bluuebunny/crossref_metadata_2025 huggingface dataset:
Axial Excitation Tool String Modelling
Authors: Forster Ian
Published: October 2015

Current types of axial excitation tool have been shown to produce beneficial results — in terms of load transfer to the bit, general reductions in string friction and reductions in drill dysfunctions — such as stick slip.

The positioning of such tools to achieve optimum benefit is therefore extremely important — in order to maximize the axial excitation to the areas of the string that require a reduction in friction, and also to minimize the axial excitation to the surface and to sensitive string tools (such as MWD) — where damage may occur.

This paper describes a string model that allows the position of axial excitation tools to be assessed — in terms of the string response — both locally and remotely from the tool.

The model breaks the string down in to springs and masses — with 10 nodes in the upper string; and 5 nodes in the BHA. Additional components can also be added to the string — such as shock tools, jars and accelerators — in terms of mass and stiffness.

The equations of motion are used to connect the nodes in terms of differential equations.

The model is Mathcad based, and as a result, executes very quickly — so allowing comparative studies to be carried out with relative ease.

Data input into the model is also achieved quickly.

The speed with which the model can be used lends itself to fine tuning input data.

The model has been compared with and ANSYS spring mass model, and good agreement has been reached.

Additionally, the model allows more than one axial excitation tool to be added to the string — in order to gauge the benefits of such a configuration.

Damping can also be varied at different locations in the string model.

The results from this model have been used to compare with field test data — derived from a string with instrumentation tool located at various points in the string.

The results show that good agreement can be reached between the model and the field test results, however, careful consideration needs to be taken of the damping assumed in the model.

The model can, never the less, be used for comparative studies — i.e. tool location, number of tools and optimum frequencies.

Further work is recommended in comparing model results with field test results — in order to get a better understanding of the effect of damping.

The damping model could be improved in the model presented here, or alternatively, the lessons learned here could be merged into an existing string model.
Experimental Investigation of Ice Mass Hydrodynamic Interaction With Offshore Structure in Close Proximity
Authors: Sayeed Tanvir Mehedi , Colbourne Bruce , Peng Heather , Colbourne Benjamin , Spencer Don
Published: October 2015

Iceberg/bergy bit impact load with fixed and floating offshore structures and supply ships is an important design consideration in ice-prone regions. Studies tend to divide the iceberg impact problem into phases from far field to contact. This results in a tendency to over simplify the final crucial stage where the structure is impacted. The authors have identified knowledge gaps and their influence on the analysis and prediction of iceberg impact velocities and loads (Sayeed et. al (2014)). The experimental and numerical study of viscous dominated very near field region is the main area of interest. This paper reports preliminary results of physical model tests conducted at Ocean Engineering Research Center (OERC) to investigate hydrodynamic interaction between ice masses and fixed offshore structure in close proximity. The objective was to perform a systematic study from simple to complex phenomena which will be a support base for the development of subsequent numerical models. The results demonstrated that hydrodynamic proximity and wave reflection effects do significantly influence the impact velocities at which ice masses approach to large structures. The effect is more pronounced for smaller ice masses.
Mathematical Model of PLEM Installation in Deepwater
Authors: Liu Ting , Ruan Weidong , Bai Yong , Qiao Hongdong , Su Feng , Fu Jianbo
Published: October 2015

The application of pipeline end manifold (PLEM) in subsea field has been increased dramatically because of its economical and convenient factors as a subsea facility, however, implementing it into the deep waters is still quite challenging. Reference software tools relating to this project should be developed to provide some theoretical technique support for the engineers. Throughout this paper, the installation of PLEM, which is welded to the lower flexible pipe and tractived by the wire rounding through the suction pile, is studied in the 3D space. The mathematical model and its dedicated algorithm of the installation system is established, and the iterative procedures to calculate the mechanical behavior of the pipeline and the tractive wire in the installation project are illustrated. The initial configuration of the system can be obtained based on the catenary theory, and its final form, when subjected to environmental loads, can be acquired through the overall iterations by satisfying the compatibility and equilibrium requirements. Results derived from the proposed method are confirmed to be accurate and reliable when compared with the ones achieved from the OrcaFlex simulation.
Design Aspects of a High Speed Monohull RoPax Ferry
Authors: Kluwe Florian , Martinsen Kay , Krüger Stefan , Lübcke Adele , Will Johannes
Published: October 2015

This paper describes the design process of a high speed mono hull RoPax ferry which operates at a Froude number of 0.4. The design task was quite challenging, as two possible transport concepts were in principle possible: Two ships were needed with a total speed of 50knots, which could result in a combination of a 30kn high speed Catamaran plus a conventional 20kn RoPax Ferry or alternatively in two identical sister vessels of 25kn each. The solution with the high speed catamaran plus the conventional RoPax-Ferry defined the total cost budget, which must not be exceeded by the design of the two sister vessels. This resulted in a tough boundary condition and made life cycle cost evaluations necessary.

Due to harbor restrictions, the length of the ships was limited by abt. 110m, resulting in a Froude number of abt. 0.4. This resulted in high costs for the propulsion system. The ferries should initially have open RoRo-Cargo spaces for cost reasons, which made the stability requirements (weather criterion plus Stockholm Agreement) quite challenging. This also strongly influenced the design of the final hull form. As the ship is very sensitive to weight, detailed steel structure optimizations had to be carried out to optimize the main grillage systems of the vehicle decks. The hull form and the appendage design required careful optimization to guarantee the required service speed with the engine power which was available in the price budget. As no vessel of comparison was available, the speed power estimation as well as all design tasks had fully to rely on numerical predictions. As the ship had further demanding requirements for course keeping and comfort in waves, the optimization of the hull form must include also these issues.

The paper shows that the design of complex ships is actually a holistic task which includes many engineering disciplines. The paper also shows that 1st principle based design methods can support the design process of specialized vessels significantly.
Design of Reelable Bulkheads: Application of Pressure Vessel and Subsea Pipeline Codes
Authors: Manouchehri Soheil , Kaye David
Published: October 2015

Midline bulkheads are often used as a restraining mechanism in High Pressure / High Temperature (HP / HT) pipe-in-pipe (PIP) systems. Their primary function is to share the loads between the inner and outer pipes and / or to minimize the extent of damage in an operational or installation incident. They can also be used during reeled pipelay to facilitate the reel-to-reel or trip-to-trip weld tie-in. Design of a reeled bulkhead, which is categorized as a “pipeline component” under DNV-OS-F101, requires the careful adaptation of a Pressure Vessel Code (PVC), a subsea pipeline code and compliance with the additional requirements of reeling, welding and fabrication. Modern pipeline codes, such as DNV-OS-F101, are LRFD based codes in which different limit states (failure modes) have been formulated and calibrated based on a given probability of failure. The PVCs, however, do not specifically address the pipeline design and careful consideration in selecting load factors, load combinations and the analysis method are necessary.

This paper summarizes a procedure that has been adopted in design of reeled midline bulkheads. The methodology can also be used in design of reeled end bulkheads. Firstly, the initial geometric dimensions of the reelable bulkhead are defined and its reelability is confirmed. Then, appropriate load combinations are identified from ASME BPVC Section VIII – Division 2. Finally, a series of Finite Element Analyses (FEA) are performed to show the fitness-for-service of the bulkhead. The importance of selecting an appropriate and justifiable “code break” is highlighted here.
Bending Analysis of a Flexible Pipe With Broken Tensile Armor Wires
Authors: de Sousa José Renato M. , Magluta Carlos , Roitman Ney , Campello George C.
Published: October 2015

In this work, the mechanical response of a damaged 2.5″ flexible pipe under combined tensile and bending loads is studied. A set of experimental tests was carried out either considering the pipe intact or with one up to four broken wires in its outer tensile armor. In these tests, the deflections along the pipe as well as the strains in its outer tensile armor wires were measured thus allowing estimating the bending stiffness of the pipe and the force distribution among the wires, respectively. The results obtained are compared to those provided by a previously presented finite element (FE) model and analytical models. The numerical and analytical predictions agreed well with the experimental measurements pointing to a negligible decrease in the stiffness of the pipe with the increasing number of broken wires and, furthermore, a redistribution of forces among the intact wires of the damaged layer with high stress concentration in the wires close to the damaged ones.
Fully-Nonlinear Simulation of the Hydrodynamics of a Floating Body in Surface Waves by a High-Order Boundary Element Method
Authors: Li Chengxi , Liu Yuming
Published: October 2015

The objective of this work is to understand and evaluate the hydrodynamics modeling of a floating rigid body in regular and irregular ocean surface waves. Direct time-domain numerical simulation, based on the potential-flow formulation with the use of a quadratic boundary element method, is employed to compute the response of the body under the action of surface waves including fully-nonlinear wave-body interaction effects associated with steep waves and large-amplitude body motions. The viscous effect due to flow separation and turbulence is included by empirical modeling. The simulation results of body motions are compared with laboratory experimental measurements. The nonlinear effects due to body motion and wave motion are quantified and compared to the viscous effect. Their relative importance in the prediction and modeling of a rigid body motion under various wave conditions is investigated. This study may provide essential information pertaining to develop effective modeling of nonlinear wave-body interactions which is needed in design of offshore structures and wave energy conversion devices.
New Form of the Hamiltonian Equations for the Nonlinear Water-Wave Problem, Based on a New Representation of the DTN Operator, and Some Applications
Authors: Athanassoulis Gerassimos A. , Papoutsellis Christos E.
Published: October 2015

We present a new Hamiltonian formulation for the non-linear evolution of surface gravity waves over a variable impermeable bottom. The derivation is based on Luke’s variational principle and the use of an exact (convergent up to the boundaries) infinite-series representation of the unknown wave potential, in terms of a system of prescribed vertical functions (explicitly dependent on the local depth and the local free-surface elevation) and unknown horizontal modal amplitudes. The key idea of this approach is the introduction of two unconventional modes ensuring a rapid convergence of the modal series. The fully nonlinear water-wave problem is reformulated as two evolution equations, essentially equivalent with the Zakharov-Craig-Sulem formulation. The Dirichlet-to-Neumann operator (DtN) over arbitrary bathymetry is determined by means of a few first modes, the two unconventional ones being most important. While this formulation is exact, its numerical implementation, even for general domains, is not much more involved than that of the various simplified models (Boussinesq, Green-Nagdhi) widely used in engineering applications. The efficiency of this formulation is demonstrated by the excellent agreement of the numerical and experimental results for the case of the classical Beji-Battjes experiment. A more complicated bathymetry is also studied.
Local Analysis of Wave Fields Produced From Hindcasted Rogue Wave Sea States
Authors: Dias Frédéric , Brennan Joseph , Ponce de León Sonia , Clancy Colm , Dudley John
Published: October 2015

Global-scale wave climate models, such as WAVEWATCH III, are widely used in oceanography to hindcast the sea state that occurred in a particular geographic area at a particular time. These models are applied in rogue-wave science for characterizing the sea states associated with observations of rogue waves (e.g., the well known “Draupner” [1] or “Andrea” [2] waves). While spectral models are generally successful in providing realistic representations of the sea state and are able to handle a large number of physical factors, they are also based on a very coarse grained representation of the wave field and therefore unsuitable for a detailed resolution of the wave field and refined wave-height statistics.

On the other hand, local wave models based on first-principle fluid dynamics equations (such as the Higher Order Spectral Method) are able to represent wave fields in detail, but in general they are hard to interface with the full complexity of real-world sea conditions. This paper displays our efforts in coupling these two types of models in order to enhance our understanding of past extreme events and provide scope for rogue wave risk evaluation. In particular, high resolution numerical simulations of a wave field similar to the “Andrea” wave one are performed, allowing for accurate analysis of the event.
Interference Assessment Between Top Tensioned Risers Using a Comprehensive Screening Approach
Authors: Sundararaman Shankar , Saldana David , Patel Suneel , Andrew Ben , Padelopoulos Pete , Karayaka Metin , Raghavan Kamaldev , Hays Paul
Published: October 2015

Interference between top tensioned risers (TTRs) is a key design challenge. Due to TTR tensioner stroke limits combined with large vessel offsets, the space out of wellheads is limited. Therefore, riser-to-riser contact is more likely to occur in extreme current conditions.

Riser clearance between adjacent risers is evaluated accounting for the effects of wake, vortex-induced vibrations, current directionality (including variation through-depth), vessel offset, riser configuration, and drilling sequence. Accounting for all of these effects simultaneously and in detail when assessing TTR interference can be challenging.

The typical TTR array interference approach consists of a combination of riser deflection shape matching and detailed wake assessment. In this paper, a revised TTR interference analysis approach is discussed, with the inclusion of an intermediate step involving screening for critical riser pairs using a simplified wake model assessment. Riser deflection shape matching ensures that the likelihood of clashing is minimized. The riser interference screening process avoids detailed wake modelling of non-critical riser pairs. The screening analysis method emphasizes avoidance of false positives (unrealistic riser clashing pairs) and false negatives (missing riser clashing pairs). It employs a simplified conservative wake model using a stratified downstream current profile to determine which riser pairs are critical and warrant detailed wake modelling. To illustrate the efficiency of the screening approach, results from this approach are compared to results from analysis with detailed wake modelling.

An implementation of this approach is presented for riser joints with fairings and strakes. Nominal drag coefficients for these joints are obtained based on experimental testing and/or computational fluid dynamics simulations. Drag amplification of the upstream riser is obtained from vortex induced vibration (VIV) analysis and is also incorporated in the analysis.
Evolution and Energy Transition of Focused Waves on Current
Authors: Bai Junli , Ma Ning , Gu Xiechong
Published: October 2015

Wave-current interaction is a common phenomenon in real sea, thus offshore structures are often exposed to combined wave and current actions. Focused wave is an unexpectedly large wave in ocean with large wave steepness and strong nonlinearity. The combined actions of focused wave and current threaten the safety of offshore structures. Due to the strong nonlinearity of the focused wave, viscous effect is important in the prediction of the flow patterns of waves which are interfered by current. The CFD method can obtain more accurate information of viscous flow field than the potential theory. Thus, RANS simulation is adopted to study the interaction between focused wave generated in a Circulating Water Channel (CWC) and current in this paper. The dynamic grid technique is adopted to simulate the motion of real wave maker in CWC while the VOF method is used to capture the free surface of wave. The transient water wave theory is used to generate deep water focused wave. Meanwhile, experiment was conducted to generate the focused wave with and without current in CWC and the numerical model is validated by the experimental data firstly. Then the effect of transient wave steepness on formation of rogue wave is studied. The effect of current on the propagation and breaking process of focused wave with special wave steepness is discussed in detail by analyzing the evolution of water surface. Accordingly, the kinetic energy of wave at different locations, the total wave energy dissipation rate and turbulent kinetic energy have been evaluated. The energy dissipation and transition in the focusing process are discussed in detail through the analyzed results of wavelet transform. From the results, wave energy has been concentrated remarkably when focused wave propagates in counter-current and the dissipation rate increase remarkably when wave breaking occurs. Wave energy transfers from low frequency to high frequency when the focused wave propagating on current.
Calibration of Stress Transfer Functions for Wellhead Fatigue
Authors: Williams Dara
Published: October 2015

In many harsh environment or high current regions (e.g. West of Shetlands, East Africa and GoM) wellhead fatigue during drilling or workover activities can be a major concern. As a result extensive wellhead and conductor fatigue assessments are required in order to predict likely fatigue damage prior to landing the BOP on to the wellhead. These pre-drilling or predictive studies are based on a number of assumptions regarding actual environmental and soil conditions. In addition the uncertainty associated with the input data requires safety factors of 10 and 20 for wave and VIV effects respectively. As a result of these assumptions the predicted levels of fatigue damage to the wellhead may be highly conservative. In cases where wellhead and conductor fatigue life is a concern the operator may choose to deploy motion monitoring systems on the BOP stack so that actual fatigue loads recorded during drilling can be calculated. These systems can be configured to provide ‘real-time’ updates on fatigue damage accumulation or can be used to record data for ‘post-drilling’ assessment. In each case the methodology applied to calculate stresses at fatigue hotspots is critical to the calculation of overall fatigue damage.

The calculation of fatigue loads based on measured data is typically carried out by applying the measured BOP motions to a global finite element model of the drilling riser and wellhead system. However due to the large amounts of data involved this can be a very time consuming exercise and is not conducive to ‘real-time’ presentation of results (e.g. for on-board systems). A proposed solution to this problem is to develop stress transfer functions (STFs) that relate BOP motion to stresses at critical fatigue hotspots. Thus ‘real-time’ BOP motions and be instantly converted to ‘real-time’ stresses. However, as is outlined in this paper, the frequency content of the system response can have a significant impact on the levels of stress calculated. If frequency dependent response is not accounted for a significant under-prediction in fatigue loads may occur.

The objective of this paper is to outline a detailed methodology to derive STFs for critical fatigue hotspots in the wellhead and conductor. This methodology accounts for capturing the non-linear frequency dependence of the system and incorporating this into the STFs. In addition a methodology for rapidly calculating ‘real-time’ fatigue damage accumulation in the wellhead and conductor system and presenting this data on-board is also outlined.
Multidisciplinary Optimization Design for Section Layout of Complex Umbilical Cables Based on Intelligent Algorithm
Authors: Yang Zhixun , Yan Jun , Ma Guojun , Lu Qingzhen , Tang Minggang , Chen Jinlong , Yue Qianjin
Published: October 2015

Umbilical which links the top floater and the subsea devices provides control functions through electrical cables and hydraulic remote transmission. They are treated as the “lifeline” of the subsea production system for offshore oil and gas exploitation. During operation, umbilical needs to undertake self-weight and periodical load due to the ocean environment. Meanwhile, the heat during power transmission in electric cable is released to the umbilical body, which influences the mechanical properties and optical transmission in the cable. However, there are a number of components and many kinds of sectional arrangement for the umbilical. So the sectional design with multiple components needs to be solved as a multidisciplinary optimization problem. From the mechanical point of view, the umbilical structure should be designed with more compacted and symmetric layout to obtain even probability of resistance to loads and reduce structural stress to improve its fatigue performance. Concerning thermal effect, these units should be arranged to dissipate the heat easily to avoid the influence on the functional and structural components. In this paper, compactedness, symmetry and temperature distribution are quantified through introducing corresponding indices. Then multidisciplinary optimization framework is established. Particle Swarm Optimization (PSO) intelligent algorithm is adopted to carry out the optimization to obtain the optimal solution, which is far superior to the initial design. The optimization design strategy is proved to be effective and efficient by some numerical examples, which provides reference for design of umbilical cables.
Dynamic Response of the Bow Flare Structure Under Slamming Loads
Authors: Ren Huilong , Yu Pengyao , Wang Qiang , Li Hui
Published: October 2015

To increase the loading capacity of large ships, the flared bow is usually adopted to ensure adequate deck area to arrange the goods or weapons, however, this type of bow structure also results in serious bow flare slamming loads. In recent years, the local structural damages of ships that were caused by local slamming loads have been heard by accident, so it’s very necessary to study on the local slamming pressure loads and structural response assessment. The ship in its whole life may encounter all kinds of wave conditions and designers are very concerned about how to determine the design slamming loads on the flare structure. 3-D linear potential flow theory and long-term analysis method of the relative slamming velocity are combined to determine the design state for assessing the slamming strength of bow flare structures. During the specific calculation, the relative motion between the hull flare section and the wave surface is simplified as relative motion between the hull section and the static water surface. And an explicit finite element method is used to predict the slamming load when the hull flare section entries into the water. By adding the slamming pressure load of each time step to the finite element model and establishing the reasonable boundary conditions, the dynamic response prediction of the bow flare structure is achieved. The calculations of different gird size are carried out to verify the convergence of the method in this paper during the prediction of local slamming pressure and structural response. The assessment results of the direct calculation method and the society classification guide are discussed to better guide the bow flare structural design.
A 3-DOF Ship-Borne Stabilization Platform
Authors: Xiao Wang , Weijia Li , Kai Zhou
Published: October 2015

In this research a 3-DOF parallel mechanism with anti-torsion arm is designed as a ship-borne stabilization platform. For adapting to the serious sea conditions, such platform needs to couple the advantages of good mechanism stiffness, high bearing capacity and large workspace. Accordingly, a kinetics and kinematics based nonlinear optimization model is developed for the optimal design of parallel mechanism. The model is applied to synthetically optimize the workspace, stiffness requirement and kinematic performance of the 3-DOF parallel mechanism. An acceleration genetic algorithm is employed to seek the global optimization solutions of nonlinear optimization model. The solution results indicate that the designed ship-borne stabilization platform can not only provide a good performance, but also meet the design requirements.
Estimating Long Term Extreme Slamming From Breaking Waves
Authors: Lian Gunnar , Haver Sverre K.
Published: October 2015

Characteristic loads for design are defined in terms of their annual exceedance probability, q. For ultimate limit state (ULS) q = 10−2, while q = 10−4 for accidental limit state (ALS). In principle a full long term analysis is required in order to obtain consistent estimates. This is straight forward for linear response problems, while it is a challenge for non-linear problems in particular if they additionally are of an on-off nature. The latter will typically be the case for loads due to breaking wave impacts.

The Contour line approach is an alternative convenient method to estimate the long term extreme response, based on short term statistics from an appropriate sea state. The consequence of very large short term variability (large coefficient of variation for 3-hour extreme value) on the application of the contour method will be discussed.

The long term integral is carried out over all sea state combinations. The lowest sea states will of course not affect the extremes. However, for the impact problem the short term variability is much larger than for most response cases. The coefficient of variation of the 3-hour maximum impact pressure is often between 0.5 and 1, while for a typical response process it is between 0.1 and 0.2. Due to the large variability, lower sea states than normal will contribute to the long term response.

In this paper the irregularity of the response surface, and the uncertainties related to the number of seeds used in each sea state is looked into. The focus is on slamming loads from breaking waves, and some results from a model test are presented. The uncertainties in long term response from slamming loads are compared to a more common response process. The effect on the long term response when integrating over a reduced area of sea states in the scatter diagram is discussed.
Studies of Extreme Energy Localizations With Smoothed Particle Hydrodynamics
Authors: Moitra Ayan , Chabalko Christopher , Balachandran Balakumar
Published: October 2015

Smoothed particle hydrodynamics (SPH) is used to simulate hydrodynamic waves and wave phenomena including focusing from wave interference. This Lagrangian based method can be used to naturally simulate hydrodynamic free surfaces, including the free surface of a breaking wave. A virtual wave tank is simulated where wave motions can be excited from either side. Wave focusing is observed at the tank center, where the waves interfere. As a measure of the interference, the wave heights that result from focusing are presented. Certain types of wave focusing are thought to lead to large ocean waves. The efficacy of SPH in modeling wave focusing is critical to further understanding and predicting extreme wave phenomena with SPH.
Re-Evaluation of Tag Safety Critical Status in an FPSO: A Case Study for Recognizing Inconsistencies
Authors: Zahirian Sina , Ratnayake R. M. Chandima
Published: October 2015

Assigning correct ‘safety critical’ (SC) status for tags in offshore production and/or in a process facility is vital for an asset owner. It minimizes not only health, safety & environmental (HSE) challenges but also the wastage in terms of cost, time and effort of maintenance activities. Correct assignment of the SC status for a tag obviates the selection of a tag which does not deserve safety critical status and vice versa. A case study has been carried out to perform a re-evaluation and audit of the safety critical status of the tags which have already been assigned by two engineering contractors (ECs) in a floating production storage and offloading (FPSO) facility designed for offshore oil and gas (O&G) industry. However, the asset owner has discovered a gap between the results of aforementioned SC status assignments which have been carried out by the initial ECs. Hence, the results of initial two SC status assignments have been handed over to a third party engineering contractor for the purpose of re-evaluation and verification. This manuscript maps the work process attached to the reevaluation and verification. Finally, it presents the reasons discovered for the inconsistencies that caused the mismatch in the initial tag SC status assignments.
Hydrodynamic Analysis of an Inverted Catenary Coldwater Pipeline of a LTTD Plant
Authors: Ajeesh M. V. , Selvam R. Panneer , Sundarvadivelu R. , Dhinesh G. , Saravanan R. , Phani Kumar S. V. S. , Ramana Murthy M. V.
Published: October 2015

Drinking water is a precious commodity with growing demand, motivating the researchers to explore innovative and cost effective measures to augment the available resources. Low Temperature Thermal Desalination (LTTD) is one among the ideas that utilizes the ocean thermal gradient for the production of freshwater. The cold water from deep sea is drawn and transported to the coast or to a floating platform to aid the conversion of surface seawater to fresh water. The behavior of this cold water pipeline in open waters is to be studied through analytical, numerical and experimental studies before actual field implementation.

This paper presents the preliminary analysis of a flexible cold water pipe made up of High Density Polyethylene (HDPE) catering for an island based desalination plant. The water is drawn at a depth of 400 m and transferred to the shore. The site with coral reef demands an inverted catenary profiled pipeline to minimize damages and is connected to sump at the shallow end and a clump weight at the deep end. Preliminary hydrodynamic analysis of the cold water pipeline is carried out using OrcaFlex software under regular waves. Obtaining the effect of flexible pipeline under regular waves besides carrying out vortex induce vibration analysis and modal analysis forms the scope of this study. Variation of VonMises stresses, and effective tension at salient points (top end, middle and bottom end) are included.
A Machine Learning Approach to the Analysis of Wave Energy Converters
Authors: Sarkar Dripta , Contal Emile , Vayatis Nicolas , Dias Frederic
Published: October 2015

The hydrodynamic analysis and estimation of the performance of wave energy converters (WECs) is generally performed using semi-analytical/numerical models. Commercial boundary element codes are widely used in analyzing the interactions in arrays comprising of wave energy conversion devices. However, the analysis of an array of such converters becomes computationally expensive, and the computational time increases as the number of devices in the system is increased. As such determination of optimal layouts of WECs in arrays becomes extremely difficult. In this study, an innovative active experimental approach is presented to predict the behaviour of theWECs in arrays. The input variables are the coordinates of the center of the wave energy converters. Simulations for training examples and validation are performed for an array of OscillatingWave Surge Converters, using the mathematical model of Sarkar et. al. (Proc. R. Soc. A, 2014). As a part of the initial findings, results will be presented on the performance of wave energy converters located well inside an array. The broader scope/aim of this research would be to predict the behaviour of the individual devices and overall performance of the array for arbitrary layouts of the system and then identify optimal layouts subject to various constraints.
Preliminary Wave Energy Converters Extreme Load Analysis
Authors: Yu Yi-Hsiang , Van Rij Jennifer , Coe Ryan , Lawson Mike
Published: October 2015

Wave energy converter (WEC) devices are designed to sustain the wave-induced loads that they experience during both operational and survival sea states. The extreme values of these forces are often a key cost driver for WEC designs. These extreme loads must be carefully examined during the device design process, and the development of a specific extreme condition modeling method is essential. In this paper, the key findings and recommendations from the extreme conditions modeling workshop hosted by Sandia National Laboratories and the National Renewable Energy Laboratory are reviewed. Next, a study on the development and application of a modeling approach for predicting WEC extreme design load is described. The approach includes midfidelity Monte-Carlo-type time-domain simulations to determine the sea state in which extreme loads occur. In addition, computational fluid dynamics simulations are employed to examine the nonlinear wave and floating-device-interaction-induced extreme loads. Finally, a discussion on the key areas that need further investigation to improve the extreme condition modeling methodology for WECs is presented.
Fatigue and Fracture Degradation Inspection of Offshore Structures and Mechanical Items: The State of the Art
Authors: Keprate Arvind , Ratnayake R. M. Chandima
Published: October 2015

A significant number of offshore structures and mechanical items installed in production systems on the Norwegian Continental Shelf (NCS) are either approaching or have exceeded their intended design life. However, with the help of the advancement of technology and analysis approaches, most of the offshore production facilities are being considered for life extension. This requires regular inspection, fitness for service (FFS) assessment, remnant life assessment, maintenance and repair (or modification). In this context, fatigue and fracture related degradation play a vital role. Hence, this paper discusses the state of the art as well as two major methodologies used for fatigue life prediction of structures and mechanical items. The first (S-N approach) is based on experimentally derived S-N curves and linear damage rule (LDR). Since LDR does not take sequence effect of loading into account the S-N approach often leads to overestimation / underestimation of fatigue life. Hence, this paper also takes into simultaneous consideration the second approach, which relies on the principles of fracture mechanics (FM) and crack growth analysis. Furthermore, the paper discusses damage tolerance analysis (DTA) and the role of Risk Based Inspection (RBI) to detect cracks before they grow to a critical level and cause catastrophic failure of the component. Thereafter, the paper discusses the reliability of Non-Destructive Evaluation (NDE) methods quantified in terms of Probability of Detection (PoD), to identify the flaw size and location. Finally, probabilistic crack growth (PCG) models used for remaining useful life estimation (RULE) and for planning inspection regimes of structural and mechanical items are discussed briefly.
Evaluation of Aged PVDF by Instrumented Indentation Technique
Authors: de Oliveira Geovanio Lima , Costa Celio A. , Costa Marysilvia F.
Published: October 2015

Instrumented indentation (IIT) is proposed as an alternative mechanical evaluation technique to identify ageing of semicrystalline thermoplastic polymers such as PVDF. This technique does not require special geometries such as standard tensile experiments and allows following changes in mechanical properties as deeper penetration depth are imposed. Therefore, it is able to detect differences in mechanical properties along the thickness of the material. For PVDF aged in crude oil, it was found that for shallow indentations the viscoelasticity effects are minimized. For deeper indentations it was possible to observe a loss of rigidity with aging time. The technique also allows to determine yield behavior.
Design and Installation of a Hybrid-Spar Floating Wind Turbine Platform
Authors: Utsunomiya Tomoaki , Sato Iku , Kobayashi Osamu , Shiraishi Takashi , Harada Takashi
Published: October 2015

A floating offshore wind turbine platform supporting a 2MW downwind-type turbine was successfully installed offshore of Kabashima Island, Goto city, Nagasaki prefecture, Japan on October 18, 2013. It has been operating since October 28, 2013 as the first grid-connected multi-megawatt floating wind turbine in Japan. The spar platform has a unique feature, that is, the upper part of the spar is made of steel (as usual) but the lower part is made of precast prestressed concrete (PC). Such a configuration is referred to as hybrid-spar. In this paper, the design methodology of the hybrid spar is presented — including environmental design conditions, DLCs (Design Load Cases), dynamic analysis, fatigue analysis, etc. Also, the installation procedure is presented briefly.
Optimization of the Cement Slurry Compositions With Addition of Zeolite for Cementing Carbon Dioxide Injection Wells
Authors: Sedić Krunoslav , Gaurina-Medjimurec Nediljka , Pašić Borivoje
Published: October 2015

Well integrity related to carbon dioxide injection into depleted oil and gas reservoirs can be compromised by corrosion which can affect casing, downhole and surface equipment and well cement. Impact on well cement can cause overall degradation of set cement and lead to migration of carbon dioxide back to the surface. Thus, special types of cements should be used. One of the acceptable solutions is application of cement blends based on a mixture of Portland cement and pozzolans. The present paper deals with optimization of the cement slurry design containing zeolite which is nowadays widely used due to its high pozzolan activity potential. Cement blends containing 20%, 30% and 40% zeolite clinoptilolite were used. Cement slurries were optimized for application in slim hole conditions on CO2 injection wells on Žutica and Ivanić oil fields in Croatia (Europe), where an old and deteriorated production casing was re-lined with new smaller sized one. Results obtained by this study suggest that cement slurry containing zeolite can be optimized for application in well conditions related to CO2 injection and underground storage, ranging from a slim hole to standard size casing cement jobs which leads to an improvement of well integrity related to CO2 injection.
Study and Simulation on the Energy Efficiency Management Control Strategy of Ship Based on Clean Propulsion System
Authors: Wang Kai , Yan Xinping , Yuan Yupeng
Published: October 2015

Nowadays, with the higher voice of ship energy saving and emission reduction, the research on energy efficiency management is particularly necessary. Energy efficiency management and control of ships is an effective way to improve the ship energy efficiency. In this paper, according to the new clean propulsion system configurations of 5000 tons of bulk carrier, the energy efficiency management control strategy of the clean propulsion system is designed based on the model of advanced brushless doubly-fed shaft generator, propulsion system using LNG/diesel dual fuel engine and energy consumption of the main engine for reducing energy consumption. The simulation model of the entire propulsion system and the designed control strategy were designed. The influence of the engine speed on the ship energy efficiency was analyzed, and the feasibility of the energy efficiency management control strategies was verified by simulation using Matlab/Simulink. The results show that the designed strategies can ensure the power requirement of the whole ship under different conditions and improve the ship energy efficiency and reduce CO2 emissions.
Numerical Simulation of Wave Interaction With Moving Net Structures
Authors: Chen Hao , Christensen Erik Damgaard
Published: October 2015

Fluid structure interaction is an important issue in design of many engineering systems including offshore fish cages. As a first step to develop a coupled CFD/FEM model, in the present study a moving porous media model has been developed in the open source library OpenFOAM. The net structure was described as a sheet of porous media with prescribed rigid body motion and mesh motion was incorporated to conform the motion of the net. Free surface wave generation and absorption framework was also introduced to simulate wave interaction with moving net structures. The results showed that mesh motion solver was coupled with free surface wave generation and absorption framework and porous media model flawlessly, and the deformed mesh remained acceptable. It was concluded that Laplace mesh motion solver with inversed quadratic diffusivity model was an appropriate option for simulating wave interaction with moving net structures.
Rotor Scaling Methodologies for Small Scale Testing of Floating Wind Turbine Systems
Authors: Martin Steven , Day Sandy , Gilmour Conor B.
Published: October 2015

Two scaling methodologies are presented to address the dissimilitude normally experienced when attempting to measure global aerodynamic loads on a small scale wind turbine rotor from a full scale reference. The first, termed direct aerofoil replacement (DAR), redesigns the profile of the blade using a multipoint aerofoil optimisation algorithm, which couples a genetic algorithm (GA) and XFOIL, such that the local non-dimensional lift force is similar to the full scale. Correcting for the reduced Reynolds number in this manner allows for the non-dimensional chord and twist distributions to be maintained at small scale increasing the similitude of the unsteady aerodynamic response; an inherent consideration in the study of the aerodynamic response of floating wind turbine rotors. The second, the geometrically free rotor design (GFRD) methodology, which utilises the Python based multi-objective GA DEAP and blade-element momentum (BEM) code CCBlade, results in a more simplistic but less accurate design.

Numerical simulations of two rotors, produced using the defined scaling methodologies, show an excellent level of similarity of the thrust and reasonably good torque matching for the DAR rotor to the full scale reference. The GFRD rotor design is more simplistic, and hence more readily manufacturable, than the DAR, however the aerodynamic performance match to the full scale turbine is relatively poor.
Multiphase Hydrate Induction Experiment in a Subsea Pipeline
Authors: Herath Dinesh , Rathnayaka Samith , Rahman M. A. , Khan Faisal
Published: October 2015

Formation of hydrates is one of the many challenges faced in the offshore oil and gas industry. It may result in blockage of subsea pipelines and equipment, which may result in flow line rupture and process accident. Although extensive experiment study is conducted to better understand the nucleation of hydrates and their slug flow behavior in gas-water/oil systems. However, there is limited understanding regarding the effects of multiphase fluid dynamics and geometric scales on the formation/growth of hydrates in subsea pipelines. In this paper a multiphase lab scale flow loop set-up is proposed to study the effects of pipe diameter, wall roughness, solid particles and hydrodynamic properties. The multiphase development length of a pipe for varying geometric and flow parameters is also analyzed considering three phase mixture properties. This study will help in identifying the accurate development length for gas/liquid/solid multiphase flow.
Ultimate Strength of Box Girders With Incline Cracks
Authors: Ao Lei , Wang Deyu
Published: October 2015

The aim of the present study is to investigate the residual ultimate strength characteristics of box girders with variable inclination cracks under torsional loading. A series of finite element models are established by changing the crack length and crack angle using a commercial FEA program, ABAQUS. The cracks are located at the center and torques are applied on both ends of the box beam. The accuracy of the nonlinear FEA results is verified by a comparison with previous predicted formulas. Based on the FEA results, the relationship between the residual ultimate strength and crack parameters can be indicated in a function with period of π in the form of Fourier series.
The Influence of Pipeline Bending Stiffness on 3D Dynamic On-Bottom Stability and Importance for Flexible Flowlines, Cables and Umbilicals
Authors: Robertson Matthew , Griffiths Terry , Viecelli Graham , Oldfield Stuart , Ma Peter , Al-Showaiter Aiman , Carneiro Daniel
Published: October 2015

In some areas of the world, subsea pipeline hydrodynamic stability is one of the most fundamental aspects of pipeline design. Several design approaches can be used to analyse pipeline stability, ranging from simple force-balance calculations to complex dynamic finite element analysis. Effective and accurate pipeline stability analysis is essential to avoid the potentially high costs associated with complex stabilisation solutions.

As it stands the current engineering practices using absolute stability or generalized stability do not explicitly account for pipeline stiffness, hence do not distinguish any difference in response between flexible and rigid pipelines of similar SG and diameter under a given metocean condition.

Observations of recent WGK dynamic stability analyses show that pipelines that have identical properties but very different stiffness will exhibit significantly different lateral response under the same metocean conditions.

Studies investigating sensitivity of predicted lateral response versus the number of storm random seed realisations have also been observed to show little or no correlation between the peak wave velocity and peak pipeline response.

Recognising that in directionally spread wave conditions, peak wave events do not tend to be associated with the widest wave crests in a seastate, this paper will investigate the nature of wave velocity and crest width characteristics leading to pipeline lateral displacement events (width and net movement) within a storm, and how this varies with pipeline stiffness.

Further understanding of the mechanisms driving pipeline response are therefore expected, based on the hypothesis that both pipeline stiffness and the crest width of waves capable of causing pipeline displacement are important parameters. It is further anticipated that this will help refine the existing design approaches for flexible pipelines which may not be adequately addressed using existing industry design methods, as well as providing recommendations for the minimum length of 3D dynamic stability models required to be considered in order to accurately capture the longitudinal length scales of importance.