recognized.
Gas Explosion Analysis for FLNG Plant Layout Design
Authors: Otsubo Kazuhisa , Masanobu Sotaro , Yaguchi Yuta , Asanuma Takayuki , Maeda Katsuya , Taniguchi Tomoki
Published: August 2013

Recently, the natural gas is expected to be an environmentally friendlier energy and alternative to the nuclear energy. In the social situation, many oil and gas and engineering companies had studied FEEDs (Front End of Engineering Designs) of FLNG (LNG-FPSO) expected to be a new production system for offshore natural gas development in the near future. The FLNG system is no proven technology and there is no FLNG operation, which can make it difficult to design FLNG. Furthermore, it is noted that FLNG designers can’t get enough information for the design.

Gas explosion is the technical problems for the topside facilities of FLNG. The objective of this paper is to study the gas explosion for the FLNG design.
Application of Breather Solutions for the Investigation of Wave/Structure Interaction in High Steep Waves
Authors: Clauss Günther F. , Klein Marco , Dudek Matthias , Onorato Miguel
Published: August 2013

During the design process of floating structures, different specifications have to be aligned such as the range of application, the warranty of economical efficiency as well as the reliability and are an inevitable integral part of the evaluation process during the design stage. The validation of the performance by means of model tests in terms of sea state behavior and the associated local and global structural loads are an important milestone within this process. Therefore it is necessary to determine an adequate test procedure which covers all essential areas of interest. Thereby one field of interest are limiting criteria of the design such as maximum local and global loads as well as maximum accelerations due to the impact of extraordinarily high waves, at which the floating structure has to survive. Different alternatives are available to conduct model tests in high, steep waves — transient wave packages, regular waves, irregular waves with random phases or more sophisticated deterministic tailored irregular wave sequences such as reproductions from numerical simulations and real-world measurements.

This paper introduces a new approach for the systematic investigation of wave/structure interaction in high, steep waves. Exact solutions of the nonlinear Schrodinger equation — the so called breather solutions — are implemented for the generation of extraordinarily high waves. Three types of breather solutions are investigated in the seakeeping basin and to cover the full range of interest, each solution has been used to generate freak waves at certain frequencies.

To evaluate the applicability of breather solutions for model tests two types of ships — a LNG Carrier and a Chemical Tanker — are investigated in the seakeeping basin. The ships are segmented and connected with strain gauges to detect the vertical wave bending moment. Furthermore, green water probes are installed on deck to evaluate the local impact on the bow of the freak waves. The obtained results are compared to investigations in regular waves with certain frequency and steepness as well as in real-world freak wave reproductions.
Effect of High-Frequency Response on TLP Tendon Fatigue
Authors: Muehlner Edmund , Banumurthy Surya , Murray John
Published: August 2013

High-frequency vibrations of Tension Leg Platforms (TLPs), commonly known as ringing and springing, have challenged TLP designers since the first full-scale TLP was installed in the North Sea in 1984. Although current design codes recognize the significance of the ringing and springing response for tendon design, no widely accepted modeling approach for their calculation has yet emerged.

This paper presents a nonlinear time-domain model of a TLP that exhibits the ringing and springing response of the vessel. The analysis model uses large displacement theory for the vessel and tendons and a semi-empirical wave model based on a modified linear wave theory. Predictions of vessel motions and tendon loads made with the analysis model were compared to model tests and were found in good agreement with the measurements.

The analysis model was also was used to investigate the fatigue damage in the tendons caused by the vessel’s high-frequency response. Tendon stress time histories were computed for nine different unidirectional sea-states. These sea-states represent a condensed wave scatter diagram for the Gulf of Mexico (GoM). The tendon fatigue was calculated from the stress histories by rainflow counting. Fatigue contributions from different frequency ranges were identified by Fourier analysis.

The analysis showed that high-frequency response was present in all sea-states even though ringing occurred only in sea-states with significant wave heights above 10 ft. Tendon fatigue damage contribution from high-frequency loads were found to be significant in every sea-state. For all sea-states combined 73% of the up-wave tendon fatigue damage was due to high-frequency response. For the down-wave and the cross-wave tendons, the high-frequency contributions were 57% and 34%, respectively.

This paper demonstrates the importance of considering high-frequency response for the fatigue design of TLP tendons. Another finding of the study is that the analysis model using a modified linear wave theory can describe the ringing and springing behavior of a TLP provided other significant nonlinearities of the system are considered.
Joint High Speed Sealift (JHSS) Segmented Model Test Results
Authors: Piro Dominic , Fu Thomas C. , Maki Kevin J.
Published: August 2013

The Joint High Speed Sealift segmented model (Model 5663) tests performed in 2007, in the Maneuvering and Seakeeping Basin at the Naval Surface Warfare Center, Carderock Division, were designed to provide a large data set for validation of numerical simulations. Model 5663 is a segmented structural ship model that has scaled longitudinal bending and torsional stiffness. The scaled stiffness is obtained by building a backspline into the model that accounts for the bending stiffness and cutting the shell in several places, segmenting it to isolate the stiffness to the backspline. The alternative way to obtain structural loads would be to build a model with scaled plates and stiffeners; however, this would be very difficult and expensive. The backspline allows the stiffness to be scaled properly while using reasonable materials and simple construction. The hull segments of the model are connected with silicone to maintain a watertight connection. The model is self propelled and steered during data collection. The test matrix spans a wide set of wave conditions, including regular and irregular seas, with heading angles spanning the possible range. A wide range of speeds are also included, with Froude numbers ranging from 0 to 0.43. This test matrix, which includes about 2000 runs, allows for validation of codes from still water test, through operational conditions, to extreme design load determination. Different aspects of the data have been studied, but much is still left to be considered. An aspect of the model tests that has not previously been considered in detail is the hydroelastic response of the vessel. Hydroelastic phenomena are a subset of fluid-structure interaction problems where the elasticity of the structure is important. The vibrational characteristics of the model are determined. The main phenomena of interest are springing and whipping, and an analysis of the springing response and the whipping response in head seas is also discussed.
Experimental Study on the Near Wake Behind Two Staggered Cylinders of Unequal Diameters
Authors: Gao Yangyang , Wang Xikun , Tan Soon Keat
Published: August 2013

The wake structure behind two staggered circular cylinders with unequal diameters was investigated experimentally using the particle image velocimetry technique (PIV). This investigation was focused on the variations of flow patterns in terms of incident angle at Reynolds number Re = 1200. Comparisons of the time-averaged flow field of two staggered cylinders with unequal diameters at different angles were made to elucidate the mean flow characteristics. The characteristics of Reynolds shear stress contours at different incident angles and spacing ratios were also investigated. The results showed that with increasing of incident angle, the scale of Reynolds stress contours behind the upstream cylinder becomes larger, as well as the effect of spacing ratio on Reynolds stress contours.
Deterministic and Wind/Wave Modeling: A Comprehensive Approach to Deterministic and Probabilistic Descriptions of Ocean Waves
Authors: Osborne Alfred R.
Published: August 2013

Deterministic Modeling of ocean surface rogue waves is often done with highly complex spectral codes for the nonlinear Schrödinger equation and its higher order versions, the Zakharov equation or the full Euler equations in two-space and one-time dimensions. Wind/Wave Modeling is normally conducted with a kinetic equation derived from a deterministic equation: the nonlinear four wave interactions are normally computed with the Discrete Interaction Approximation (DIA) algorithm, the Webb-Resio-Tracy (WRT) algorithm or the full Boltzmann integral. I give an overview of these methods and show how a fully self-consistent approach can simultaneously yield all of these methods while computing a multidimensional Fourier series that contains rogue wave packets as “coherent structures” or “nonlinear Fourier components” in the theory. The methods also lead to hyperfast codes in which deterministic evolution is millions of times faster than traditional spectral codes on a large multicore computer. This method could lead the way to an ideal future in which there are single codes that can simultaneously compute the deterministic and probabilistic evolution of surface waves.
Damping Characteristics of Heave Plates Attached to Spar Hull
Authors: Philip Nimmy Thankom , Nallayarasu S. , Bhattacharyya S. K.
Published: August 2013

Damping elements are used in ships and offshore structures as response reduction devices for maintaining the hydrodynamic response within acceptable limits. The use of such elements so far has been based on past experience or using empirical based design approach. An investigation of the wave excitation forces acting on these structures with and without damping elements may give an insight into the damping effects of such elements. Also understanding the flow pattern and the magnitude of flow variation around these elements may be useful to improve the design methods. In the present study, measurement of damping forces and response were carried out to establish the relationship between wave frequency amplitude and damping ratio. It was observed that the damping varies with wave period non-linearly and linearly with wave height. Also it was observed that increase in added mass combined with viscous damping is a key factor for the effectiveness of heave plates in reducing motion response of spar platforms. Of the various configurations tested, the spar with double heave plate at the bottom has higher damping compared to other configurations.
On-Bottom Stability Analysis for a Pipeline on a Mobile Seabed
Authors: McMaster Scott Y. , O’Brien Dermot , Scholtz David E. , Ryan Jay R.
Published: August 2013

This paper describes the work performed to assess the stability of an existing pipeline off the North Western coast of Western Australia. Originally, the pipeline was post-trenched (for stability), by ploughing. The plough formed an open v-shaped trench below the pipeline. Several years after the pipeline was installed, a severe cyclone caused significant changes to the seabed along the pipeline. The v-shaped ploughed trench is generally no longer visible with the trench backfilled in places and the pipeline exposed in others. In addition to this, the embedment profile either side of the pipeline was often found to be assymetrical, with higher embedment on one side of the pipeline relative to the other.

Current on-bottom stability guidelines and recommended practices do not account for the effects of assymetrical embedment and seabed mobility (Ref. 5). These two factors are believed to contribute significantly to the overall stability of the pipeline. Consequently, it was decided to use physical model testing to obtain a better understanding of hydrodynamic loading and changes in embedment over the duration of a design storm. The physical model testing was performed for various pipeline embedment profiles. The results of these 2D physical model tests were then applied to the 3D FEA on-bottom stability software, CORUS 3D.

This paper provides a detailed description of the physical model testing program performed as part of the pipeline stability assessment. The method used to combine the results of the physical model testing and apply them to the 3D FEA on-bottom stability software is also presented.
Nonlinear Structural Consequence Analysis of FPSO Topsides Under Fire
Authors: Kim Jeong Hwan , Kim Du Chan , Kim Cheol Kwan , Islam Md. Shafiqul , Paik Jeom Kee
Published: August 2013

This study aims to develop a practical procedure for the nonlinear structural consequence analysis of structures under fire. The thermal and structural response analysis have been performed in this study using a commercial nonlinear Finite Element Analysis (FEA) code. The results of the structural response analysis are then compared to the experimental results. This study concludes by presenting methods for fire load applications and nonlinear material modeling. The insights offered by the modeling techniques and analysis procedures presented in this study should be very useful and practical in the fire risk assessment of offshore structures.
State of Art in Life Extension of Existing Offshore Structures
Authors: Nezamian Abe , Nicolson Robert J. , Iosif Dorel
Published: August 2013

A large number of the old oil and gas facilities have reached or exceeded their initial design life. With a continued requirement to produce oil or gas, either from the original fields or as a base for neighbouring subsea completions, many of these respective offshore installations are likely to remain operational for a period of time in the foreseeable future. The ageing offshore infrastructure presents a constant and growing challenge. Ageing is characterised by deterioration, change in operational conditions or accidental damages which, in the severe operational environment offshore, can be significant with serious consequences for installation integrity if not managed adequately and efficiently.

In order to ensure technical and operational integrity of these ageing facilities, the fitness for service of these offshore structures should be maintained. The maintenance of structural integrity is a significant consideration in the safety management and life extension of offshore installations. Detailed integrity assessments are needed to demonstrate that there is sufficient technical, operational and organisational integrity to continue safe operation throughout a life extension. Information on history, characteristic data, condition data and inspection results are required to assess the current state and to predict the future state of the facility and the possible life extension.

This paper presents state of art practices in life extension of existing offshore structures and an overview of various aspects of ageing related to offshore facilities, represented risk to the integrity of a facility and the required procedures and re assessment criteria for deciding on life extension.

This paper also provides an overall view in the structural requirements, justifications and calibrations of the original design for the life extension to maintain the safety level by means of a maintenance and inspection programs balancing the ageing mechanisms and improving the reliability of assessment results.
Basic Design of a Semi-Submersible Rig for Experimental Drilling of Methane Hydrate by “Harmonic Design Tool”
Authors: Taniguchi Tomoki , Yukawa Kazuhiro , Otsubo Kazuhisa , Sato Hiroshi
Published: August 2013

For the growth of demand for utilization of the vast Japanese Economic Exclusive Zone (EEZ), Ministry of Land, Infrastructure, Transport and Tourism of Japan (MLIT) started the R&D project of offshore platform technologies in 2007. National Maritime Research Institute (NMRI) had developed the integrated design support tool, called “Harmonic design tool” for evaluating economical, safety and environmental aspects of the offshore platforms. The tool conducts conceptual design of offshore platforms for seafloor massive sulphides mining, experimental drilling of methane hydrate, combined application of food and energy production. Using the tool, basic design of semi-submersible rig type platform for experimental drilling of methane hydrate is carried out and optimum hull size, predicted steady environmental forces acting on the platform (current, wave, wind forces), thruster specification for Dynamic Positioning System (DPS) such as required thrust force, construction cost are estimated.

The purpose of this paper is to evaluate the feasibility of proposed basic design of the platform aspects of stability and position keeping ability. From results of experiments and numerical calculation, it is confirmed that the basic design of the platform proposed by the “Harmonic design tool” is appropriate.
Offloading Operability With Heading Control of Side-by-Side Moored FLNG and LNGC
Authors: Kim Hyun Joe , Kwak Hyun-Uk , Lee Jonghun , Kim Se Eun , Seo Jong Soo
Published: August 2013

In this study, a series of model tests was performed and the effect of the heading control on the offloading operability of side-by-side moored vessels in multidirectional environments was investigated. During the tests, hawser tensions, fender loads, and relative motions between two vessels were focused, which are the key factors defining the offloading operability. Basically, the heading control is designed to maintain the FLNG (LNG FPSO)’s heading at between wind wave and swell. In the model tests, several heading angles were selected to investigate the impact of the heading control on the offloading operability, which includes the heading angles aligned with swell and between swell and wind wave. The loading conditions of the FLNG and LNGC were chosen to have a similar roll natural period, and the period of swell was also selected close to the roll natural period, which realizes an worst situation.

The model tests proved that heading control improves the offloading operability in the multi-directional environments. However, in the test, as vessel’s heading angle approaches to the direction of swell, the LNGC is exposed to wind wave as much as to increase the relative motion between the two vessels and deteriorate the offloading operability. In the model test campaign, the motion RAOs and horizontal drift forces/moment due to waves for the side-by-side moored vessels were measured and compared with the analytical calculations, which show the strong shielding effect on the wind wave by the FLNG.
Monitoring Campaign on Sub-Sea Installation
Authors: Navarro Jesus Rodriguez , van Drunen Joris , de Bruin Ruben
Published: August 2013

The Deepwater Construction Vessel (DCV) Balder performed an one-and-half year deep water construction project in west Africa. The project started in 2010 and consists of multiple oil fields located in water depths ranging from 1800m to 2100m. Heerema Marine Contractors (the Contractor) has successfully installed a large number of sub-sea structures, flow lines and Risers. Installation procedures were optimized, new deck equipment has been installed and passive heave compensation is designed. These measures enabled the crew to achieve an excellent performance in the swell dominated seas West of Africa. To accurately capture the experiences with the new equipment and procedures, an extensive Offshore Monitoring Campaign (OMC) was performed.

To achieve this goal three areas of interest were identified:

• Lowering structures to the seabed at deepwater

• Offloading of barges alongside the DCV Balder

• Motion behavior of DCV Balder in swell environment

The objectives of the monitoring campaign were:

• Validation of the models and performance of the equipment

• Verification of the operability limits for different installation activities

• Transfer of knowledge between engineering and operations

The paper describes how the Contractor executed the different measurements and how the results from the measurements were fed back into the execution of the project. The challenges which were faced to obtain high quality measurement data will be dealt with. Finally it shows the benefits of performing full-scale measurement to engineer safe and efficient deep water operations.
Evacuation From Petroleum Facilities Operating in the Barents Sea
Authors: Jacobsen Sigurd R. , Gudmestad Ove T.
Published: August 2013

The purpose of this paper is to examine conditions relevant to evacuation and rescue of personnel from facilities operating in the Barents Sea. The paper considers the area from the Norwegian coast to Bjørnøya (Bear Island) in the north and the new border with Russia in the east. This corresponds roughly to the area that is open for exploration and exploitation of petroleum resources in the Norwegian sector of the Barents Sea.

Pertinent meteorological observation data is collected from the Norwegian Meteorological Institute. The data is used to evaluate evacuation and rescue under the observed conditions. The probability and effect of ice accretion on vessels, in particular lifeboats, is considered. Ice accretion on lifeboats is possible and could threaten stability if the lifeboat has to ride off a storm while waiting for a weather window that allows rescue of the passengers.

Meteorological conditions in the Barents Sea are such that existing equipment like life rafts, escape chutes, davit launch lifeboats and 1st and 2nd generation standby vessels may not be appropriate for the prevailing conditions during winter. Access to reliable weather forecasts is paramount for operating in the Barents Sea. Responsible personnel onboard facilities operating in the Barents Sea should be competent in the interpretation and understanding of weather forecasts and the implications the conditions may have in an evacuation and rescue situation.
Primary Conversion Efficiency of OWC Type WECs Installed on a Large Floating Structure
Authors: Ikoma Tomoki , Masuda Koichi , Rheem Chang-Kyu , Maeda Hisaaki , Watanabe Yuka
Published: August 2013

This paper describes the performance of wave power conversion. A large scale floating system on which oscillating water column type wave energy convertors installed is proposed and the efficiency of primary conversion of wave power is estimated.

In this paper, a boundary condition due to OWC is theoretically modeled in order to consider influence of wave energy absorption to hydrodynamic forces on the floating structure into the three-dimensional singular point distribution method based on the linear potential theory. In the modeling, a boundary condition on a free water surface and an equation of state within an air-chamber above OWC are mathematically and linearly formulated. Air-pressures and variation of water elevation within OWC areas can be simultaneously and directly solved by setting both the variables and by solving the simultaneously equations of the air-pressure and the vertical displacement.

The damper effect of OWC devices is taken into account. The validity of the prediction method proposed is proven by comparing with experimental results. There are three types of large scale wave energy platform which is a pontoon type in the paper. Each performance of the efficiency of primary conversion of wave power is estimated. It is shown that the performance is very good.