bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical and Experimental Wind Tunnel Analysis of Aerodynamic Effects on a Semi-Submersible Floating Wind Turbine Response
Authors: Fontanella Alessandro , Bayati Ilmas , Taruffi Federico , Facchinetti Alan , Belloli Marco
Published: November 2019

This paper presents the main results of an experimental campaign about the DeepCwind semi-submersible floating offshore wind turbine (FOWT), that was carried out at Politecnico di Milano wind tunnel, adopting a hybrid hardware-in-the-loop (HIL) testing technique. Differently from previous works by the authors, this further analysis herein reported, is specifically focused on evaluating the effects of aerodynamic loads on the FOWT platform motions. In order to reproduce the FOWT response to combined wind and waves in a wind tunnel, exploiting the high-quality flow, a HIL system was used. The aerodynamic and rotor loads were reproduced by means of a wind turbine scale model operating inside the wind tunnel and were combined with numerically generated wave loads for real-time integration of the FOWT rigid-body motion equations. The resulting platform motions were imposed to the wind turbine scale model by a hydraulic actuation system. A series of HIL tests was performed to assess the rotor loads effect on the FOWT response. Free-decay tests in still water under laminar un-sheared wind were carried out to evaluate how the aerodynamic forcefield modifies the platform modes frequency and damping. Irregular wave tests for different steady winds were performed to investigate the dependency of platform motion from the wind turbine operating conditions. A FAST v8 model of the studied floating system was developed to support the analysis and numerical simulations were performed to reproduce environmental conditions equivalent to those of the experimental tests. The FAST model prediction capability is discussed against HIL wind tunnel tests results.
Increased Recovery Using Autonomous Inflow Management
Authors: Aadnøy Bernt S. , Al Furati Beder
Published: November 2019

Inflow control devices (ICD) were first introduced 26 years ago on the Troll field. The main purpose was to reduce water coning to delay water production. This technology is commonly used in long horizontal wells. An ICD is a passive orifice.

More recently several types of active devices have been developed. The choking effect here depends on viscosity, fluid density or pressure contrasts. They are called autonomous devices as they react on changes inside the reservoir, without signal from surface.

The main objective is to maximize oil recovery, before water production is so large that the wells are abandoned. A master thesis study conducted at the University of Stavanger together with Neptune Energy has investigated the applications of passive and autonomous inflow devices, to see which tool actually provides the highest oil recovery. The analysis was based on existing products and tools under development. Areas where a specific tool works most optimally were identified. Wells from a producing field were used as candidates for the analysis. A considerable portion of the work was to build a realistic reservoir simulator from production data. This paper will present the work and discuss the results of the study.
Numerical Modelling of Wave Interaction With an FPSO Under Different Incident Wave Conditions
Authors: Kamath Arun , Martin Tobias , Bihs Hans
Published: November 2019

Numerical modelling of wave interaction with offshore structures provides deep insights into the complex wave hydrodynamics and the wave-structure interaction problem. Offshore structures such as an FPSO vessel are susceptible to continuous wave loading under operational conditions and catastrophically large wave loadings under extreme conditions. It is essential to obtain accurate information on the environmental forcing due to waves on an FPSO to reliably predict the operational conditions to optimise the operational times while considering operational safety. In addition, the wave hydrodynamics involved under extreme wave conditions also have to be evaluated in order to predict the survivability of the vessel and avoid major accidents at sea.

In the current study, the open-source hydrodynamic model REEF3D is used to study the wave interaction with an FPSO structure. The numerical results are compared to available experimental data for the free surface around the structure, the pressure due to wave incidence and the motion of the structure. The wave interaction with the structure is studied for a fixed structure and considering the six degrees-of-freedom for a floating object. Different incident waves are simulated including irregular waves using the JONSWAP spectrum and phase-focussed irregular waves. The model solves the incompressible Reynolds-averaged Navier-Stokes equations with high-order discretization schemes for convection and time discretization. The local directional immersed boundary ghost cell method extended to three dimensions is used to represent the boundaries of complex objects. The level set method is used to obtain a sharp free surface. The Poisson pressure equation is solved using the multi-grid preconditioned BiCGStab algorithm provided by the Hypre high performance solver library. The numerical model is fully parallelised using the MPI library.
Experimental Study on the Hydrodynamic Characteristics of Artificial Reefs
Authors: Zhao FenFang , Ong Muk Chen , Tang Yanli , Wang Xinmeng
Published: November 2019

Artificial reefs (ARs) are structures constructed on the seabed to attract and concentrate fish and to potentially improve and rehabilitate coastal ecosystems. In order to investigate the hydrodynamic characteristics of ARs, a series of model experiments of cubic artificial reefs are carried out in the flume. The model reefs are made of acrylic material with different shapes of opening and various open-area ratios. The hydrodynamic forces of the models are measured in the experiments. The changes of drag coefficient with respect to the open-area ratio and the flow-facing angle of attack are investigated and discussed respectively.
Virtual Prototyping and Simulation of Multibody Marine Operations Using Web-Based Technologies
Authors: Fonseca Ícaro A. , de Oliveira Felipe F. , Gaspar Henrique M.
Published: November 2019

This paper focuses on virtual prototyping and simulation of marine operations based on web technologies. The ship is represented as a digital object, which can be used to perform different types of analyses and simulations. The presented simulations are: motion of a single hull and of multiple hulls in regular waves calculated with closed-form expressions, induced pendulum motion response to a lifted load, and motion of a barge with initial movements in still water calculated with equations of motion.

The simulations are developed as web applications in JavaScript and HTML, with graphical user interfaces and 3D renders of the operations. Relevant parameters of the simulations such as wave characteristics and design dimensions are linked to interactive dashboards, allowing the user to modify them and visualize the results in real-time. The applications are lightweight enough to be executed locally in the web browser of most modern devices.

The work employs an open source approach, relying most notably on the Vessel.js library. This aims to foster reuse of models and collaboration with external contributors.
Three-Dimensional Dynamic Analysis Method of Multi-Component Mooring Lines
Authors: Hermawan Yuda Apri , Furukawa Yoshitaka
Published: November 2019

Complicated mooring system well-known as a multi-component mooring line is highly required owing to the deep depth of water and severe sea conditions. Since the dynamic behaviors of such mooring line are quite complex, proper numerical method is indispensable to predict the dynamic behaviors of a multi-component mooring line efficiently and precisely. In this paper, a numerical method improving the lumped mass method is proposed to introduce the three-dimensional dynamic analysis of multi-component mooring line with the motion of an anchor and clump weights. The mooring line is regarded as a multi-component object which has nonuniform segment line characteristics. In this method, lumped mass technique is developed to represent the three-dimensional dynamic behavior of each segment individually, allowing the motion of bottom-end segment as well as the anchor. Then, the motion of the end-segment is regarded as the motion of the upper-end of lower segment. Meanwhile, calculation method of initial condition for dynamic calculation is developed by adopting the basic principle of multi-component mooring line catenary equations. The results of time histories representing the three-dimensional dynamic analysis of mooring line are obtained and compared with other numerical and experimental results presented in published papers. The results show good agreement with both numerical and experimental results.
A Method for Designing the Backbone for the Segmented Model of an Ultra-Large Container Carrier
Authors: Li Hui , Zou Jian , Sheng Weijia , Hu Xuecong , Hu Wenjia
Published: November 2019

The segmented model test is often used to study the wave load characteristics of large ships as it can account for the hydroealstic effect. The vertical bending moment (VBM) is of crucial importance in ensuring the safety of ocean-going vessels in rough seas, and there exists in the literature a large number of experimental studies of the VBM. For ships with large openings in the deck, for instance, container ships, the lateral wave loads, such as horizontal bending moment (HBM) and torsional moment (TM) in quartering seas, are as important as VBM. There are, however, few studies on the measurement of the coupled horizontal-torsional vibrations of such ships in model tests. In the paper, a method is proposed for designing flexible backbone models that satisfy the similarities of vertical and horizontal bending stiffness as well as the torsional stiffness, and the measurement of the wave load components is also described. In order to meet the similarity of the hull girder stiffness, the backbone cross-section of a complex form is designed. Finite element method (FEM) is used to calculate the natural frequencies and mode shapes of the segmented model. Measurement of the vertical bending moment, horizontal bending moment and torsional moment are calibrated by applying various combinations of loads.
The Use of Ensemble Forecast in Defining Offshore Installation Operability: A Case Study on Umbilical Shore Float-In Operations
Authors: Tinoco Francisco , Ting Kee Chien , Chavan Kishor
Published: November 2019

To address the uncertainty in both the monitoring and the forecasting of the environmental conditions for offshore operations, DNV recommends the use of Alpha factors. These factors are defined for the environmental conditions in North Sea and the Norwegian Seas. Since no localised Alpha factors are available for regions outside of North Sea, these factors are conservatively applied everywhere in the world. In a region with benign weather characteristics such as the Mediterranean, this could be overly conservative for the selection of operational weather windows. A deterministic weather forecast is sufficient to assess weather risk for short term operations; nonetheless, uncertainties of longer term predicted weather should still need to be addressed.

Shore float-in of umbilicals and cables requires a number of sequential activities with associated durations that build up to a total timeframe. This timeframe is beyond what can be accurately assessed using only deterministic weather forecasts. The operational activities are also very sensitive to tidal, current & sea-state variation; particularly associated with the use of personnel in the water like divers and waders for the removal of floats and the use of small crafts for control of the of the product alignment.

The use of Ensemble Forecast which utilises probabilistic approach compared to the deterministic approach, addresses the uncertainty of long term weather forecasting and remove artificially added conservatism such as the Alpha factor. This forecast is derived from multiple simulations, each with minor variations of initial conditions and slightly modified weather models.

Ensemble Forecast has been applied in offshore operation planning and execution in a project offshore Egypt in Mediterranean; particularly for a shore float-in of a main umbilical. It was used to define the main operational window, the start of the operation and weather risk beyond deterministic weather forecast.

The use of the Ensemble Forecast for selection of overall weather window has been shown to optimise operability, reduce the risk of product being damaged, provide added confidence in the offshore operational safety for the personnel involved while minimising waiting on weather.

The strategy of defining storm riding configuration for umbilical could enhanced overall operability. When the operational restrictions relating to limiting activities are anticipated to be surpassed but remains still within the storm riding sea-state, these activities could be safely halted and product held in place in stand-by mode until the weather conditions improved without affecting its integrity or overloading installation equipment.
A Numerical Method for Calculation of Ship-Ship Hydrodynamics Interaction in Shallow Water Accounting for Sinkage and Trim
Authors: Ren Huilong , Xu Chen , Zhou Xueqian , Sutulo Serge , Guedes Soares Carlos
Published: November 2019

Sinkage and trim, which often occur to ships moving in shallow water, do not only have an effect on the ship-ship hydrodynamic interaction forces, but also increase the risk of grounding. An algorithm based on the potential theory has been devised for real-time simulation of the hydrodynamic interaction between two ships in shallow water accounting for sinkage and trim. The shallow water condition is modeled using the mirror image method; while the sinkage and trim are solved iteratively based on the principle of hydrostatic balance, where a mesh trimming procedure is performed when the waterline is changed. Simulations are performed with and without accounting for the sinkage and trim, and comparison with experimental results shows a fair agreement.
Out-of-Roundness of the TurkStream Project Line Pipe
Authors: Selker Ruud , Liu Ping , Jurdik Erich , Chaudhuri Jay
Published: November 2019

When using stochastic data in a probabilistic engineering assessment it is common practice to fit the central portion of the data, using for example the Normal distribution. This generally works for predicting expected behaviour; however, it does not necessarily describe extreme behaviour very well.

Extreme value theory and more specifically the peaks-over-threshold method [1] is adopted to assess the extreme behaviour represented by the distribution’s tail. A generalized Pareto distribution is used to fit all samples exceeding a certain threshold value. Estimation of the distribution’s shape parameter provides valuable information on whether the upper tail of the fitted distribution has “finite” (having an endpoint) or “infinite support”.

Line pipe out-of-roundness affects weldability, fatigue performance and collapse resistance. Especially for deep water pipelines it is important to meet tight tolerances. Out-of-roundness data were evaluated and show finite support of the upper tail; however, the evidence is not as strong as the evidence that was found for the line pipe material strength [7]. Hence it is important to measure and confirm the out-of-roundness of every pipe joint during manufacture.

The degree of tail support can have a major effect on the calculated return period of failure events, particularly when related to low occurrence probabilities.
Numerical Study on Seakeeping Performance of a Damaged Ship
Authors: Cui Lu-Ning , Zheng Yi , Li Yinggang , Zhu Ling , Chen Mingsheng
Published: November 2019

Ships sailing in the sea may encounter collision, grounding or projectile impacting accidents, which may cause hull damage and subsequent compartment flooding. Due to the effect of the flooding water induced moment and the restoring moment, the damaged ship may have inclination and rolling motion. When the inclination or the rolling motion is too large, it may affect the safety and survivability of ship in navigation and cause severe casualties and property losses. In order to increase the navigation safety and survivability of the damaged ship, a numerical model is established based on the potential flow theory to investigate the seakeeping performance of the damaged ship in two scenarios, i.e., the case before ship damaged, and the case when the damaged ship reaching a relatively stable floating state. The heave, pitch and roll motion responses and corresponding wave-induced loads acting on the ship are analyzed in regular waves. In addition, the effects of the navigation speed and the wave direction on the seakeeping performance are also investigated.
Coupled Thermo-Elastic Analysis on Cross-Section of Umbilical Cables
Authors: Yan Jun , Hu Haitao , Su Qi , Lu Qingzhen , Yang Zhixun , Yue Qianjin
Published: November 2019

Umbilical cable is composed of electronic cables, optical cables, steel tubes and structural strengthening components, which can be regarded as a key industrial equipment integrating mechanical and electronic functions. Especially, when it is oriented at the power supply with a relatively high rated voltage, the power transmission will produce a large amount of heat with the sectional temperature rising up, which impacts on the material performance and mechanical responses of the cable and even the whole umbilical. Therefore, the thermo-elastic analysis is the critical technology in the cross-sectional design of umbilical cable. Analytical and numerical methods are proposed to conduct the thermo-elastic analysis of the cross-section. Firstly, the steady-state thermal analysis of cross-section of the umbilical cable is implemented, and the thermal field distribution with different cable ampacity is obtained. Then, the thermo-elastic coupled analysis of the cross-section is presented. It is found that the results are quite different from that of static mechanical analysis, which provide a helpful guide for the design of umbilical structures.
Wellhead Fatigue: Benefits of Structural Reliability Analysis Applied to Groups of Wells
Authors: Hørte Torfinn , Macke Michael , Aardal Andreas Buvarp , Reinås Lorents , Bjønnes Pål , Frimanslund Erik Kristian
Published: November 2019

This paper outlines the methodology when performing structural reliability analysis (SRA) for wellhead fatigue of a complete field compared to individual wells. The field consist of several well and template systems from different suppliers. Further this paper will emphasize on added knowledge that can arise from the field vs. well analysis approach. Finally, some advice and guiding on necessary prerequisites for launching a field SRA will be shared.

The methodology followed previous advices, with the added element that all wells were categorized depending on similarities in well design. Furthermore, a number of local finite element analyses for a specified systematic set of variations in input parameters have been performed by relevant equipment manufacturers to calculate the fatigue stress at critical hot spots (load-to-stress curves).

Similar actions on loads had to be performed. A comprehensive set of global load analyses for each of the different rigs that have been operating on the field where needed to assess the cyclic fatigue loads from historical operations on each well. Finally, global and local analysis results were then gathered, and used as input to the SRA combined with a response surface technique.

The additional effort for making the underlying global and local analyses covering all wells, and not only a single well, is limited for a given type of wellhead system. The field results now provide the operator with an improved basis for planning of future operations on the field, and enables optimization of the drilling plan and utilization of rigs throughout the varying environmental conditions for different seasons of the year and different operations, while managing the associated risk.
Biofouling Characterization and its Effect on Resistance of Surface Ship
Authors: Thomas Della , Surendran S. , Vasa Nilesh J.
Published: November 2019

Manufacture of coatings to tackle the biofouling menace has been of prime concern for the shipping industry since industrial revolution. Even though many types of coatings exist in the market, they have not been able to wipe out the problem of biofouling completely. The proposed approach which combines both the resistance due to biofouling on ships, as well as biofouling characterization, will surely serve as a critical input for the manufacture of efficient anti-fouling coatings. Resistance simulations have been carried out for a tanker ship by Maxsurf® wherein the hull fouling effect has been accounted for by modified ITTC 1978 empirical formula considering roughness. Different draft conditions were relied upon to find out the resistance at various Froude numbers. The fouling growth conditions considered are small and medium calcareous fouling. The simulated results also have been validated by towing tank experiments, for no-foul condition, for speeds varying from 0.3 to 1.1 m/s and different drafts. The additional power requirement was determined for various loading conditions and speed ranges.
Review of the In-Air Fatigue Behaviour of CRA Clad and Lined Pipe
Authors: Johnston Carol , Crump Jenny
Published: November 2019

Pipe internally clad or lined with corrosion resistant alloy (CRA) is used offshore when production fluid contains H2S and/or CO2 to protect the carbon-steel pipe from corrosion. Girth weld classification in standards such as BS7608 and DNVGL-RP-C203 do not specifically include clad and lined pipes but their fatigue performance is relevant to designers of these pipelines. This paper first reviews available literature on the in-air fatigue performance of clad and lined girth welds. Data from the literature as well as full scale resonance fatigue results from tests carried out at TWI are then analysed so that recommendations can be made on a suitable fatigue classification. Finally, areas are highlighted where more research into clad and lined pipe is needed.
Axial and Moment Carrying Capacity of Split Sleeve Grouted Connections for Repair of Tubular Members
Authors: Vignesh Chellappan N. , Nallayarasu S.
Published: November 2019

The tubular members damaged by ship impact or falling objects require repair and rehabilitation in offshore structures. The repair of damaged underwater tubular member using welding is hazardous and expensive and hence alternative connection methods such as grouted clamp techniques have been in use for many decades. The existing guideline on the design of grouted connections especially under axial tension and moment is very limited and requires further study. The load transfer mechanism of grouted clamps depends on various geometric parameters and bond between clamp and parent member. A comprehensive study on split sleeve grouted connection for load transfer between two parts of tubular members has been investigated and presented.

Numerical simulation of split sleeve grouted connection has been carried out using finite element model of tubular member – sleeve through bond strength of grout. The numerical model has been validated using existing guidelines and further parametric study has been carried out. The parametric study includes geometric parameters such as diameter to wall thickness ratio of split sleeve, sleeve friction length, grout strength and grout shrinkage. The simulations have been carried out for combination of axial tension and moment loading.
An Investigation of the Effect of Biomimetic Tubercles on a Flat Plate
Authors: Marino Alessandro , Atlar Mehmet , Demirel Yigit K.
Published: November 2019

This paper describes the investigation, by means of Computational Fluid Dynamics (CFD), of the effect of biomimetic tubercles on the hydrodynamics of a fully submerged flat plate. The application of these tubercles takes inspiration from the features of the humpback whale (Megaptera Novaeangliae). These huge marine mammals are capable of quick and agile movements in the water, despite their bulky bodies. Researchers investigated the causes of this ability by studying some peculiar somatic characteristics of these animals, in particular the tubercles on the leading edge of their pectoral fins. These tubercles were applied in the form of sinusoidal perturbations of the leading edge of wing profiles and foils, and they proved to cause a positive effect on the overall performance.

The aim of this paper is to investigate another type of tubercles, which appear in the shape of bumps on the whales head. The effect of these tubercles has not been studied yet, and this paper presents a study on the fundamental phenomena they generate in the water flowing on the surface of a flat plate. The tubercles are modelled as axisymmetric sinusoidal bumps placed on the flat plate. Different combinations of these tubercles are studied, in order to assess what the effect of a single tubercle is, and how more tubercles interact when they are placed closed to each other, in different configurations (number of tubercles and relative position). In addition, a systematic study of the effect of a single row of tubercles spanning perpendicularly to the flow is carried out. The tubercles change systematically in amplitude and position along the plate.

One further objective of this paper is to investigate if an optimised application of the biomimetic tubercles can lead to a drag reduction for the flat plate. Preliminary simulations show that the rows of tubercles interact with the boundary layer by modifying the pressure distribution, velocity and direction of the flow. The tubercles appear to generate vortices that are similar to those created by sinusoidal tubercles on the leading edge of foils, which tend to thin the boundary layer. A change in the total drag of the plate with tubercle is also noticeable, which even decreases from the baseline (flat plate with no tubercles), at certain combinations of position and tubercle amplitude.
Automated Subsea Architecture Optimization Using Low-Dimensional Multiphase Flow Models
Authors: Khan Zurwa , Meziou Amine , Tafreshi Reza , Franchek Matthew , Grigoriadis Karolos
Published: November 2019

Due to the global increase in energy demand, the need for economic oil and gas production is rising more than ever. Therefore, it is necessary to ensure that subsea architecture designs are economical and safety oriented. While numerous challenges are encountered during subsea system’s installation and operation phases, most of these challenges can be avoided by ensuring an economical and reliable design. For a safe and cost-effective design and operating scenario, it is essential to predict the hydraulic and thermal behavior of multiphase fluid encountered in petroleum pipelines for a range of conditions. This cannot be accomplished by empirical models, which are dependent on limited data available. Consequently, mechanistic low-dimensional models have been used for two-phase gas-liquid steady-state flow. However, mechanistic low-dimensional models assume adiabatic conditions, which is rarely the case in subsea architectures, which encounter cold surroundings. Therefore, to predict the temperature-based characteristics of multiphase flow in environments with thermal gradients, a thermal model has been developed and validated with experimental data. 80% of the validation data was predicted by this developed thermal model with error difference of less than 30%. The developed two-phase gasliquid thermal model was merged with Beggs and Brill hydraulic multiphase flow model to predict the overall behavior of two-phase gas-liquid flow, and used to develop an optimal model-based multi-well subsea architecture design. A case study of a four-well subsea system was used to demonstrate the automated subsea architecture optimization technique. Through this case study, it was shown that approximately 23% of savings in pipelines procurement could be made relative to the conventional designing approach. Industry standards, safety factors, and multiphase flow models were used to design jumpers and place the manifold for a subsea multi-well system. Merging hydraulic and thermal multiphase flow models showed the effect of temperature on the flow, which led to an optimized design for the subsea insulation in which issues such as wax deposition can be prevented. The resulting optimized subsea architecture was then implemented in Simscape® environment to obtain the transient response. Along with optimized subsea architecture automated design, the developed thermal model has the potential to be used for real-time prediction of two-phase flow rate, pressure drop and void fraction as virtual sensors to provide economical alternative to expensive and impractical hardware sensors. Furthermore, the developed model can also be used to design effective control strategies for multiphase flow regulation in jumpers and prevention of backflow at the manifold.
CFD Simulation of Two-Phase Vertical Annular Flow in Both Upward and Downward Direction in a Small Pipe
Authors: Abobaker Ekhwaiter , Elsanoose Abadelhalim , Shirokoff John , Rahman Mohammad Azizur
Published: November 2019

Computational fluid dynamics (CFD) simulation is presented to investigate the annular flow behavior in the vertical pipe by using ANSYS Fluent platform 17.2. The study was analyzed complex behavior of annular flow in two cases (upward and downward flow) for different air superficial velocities and range of Reynolds number for water, in order to obtain the effect of orientation flow and increasing superficial gas and liquid velocities on the base film, mean disturbance wave thickness, the average longitudinal size of disturbance wave as well as pressure gradient. For multiphase flow model, the volume of fluid method (VOF) for two-phase flow modelling was used and coupled with RNG k-ε turbulence model to predict fully annular flow structures in the upward and downward flow direction. From CFD simulation results, it is clear to see how increases in air velocity result in reductions in film thickness and increase in pressure gradient. Additionally, the results showed monotonic enhancement of film thickness occurring in tandem with increases in the liquid flow rate. However, due to the effect of gravitational force and interfacial friction, the film thickness and pressure gradient are slightly larger for the upward flow than for the downward flow. The results agree with the recent experimental data that studied the annular flow behavior and pressure drop in the upward and downward flow direction. This study will be very helpful in understanding multiphase flow behavior in natural gas wells.
Experimental Validation of Towed Underwater Cable Model
Authors: Grindheim Jan Vidar , Fernandes Antonio Carlos , Sales Junior Joel Sena , Revhaug Inge
Published: November 2019

Towed underwater cable models have been validated using experimental results performed in the current channel at Laboratório de Ondas e Correntes (LOC) at COPPE/UFRJ, Rio de Janeiro. The numerical simulators utilize a Finite Difference Method to solve the Partial Differential Equations describing the dynamics of a towed underwater cable under tension. A non-dimensional analysis of the system dynamics for the two-dimensional case has been performed, with non-dimensional governing equations being presented. The experimental setup consists of two cable sections of ∼1.5 m length each, the first having 3 mm diameter and slightly positive wet weight while the second section has 2.5 mm diameter and slight negative wet weight. With the cable in steady-state condition, the towpoint is moved 0.50 m sideways and the time for the cable to return to straight tow is measured. Further, the cable depths at midpoint and tail are measured in steady-state. Experiments are performed at currents ranging from 0.17 to 0.47 m/s. The presented experimental results are compared to the numerical results. Reasonable agreements are obtained.
Power Take-Off Selection for a U-Shaped OWC Wave Energy Converter
Authors: Romolo Alessandra , Henriques João C. C. , Gato Luís M. C. , Malara Giovanni , Laface Valentina , Gomes Rui P. F. , Portillo Juan C. C. , Falcão António F. de O. , Arena Felice
Published: November 2019

The REWEC3 (Resonant Wave Energy Converter) is a fixed oscillating water column (OWC) wave energy converter (WEC) incorporated in upright breakwaters. The device is composed by a chamber containing a water column in its lower part and an air pocket in its upper part. The air pocket is connected to the atmosphere via a duct hosting a self-rectifying air turbine. In addition, a REWEC3 includes a vertical U-shaped duct for connecting the water column to the open sea (for this reason it is known also as U-OWC). The working principle of the system is quite simple: by the action of the incident waves, the water inside the U-shaped duct is subject to a reciprocating motion, which induces alternately a compression and an expansion of the air pocket. The pressure difference between the air pocket and the atmosphere is used to drive an air turbine coupled to an off-the-shelf electrical generator connected to the grid.

The main feature of the REWEC3 is the possibility of tuning the natural period of the water column in order to match a desired wave period through the size of the U-duct. The REWEC3 technology has been theoretically developed by Boccotti, later tested at the natural basin of the Natural Ocean Engineering Laboratory (NOEL, Italy), and finally proved at full scale with REWEC3 prototype built in the Port of Civitavecchia (Rome, Italy).

The objective of this paper is to select and optimize a turbine/generator set of a U-shaped OWC installed in breakwaters located in the Mediterranean Sea, such as the Port of Civitavecchia, where the first prototype of REWEC3 has been realized, or the Port of Salerno or Marina delle Grazie of Roccella (Italy). The computations were performed using a time domain model based on the unsteady Bernoulli equation.

Based on the time-domain model of the power plant, the following data is computed for the turbines: i) the ideal turbine diameter; ii) the generator feedback control law aiming to maximize the turbine power output for turbine coupled to the REWEC3 device for Mediterranean applications.
Fatigue Approaches for Mooring Chains Subjected to Wear Degradation
Authors: Gemilang Gilang M. , Reed Philippa A. S. , Sobey Adam J.
Published: November 2019

There are currently 365 FPSOs in service around the world. These vessels all use mooring lines to maintain position and provide stability, keeping the vessel and cargo safe. However, more than 21 failures have occurred between 2001 and 2011 and approximately 50% of the reported failures occurred in the first 3 years of 20-year design life. Each mooring line failure represents the potential for serious environmental and economic consequences. Based on industry surveys, the most common failure mode is fatigue failure. In the current offshore standards, the surface degradation due to wear and corrosion is modelled as a diameter loss at a standards rate. To assess whether the uniform reduction in chain diameter suggested in the offshore standards is able to explain the early chain failures seen in service, this paper incorporates two wear rates into a fatigue life calculation; one wear rate is taken from DNV-OS-E301 and is compared against one taken from NORSOK M-001. Three fatigue life estimation approaches: tension, nominal stress and hotspot, are used to compare the differences in fatigue method. The stress in the chain is calculated using an analytical model, which is verified against an FE model. The effect of wear degradation on the ultimate strength of the chain is calculated based on the minimum breaking load. The results show that the diameter loss rates suggested in the offshore standards are not able to explain the early mooring chain failures seen in the past and that the reduction of diameter cannot solely explain the early failures seen in service. The hotspot approach, not often used in mooring line predictions, is best able to predict these shorter lives, as it offers more accurate fatigue predictions by considering high peak stresses compared to standard methods such as tension and nominal stress approaches.
Methodology Proposal for Corrosion-Fatigue Assessment for Flexible Pipes Tensile Armour in Aggressive Environments
Authors: Favaro Borges Marcelo , dos Reis Tagliari Mariana , Gonçalves Rafaela , Fortis Kwietniewski Carlos Eduardo
Published: November 2019

Flexible risers are the main way to connect wells and floaters for petroleum and gas applications on the Brazilian coast. The failure of those risers in operation can generate huge irreversible economic and environmental expenses. Statistics indicate that there is a predominance of premature failure associated with the flooding of the annular space. When cyclic loads are present, this condition will introduce corrosion-fatigue of the metallic reinforcements. Therefore, to ensure the operational safety, their design should consider the combined action of fatigue and corrosion phenomena. The experimental test procedure to be applied for generating S-N curves and qualifying the steel materials with respect to corrosion-fatigue face different challenges. One of the important ones is to introduce representative surface defects that reflects the exposure time of the real life corrosion process as this will have a strong influence of the slope parameter of the S-N curves. The aim of this work is to propose a new methodology to obtain S-N design curves allowing for the samples to be pre corroded to obtain realistic surface defects. The pre corrosion procedure is made to ensure that all samples have the same defects caused by localized corrosion and that those defects are representative of those presented in real risers with flooded annulus. Considering the effect of the defects aspect on fatigue evaluation allows a better prediction of the remaining life of pipes operating in flooded annulus conditions. Two S-N curves were compared allowing for proving the potential of the methodology and promising results were obtained, giving more reliable S-N curves. Furthermore, it was possible to conclude that under the assessed conditions the fatigue in air tests of pre-corroded wire during 60 days showed the same performance as the new one (non-pre-corroded) wires tested in corrosion-fatigue.
The Impact of Second-Order FPSO Motions on the Fatigue Performance of Large Diameter SCRs
Authors: Hejazi Rasoul , Grime Andrew , Randolph Mark , Efthymiou Mike
Published: November 2019

Large diameter steel catenary risers (SCRs) are considered a cost efficient export riser solution for gas field developments at deep-water sites. However, SCRs are prone to fatigue damage at their interface with the seabed (the touchdown zone, TDZ) and hence accurate estimation of their fatigue life is crucial. The major source of TDZ fatigue damage is the motions of the host vessel subjected to irregular ocean waves. The first-order interaction between the host vessel and the ocean causes oscillations in all degrees of freedom at the same frequencies as the incident waves. The second-order interactions result in a mean-drift offset from the static equilibrium position in the horizontal plane and slowly-varying cyclic motions about that offset position. This paper investigates the effects of second order motions on the fatigue life of a 26” SCR connected to a representative Floating Production Storage and Offload vessel (FPSO), using realistic environmental conditions relevant for a deep-water site on the Australian Northwest Shelf. A diffraction analysis was performed to obtain the hydrodynamic characteristics of the ship-shaped vessel which was subsequently used as the input into a fully coupled response model consisting of the floater, mooring lines and the SCR. A realistic fatigue wave scatter diagram was adopted, consisting of 100 sea-states combining irregular seas, swell, current and winds. This was combined with dynamic time-domain motion analysis and a rainflow cycle counting algorithm in order to determine the fatigue damage within the SCR TDZ due to the host FPSO motions. The results shows that for this representative system the second-order cyclic low frequency (LF) motions have beneficial impacts on fatigue life of the large diameter SCR. Similarly, mean-offsets of the FPSO have a beneficial effect due to changes in the fatigue hotspot location along the SCR within the TDZ for each sea-state. Finally, a simplified method is presented to capture these beneficial effects at the early design stages.
The Analysis of the Joint Limitation Condition of Wave Height-Period on the Floating Crane Lifting Operation
Authors: Wang Xue-gang , Ying Zong-quan , Chen Ze-cong
Published: November 2019

Considering the influence of wave period on the motion of ship, the hydrodynamic model of the floating crane-lifting objects coupling system is built. This model can calculate the motion response of floating crane and lifting object under wave conditions with different wave heights and periods. And it takes stability and sea-keeping of ship, personnel comfort as well as safety of equipment into full account. By comparing swing amplitude and acceleration amplitude of the floating crane and the lifting object, the limit working condition including both wave height and period for hoisting operation of the floating crane is determined. The method for limitation operation condition analysis of the floating crane not only offers calculating foundation for the construction operating adaptability of the engineering ship and the allowable working time window of the construction sea area, but also provides a new technical approach to the arrangement for the construction plan.
REEF3D::FNPF: A Flexible Fully Nonlinear Potential Flow Solver
Authors: Bihs Hans , Wang Weizhi , Martin Tobias , Kamath Arun
Published: November 2019

In situations where the calculation of ocean wave propagation and impact on offshore structures is required, fast numerical solvers are desired in order to find relevant wave events in a first step. After the identification of the relevant events, Computational Fluid Dynamics (CFD) based Numerical Wave Tanks (NWT) with an interface capturing two-phase flow approach can be used to resolve the complex wave structure interaction, including breaking wave kinematics. CFD models emphasize detail of the hydrodynamic physics, which makes them not the ideal candidate for the event identification due to the large computational resources involved. In the current paper a new numerical wave model is represented that solves the Laplace equation for the flow potential and the nonlinear kinematic and dynamics free surface boundary conditions. This approach requires reduced computational resources compared to CFD based NWTs. In contrast to existing approaches, the resulting fully nonlinear potential flow solver REEF3D::FNPF uses a σ-coordinate grid for the computations. Solid boundaries are incorporated through a ghost cell immersed boundary method. The free surface boundary conditions are discretized using fifth-order WENO finite difference methods and the third-order TVD Runge-Kutta scheme for time stepping. The Laplace equation for the potential is solved with Hypres stabilized bi-conjugated gradient solver preconditioned with geometric multi-grid. REEF3D::FNPF is fully parallelized following the domain decomposition strategy and the MPI communication protocol. The model is successfully tested for wave propagation benchmark cases for shallow water conditions with variable bottom as well as deep water.
Efficient Anchoring System for FPSO in Arbitrary Waters
Authors: de Araujo Jairo B. , Fernandes Antonio C. , Sales Joel S. , dos Santos Mario Jorge C. , Thurler Ana Clara S.
Published: November 2019

Oil offshore Brazil, as well as in other continents, has demanded floating units for oil production. Typical anchored production units are the semi-submersible platforms, FPSO, Spar Buoys and TLPs. All of them use several anchoring lines for station keeping.

In the offshore Brazil scenario, the last twelve years the FPSOs adopts the torpedo piles and polyester ropes in the anchoring lines in deep and ultra-deep waters. The FPSO “Cidade de Angra dos Reis” was the first to operate in pre-salt area in October 2010. From then on, it became common the production units with VLCC size, oil production of 150,000 barrels per day and 24 anchoring lines. The reason for such large number of anchors is the 1,200 tonnes torpedo piles holding power restriction.

The proposition developed in this paper reduces the number of anchor lines using well know offshore equipment such as: driven piles, mooring chains, polyester ropes and conventional anchoring deck equipment. The main difference is the use of driven piles with holding power above 1,800 tonnes and anchor lines with a breaking strength of more than 2,000 tonnes. It is shown that a decrease of number of anchoring lines from 24 to 12 can be achieved. Consequently, it will decrease also the space occupied by the anchoring lines in the seabed by reducing the subsea layout of risers, flow lines and well head. This allows a technical and economical scenario more feasible when compared with the currently used on the Brazilian oil & gas market.
Dynamic Response of a Gangway Between Two Floating Bodies in a Side-by-Side Configuration
Authors: Dong Qing , Yang Jianmin , Lu Haining , Li Xin , Lu Wenyue , Liu Lei
Published: November 2019

A number of tender assisted drilling platforms (TAD) are built to offer comfortable accommodation for crew, onsite construction and maintenance for production platforms. The two platforms are usually connected by a telescopic gangway. The safety and feasibility of gangway are essential during personnel transfer operation. In this paper, a TLP-TAD coupled system was investigated numerically and experimentally to study the gangway motion responses. The numerical model was established in frequency domain using 3D multi-body diffraction/radiation wave theory. Then time domain simulation was performed to consider the second-order drift loads and nonlinearity of mooring lines and hawsers. A systematic model test was also conducted for the coupled system. The connection point between gangway and platform was regarded as a joint, so gangway motion could be derived by global motions of TLP and TAD. Both single-body and multi-body cases were studied for comparison. The differences of global responses between two cases were relatively small, indicating that multi-body hydrodynamic interaction was not quite significant. The interaction aroused sway-surge and sway-heave coupling, and shielding effect also existed. Based on the above, gangway extension was investigated to analyze which degree of freedom (DOF) of global motion governed it.