bluuebunny/crossref_metadata_2025 huggingface dataset:
Irregular Wave and Current Loads on a Fish Farm System
Authors: Fredriksen Arnt G. , Bonnemaire Basile , Nilsen Øyvind , Aspelund Leiv , Ommundsen Andreas
Published: September 2018

Accurate calculation of the design mooring loads on an aquaculture fish farm mooring system is often a difficult task. The fish farm system has a large horizontal extension with variable environmental conditions across the entire structure. In addition, the drag loads on the fish nets are thought to be the governing environmental force. This means that the mean position of the fish farm is a function of the mean of the fluid particle velocity squared, where the fluid particle velocity must be taken as the sum of current and wave induced fluid particle velocities. Additional offsets will be slowly varying, where the response time will depend on the total mooring stiffness. The magnitudes depend on the height and length on wave groups in the irregular sea state. The paper presents simulations of the response of such a system to a set of combined irregular waves and current conditions. The response evolution in time is discussed as well as parameters affecting the maximum responses in the systems (displacements and loads). Finally, the resulting loads on the fish farm in irregular waves are compared to loads obtained in equivalent regular waves, as this is an often used engineering practice when analyzing the response and mooring loads of a fish farm.
Change of Regime of Air-Sea Dynamics in Extreme Metocean Conditions
Authors: Babanin Alexander V.
Published: September 2018

As a reference point for the extreme Metocean conditions, the hurricane-scale classification is often used: that is a tropical storm becomes a hurricane if the wind speed reaches U ∼ 33m/s. In this paper, it is argued that such classification is not arbitrary, and indeed signifies change of the physical regimes in all environments near the air-sea interface: in the atmospheric boundary layer, at the surface, and through the upper ocean. This threshold is approximately the wind speed at which the drag coefficient was found to saturate in the field observations (U10 ≈ 32–33m/s), which saturation has received a lot of attention. Less known are the observations that below the surface, change of the upper-ocean mixing mechanism and of bubble dynamics occur at U10 > 35m/s. Directly at the surface, wave dynamics also undergoes essential transformations, from wave breaking (dissipation) being driven by evolution of nonlinear waves, to the breaking being forced directly by the winds, at U10 ≈ 34 m/s. It is therefore argued that the simultaneous change of physical regime in all the three air-sea environments cannot be coincidental, and consequences of the regime change for the Metocean modelling are discussed. As an important byproduct, parameterisation of wave-breaking probability is obtained in terms of the mean symmetry of surface waves. Such parameterisation allows us to estimate frequency of breaking events, based on time series of surface elevations, without explicitly detecting the breaking waves.
Numerical Investigation of Effects of Bow Flare Angle on Greenwater Overtopping a Fixed Offshore Vessel
Authors: Zhang Xiantao , Draper Scott , Wolgamot Hugh , Zhao Wenhua , Cheng Liang
Published: September 2018

The effect of bow flare angle on greenwater overtopping is numerically investigated using CFD with a VOF surface capturing scheme. A two dimensional fixed rectangular box with a flare angle is adopted to represent a Floating, Production, Storage and Offloading Unit (FPSO) with flare. Deep water conditions are assumed. The incident waves are focused wave groups based on the New Wave formulation, which represents the most probable free surface elevation around a large crest and includes much of the spectral properties of the underlying random sea state. Bow flare angles of 10, 30 and 50 degrees are analysed, which cover the typical range of a FPSO unit. For the box model, the length L, draft D and freeboard f are modelled as L/D = 15 and f/D = 0.25, which are typical dimensions for a FPSO. A range of wave steepness and relative length are considered, which lead to a variety of overtopping conditions including the hammer fist (HF), plunging plus dam break (PDB) and dam break (DB) greenwater events observed for vessels with no bow flare angle. The numerical simulations are performed using the open source CFD toolbox OpenFOAM of version 2.4.0 together with a fully nonlinear wave generation and absorption utility waves2Foam. The effect of the bow flare angle is quantified by analyzing changes in freeboard exceedance, overtopping volume and horizontal momentum flux of water on deck flow for different bow flare angles. The results indicate that, for the special case in which the vessel is fixed, the bow flare angle can significantly reduce each of these quantities. It is also shown that trends in the overtopping volume and momentum flux can be explained reasonably well using the modified dam break model proposed by Zhang et al. [14].
Numerical Integration of the AUSMV Scheme With a Dynamic Wellbore Temperature Model
Authors: Udegbunam John Emeka , Fjelde Kjell Kåre , Sui Dan
Published: September 2018

The AUSMV scheme is a hybrid transient model derived from Advection-Upwind-Splitting Method (AUSM) and Flux Vector Splitting (FVS) method. The two-phase flow model was formulated under isothermal condition. This neglected the behavior of temperature during transient scenarios, for instance, unloading and drilling fluid circulation. In contrast to this assumption, wellbore temperature changes locally with time under such dynamic conditions.

The numerical accuracy of the AUSMV scheme can be improved in two ways. The scheme can be reformulated by including energy equation in the system of governing conservation laws. This option, however, is computationally rigorous and expensive. A better alternative is to develop a separate dynamic temperature model that will calculate wellbore and formation temperatures. Then the two dynamic models — the AUSMV scheme and temperature model — are numerically coupled into a thermohydraulic simulator.

The present work will include a brief introduction to the AUSMV scheme, followed by the description of the temperature model. In addition, how the two transient models are integrated will be presented.

Simulation cases, demonstrating the improved modeling capability of the scheme for a drilling situation, will be shown.
Collapse Analysis of Ship Hull Girder Using Hydro-Elastoplastic Beam Model
Authors: Htoo Ko Han Htoo , Tatsumi Akira , Iijima Kazuhiro , Fujikubo Masahiko
Published: September 2018

A method of time-domain collapse analysis of ship hull girder considering the interaction between elastoplastic deformation and hydrostatic/dynamic forces is developed. Ship hull girder is longitudinally divided by conventional beam elements, and progressive collapse behavior of cross sections is simulated by Smith method considering material yielding, buckling and post-buckling of structural elements. Average stress–average strain relationship of structural elements is transformed to average stress–average plastic strain relationship so that it can be treated as pseudo strain-hardening/softening effects. Strip method is used for the calculation of hydrodynamic forces on the hull girder. Hydrodynamic coefficients for cross-sections are calculated by 2D-BEM. In-house analysis code is developed and applied to the collapse analysis of a uniform hull-girder model under impulsive bending loads. The effects of load duration time on the dynamic collapse behavior of the hull girder are discussed.
The Impact of a High-Fidelity Wave-to-Wire Model in Control Parameter Optimisation and Power Production Assessment
Authors: Penalba Markel , Ringwood John V.
Published: September 2018

Accurate control parameter optimisation and power production assessment is essential to evaluate the performance of a wave energy converter (WEC). However, commonly used numerical models excessively simplify the power take-off (PTO) system of the WEC, which may strongly affect power production predictions. Therefore, the present paper compares a commonly used WEC model that includes nonlinear viscous losses and an ideal PTO system model, referred to as NLideal, with a high-fidelity wave-to-wire model (HFW2W) model. Results show the incapacity of the commonly used NLideal model to accurately optimise control parameters, particularly using reactive control. Likewise, the annual mean power production (AMPP) predicted using the NLideal model is significantly overestimated, with differences of up to 160% with respect to the more realistic HFW2W model. More dramatically, the use of control parameters optimised with the NLideal model in the HFW2W model results in negative AMPP, meaning that the WEC consumes more energy than it produces.
Experimental Study of Ship Response due to Internal Viscous Cargo Motions
Authors: Baudry Virginie , Rousset Jean-Marc
Published: September 2018

Potential liquefaction of some cargoes (Nickel ore, iron ore, ...) is a major risk for the maritime industry. The difficulties to simulate accurately the behaviour of these materials as well as their interaction with a bulk carrier model leaded us to use a non-Newtonian highly viscous fluid to model a liquefied ore. An experimental approach is presented in this paper. Roll responses of a ship model as well as details on the internal free surface behaviours are investigated for different loading conditions: solid cargo, fresh water and viscous fluids.
Experimental and CFD Study of Circulation Efficiency in Simulated Irregular Annulus
Authors: Divyankar Shreyansh , Khatibi Milad , Time Rune Wiggo , Skadsem Hans Joakim , Aasen Jan Aage
Published: September 2018

Primary cementing is a critical well construction operation that should ensure annular zonal isolation over the life cycle of the oil well. Efficient conditioning and mobilisation of mud prior to cementing is important to ensure that residual mud, especially in washout zones, does not contaminate the cement slurry. Therefore, experiments are carried out to better understand the flow dynamics and associated phenomenon in irregular geometry.

The narrow annulus between casing and formation is approximated by a rectangular slot, in order to aid instrumentation and data acquisition purposes. This methodology can be justified to reproduce flow in narrow annuli fairly well, and is considered a very useful technique for measurement in narrow geometries.

Particle image velocimetry (PIV) is used to obtain the liquid velocity profiles in regular and irregular sections of the experimental setup. Analysis of velocity profiles and vector fields provides information on regions of flow in the vicinity of the irregularity, and this allows a systematic study on the effects of flow rate in regular and irregular wellbore geometries. Entry effects from regular to irregular section and the development of areas with recirculation zones are investigated in irregular section. This enables us to estimate variability of circulation efficiencies in the irregularity. The experimental results are compared with numerical simulations in corresponding irregular geometry.
Effect of Buoyancy and Inertia on Viscoplastic Fluid: Fluid Displacement in an Inclined Eccentric Annulus With an Irregular Section
Authors: Kragset Steinar , Skadsem Hans Joakim
Published: September 2018

Primary cementing is an important well construction process that should establish well control barriers and zonal isolation. Critical for primary cementing is the successful displacement of drilling fluid from the annulus between casing and formation by a sequence of spacer fluids and cement slurry. Failure to displace the drilling fluid may compromise the annular cement integrity and result in contaminated cement with degraded mechanical properties. Issues such as eccentricity, washouts and other geometric irregularities in the wellbore can complicate the displacement processes, and their effect on the quality of the cementing job and the final result is linked to uncertainty.

We present numerical simulations of the displacement process between two viscoplastic fluids in the vicinity of a symmetric local hole enlargement. The study is limited to laminar flow regimes in the regular part of the annulus, and we focus on a near-horizontal section with significant eccentricity and small annular clearance. We vary the volumetric flow rate and the mass density difference between the fluids, and study how the irregularity affects the displacement efficiency and the presence of residual fluid in and after the irregularity.

In the regular part of the geometry, eccentricity favors flow in the wider, upper part of the annulus, while density difference leads to azimuthal flow from the top to the low side of the annulus. The results support the assumption that increasing the mass density difference improves the displacement efficiency. In the laminar regime, lower flow rates can be favorable over higher ones in terms of efficiency measured as a function of volume that is pumped into the enlarged section.

Displacement of drilling fluids for primary cementing is a rich flow problem involving different non-Newtonian fluids and possibly irregular geometry. Simulations of the displacement process can aid in optimizing fluid properties and injection rates for primary cementing operations, and assist cement log interpretation after the operation.
Resonant Steady-State Sloshing in Upright Tanks: Effect of Three-Dimensional Excitations and Viscosity
Authors: Timokha Alexander N. , Raynovskyy Ihor A.
Published: September 2018

Bearing in mind recent experimental and theoretical results showing that viscous damping can qualitatively affect resonant sloshing in clean tanks, the Narimanov-Moiseev multimodal sloshing theory for an upright circular container is revised to analytically analyze steady-state surface waves when the container performs a small-amplitude sway/roll/pitch/surge prescribed periodic motion with the forcing frequency close to the lowest natural sloshing frequency. The revised theory is applicable for the radius-scaled mean liquid depths h > 1 providing the secondary resonance phenomenon does not occur at the primary resonance zone. A focus is on how the damping influences the phase lag as well as on the amplitude response curves versus the forcing type, which can in the lowest-order approximation be treated as if the container translatory moves along an elliptic orbit in the horizontal plane. The analytical results are compared with existing experiments for longitudinal and circular orbital tank excitations. Whereas a good agreement is found for longitudinal excitations, a discrepancy is detected for the circular orbital forcing. The discrepancy may, most probably, be explained by the wave breaking and mean angular mass-transport (Ludwig Prandtl, 1949) phenomena. Occurrence of the Prandtl phenomenon makes inapplicable the existing analytical inviscid sloshing theories, even if they are modified to account for damping.
Considerations in Design of Centralizers for Pipe-in-Pipe Systems
Authors: Manouchehri Soheil
Published: September 2018

For un-bonded (sliding) Pipe-In-Pipe (PIP) systems, one of the main components is the centralizers (also called spacers). The main functions of the centralizers are to centralize the inner pipe inside the outer pipe, to transfer the loads between inner pipe and outer pipe and to safeguard the insulation material in the annulus from excessive compression during fabrication, installation and operation.

Centralizers must also have good thermal insulation properties so that the heat loss is minimized. Different designs are now available for centralizers but the majority are based on two half shells which are bolted together. During fabrication, installation and operation, centralizers subject to different loads under which they are required to continue functioning properly.

This paper provides an overview of centralizer design aspects and then focuses on the loading history during installation using reeling method. The main contributing parameters to centralizer loading during reeled installation technique are discussed and conclusions are drawn. It is believed that this will enable Pipeline Engineers to select the most appropriate material and design for centralizers.
Thermal Elasto-Plastic Analysis of Welding Sequence for Least Distortion of Overlay Welded Structure
Authors: Kang Sungwook , Kim Jaewoong , Kim Dongjoo , Jang Youngjae , Cho Jaeyoung
Published: September 2018

Generally, equipment or mechanical parts are worn out by abrasion, impact, erosion, and environmental corrosion. To reduce these losses, the welding that wears the surface of the material with abrasion or corrosion resistance is called overlay welding. Corrosion and high temperature oxidation are very important factors in overlay welding material selection because they have a great effect on wear rate. Typical equipment is made of carbon steel, stainless steel or a material that is not wear resistant. Therefore, overlay welding is applied to parts where the parts themselves are subject to severe wear or corrosion. The diffusion of chlorine and alkaline ash during the combustion process causes foiling, corrosion, and erosion in the water wall panel & tube which is a heat transfer device. In order to protect the water wall panel & tube from various chemical and physical phenomena occurring during the combustion process, heat-resistant alloy steel such as Inconel is overlay welded on the outer surface of carbon steel pipe. However, since the length of the water wall panel & tube is more than 7 meters, the deformation after overlay welding is very large and the straightening work requires a lot of time and money. Also, since cooling water flows through the pipe during welding, the temperature of the cooling water also affects the welding deformation. In this study, welding process sequence and coolant temperature are optimized to minimize welding deformation during overlay welding.
The Investigations of Two Analytical Heat Transfer Modelings to Predict the Temperature Profile in the Wellbore
Authors: Feng Ming , Teodoriu Catalin
Published: September 2018

The temperature is very interesting phenomena in the well system. Due to the geo-thermal gradient induced by the core of the earth, deeper well means higher temperature. The wellbore temperature during drilling depends on many factors such as wellbore geometry, wellbore depth, flow rate, the properties of the formation, the properties of the fluid, the geothermal gradients etc. Analytical modeling is very helpful to understand the heat phenomena in the well system and how the thermal effects are working in the well system. In this paper, two analytical modelings are investigate to discover how the temperature distributes in the wellbore. Some sensitivity analyses are calculated.
Comparison of Wind and Wave Extremes in Very Long-Term Climatic Scales
Authors: Stefanakos Christos N. , Vanem Erik
Published: September 2018

The study of very long-term ocean climate is of great interest in a number of different applications. In a climate change perspective, estimations of return values of wind and wave parameters to a future climate are of great importance for risk management and adaptation purposes. However, there are various ways of estimating the required return values, which introduce additional uncertainties in extreme weather and climate variables pertaining to both current and future climates. The different approaches that are considered in the present work include the annual maxima approach, the block maxima approach, and the MENU method which is based on the calculation of return periods of various level values from nonstationary time series data. Furthermore, the effect of different modelling choices within each of the approaches will be explored. Thus, a range of different return value estimates for the different data sets is obtained for a field of datapoints. Long-term datasets for an area in the North Atlantic Ocean are used in the present study, derived for project ExWaCli, comprising of 30 years in the present (historic period) and two sets of 30 years in the future (future projections). The comparison between the results of the various approaches reveals a variability of the return period estimates, and an assessment of this is given. Moreover, it seems that a slight shift towards higher extremes in a future wave climate might be possible based on the particular datasets that have been analysed.
Dynamic Response Analysis of a Floating Bridge Subjected to Environmental Loads
Authors: Cheng Zhengshun , Gao Zhen , Moan Torgeir
Published: September 2018

Designing floating bridges for wide and deep fjords is very challenging. The floating bridge is subjected to wind, wave, and current loads. All these loads and corresponding load effects should be properly evaluated, e.g. for ultimate limit state design check. In this study, the wind-, wave- and current-induced load effects of an end-anchored floating bridge are numerically investigated. The considered floating bridge, about 4600 m long, was an early concept for crossing Bjørnafjorden, Norway. It consists of a cable-stayed high bridge part and a pontoon-supported low bridge part, and has a number of eigen-modes, which might be excited by the relevant environmental loads. Numerical simulations show that the sway motion and strong axis bending moment along the bridge girder are primarily induced by wind loads, while variations of heave motion and weak axis bending moment are mainly induced by wave loads. Current loads mainly provide damping force to reduce the variations of sway motion and strong axis bending moment. Turbulent wind can cause significantly larger low-frequency resonant responses than second-order difference-frequency wave loads.
Tracking Fluid Interface in Carbon Capture and Storage Cement Placement Application
Authors: Frigaard Ian , Maleki Amir
Published: September 2018

One current methodology for Carbon Capture and Storage (CCS) involves pumping carbon dioxide (CO2) into a depleted oil and gas reservoir, usually via an existing well. Permanence of the storage in this case relies on the integrity of the reservoir and also the avoidance of leakage at the points of entry. Two different cementing procedures are involved in the latter problem: primary cementing and squeeze cementing. Here we consider how to track the interface between two fluids during primary cementing. The main idea is to exploit the density difference between successive fluids pumped in order to design a tracer particle to sit at the interface. Although apparently trivial, such particles must also overcome strong secondary flows in order to remain in the interface. We provide a proof of concept analysis of this situation assuming the displacement involves laminar flows of two Newtonian fluids in a narrow vertical annulus and demonstrate its feasibility.
Numerical Prediction and Experimental Validation of Slamming Load for Large Container Ship Undergoing Parametric Rolling Motion
Authors: Lin Yuan , Ma Ning , Wang Deyu , Gu Xiechong
Published: September 2018

This article proposes an approximate prediction method for slamming loads in parametric rolling condition for large container ships and the method has been validated through model experiments. Up to now, there have been some studies focused on two-dimensional asymmetric slamming analysis. Nevertheless, slamming load prediction in parametric rolling condition should consider not only heave and pitch motions, but also large amplitude roll motion which is usually neglected in analysis. For this purpose, a 6-DOF weakly nonlinear time domain model is adopted to predict the ship motions including parametric roll motion. The consequent roll motions obtained by the proposed model are incorporated in the calculation of impact angle and relative vertical velocity between ship section on the bow flare and wave surface, according to an asymmetric water entry assumption. Slamming impact loads and occurrence probability of slamming are analyzed by the Wagner model. To validate the numerical method, the segmented model experiment of a 10000 TEU container ship was executed and the slamming impact pressures and bending moments were measured for the wave condition and ship forward speed. The calculated pressures are compared with experiments. Based on numerical simulations, the maximum flare slamming pressures and slamming occurrence probability in different speed and wave conditions are investigated. The results indicate that flare slamming pressure is smaller than bottom slamming, but possessing longer lasting time and the occurrence of flare slamming is associated with the cycles of parametric rolling motions. Furthermore, the relationship between slamming pressure and 3-DOF motions namely roll, pitch and heave in the simultaneous simulation is given and the mechanism of flare slamming phenomena in parametric rolling condition is elaborated.
Experimental and Numerical Evaluation of Manoeuvring Capability of an AHTS Using Free-Running Tests
Authors: Swan de Freitas Caio , Vileti Vinicius L. , Esperança Paulo de Tarso T. , Sphaier Sergio H.
Published: September 2018

Investigating the hydrodynamics of a ship manoeuvring is a continuing concern for researchers. In recent years, with the growth of the operations complexity, understanding the hydrodynamics of a ship manoeuvring has become a central issue for supply vessels operating in close proximity of oil platforms. One of the main obstacles in understanding this problem is the difficulty of reproducing the manoeuvring on real ships with acceptable measurements, uncertainties and environmental control, not to mention the cost involved. A natural approach to address the issue is to run model tests, where it is possible to control a great number of variables.

In the desire to develop new methods to evaluate hydrodynamic coefficients and to improve the understanding of the phenomena, this paper proposes different types of free-running tests to be conducted in an experimental ocean basin. An Anchor Handling Tug Supply Vessel (AHTS) scaled model was used to perform the classic turning circle and the novel turning eight, a substitute to the zigzag that fits in limited manoeuvring facilities. Optimizations of mathematical manoeuvring models were applied to estimate the hydrodynamic coefficients with a new proposed metric of manoeuvres comparison. Simulations results were compared with the experimental measurements of the model during the tests. The experimental tests were performed at LabOceano, the Ocean Technology Laboratory of the Federal University of Rio de Janeiro.
Effect of the Free Surface on the Drag Forces on a Flat Plate Translating Normal to the Flow
Authors: Satheesh Sukruth , Haëck Clément , Huera-Huarte Francisco J.
Published: September 2018

A series of experiments were carried out with a flat plate towed normal to the flow in quiescent fluid. The focus was given to the analysis of the drag force seen by the plate as a function of its aspect ratio and hydraulic diameter. The effect of towing the plate near the water free surface was also investigated thoroughly. Plates of aspect ratio ranging from 0.25 to 4 were towed in a still water tank at different Reynolds numbers in the range from 15000 to 60000. Submergence depth was measured from the upper edge to the free surface and varied from zero to the centre of the tank. Forces on the plates were measured using a submersible bending beam load cell and the carriage motion was monitored by a rotary potentiometer. It was found that the drag increases abruptly prior subsiding with increasing submergence depth, with this effect being more dominant in lower aspect ratio plates. The abrupt rise in the drag is due to the interaction of the upper edge of the plate with the free surface resulting in a large shrinkage of the recirculation zone. The non-unit low aspect ratio plates also showed another drag peak around 50% depth, especially at lower speeds. Overall, the trends were Reynolds number independent, except when the aspect ratios was in the range from 0.75 to 1.33 and the plate was near the free surface.
ECAs, Probabilities and Axial Misalignment
Authors: Cosham Andrew , Macdonald Kenneth A.
Published: September 2018

An engineering critical assessment (ECA) is commonly conducted during the design of an offshore pipeline in order to determine the tolerable size of flaws in the girth welds. API 579-1/ASME FFS-1 2016 and BS 7910:2013+A1:2015 Incorporating Corrigenda Nos. 1 and 2 give guidance on conducting fitness-for-service assessments of cracks and crack-like flaws. The essential data required for an assessment (nature, position and orientation of flaw; structural and weld geometry; stresses; yield and tensile strength; fracture toughness; etc.) is subject to uncertainty. That uncertainty is addressed through the use of bounding values. The use of extreme bounding values might be overly-conservative. A sensitivity analysis is one way of investigating the sensitivity of the results of an assessment to the input data. A structural reliability-based assessment (a probabilistic assessment) is an alternative. A probabilistic assessment is significantly more complicated than a deterministic assessment.

API 579-1/ASME FFS-1 and BS 7910:2013 note that a sensitivity analysis, partial safety factors or a probabilistic analysis can be used to evaluate uncertainties in the input parameters. Annex K of BS 7910:2013 gives partial safety factors for different combinations of target reliability and variability of input data. ISO 16708:2006 gives guidance on the use of structural reliability-based limit-state methods in the design and operation of pipelines.

The structural reliability-based assessment of circumferentially-orientated, surface crack-like flaw in a girth weld in a pipeline is used to illustrate the significance of the distributions of the difference between the wall thickness and the ovality (out-of-roundness) of two pipes when calculating the bounding value of the stress concentration factor due to axial misalignment. The (assumed) distributions of diameter, wall thickness, out-of-roundness, yield strength, etc. are based on Annex B of ISO 16708:2006. The (nominal) probability of failure is calculated. It is then used to inform the choice of an appropriate bounding value (i.e. a characteristic value) for axial misalignment.
Wood Fibre Based Lost Circulation Materials
Authors: Saasen Arild , Hodne Helge , Ronæs Egil , Aarskog Simen André , Hetland Bente , Løvereide Marie Bjørdal , Mohammadi Rahmat
Published: September 2018

In this paper both a coarse and fine wood fibre type of Lost Circulation Material (LCM) is tested in the laboratory. It is shown how these fibre treatments work. The fibre type is partially oil wetting making them suitable for application in oil based drilling fluids. The fine material helps stopping small drilling induced fractures, while the coarse helps stopping lost circulation into several natural fractures or coal or conglomerate formations. In the article, the selection of wood fibres is described in more detail.

Testing of the fine materials were conducted conventionally by pumping drilling fluid volumes with LCM onto slotted disks in fluid loss apparatuses. The coarse fibres are too large to be tested in these apparatuses. Therefore, gravel with grain diameter around two centimetres was filled into transparent cylinders. The pore throats created by these gravel particles were above half a centimetre. For both of these LCMs the experiments show the sensitivity of the LCM concentration in the drilling fluid to stop the lost circulation. Also, it is shown the effect of the LCM on viscous properties of the drilling fluids. Not all LCMs can be pumped through the bit. The article describes the need for circulation subs in the bottom hole assembly (BHA) to hinder the LCM blocking the entire BHA.
Detection of Plunging Breaking Waves Based on Machine Learning
Authors: Tu Ying , Cheng Zhengshun , Muskulus Michael
Published: September 2018

Plunging breaking waves that occur in the vicinity of offshore structures can lead to high impulsive slamming loads, which are significant for the structural loading. The occurrence of plunging breaking waves is usually identified based on criteria that are derived from theoretical analyses and experimental studies. Given a large amount of data, detecting plunging breaking waves can be treated as a typical classification problem, which can be solved by a machine learning approach. In this study, logistic regression algorithm is used together with the experimental data from the WaveSlam project to train a classifier for the detection. Three normalized dimensionless features are introduced based on the measured data for the training. A classifier with respect to four wave parameters (i.e. water depth, wave height, crest height and wave period) is then explicitly developed for detecting plunging breaking waves. It is found that the trained classifier has an accuracy of 98.7% and F1 score of 99.2% for the tested data. Among the three dimensionless parameters, the ratio of wave height to water depth, H/d, is the most decisive factor for the detection of plunging breaking waves.
Comparison and Validation of Hydrodynamic Load Models for a Semi-Submersible Floating Wind Turbine
Authors: Hegseth John Marius , Bachynski Erin E. , Karimirad Madjid
Published: September 2018

In global aero-hydro-servo-elastic analyses of floating wind turbines (FWTs), the hydrodynamic loads are usually found from potential flow theory and applied in a single point of a rigid hull. When the hull is relatively stiff, this approach ensures correct behaviour for the six rigid body degrees-of-freedom (DOFs), but provides no information about the internal loads in the hull. The current work considers a simplified method to include distributed, large volume hydrodynamics in the global analysis, where frequency-dependent loads from potential theory are applied on a finite element (FE) model of the hull in a strip-wise manner. The method is compared to conventional load models for a braceless 5MW semi-submersible FWT, and validated against experimental results from model tests with focus on internal loads and rigid body motions in the main wave-frequency range. The global motions are accurately predicted by the distributed model for all investigated load cases. Good agreement with experimental results is also seen for the column base bending moment in wave-only conditions, although extreme values are not captured correctly due to limitations in linear theory. In combined wave-wind conditions, the measured bending moments are significantly increased because of the wind-induced mean angle of the platform. This effect is not considered in the numerical model, which therefore underestimates the moment response. However, an approach which calculates the loads in the actual mean configuration of the hull is found to give reasonably accurate results, at least in moderate wave conditions.
Multi-Platform Concepts for Combining Offshore Wind Energy and Fish Farming in Freezing Sea Areas: Case Study in the Gulf of Bothnia
Authors: Mikkola Eeva , Heinonen Jaakko , Kankainen Markus , Hekkala Toni , Kurkela Juha
Published: September 2018

Climate change together with increasing demand for space pose a challenge to energy and food production at sea areas. Co-location of offshore renewable energy production, aquaculture and other blue economy activities can answer the need for more sustainable marine space utilization while reducing and dividing costs of the different activities. This paper presents a case study of a multi-use platform that combines wind energy and fish farming in the Gulf of Bothnia, where the sea freezes every winter. A marine spatial planning (MSP) tool is used to find suitable locations for the multi-use platform with respect to economic potential of energy and nutrient production, structural solutions and environmental impact. The tool is used to visualize site selection criteria based on geographic information system (GIS) data such as seabed data, wind and wave data and ice data as well as protected marine areas. Production potential scenarios for the multi-use platform at different scales are calculated. Synergies of combining offshore wind energy and fish farming are discussed.
Numerical Simulations of Peregrine Breathers Using a Spectral Element Model
Authors: Koukounas Dimitrios , Eskilsson Claes , Engsig-Karup Allan P.
Published: September 2018

Breather solutions to the nonlinear Schrödinger equation have been put forward as a possible prototype for rouge waves and have been studied both experimentally and numerically. In the present study, we perform high resolution simulations of the evolution of Peregrine breathers in finite depth using a fully nonlinear potential flow spectral element model. The spectral element model can accurately handle very steep waves as illustrated by modelling solitary waves up to limiting steepness. The analytic breather solution is introduced through relaxation zones. The numerical solution obtained by the spectral element model is shown to compare in large to the analytic solution as well as to CFD simulations of a Peregrine breather in finite depth presented in literature. We present simulations of breathers over variable bathymetry and 3D simulations of a breather impinging on a mono-pile.
Drilling String Vibration: Modelling and Simulation
Authors: Amat Sergio , Henríquez Velez Gustavo E. , Legaz María J.
Published: September 2018

The offshore industry has a significant impact on the global economy and it is expected to grow over the next 4 years. A crucial part of an offshore structure is the drill string. Drill strings can be seriously damaged by vibration during drilling operations. The control of vibrations in drill strings is essential. In drilling wells, this is useful to minimize the risks of well loss and also to improve drilling performance. The objective of this study is to model and simulate the vibrations in drill strings. This is made to know the region of stabilization of the system and to establish the optimal parameters of drilling which can be manipulated from the driller console, such as angular velocity of the drill string on the surface and the weight on the drill bit. The simulation of these vibrations is made by solving the differential equations that describe these phenomena. An interface for the manipulation of variables involved in the models of the drill string has been designed and shown in this work.
Experimental Assessment of the Performance of CECO Wave Energy Converter in Irregular Waves
Authors: Rodríguez Claudio A. , Taveira-Pinto F. , Rosa-Santos P.
Published: September 2018

A new concept of wave energy device (CECO) has been proposed and developed at the Hydraulics, Water Resources and Environment Division of the Faculty of Engineering of the University of Porto (FEUP). In a first stage, the proof of concept was performed through physical model tests at the wave basin (Rosa-Santos et al., 2015). These experimental results demonstrated the feasibility of the concept to harness wave energy and provided a preliminary assessment of its performance. Later, an extensive experimental campaign was conducted with an enhanced 1:20 scale model of CECO under regular and irregular long and short-crested waves (Marinheiro et al., 2015). An electric PTO system with adjustable damping levels was also installed on CECO as a mechanism of quantification of the WEC power. The results of regular waves tests have been used to validate a numerical model to gain insight into different potential configurations of CECO and its performance (López et al., 2017a,b).

This paper presents the results and analyses of the model tests in irregular waves. A simplified approach based on spectral analyses of the WEC motions is presented as a means of experimental assessment of the damping level of the PTO mechanism and its effect on the WEC power absorption. Transfer functions are also computed to identify nonlinear effects associated to higher waves and to characterize the range of periods where wave absorption is maximized. Furthermore, based on the comparison of the present experimental results with those corresponding to a linear numerical potential model, some discussions are addressed regarding viscous and other nonlinear effects on CECO performance.
Phenomenological Modelling of Cylinder VIV With Contributions From Oscillatory Flows
Authors: Opinel Pierre-Adrien , Srinil Narakorn
Published: September 2018

This paper presents a numerical phenomenological model for a two-degree-of-freedom VIV of a flexibly mounted circular rigid cylinder subject to sinusoidal oscillatory flows. This prediction model is based on the use of double Duffing-van der Pol (structure-wake) oscillators which capture the structural geometrical coupling and fluid-solid interaction effects through system cubic-quadratic nonlinearities. Empirical coefficients are calibrated based on computational fluid dynamics results in the literature for the Keulegan-Carpenter numbers (KC) of 10, 20 and 40, satisfying a reasonable correspondence in amplitude and frequency responses. For KC = 10, the cross-flow vibrations present a single-frequency response. For KC = 20 and 40, cross-flow vibrations have multi-frequency responses. The primary frequency of the response in the cross-flow direction decreases with increasing reduced velocity, except for small values of the reduced velocities. In all KC cases, the in-line vibrations exhibit mostly a single frequency. Overall, parametric studies capture the dependence of response characteristics on the KC, reduced velocity, mass ratio, frequency ratios and empirical coefficients.
Comparison Between Experiments and a Multibody Weakly Nonlinear Potential Flow Approach for Modeling of Marine Operations
Authors: Wuillaume Pierre-Yves , Ferrant Pierre , Babarit Aurélien , Rongère François , Lynch Mattias , Combourieu Adrien
Published: September 2018

This paper presents validation tests for a new numerical tool for the numerical simulation of marine operations. It involves multibody dynamics modeling, wave-structure interactions with large amplitude body motion and cable’s dynamic modeling. Hydrodynamic loads are computed using the WS_CN weakly nonlinear potential flow solver, based on the weak-scatterer hypothesis. Large deformation of the wetted body surfaces can be taken into account. Firstly the ECN’s WS_CN solver capabilities are extended to multibody simulations. A first validation test is performed by comparing numerical results to the experimental data of [1]. Then, a second validation test is proposed. It consists in the ballasting operation of a spar. The experimental set-up is described.
Insights of the Vortex-Induced Vibration Phenomenon: Ideal Models Versus Reality
Authors: Zueck Robert F.
Published: September 2018

Induced vibrations are three-dimensional oscillations in a structure, whereby maximum amplitude is mostly perpendicular to sustained action. In this paper, we discuss the specific physics for how induced-vibrations evolve with space and time in a few example structures. We demonstrate how a sustained action (particularly fluid drag and gravity loading) rotates and reshapes these slender structures. We demonstrate how this then shifts and expands the dynamic nature of the structure, making the structure more receptive to vibrational inducements of any kind. Contrary to historical focus, the structure (not the fluid) primarily determines the physical nature of any induced vibrations, including fluid-induced vibrations.
Experimental Study on the Wave Loads on Monopile and Jacket Type Support of Offshore Wind Turbines
Authors: Zhang Jing , Liu Qin , Shi Xing Hua , Guedes Soares C.
Published: September 2018

As the offshore fixed wind turbine developed, more ones will be installed in the sea field with the depth 15–50 meters. Wave force will be one of the main forces that dominate the design of the wind turbine base, which is calculated using the Morison equation traditionally. This method can predict the wave forces for the small cylinders if the drag and inertia coefficients are obtained accurately. This paper will give a series scaled tests of monopile and jacket type base of the offshore wind turbine in tank to study the nonlinear wave loads.
Mitigation of Pipeline Free Span Fatigue due to Vortex Induced Vibration Using Longitudinally Grooved Suppression
Authors: Jayasinghe Kanishka , Marcollo Hayden , Potts Andrew E. , Dillon-Gibbons Craig , Kurts Phillip , Pezet Peter
Published: September 2018

Irregular seabed bathymetry around subsea pipelines can lead to the formation of pipeline free spans. When exposed to on-bottom currents these free spans can be subject to Vortex-Induced Vibration (VIV), with consequential effects on the fatigue life of the pipeline. Traditional VIV suppression technologies such as strakes and fairings present installation challenges and durability concerns due to the significant increase in overall diameter associated with the geometric profiles of strakes and fairings.

Longitudinally Grooved Suppression (LGS) technology was developed from a concept stage through to field deployment on active drilling risers (Johnstone et. al., OMAE 2017) [1]. The low profile and VIV suppression abilities of LGS present an opportunity for a more effective and operationally beneficial VIV suppression solution for pipeline free spans. Based on existing Class guidance for assessing pipeline free spans, a simplified framework for assessing free spans with LGS under a response based approach is presented. The simplified assessment implied a suppression efficiency (reduction in vibration amplitude) of up to 80%. An alternative comparative analysis using a force based approach was also performed in SHEAR7 of a bare pipeline and a LGS-wrapped pipeline. The requirements for qualification of new VIV mitigation technologies are also addressed and an example of an actual field installation of the device is presented, on an existing pipeline free span with low seabed clearance.
Mathematical Modeling and Analysis of Riser Gas Unloading Problem
Authors: Kiran Raj , Salehi Saeed
Published: September 2018

Pushing the boundaries of oil and gas exploration and development to new frontiers have led to exposure and more significant uncertainties, which necessitates robust strategies and techniques. With the increasing water depth, longer risers, and harsh pressure and temperature conditions; the risk of riser gas getting undetected get bigger. The lack of an integrated system to anticipate the controlling parameters at the choke below the BOP constrains the tackling operations and exacerbate the side effects of oil and gas well blowouts. This leads to an urgent need for an extensive study to address the riser gas unloading (RGU) events.

This study encompasses the development of a robust model that can characterize the effect of different parameters such as temperature, mud types, back pressure, and solubility in RGU events. It also presents comparative results of oil-based and water-based mud systems, using a novel tool based on analytical and numerical models. The analytical model is constructed using combined gas law, heat transfer mechanism, and gas solubility and bubble point pressure concepts.

Results suggest that the oil-based mud (OBM) takes more time for gas unloading in comparison to the water-based mud. Also, a significant deviation was observed in unloading patterns while considering temperature effect. For the drilling fluid without temperature consideration, the gas unloading occur in a smaller span of time and at a higher depth. Overall, this paper will demonstrate the effect of different parameters affecting the gas unloading in the riser, and present a comparative study of different parameters using an analytical which can be used in the field to get an idea of gas prior to any response for abnormality.