bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of the Concentration of Viscosifier on Desorption Kinetics of Methane From Synthetic Based Fluids
Authors: Ojedeji Damilola , Chen Yuanhang
Published: December 2020

Synthetic-oil based drilling mud is currently the most commonly used type of drilling fluid for offshore drilling in the Gulf of Mexico, due to the environmental regulation in the area, as well as the numerous operational benefits they provide. However, early kick detection and well control decision-making are more challenging due to the solubility of formation gas in synthetic-based fluids. This partially contributes to the poor understanding of the mass transfer kinetics of formation gas in and out of synthetic fluids during these well control events. The objective of this work was to better understand the mass transfer of gas from a solution by evaluating the influence of viscosifier concentration on the desorption kinetics of methane from pure internal olefin and internal olefin-viscosifier mixture. The desorption coefficients were determined from a custom-built mass transfer apparatus. Different suspentone concentrations ranging from 0 to 5wt% by volume of liquid were used to investigate the influence of viscosifier concentration on the desorption coefficient. It was observed that the presence of suspension agents in the liquid phase decreased the mass transfer coefficient. This decrease could be due to an increase in the resistance to the flow of gas bubbles evolving from the liquid phase.
S-N Curve Model for Assessing Cumulative Fatigue Damage of Deep-Water Composite Riser
Authors: Singh Manander , Ahmad Suhail , Jain A. K.
Published: December 2020

Random stress produces fatigue damage due to fluctuation in the presence of high bending stresses gets enhanced and leads to the primary failure of the production composite riser. The main problems with the fatigue deterministic model are related to the fact that not all waves have the same period. The second problem is due to the assumption that all waves are regular and do not into account the stochastic nature of the marine environment. The wave data is provided on a statistical basis. Work attributed to damage related stress distribution and interaction of composite along with integral difficulties in fatigue is challenging task. Resulting flexural stress history is random in nature. The purpose of this study is to present a methodology for assessing cumulative fatigue damage using MATLAB of the deep-water composite riser. The simulation of 12 random sea states is examined for the purpose of estimation of accumulated damage. Using the recommended method by DNV-OS-C501, fatigue life of composite riser is determined for different conditions. Damage induced by stress signals of varying amplitudes is calculated using Palmgren-Miner’s rule. Total fatigue life for stress history is estimated by the summation of damage fractions. Estimation of fatigue damage over riser’s entire service life is done taking into consideration the entire expected sea area. Extreme flexural stress random fluctuations are estimated with and without currents.
Fighting Invisible Kicks: Study of Time Dependent Vaporization of Methane Gas Dissolved in Base Oil
Authors: Nilsen Marius Staahl , Hovda Sigve
Published: December 2020

Understanding the interaction between the drilling fluid and the natural gas from a gas kick may be of great importance when predicting how a well control incident evolves during drilling operations. This is especially true for oil based mud, which has the ability to dissolve large quantities of gas under high pressure, thus potentially hide any volumetric impact of a gas kick. When the pressure of the dissolved gas decreases below the bubble pressure, free gas will start to emerge. Dangerous situations can occur if the bubble point pressure is low and located close to the surface. This may result in a rapid volumetric expansion of the free gas, as it emerges from solution, thus little to no time to react and initiate proper well control procedures.

Most conventional well control simulators that takes gas solubility into consideration assumes an instantaneous vaporization of gas as the vapour-liquid phase equilibria changes. However, this assumption might not always be realistic. It may take some time before a new equilibrium is reached when the conditions are changed. This will thus affecting the rate of gas liberation from the liquid.

To better understand this complex issue, an analytical expression for the transition rate of dissolved gas to free gas in a supersaturated liquid has been derived for low pressure systems. The analytical model is strongly dependent on the solubility coefficient, Kh, and the transition rate factor, γ, and follows an exponential curve. In this expression, Kh is a measure of how much the liquid is supersaturated at any given time and controls how much gas that will be liberated. γ determines how fast the system will reach a new equilibrium, i.e. how fast the gas will be liberated based on the size of the supersaturation. Both Kh and γ are thought to be values given for a specific gas-liquid combination.

In order to verify the analytical expression, experimental testing has been conducted. The experiment is carried out by pressurizing a tank partly filled with the base oil Exxsol D60 by feeding it with methane gas. Some of the gas will dissolve into the liquid. The rest will flow to the top as free gas and pressurize the tank. By quickly removing some of the free gas, thus depressurize the tank, the liquid will instantaneously become supersaturated, hence triggering liberation of free gas from the solution until a new equilibrium is established. By measuring the tank pressure throughout the degassing phase, values for Kh and γ can be estimated and compared to the analytical model.
Design and Fabrication of a Fully Elastic Ship Model
Authors: Chen Yingying , Zhang Shanli , Chen Wei Kean , Magee Allan
Published: December 2020

Ship structures are subjected to various deteriorating mechanisms throughout their service life. Continuous awareness of the vessel’s structural health is a critical aspect of the overall situational awareness. Model tests are often used to validate software which predicts the vessels’ hydrodynamic loading and dynamic structural response. Generally, two different methods can be used to model the flexibility of the ship. The most common method is to sub-divide the hull into a number of rigid segments that are interconnected by a flexible backbone beam. The elasticity of the model is represented by the elastic beam to which rigid segments are connected. However, the segmented model limits the measurements to prescribed locations between segments. The other method is to fabricate the model using a continuous elastic material. In this paper, a new method for fabricating a fully elastic model is introduced as part of a structural health monitoring system. Since the model is built from continuous materials with known elastic properties, it can be instrumented to measure strain at a larger number of locations. A suitable material for construction of the elastic model has been identified. Material tests are conducted to better understand the static and dynamic behavior of the elastic material. The material shows linear stress-strain relationship and stable mechanical properties within the loading range. Due to the low elastic modulus of the material, the strain gauge stiffening effect is obvious and has been taken into account in the calibration process. Using the elastic material, a fully elastic model of the S175 midship section is designed. As a first step trial, the middle part of the model representing a three-cargo hold is manufactured. Static bending tests are conducted to examine the elastic characteristics of the fabricated model. Wave experiments are carried out. The results from these experiments are compared to numerical simulations.
Loads on a Point-Absorber Wave Energy Converter in Regular and Focused Extreme Wave Events
Authors: Katsidoniotaki Eirini , Ransley Edward , Brown Scott , Palm Johannes , Engström Jens , Göteman Malin
Published: December 2020

Accurate modeling and prediction of extreme loads for survivability is of crucial importance if wave energy is to become commercially viable. The fundamental differences in scale and dynamics from traditional offshore structures, as well as the fact that wave energy has not converged around one or a few technologies, implies that it is still an open question how the extreme loads should be modeled. In recent years, several methods to model wave energy converters in extreme waves have been developed, but it is not yet clear how the different methods compare. The purpose of this work is the comparison of two widely used approaches when studying the response of a point-absorber wave energy converter in extreme waves, using the open-source CFD software OpenFOAM. The equivalent design-waves are generated both as equivalent regular waves and as focused waves defined using NewWave theory. Our results show that the different extreme wave modeling methods produce different dynamics and extreme forces acting on the system. It is concluded that for the investigation of point-absorber response in extreme wave conditions, the wave train dynamics and the motion history of the buoy are of high importance for the resulting buoy response and mooring forces.
Experimental Assessment of the IFPEN Solution to the WEC Control Competition
Authors: Tona Paolino , Sabiron Guillaume , Nguyen Hoai-Nam , Mérigaud Alexis , Ngo Caroline
Published: December 2020

This paper describes the experimental assessment of the solution developed by IFP Energies nouvelles (IFPEN) for the WEC Control Competition (WECCCOMP), a benchmark devised to compare energy-maximizing controllers for wave energy converters (WECs). For the first round of the competition, carried out in simulation with WEC-Sim, IFPEN had submitted an MPC-based solution, which eventually scored best among the contestants, with respect to the energy-related criterion defined by the organizers and computed on a set of six irregular waves. For the second round of the competition, the performance of this MPC solution has been assessed in a tank test situation, on the same Wavestar-like scale device that had been simulated in WEC-Sim.

The paper first recalls the features of the control solution implemented for the competition, an offline-tuned weighted-QP MPC algorithm using short-term prediction from present and past wave excitation force estimates. Then, the major steps of the test plan are described and control design choices are explained.

Finally, the experimental results in closed loop are presented. Despite noise and an imperfectly controlled PTO actuator, IFPEN’s MPC solution proves efficient and robust: its experimental performance in terms of harvested electrical energy is in accordance with the nominal results obtained in simulation using the linear design model, and this, for different realizations of each sea state. This performance has allowed the IFPEN team to win the competition also at the experimental stage.
Nearshore Wave Modelling in a Norwegian Fjord
Authors: Stefanakos Christos N. , Furevik Birgitte R. , Knutsen Øyvind , Christakos Konstantinos
Published: December 2020

Phase averaged wave models is a good supplement of in situ measurements for the study of wave climate in a specific location. In spite of having been tested in smoothly varying coastal areas, they haven’t previously been systematically validated in complex topography (coastline and bathymetry) such as Norwegian fjords, due to lack of measurements. However, in planning for large fjord crossings, the Norwegian Public Roads Administration have launched a number of buoys which allow for validation of model setup.

In the present work, nearshore wave conditions in the area of Sulafjord, central Norway, are investigated as derived from numerical modelling with several different setups, and are compared against in situ buoy measurements with good accordance. The analysis is carried out by transferring offshore wave conditions to the nearshore area by successive applications of the well-known third-generation wave model SWAN. As input has been used a very detailed bathymetry of the area, and time series of wind and wave parameters derived from ERA5 and NORA10 datasets. Various scenarios reconstructing the wave input spectra have been considered.
Application of Fractional Calculus in Modelling Viscoelastic Foundation of Ship Structures for Passive Vibration Control
Authors: Praharaj R. K. , Datta N.
Published: December 2020

Viscoelastic materials (VEM) are often used in the ship floors to dampen the noise and vibration induced by machinery (engines, pumps, alternators) and live loads (human traffic and liquid movement in pipelines). Therefore, accurate modeling of such materials is important for the iterative design-analysis process. VEM exhibits both time and frequency-dependent properties. Traditionally, integer-order viscoelastic mechanical models are used to describe the rheological properties of the viscoelastic materials. However, such models find difficulties in predicting the rheological characteristics since their kernel functions are a combination of exponential functions. For instance, the integer-order Maxwell model is good at describing the stress relaxation behavior while is poor in capturing creep, and vice-versa for the integer-order Kelvin-Voigt model. However, the fractional-order mechanical model can overcome the above problem with a fewer number of model parameters. Therefore, in this paper, the fractional derivative-based Kelvin-Voigt mechanical model is employed to describe the time-dependent vibratory behavior of the VEM. To validate the effectiveness of the above model, a thin elastic plate bonded with a thin layer of VEM, subject to a concentrated impact load, is studied. Galerkin’s method and Triangular strip matrix approach are used to solve the partial fractional differential equations of motion. The semi-analytical approach of modal superposition is used to generate the response, whose first dynamic overshoot of displacement is crucial to contain the dynamic stress within the working stress level. This bypasses the computationally expensive Finite Element Method. Additionally, ANSYS is limited to the integer-order damping model only. This analysis gives insights into the efficacy of the VEM chosen for passive vibration control of structural components of ship hulls. A case study for free vibration is done with a commercially available VEM, which is used as surface flooring to cover steel plates of ship hulls, thereby acting as vibration dampers. Free vibration results show that the damping coefficient of the plate foundation system increases with increasing the order of the derivative. In addition, the amplitude of the transient response decreases with the order of the derivative. Thus, the classical integer-order mechanical model overestimates the damping of the viscoelastic materials, which leads to underestimating the displacement and associated stresses. The results are verified with literature and ANSYS.
Machine Learning for Subsea Pipeline Reeling Mechanics
Authors: Giry Eric , Cocault-Duverger Vincent , Pauthenet Martin , Chec Laurent
Published: December 2020

Installation of subsea pipelines using reeling process is an attractive method. The pipeline is welded in long segments, typically several kilometers in length, and reeled onto a large diameter drum. The pipeline is then transported onto such reel to the offshore site where it is unreeled and lowered on the seabed. The deformation imposed on the pipeline while spooled onto the drum needs to be controlled so that local buckling is avoided.

Mitigation of such failure is generally provided by proper pipeline design & reeling operation parameters. Buckling stems from excessive strain concentration near the circumferential weld area resulting from strength discontinuity at pipeline joints, mainly depending on steel wall thickness and yield strength. This requires the characterization of critical mismatches obtained by trial and error. Such method is a long process since each “trial” requires a complete Finite Element Analysis run. Such simulations are complex and lengthy. Occasionally, this can drive the selection of the pipeline minimum wall thickness, which is a key parameter for progressing the project. The timeframe of such method is therefore not compatible with such a key decision.

The paper discusses the use of approximation models to capitalize on the data and alleviate the design cost. To do so, design of experiments and automation of the computational tool chain are implemented.

It is demonstrated that initial complex chain of FEA computational process can be replaced using design space description and exploration techniques such as design of experiments combined with advanced statistical regression techniques in order to provide an approximation model. This paper presents the implementation of such methodology and the results are discussed.
Pressure Testing of Barrier Integrity
Authors: Raaen Arne M. , Fjær Erling
Published: December 2020

Plug and abandonment of a well is expected to ensure that the sealing integrity of the cap rock is restored. This also implies that the annulus between casing and rock is sealed off. Some formations may under the right conditions close in on the casing, providing a highly valuable contribution to the sealing. Although acoustic tools may be used to evaluate whether there is casing contact, the hydraulic integrity of such a barrier must be established by field testing for all potential formations.

While the main focus here is on natural shale barriers, the analysis and conclusions apply equally well to testing of cement based and other barriers.

This paper discusses two pressure integrity tests that are used for this purpose, the XLOT-type test and the communications test. The test procedures are outlined, and the expected sensitivity is estimated. In particular, it is shown that temperature effects may complicate test interpretation, possibly giving both false acceptance and false rejections of tests. Hence, it is of vital importance to include temperature measurements in the evaluation of the tests.

It is estimated that the leakage detection limit of both tests is significantly higher than the target value needed to ensure that sealing integrity has been restored. This calls for improvements of the testing procedures.
Accuracy of Simplified Methods for Fatigue Damage Estimation of Exposed Fish Farms
Authors: Slagstad Martin , Takle Bore Pål , Amdahl Jørgen
Published: December 2020

Fish farms are being placed in more exposed locations than earlier, encouraged by the “development licenses” that the Norwegian Directorate of Fisheries have introduced. Traditional design methods for estimating fatigue damage for fish farms are based on formulations given in the code NS - 9415 Marine fish farms. These methods are initially developed for sheltered areas and may not give an adequate level of safety in more exposed locations, where the dynamic response from waves is of greater importance. Accurate calculations using state of the art methods are very time consuming both with respect to man hours, but especially with regard to CPU consumption. Hence, for practical design, simplified procedures, such as combination of the design wave method with an assumed Weibull distribution, are often used to limit the complexity of the analysis as well as the costs. To the authors knowledge, the accuracy of such simplified methods is not well documented for exposed fish farms and with limited full-scale experience it is difficult to conclude that the desired safety level is achieved. This paper addresses this problem by investigating the accuracy of simplified methods to estimate the fatigue damage. A case study of a modern fish farm concept for exposed waters is performed where the simplified methods are compared to more complex time-domain analyses using state of the art modeling techniques.
Numerical Simulation of Cone Penetration Tests Inside Suction Caisson Foundations in Sand
Authors: Stapelfeldt Marc , Alkateeb Diaa , Grabe Jürgen , Bienen Britta
Published: December 2020

Offshore wind is increasingly utilised as a renewable energy source. A growing number of bottom fixed wind turbines installed offshore are supported by suction caisson foundations. The suction-assisted installation remains a source of uncertainty towards the in-service performance due to the unknown post-installation soil plug state. Cone penetration tests within the suction caisson can help to improve the reliability. Therefore, cone penetration tests were employed in centrifuge tests to investigate the plug state in a previously installed suction caisson. However, the performance of a cone penetration test in a small-scale experiment is connected to uncertainties: A relatively large diameter device is required to conduct the cone penetration test — especially in a centrifuge test. Different finite element models are developed in order to visualise and investigate a cone penetration test inside a suction caisson. The numerical analysis results are validated through the back-calculation of centrifuge cone penetration tests. The results of the simulated cone penetration tests inside a suction caisson are evaluated and compared to the centrifuge experiments. This investigation reinforces the scope of the centrifuge experiments and emphasises a considerable effect of the pressure transferral through the caisson lid in the soil plug state. Hence, the results of this study reduce existing uncertainties regarding possible suction installation effects on the in-service performance of caisson foundations.
Collision Experiments of Ship Models in Water Tank
Authors: Xu Lijun , Zhu Ling , Wang Xiangbiao , Pedersen Preben Terndrup
Published: December 2020

The consequences of ship collision could be very serious, causing lots of human casualties, environmental pollution and huge economic losses. It is essential to study the collision process including two ships in water. In the past, most ship collision tests are based on the study of collision damage of local structures and there are few experiments considering the motion response of ships during the collision process. Actually, the interaction between the fluid and structure does have effects on the collision consequences. In this paper, the collision experiments of ship models are conducted in a water tank, with particular attention on structure in the collision region. Considering the coupling effect of external dynamics and internal mechanics, the dynamic responses of ships during collision are studied. The failure mode and deformation damage characteristics of ship’s side structure in collision region are also assessed. On this basis, the time history of collision forces, the damage extent of the struck structure and the energy absorption are analyzed and then the influence of velocity and ship’s mass on the results are evaluated. It provides valuable test data for validation of numerical simulation and theoretical studies.
Methodology for Mitigation of Armour Wire Bird Caging in Offshore Wind Export Cables
Authors: Connaire Adrian , Killeen Caitríona , Savitsky Ivan , Anwasi Richard , Nestor Ruairí
Published: December 2020

Subsea export power transmission cables for offshore wind farms are being installed more extensively year-on-year due to the increasing demands for power output from renewable sources. With the increasing number of installations, the number of cable failures during installation has increased. One failure mode involves the temporary or permanent radial deformation of armour wires otherwise known as armour wire bird caging which occurs from a combination of twist, bending and compression loads which build up in a cable. This failure mode can lead to significant remediation costs and schedule delays for projects affected.

In this paper, the authors present a method for predicting armour wire bird caging for generic installation configurations based on a review of the root causes from several historical bird caging failure instances. Various numerical models and analyses which simulate the installation conditions are described. The models simulate key response mechanisms including bending-induced twist and inter-layer separation within a cable. Cable loading conditions are compared with cable bird caging limits and the parameters which influence the onset of bird caging are identified. Based on a range of sensitivity analyses, handling curves to assist with installation are developed and a full-scale test validation programme is proposed. This work was performed for a project which received financial and technical support from The Carbon Trust’s Offshore Wind Accelerator (OWA), a collaborative R&D programme funded by nine leading offshore wind developers (EnBW, Equinor, Innogy, Ørsted, RWE, SPR, Shell, SSE, Vattenfall) and the Scottish Government.
Ship As a Wave Buoy: Estimating Full Directional Wave Spectra From In-Service Ship Motion Measurements Using Deep Learning
Authors: Scholcz T. P. , Mak B.
Published: December 2020

The ocean wave directional spectrum is an important wave characteristic for maritime safety and navigation. Accurate estimation of directional spectra in real-time is a challenge. In this study we aim to reconstruct the directional spectra from ship motions using a deep convolutional encoding-decoding neural network. In-service measurements of ship motions and wave spectra from a WAMOS II wave scanning radar were used to train the neural network. The data was collected from a frigate type ship for a period of two years. We demonstrate that the deep convolutional encoding-decoding neural network is successful in predicting the directional spectra in real-time. At the same time, we conclude that more data is needed for a better prediction performance, including a more complete coverage of operational conditions.
Experimental Study on the Wall Factor for Rod-Shaped Proppant Settling in Vertical Fracture
Authors: Zhu Zhaopeng , Song Xianzhi , Yao Xuezhe , Zhu Shuo , Jing Silin
Published: December 2020

Hydraulic fracturing is an important technology to improve oil and gas production. In recent years, rod-shaped proppant has received increasing attention for its advantages in avoiding fracture closure and enhancing conductivity. Due to its special shape, the settling process in the fracture is more complicated than that of a spherical proppant. Accurate description of the wall factor of fracture on the settling rod-shaped proppant is pivotal in predicting the transport distance of rod-shaped proppant and improving the effect of fracturing. However, few researches have been reported about the fracture wall factor on the settling rod-shaped proppant. In this study, the transparent fracture model with different width and a high-speed camera were used to record the settling process of the rod-shaped proppant in the fracture. A total of 215 tests were carried out to analyze the effects of fluid properties, the equivalent dimensionless diameter, sphericity, and Reynolds number on the wall factor, involving the ranges of the equivalent dimensionless diameter and the particle Reynolds number are 0.03 to 1.47 and 0.03–1354.14, respectively. The settling processes of rod-shaped proppant under horizontal and vertical states were studied, and two wall factor models for the two states were established, respectively. The results show that the wall factor is a function of both the equivalent dimensionless diameter and Reynolds number. Finally, the prediction models of wall factor with the prediction error of 1.70 and 4.44% are established for these two Reynolds number regions, respectively. The results of this study can further improve the performance of rod-shaped proppant in hydraulic fracturing.
Stinger Structural Analysis Using Fully Coupled Model for Pipelay Operations
Authors: Gupta Anupam , Tallavajhula Sudhakar , Mathakari Sachin
Published: December 2020

With the growing demands and increasing challenges in deep-water pipeline installation, it is increasingly important to optimize stinger capacity assessment procedure for higher level of accuracy and cost-efficiency as compared to the traditional approach which is highly conservative.

The traditional approach combines maximum stresses from three different analyses for environmental loads, vessel motions and pipeline forces to calculate the dynamic performance of a stinger. This approach ignores the coupled behavior between stinger, vessel and pipeline leading to conservative results and over-design which significantly underestimates the operational limits of the stinger structure.

The main objective of this study is to develop a simple yet optimized and accurate stinger design and analysis procedure by considering the combined effect of vessel motion, pipeline forces and environmental loading on stinger structure. This is achieved by performing a Fully Coupled Analysis (FCA) in time domain, with capability to capture the impact of stiffness and hydro-elastic properties of stinger and pipeline. The fully coupled model also allows inclusion of hydrodynamic loads on installation vessel as compared to use of vessel motion RAOs in the traditional approach. Forces from this time-history analysis are extracted and mapped onto the structural model to check for structural strength using API and AISC codes.

This paper presents a comparison between structural analysis results obtained from Fully Coupled Analysis (FCA) and traditional approach. Results from the FCA procedure have shown significant improvement in the operational limits of stinger.
Monitoring and Performance Evaluation of Plastic Cleanup Systems: Part I — Description of the Experimental Campaign
Authors: Sainte-Rose Bruno , Wrenger Hendrik , Limburg Hans , Fourny Arthur , Tjallema Arjen
Published: December 2020

The Ocean Cleanup Foundation is developing floating barrier systems to concentrate and extract buoyant plastic from the global accumulation zones located in the subtropical convergence zones in the world’s oceans. In that context, two cleanup systems have been designed, built and deployed in the Great Pacific Garbage Patch since 2018 to evaluate their performance in the field. During those campaigns, a large amount of data in terms of system displacement and environmental conditions has been collected. This data serves to further validate and calibrate numerical models that will be used to develop future generations of cleanup systems.

The main performance indicator in scrutiny is the field efficiency which can be derived from on the one hand the barrier horizontal motion in terms of displacement and span heading and on the other hand the plastic transport around and inside the barrier. This information then needs to be correlated to the background environmental conditions to evaluate the barrier’s efficiency over a longer deployment period and allow multi-decadal hindcast analysis. describe both the experimental and numerical work, this paper is split in two parts. This first part is dedicated to the presentation of the experimental setup in terms of barrier displacement, environmental conditions and surface transport measurements.

This paper details the techniques employed to carry out those measurements, how they are processed but also the short comings of some measurement methods.
Energy Harvesting From the Tidal Currents Using a Mangrove-Like System
Authors: Kazemi Amirkhosro , Curet Oscar , Gómez Daniel , Castillo Eduardo E.
Published: December 2020

Tidal energy has the potential for future electricity generation to be widely used in intercostal areas during the rise and fall of tides. Inspired by mangrove roots, we designed a novel device to harvest hydrokinetic energy from tidal currents. This device consists an oscillating cylinder, partially submerged in a flow of water and an electric generator composed of a fixed magnet and a coil attached to the cylinder pivoted at its top by a thin flexible steel plate. This energy harvesting system is considered as one-degree-of-freedom vortex-induced vibration (VIV). The oscillation amplitude of the cylinder tip was recorded with a high-speed camera and 2-D PIV measurements were made to explore the hydrodynamic interaction within the devices for Reynolds numbers ranging from 200 to 1500 (based on cylinder diameter) consistent with biological velocity in tidal flows. We analyzed the kinematics as well as the power generation of the device for different stiffness of the plate. We observed that the cylinder was unmoved for low water velocities; however, by increasing the flow velocity the oscillations increase and reached a maximum value; similar fashion was observed for all stiffness. It was found that for a specific range of reduced velocity (0 < Ur < 3.5) the device worked in its optimal range in which the amplitude of oscillations and the efficacy of the system reach the highest values. This analysis of VIV correlated with oscillations will be fundamental for future bio-inspired energy harvesting devices. These renewable energy devices could have applications to power small actuators or sensors to monitor coastal infrastructure.
Clamping Effect on the Welding Deformations in Dissimilar Welded Pipes (Joints) With Varied Thicknesses: Axial and Radial Shrinkage
Authors: Kogo Bridget , Wang Bin , Chizari Mahmoud
Published: December 2020

This study investigates and evaluates the welding residual stresses and deformations in the dissimilar material MSSS metals in order to verify the clamping effect on the residual stresses and deformations and entails comparison with the finite element simulation, the critically reflected longitudinal ultrasonic stress measurement and the hole-drilling residual stresses in a Butt-welded plate courtesy of Javadi et al [1]. The angular shrinkage measurement and vertical displacement were used to achieve this objective. The outcome of the study proved that the measurement of residual stress using protractor is an effective way of differentiating the influence of clamps on the longitudinal stresses.
Waves in the Great Pacific Garbage Patch: Cross Validation of Three Measurement Techniques and Two WW3 Models
Authors: Wrenger Hendrik , Sainte-Rose Bruno , Soares Ivan
Published: December 2020

The Ocean Cleanup Foundation is developing floating barrier systems to concentrate and extract buoyant plastic from the global accumulation zones. To analyze and improve the efficiency of such cleanup systems, access to accurate Metocean conditions is inevitable. The Ocean Cleanup runs its own implementation of WAVEWATCH III® to have full control over the spatiotemporal resolution and to have access to all critical output, such as 2D wave spectra or Stokes drift at the surface to only name two examples.

We present here an experimental campaign that was carried out during the deployment of the first two systems in the Pacific to A) validate the performance of both a GFS and CFSR forced wave model, 1000+nm from available NOAA buoys and B) intercompare the wave measurements from an unmanned surface vessel (USV) with those from wave buoys and a vessel based X-Band radar. The USV is the most feasible tool to collect long periods of data along the system trajectory and our preferred option in the long run. We zoom into one day to review in detail the various data sources, their comparability and employed post processing techniques. We present a preliminary comparison of integral wave parameters for the entire campaign (2272hrs/data points) and discuss further steps towards a more complete investigation.

The preliminary comparison suggests that the USV and wave buoy data compare rather well. The largest differences are seen in wave periods. We obsen’e small differences between the GFS and CFSR forced model, both compare well with the field measurements.
High-Fidelity Modelling of Floating Offshore Wind Turbine Platforms
Authors: Burmester Simon , Vaz Guilherme , el Moctar Ould , Gueydon Sebastien , Koop Arjen , Wang Yu , Chen H. C.
Published: December 2020

There are already several examples of commercially operating floating offshore wind turbines (FOWTs) delivering electricity to the grid. These devices operate usually in harsh environments due to high waves, strong winds and currents. Guaranteeing the survivability of these complex and expensive constructions is crucial but not straightforward, and this may be partially controlled by the associated hydrodynamic damping. FOWT structures are still designed and analysed using potential-flow and empirical tools. However, nowadays, almost all fields of maritime problems are seeing an increased use of higher-fidelity viscous-flow tools (CFD, or Computational Fluid Dynamics), due to its increased modelling accuracy. Therefore, in this work, a FOWT is analysed in detail using CFD with emphasis on the hydrodynamic damping. This damping arises due to wave radiation, skin friction, eddy making and lift, and drag of the mooring lines. The CFD associated turbulence model influences largely the eddy making and skin friction components of the damping. Moorings are also a critical issue when assessing the damping of a floater. Catenary moorings are towed along with the motion of the floater and induce loads (drag and inertia) on the lines, effects which cannot be represented by static or quasi-static approaches. And numerical errors, often neglected or not quantified, can influence all the previous aspects and the final accurate prediction of the structure damping. In this paper, all these aspects are investigated by performing CFD decay tests, and their contribution quantified in terms of linear and quadratic damping components. The following issues were found to have a major contribution to the hydrodynamic damping and consequently on the motions of the platform: numerical discretisation (temporal and spatial), Reynolds number, free surface, turbulence and accurate mooring modelling.
Additive Manufacturing: Challenges and Solutions for Marine and Offshore Applications
Authors: Qiao Dongchun , Wang Bo , Gu Hai
Published: December 2020

As a rapidly developing technology, the expanded capability of Additive Manufacturing (AM) allows implementation into the marine and offshore industries. Qualification of metal AM part may need detailed test plans and test results in order to meet the design requirements with a certain level of reliability. Approval may also be needed in accordance with regulation codes and class rules.

However, challenges and gaps remain between the current metal AM technology level and marine and offshore applications. In this paper, two major categories of metal AM processes are discussed: powder bed fusion (PBF) and directed energy deposition (DED) with powder or wire as raw materials and laser, electronic beam (E-beam) or arc as heat source. The main challenges are also investigated in this study as follows: 1) Optimization of build cost and part quality by selection of appropriate process; 2) Performance of the part under static and cyclic loading; 3) Evaluation of the effects of corrosive environments (seawater); 4) Lack of standardization of the AM process for quality control. This paper addresses possible solutions for these challenges and provides qualification framework for metal AM parts to be implemented into marine and offshore applications.
Generation of 3-hr Long-Crested Waves of Extreme Sea States With HOS-NWT Solver
Authors: Canard Maxime , Ducrozet Guillaume , Bouscasse Benjamin
Published: December 2020

The accurate control of wave fields generated in experiments and numerical simulations is of great interest for the ocean engineering community. In the context of wave-structure interactions, the recommended practices of classification societies are indeed based on the definition of a wave spectrum, that needs to be reproduced. The present work intends to address this problem from the numerical point of view, using a Numerical Wave Tank equipped with a wavemaker and an absorbing beach, based on the High-Order Spectral method (HOS-NWT). The challenging case of the generation of 3-hours long-crested extreme sea states is studied in details. An iterative procedure to reproduce a target wave spectrum at a given distance from the wavemaker is proposed. The quality of the sea state obtained is evaluated using several criteria defined from spectral quantities. A validation is first performed with a highly nonlinear but non-breaking sea-state. Statistical crest distributions obtained are compared with the Forristall and Huang distributions [1,2]. Then, the Gulf of Mexico 1,000 Year Return Period wave condition is generated. This corresponds to an extreme sea state with significant wave breaking occurrence. The numerical solver needs to be able to account for this phenomenon [3]. The Tian breaking model [4, 5] is calibrated to realistically reproduce the dissipation due to breaking, with particular attention paid to the spatial discretization, enlightening its significant effect on breaking model actions. Consequences on the iterative correction process are studied. The computed statistical quantities appear to be significantly different changing the spatial discretization, while the wave energy spectrum stands the same. It questions the relevance of the characterization of a sea state with the sole wave energy spectrum.
How Reliability Based Design Can Increase Operating Window for 20k Drilling Risers
Authors: Pilisi Nicolas , Lewis David
Published: December 2020

The objective of this paper is to answer the questions posed and in doing so create an awareness of the necessity to use reliability based design when looking at vessel operating windows rather than working stress design which has been used for marine drilling riser analysis and design for many decades. In answering, this paper presents several case studies of drilling riser stresses and operating windows when drilling to access reservoirs with pressure greater than 15,000 psi and up to 20,000 psi. This paper examines the need for full and proper use of a more probabilistic design over working stress design for a set of water depth, subsea equipment, environments and loading conditions.
Hydrodynamic Study on the Damping Characteristics of Cylindrical FPSO With the Heave Plate
Authors: Jiang Meirong , Li Da , Wang Zhongchang , Jia Xu , Yu Yang
Published: December 2020

The heave plate setting up on the hull can effectively improve the movement performance of the cylindrical FPSO. In the research process of its damping effect, a reliable model is also needed to do the further study besides the physical model test. The objective of the present work is to establish a reasonable hydrodynamic model to investigate the damping behavior of the cylindrical FPSO with the heave plate. Based on the k-ε turbulence model and the VOF method, a three-dimensional hydrodynamic model are established to simulate the movement of the cylindrical FPSO in the offshore environment. A typical annular thin plate is set up on the bottom side of the hull as the damping structure. The available experimental data are adopted to preliminarily verify the established model. The structural pattern of the damping plate is optimized considering different widths of the plate and excitation modes. The motion amplitude, natural period and damping coefficient can be extracted from the computation and be analyzed to obtain the hydrodynamic performance of the cylindrical FPSO. From the simulation and verification, the numerical results are agreed quite well with the measured values in the model test, which proves that the model is reliable. When the cylindrical FPSO oscillates in the water body, the vortexes are generated around the heave plate and shed from it, which are the primary reason for the increase of the viscous damping. As the width of the heave plate increases, the damping coefficient and ratio increase gradually, but the natural period of the structure is basically unchanged. As for the amplitude of the motion, the variation of the damping coefficient is not obvious, and the natural period of the structure is also basically unchanged. When the motion amplitude is relatively small, the damping ratio increases gradually with the increase of the motion amplitude; while the motion amplitude is large, the amplitude increment of damping ratio tends to be flat, or even decreases. This hydrodynamic study is meaningful for the researcher and engineer to further understand the damping characteristics of the cylindrical FPSO. It can also provide some technical supports for the structural optimization design of the heave plate.
Fatigue Design of Hybrid Welded Steel Beams
Authors: Van Wittenberghe Jeroen , Van Den Abeele Filip
Published: December 2020

Hot rolled sections such as I- and H-beams are commonly used as structural elements in offshore platforms. They exhibit a superior fatigue life compared to welded sections due to smooth corners and the absence of welds between the flange and web plates. Hence, when loaded in fatigue, the butt welds joining different sections are the hotspots in the design.

Due to the size of the rolling installations, the maximum height for such sections is limited. To overcome these limitations, a hybrid welded beam concept is studied. This concept consists of 2 hot rolled T-sections with a plate welded in between to form an H-beam with increased section height. The advantage of this approach is that it maintains the smooth corners of the hot rolled sections and places the longitudinal welds closer to the neutral axis of the beam where the stress levels in bending are reduced compared to the flange-web interface. Hence, the butt weld between different beam sections remains the hotspot instead of the longitudinal weld.

In the present work the fatigue design of such hybrid welded beams is studied. As part of this research, an efficient experimental assessment methodology is proposed. Fatigue tests are performed on a large-scale resonant bending test setup allowing a testing frequency of around 30 Hz. Tests are performed on HISTAR HE girders with a section height of 696 mm and 524 mm. As a reference, fatigue tests are performed on hot rolled HE beams joined by butt welding. Hybrid welded beams will be tested in the same setup at a later stage. During the tests, fatigue crack initiation is detected using an acoustic emission system. Further crack propagation is monitored through local strain measurements and by intermediate inspections during the tests. In addition, the beach marking method is applied, allowing to investigate the evolution of the crack front by post-mortem analysis.
Anomaly Criteria for General Visual Inspection of Subsea Flexible Pipes
Authors: Tadjiev Damir
Published: December 2020

For flexible pipes in subsea applications, General Visual Inspection (GVI) by Remotely Operated Vehicles (ROV) remains the most common inspection method that is used on a routine basis. It enables verification of pipe configuration or layout and also helps to identify any areas of concern indicative of an increased risk of in-service failure.

The success of ROV GVI chiefly relies on the anomaly criteria used, these help inspectors to identify any areas of concern, which can then be assessed by a competent person to ensure any threat to the integrity of an inspected component is identified and addressed.

Currently there are no commonly accepted anomaly criteria for ROV GVI of flexible pipes. As a result there is no consistent approach between different operators and experience shows that the inspection approach and anomaly criteria are often adopted from what has traditionally been used for rigid pipes. Since flexible pipes have different design and associated failure threats and mechanisms to rigid pipe, use of this approach may result in under or over inspection of flexible pipes.

This paper presents a set of anomaly criteria for ROV GVI of flexible pipes. The criteria were developed using the experience and lessons learned from a population of approximately 350 flexible pipes from two different manufactures operating in deep waters of the UKCS for over a period of 20 years. The criteria cover dynamic flexible risers and associated ancillary equipment, seabed flexible flowlines and jumpers. The applicability of the proposed anomaly criteria to other systems, the benefits of having commonly accepted anomaly criteria, the anomaly detection capability of ROV GVI and the reporting of anomalies are also discussed.