bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental and Numerical Investigations on VIV Response of a Pipe in Shear Flow
Authors: Karanjai Dipanjan , Nallayarasu S. , Bhattacharya S. K.
Published: December 2020

The vortex induced vibration of slender cylindrical structures is common in offshore structures and marine applications such as risers, towing cables, etc. The VIV response of such slender elements in steady uniform current has been investigated in the past using numerical and experimental studies. Though few numerical studies exist for varying current (sheared flow), experimental studies are limited. Hence, the experimental studies are an essential part of VIV investigation, especially for sheared flow.

The experiments were conducted using a specially fabricated circular steel tank of diameter 2.4 m with a central hinge to rotate the pipe horizontally in a water pool of depth 0.7 m. Shear current is simulated by rotating the pipe about the hinge. A pipe of diameter 25 mm (= D) and length 1 m (= L) was fixed at one end of the rotating cable support, and the other end was passed over a pulley inside a rotating cylinder. The rotating cylinder is provided with a pulley at the top to tension the pipe. A shear current with a maximum velocity of 1.3 m/s and a minimum velocity of 0.1 m/s is generated using the set up. The VIV response of the pipe was measured using electrical resistance-type strain gauges pasted along the length. The measured axial strain was used to obtain transverse displacements, which was used to determine the response frequency, amplitudes, and forces. The Strouhal number was calculated. The VIV response and the fluid force coefficients obtained from the experiments were compared with Shear7 results.
Motion Characteristics of a Spar-Buoy With Ring-Fin Motion Stabilizer in Deep Sea
Authors: Katayama Toru , Yamamoto Yusuke , Morimoto Taishi , Goto Masahiro
Published: December 2020

The main topics of this paper is the investigation of motion characteristics of the spar-buoy with ring fin motion stabilizer, which is developed to use as a platform for wind observation with a doppler lidar in shallow sea, some mooring conditions to apply it for deep sea. Four different mooring conditions are applied the spar-buoy and it motion in a designed sever wave condition are measured in the towing tank of Osaka Prefecture University.

The results show that the amplitude of pitching in deep sea is smaller than the results in shallow sea, even if the wave condition in deep sea are severer than that in shallow sea. From further investigation about the effects of four different mooring conditions on motion characteristics, it has that the center of pitching at four mooring conditions are almost same but the amplitude of pitching is smaller by changing the mooring point and the intermediate sinker position.
Experimental Investigation of Vortex Induced Vibration of Cross Flow Response for a Flexible Cable Under Uniform Current
Authors: R Prethiv Kumar , Nallayarasu S.
Published: December 2020

Vortex Induced Vibration (VIV) of slender cylindrical structures subjected to uniform flow is subject interest since the use of such elements is common in the offshore and naval industry. The numerical and experimental investigation of VIV for slender, flexible cable member in uniform current has been carried out in this study. The experiments were conducted in a towing tank of 85 m length and 2.8 m water depth mounted with the facility to control speed, The carriage has a range of velocity up to 5 m/s. A flexible cable of diameter 15 mm with an aspect ratio of approximately 100 has been used for the present study. The lateral displacement of cable has been measured using the strain gauges mounted along the length. The strain gauges measure the axial strain due to lateral bending of the cable and in turn, converted to lateral displacement. Experiments were carried out for a wide range of Reynolds number (Re) ranging from 3000 to 15000. From the measured responses, the Strouhal number (St) is back calculated. It is observed that the Strouhal number for flexible cable ranges from 0.13 to 0.17 for low Reynolds number from 3000 to 15000 and it falls within the expected range. The normalized RMS displacements (RMS A/D) have been obtained for specified range of Re. The lift coefficient (CL) attains a stable value of around 0.10 to 0.25 for reduced velocity (Vr) greater than 5.33 for different locations. The lift coefficient follows a similar trend along the span for different Vr. Numerical simulation has been carried out using Shear7 software with results matching reasonably well.
Experimental and Numerical Assessment of Both Slug and Vortex Induced Vibrations on a Spool Model
Authors: Minguez Matthieu , Le Prin Kevin , Liné Alain , Lafon Vincent , Pétrié François , Rippol Thierry
Published: December 2020

The paper addresses the flow-induced response of a rigid spool/jumper. It mainly focuses on its mechanical response resulting from internal intermittent slug flows but also addresses potential coupling with an external excitation due to vortex shedding. These works provide quantitative experimental data that match quite well with existing empirical correlations in terms of slug flow properties. The repeatability of the measurement system has been experienced and underlines promising capabilities. The tests provide exhaustive complementary data regarding the slug flow properties (e.g. pocket length) which will be reused for numerical modelling purpose. The mechanical response of the spool is exhaustively addressed for different regular slug flows. Some correlations are proposed aiming at describing the mechanical responses. The coupling with an additional external current solicitation and Vortex Induced Vibrations (VIV) is discussed and characterized for some conditions. Finally, a discussion on the current Industry Best Practices is introduced in order to challenge the capability of the proposed approaches to described and recover such complex phenomenon. The comparisons underline the weak agreement between experiments and numerical models, opening discussion on the best way to address this physics and the next developments.
Sensitivity Analysis of Parameters for FPSO Model Updating
Authors: Wu Qihao , Zhang Min , Yang Tian’en , Xu Nuoya , Wang Junrong , Du Junfeng , Fonseca Nuno , Tahchiev Galin
Published: December 2020

This paper presents parameter sensitivity analysis for a FPSO numerical model updating. Generally, model test data are considered a better presentation of physical phenomena than its numerical counterpart. To minimize the discrepancy, model updating is of pragmatically importance. Model updating of a certain FPSO can be achieved by specific steps. In each step, the required properties of numerical model and test results are matched by means of tuning of the related parameters.

To avoid inefficiency and physical meaning loss resulting from large modification of parameters which are insensitive to objective properties, parameter sensitivity analyses using the direct method are conducted in this paper. The investigated parameters mainly are the FPSO’s mooring line length, mooring line mass per unit length, mooring line cross-sectional area, fairlead position, FPSO hydrostatic stiffness, FPSO mass properties, linear and quadratic damping coefficients. According to the different stages of FPSO model updating, the objective functions are set to be the FPSO’s mooring line pretension, mooring system horizontal restoring force, the natural periods of the FPSO’s 6 degree of freedom motions and the standard deviation of motion response spectra under irregular waves.
Geotechnical Study to Design a GIS Platform Subjected to Pipeline in Permafrost Area
Authors: Kim Sewon , Kim YoungSeok
Published: December 2020

Construction in the extreme cold regions becomes one of the important businesses in the world. Geophysical surveys, which are effective in providing consecutive ground information, have been widely applied to the civil engineering and environmental sectors. The permafrost area covers about 14 percent of the world’s land area and the global construction market for such area is rapidly expanding. Where developed countries have already recognized the need for research of the coldest places and therefore have invested heavily in technology development, the non-arctic countries are still developing and related research has rarely been performed. The purpose of this study is to design the GIS (Geophysics Information System) platform for a construction environment with a 2,000 km-class pipeline (overland) operated under the permafrost temperature conditions (−40°C∼+20°C). The GIS platform when subjected to geotechnical engineering provides an understanding of the frozen ground and information about the geo-environment of the extreme cold regions. This paper shows the characteristics and procedures of Environmental Impact Assessment (EIA) for developing Arctic resources, and reviews how to construct a system for the systematic management of the necessary information to consider building the GIS platform. In addition, this study introduces the GIS algorithm to build the route for the pipeline path, and factors which should be considered first are safety, environment, economic feasibility and technical feasibility.
Influence of Mean Tension on Mooring Line Fatigue Life
Authors: Lone Erling N. , Leira Bernt J. , Sauder Thomas , Aksnes Vegard , Gabrielsen Øystein , Larsen Kjell
Published: December 2020

Studies published in recent years have documented a significant mean load effect on fatigue capacity for offshore mooring chain, and show that a reduction of the mean load gives an increase in fatigue life. However, current S-N design curves are based on fatigue tests performed at a mean load of 20% of minimum breaking load (MBL), which is well above the typical mean loads for most mooring systems.

This paper investigates the mean loads experienced during fatigue damage accumulation for the mooring system of a typical production semi-submersible, operating in Norwegian Sea conditions. The study is based on numerical, time-domain simulations, using environmental conditions defined from a series of hindcast data. A parameterized S-N design curve suggested by Fernández et al. (2019), incorporating a Smith-Watson-Topper mean stress correction model, is applied for fatigue damage calculation and compared to results for the S-N design curve prescribed by current standards.

For the semi-submersible unit considered there is negligible difference in basing the correction on 3-hour mean load compared to the mean load of individual stress cycles, due to small low frequency tension variations. On this basis, a single correction factor is proposed to allow for mean load correction based on results available from a standard fatigue analysis.
Experiment-Scale Multi-Vessel Dynamic Positioning System for the Twin-Lift Decommissioning Operation
Authors: Fu Zhenqiu , Li Wei , Sun Hao , Yuan Meng , Li Xin , Cai Liancai , Wang Andy , Guo Xiaoxian , Wei Handi , Tian Xinliang
Published: December 2020

In general, the lifetime of the offshore oil and gas platform is about 20–30 years, and a large number of oil and gas platforms is approaching the end of their service. So, how to decommission these oil and gas platforms safely and cleanly becomes an urgent problem for the oil and gas industry and related organizations. And the costs for decommissioning equipment and services are expected to be as low as possible. It is important to develop the innovative removal techniques to reduce the overall cost of decommissioning. In this paper, we propose a new concept to remove large and heavy structures with a single lift, utilizing three semi-submerged vessels. The main process of the twin-lift solution for decommissioning includes two semi-submerged vessels lifting the topside together, and then carrying the topside to the third semi-submerged vessel. This method is simple in principle and does not require the manufacture of new special vessels, only needs to make simple modifications to existing semi-submerged vessels. It requires high positioning accuracy especially under environmental disturbances to ensure the safety of transportation. To ensure efficiency and safety, we develop a dynamic positioning (DP) system and perform a wave tank test for such a twin-lift decommissioning solution. Some aspects of an experimental facility set-up for scaled model test with dynamic positioning system are described. It includes the topology of the experiment and details of the deck mating, DP and monitoring system.
Optimization-Based Multi-Attribute Decision Making for the 7th Generation Semi-Submersible Drilling Unit
Authors: Meng Xun , Tang Pin , Li Dejiang , Xu Yu , Fu Qiang
Published: December 2020

The 7th generation semi-submersible drilling units (CSDU) are characteristic of deeper drilling depth, site locations and higher operational efficiency, compared with the last generation ones. Given the enormous live loads change and increasing trend of main size dimensions, considerable optimization should be deployed to achieve a balance of economy, safety and good work performance. Trial calculation and definite assessment to check whether alternative schemes meet the requirements turns out to be ineffective, for the case by case study of hydrodynamic and structure strength analysis is time consuming. In the paper, an integrated optimal design model is formulated by merging multi-objective optimization and multi-attribute decision making into one. A predesigned parametric Finite Element Analysis (FEA) structural model of CSDU is developed and validated and then coupled with detailed hydrodynamic analysis. Three mutually conflicting design objectives are arrived by hydrodynamic solutions. They are stability, hydrodynamic performance and steel consumption, which are screened to obtain Pareto optimality. The Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) is applied to incorporate these optimal attributes into decision-making process, considering all criteria in terms of quantitative stability, hydrodynamic performance and qualitative economy. The objective entropy coefficient measuring the importance of different attributes is introduced into weight selection for the purpose of avoiding non-determinacy and optional judgements. The optimal solutions are further verified with main dimensions of CSDUs in service and also could give predictive suggestion for the new CSDUs. The study provides a more objective way of benchmarking different structural schemes of CSDU by considering multiple criteria simultaneously. It is demonstrated that the proposed structural optimization model is capable of effectively and accurately determining the optimal design of CSDU.
A Modelling System for Power Consumption of Marine Traffic
Authors: Tvete Hans Anton , Guo Bingjie , Liang Qin , Brinks Hendrik
Published: December 2020

The International Maritime Organization (IMO) has enforced stricter limit on the Greenhouse Gas (GHG) emission due to environment and climate concern. The measures to reduce GHG emissions from shipping can be divided into two groups. The first one is to improve ship energy efficiency through new technology, optimized operation and logistics, and the second one is to introduce alternative fuels with lower carbon intensity. However, the effectiveness and applicability of any measure depend on ship type, ship size, operation type, operation environment (wind and wave condition), as well as the cost of the measure. It is necessary to evaluate new measures in real shipping scenario. Modeling of ship fuel consumption and emission are fundamental input to evaluate the impacts of shipping on the environment and climate, and to evaluate new measures for reducing GHG emission. A modeling system is developed to estimate ship fuel consumption and emission, based on ship hydrodynamical models, information from Automatic Identification System (AIS), the IHS Fairplay database and metocean data. The modeling system aims to cover most of the ship types equipped with AIS transponders, and it will provide different hydrodynamical models to calculate fuel consumption based on the available ship information. In the paper, the modeling system will be described, and the power consumption from the modeling system are compared with the measurement data on one container ship. The comparison shows that the power consumption predicted with the modeling system agrees with measurement data well. The effects of weather data and measured speed on predicted power consumption are also analyzed.
An Analytical Method for Calculating Hydrostatics for Jacket Structures
Authors: Last Irene , Krüger Stefan
Published: December 2020

In response to the continuing importance of offshore structures a framework for the simulation of offshore platforms is implemented in the open software environment E4. The non-commercial suite contains various methods for the ship design so that the newly developed procedures benefit from the existing knowledge. Jacket structures are installed as fixed platforms or compliant towers and play a significant role in the offshore exploitation. They are made out of hundreds of tubular steel members and each of the cylinders or cones can be seen as a simple body compared to ships. Besides the dynamic behaviour and fatigue analysis, the examination of the installation procedure especially the launch operation is of high interest. This article presents an analytical approach for the hydrostatic calculations of partially submerged cylinders. Hydrostatic forces on jacket structures are calculated by superimposing single cylinders and the results of the newly developed method are compared with a commercial simulation software. The aim is to speed up the computation time in comparison to numerical methods which is a significant advantage in the early design stage. The presented method will be the foundation for the simulation of jacket launching manoeuvres in time domain.
Scale Effects in Heave Plates: PIV Investigation
Authors: Anglada-Revenga Elena , Bezunartea-Barrio Ana , Maron-Loureiro Adolfo , Molinelli-Fernandez Enrique , Oria-Escudero Julio , Saavedra-Ynocente Leandro , Soriano-Gomez Cristina , Duque-Campayo Daniel , Gomez-Goni Jesus , Souto-Iglesias Antonio
Published: December 2020

Bezunartea et al. Scale Effects on Heave Plates for Semi-Submersible Floating Offshore Wind Turbines: Case Study With a Solid Plain Plate (Journal of Offshore Mechanics and Arctic Engineering, Vol. 11, pp 1–14, 2019) studied scale effects on the hydrodynamics of heave plates by conducting model scale experimental tests. Three models with different scales of one leg of the HiPRWind platform, equipped with a plain heave plate, were built and tested with forced harmonic heave motions. They showed that the influence of the model scale on the coefficients is clearly weaker than that of the heave motion amplitude. Among the pending tasks in that work was to conduct velocity field measurements of the flow with Particle Image Velocimetry techniques (PIV). This paper discusses some of the results of such measurements, carried out forcing vertical harmonic motions. In these measurements, it is assessed how significant scale effects are also in the local kinematics of the flow, thus complementing the global ones carried out in the referred work. From velocity and vorticity graphs, the flows show great similarities across the three scales, consistent with the ones found in the prequel paper. A metric has been developed whose outcomes are aligned with these results but further work is necessary to develop key indicators to compare flow fields from PIV across different scales.
Numerical Investigation of Heaving Hydrodynamic Behavior of a Single Cylinder and a Dual Coaxial-Cylinder System Using CFD
Authors: Zhi Pengfei , Zhang Xinshu , Chen Ke , Yeung Ronald W.
Published: December 2020

In this paper, we apply a CFD computer code to study the hydrodynamic behavior of a stand-alone cylinder and a dual coaxial-cylinder system (DCCS) via free-decay motion tests. The geometric proportions of a stand-alone cylinder and the inner and outer cylinder of the DCCS are chosen to be the same as those in [1] and [2], respectively, as ocean wave-energy converter (WEC) devices. Overset mesh based on the commercial code ‘Star-CCM+ 11’ is used to simulate the free-decay motion of the two systems. Five parameters chosen for the CFD implementation are: turbulence model, initial displacement, time step, number of prism layers and mesh size. Results obtained from using different values of these parameters are compared so as to confirm the validity of choices made. The hydrodynamic performance of the stand-alone cylinder and outer cylinder in the DCCS are compared with the experimental results to assess and validate the CFD models. In addition, the heave hydrodynamic coefficients, namely, the added mass and total damping, and ‘resonance’ frequency of the stand-alone cylinder and those of the inner cylinder of the DCCS, with the outer cylinder being fixed, are obtained by using the CFD procedure. The hydrodynamic coefficients of another stand-alone cylinder with the same dimensions as the inner cylinder of the dual-coaxial cylinder are also obtained by simulations. The vorticity-contour plots for the stand-alone cylinder and the outer cylinder in the DCCS in free-decay motion are presented and analyzed. Finally, the results of the three cases are compared to examine the effect of the outer cylinder on the heave hydrodynamic coefficients of the inner cylinder.
Tidal Turbine Load Variability in Following and Opposing Irregular Wave Conditions
Authors: Draycott Samuel , Steynor Jeffrey , Nambiar Anup , Sellar Brian , Venugopal Vengatesan
Published: December 2020

Waves induce large loads on tidal turbines, yet the effect of irregular waves, particularly those which oppose the current direction, have received little attention. Here we experimentally assess the loading resulting from 24 irregular combined wave-current cases, statistically presenting the load variability acting on a 1:15 scale tidal turbine. Comparisons are made between following and opposing conditions of equivalent wave parameters, and the effect of varying frequency and amplitude is assessed. Example load distributions are also shown, along with streamwise variation of significant wave height around the turbine. It is concluded that both opposing and following conditions must be assessed to effectively de-risk tidal turbine design: large differences are observed both in terms of the nature & magnitude of the resulting loads and the wave interaction with the surrounding flow field.
Displacement Efficiency in Eccentric Annuli
Authors: Lund Bjørnar , Ytrehus Jan David , Taghipour Ali , Saasen Arild
Published: December 2020

A successful primary cementing job requires an efficient fluid displacement process in order to place cement in the annulus between a casing or liner and the formation around the wellbore. This operation is a critical part of the well construction and due to the hardening properties of cement there will be no second chance with this operation. Once hardened, the cement should provide zonal isolation and pressure containment and should do so through the lifetime of the well. Furthermore, the cement shall anchor and support the casing string and protect it against corrosion due to formation fluids. A good quality cement job is important both from an environmental and from an economical perspective.

In order to improve recommended cementing practices, and thus also the quality of cementing process, it is necessary to develop cementing simulation software for proper engineering. Detailed data on the displacement efficiency from practical operations are difficult or impossible to obtain. Furthermore, field operations do not allow for a systematic change in fluid properties. To be able to validate the engineering tools it is therefore necessary to perform laboratory measurements with fluids with relevant properties. It is also important to perform these experiments in laboratory equipment of sufficient diameter dimensions to facilitate scale up of the results.

The purpose of the present article is to give an overview of cement displacement experiments which were conducted using a large diameter annular flow loop using model fluids with relevant rheological properties. While some results from these experiments have been presented in previous articles, this article is intended to give an overview of all the experiments, including the setup and experimental approach, and provides in more detail results not presented previously. In particular we show results from video recordings which confirm previous conclusions based on conductivity measurements.

It is found that eccentricity affects the displacement physics significantly. In washout sections the effects of eccentricity are different and generally smaller since the eccentricity is also reduced. Drill string rotation improves displacement efficiency in all tested cases both within and outside the washout cavity.

Since the amount of publicly available experimental data of this type is very limited, the detailed information presented here should be of great interest and relevance for the development and validation of industrial cementing simulation software.
Analysing Supply Chain Phases for Design of Effective Autonomous Ship Technology in New Transport System Solutions
Authors: Wennersberg Lars Andreas Lien , Nordahl Håvard , Rødseth Ørnulf Jan , Bolbot Victor , Theotokatos Gerasimos
Published: December 2020

Maritime autonomous surface ships provide new capabilities for transport systems design, and by that the potential to dramatically change ship logistics. Advantages of autonomous ships include operational flexibility with more and smaller ships, combined with new ship hull and superstructure designs, allowing more cargo to be transported at lower cost and reduced emissions. It is also easier to operate smaller ships with alternative energy sources such as batteries, and by that further reducing the emissions. However, the investment cost of an autonomous ship system is perceived to be higher compared with conventional ships. Combined with limited operational experience, this creates a challenging threshold for launching new projects.

The purpose of this work is to lower this threshold by providing a better understanding for the analysis tools employed in new transport systems design. Structured analysis of the supply chain phases and tasks enables the involved stakeholders to evaluate the cost and benefits associated with the increased automation in a specific phase of the transport system. This will additionally form a basis for developing business models and priorities for gradual introduction of key enabling technologies through retrofitting of conventional ships or designing and building new smart ships.

A scalable method for the systematic supply chain analysis of waterborne transport systems is introduced. The method is developed through a mapping and analysis of one inland waterways use-case and one short sea shipping use-case, and used as a basis for the systematic identification of all the supply chain phases and all tasks carried out in each phase of the transport system. A design process for autonomous ship systems consisting of the supply chain analysis, the task distribution and the cost benefit analysis is also presented. The effectiveness of this process in the design and incorporation of autonomous ship technologies in new transport systems solutions is demonstrated.
Removal of Magnetic Contamination in Drilling Fluids: Effect on Directional Drilling
Authors: Saasen Arild , Poedjono Benny , Ånesbug Geir Olav , Zachman Nicholas
Published: December 2020

Magnetic debris in a drilling fluid have a significant influence on the ability of the drilling fluid to maintain its function. Down hole logging can suffer from poor signal to noise ratios. Directional drilling in areas close to the magnetic North Pole, such as in the Barents Sea, Northern Canada or Russia can suffer because of magnetic contamination in the drilling fluid. Magnetic particles in the drilling fluid introduce additional errors to the magnetic surveying compared to those normally included in the ellipsoid of uncertainty calculation.

On many offshore drilling rigs, there is mounted ditch magnets to remove metallic swarf from the drilling fluid. These magnets normally only remove the coarser swarf. In this project, we use a combination of strong magnets and flow directors to significantly improve the performance of the ditch magnets. This combination, together with proper routines for cleaning the ditch magnets, significantly helps to clean the drilling fluid. Through the combined use of flow directors and ditch magnets, it was possible to extract more than five times as much magnetic contamination from the drilling fluid as normal compared with other proper ditch magnet systems. This is verified by comparing the ditch magnet efficiencies from two drilling rigs drilling ERD wells in the North Sea area.

In the paper, it is discussed how the accuracy of directional drilling and well position effected by various interferences can be improved by the use of a drilling fluid with minimal effect to the MWD measurement.
Dynamic Analysis on Critical Responses of Pipeline and Cable During Pipeline End Termination Installation
Authors: Han Duanfeng , Huang Kuo , Zan Yingfei , Yuan Lihao , Wu Zhaohui
Published: December 2020

In order to figure out the dynamic characteristics of the pipeline and cable during pipeline end termination (PLET) installation based on S-laying, numerical simulation is carried out based on a practical operation project performed at Liwan oil and gas fields in the South China Sea. Four scenarios are selected from the PLET installation process in sequence for simulation. Critical responses of the pipeline and the cable in different scenarios of the operation are analyzed in this paper with a coupled model using RIFLEX module of SIMA software. Both the pipeline and the cable are modeled by the finite element method, and the pipelaying vessel is controlled by a dynamic positioning system. The simulation results are validated by the commonly used OrcaFlex software. The critical responses analyzed include static configuration, time-domain variation of axial tension at the top of the cable and bending moment variation near the touchdown point (TDP) of the pipeline. Furthermore, the time-domain variation of the tension at the top of the cable under different wave and current directions are also compared and analyzed, in order to study the effect of sea environment on the pipeline and cable during PLET installation operation. The results show that the responses of pipeline and cable vary in different operation scenarios, and the sea environment has remarkable effect on the pipeline and cable. The study in this paper is of value to the design of PLET installation based on pipelaying and can help predict the response of pipeline and cable during the operation.
Data Assimilation for Phase-Resolved Ocean Wave Forecast
Authors: Wang Guangyao , Pan Yulin
Published: December 2020

The phase-resolved prediction of ocean waves is crucial for the safety of offshore operations. With the development of the remote sensing technology, it is now possible to reconstruct the phase-resolved ocean surface from radar measurements in real time. Using the reconstructed ocean surface as the initial condition, nonlinear wave models such as the high-order spectral (HOS) method can be applied to predict the evolution of the ocean waves. However, due to the error in the initial condition (associated with the radar measurements and reconstruction algorithm) and the chaotic nature of the nonlinear wave equations, the prediction by HOS can deviate quickly from the true surface evolution (in order of one minute). To solve this problem, the capability to regularly incorporate measured data into the HOS simulation through data assimilation is desirable. In this work, we develop the data assimilation capability for nonlinear wave models, through the coupling of an ensemble Kalman filter (EnKF) with HOS. The developed algorithm is validated and tested using a synthetic problem on the simulation of a propagating Stokes wave with random initial errors. We show that the EnKF-HOS method achieves much higher accuracy in the long-term simulation of nonlinear waves compared to the HOS-only method.
A Transient Thermal Flow Model for CO2 Foam and N2 Foam With Phase Transition Effect During Horizontal Well Drilling
Authors: Xu Zhengming , Song Xianzhi , Zhu Zhaopeng
Published: December 2020

Foam is one kind of widely used drilling fluids for underbalanced drilling due to its high cuttings-carrying capacity and low lost circulation rate. However, temperature and pressure change continuously during drilling operations, making the accurate prediction of the bottomhole pressure more difficult. Besides, for fluids near the bottom, CO2 or N2 could change phase state among gas, liquid, and supercritical condition. Thus, density, rheology, and thermal properties of foam fluids could change significantly.

In this study, a transient thermal model is developed to predict pressure and temperature profiles in horizontal wells. The velocity and pressure are calculated based on continuity and momentum equations. Temperature profiles of foam and formation are obtained by simultaneously solving energy equations for different thermal regions in the wellbore-formation system. Foam properties are calculated using the state-of-the-art equation of state with the effect of phase transition.

The effects of four important parameters on the wellbore temperature and pressure distributions are analyzed. As the annulus outlet foam quality and geothermal gradient increase, wellbore pressure decreases accordingly for both CO2 foam flow and N2 foam flow. For both CO2 and N2 foam flow in horizontal wells, the phase state of CO2 and N2 phases are the gas state and supercritical state. And the critical well depths for CO2 and N2 foam flow are different due to their different critical temperature values.

This work provides a transient model to simulate flow and thermal behaviors of CO2 foam and N2 foam during both drilling and making a connection in the horizontal wellbore. Operators may use this model to select the most suitable foam fluids to optimize injection parameters.
Bayesian Multi-Level Modelling for Improved Prediction of Corrosion Growth Rate
Authors: Di Francesco Domenic , Chryssanthopoulos Marios , Havbro Faber Michael , Bharadwaj Ujjwal
Published: December 2020

In pipelines, pressure vessels and various other steel structures, the remaining thickness of a corroding ligament can be measured directly and repeatedly over time. Statistical analysis of these measurements is a common approach for estimating the rate of corrosion growth, where the uncertainties associated with the inspection activity are taken into account.

An additional source of variability in such calculations is the epistemic uncertainty associated with the limited number of measurements that are available to engineers at any point in time. Traditional methods face challenges in fitting models to limited or missing datasets. In such cases, deterministic upper bound values, as recommended in industrial guidance, are sometimes assumed for the purpose of integrity management planning. In this paper, Bayesian inference is proposed as a means for representing available information in consistency with evidence. This, in turn, facilitates decision support in the context of risk-informed integrity management.

Aggregating inspection data from multiple locations does not account for the possible variability between the locations, and creating fully independent models can result in excessive levels of uncertainty at locations with limited data. Engineers intuitively acknowledge that the areas with more sites of corrosion should, to some extent, inform estimates of growth rates in other locations. Bayesian multi-level (hierarchical) models provide a mathematical basis for achieving this by means of the appropriate pooling of information, based on the homogeneity of the data.

Included in this paper is an outline of the process of fitting a Bayesian multi-level model and a discussion of the benefits and challenges of pooling inspection data between distinct locations, using example calculations and simulated data.
Tuning of Nacelle Feedback Gains for Floating Wind Turbine Controllers Using a Two-DOF Model
Authors: Lenfest Eben , Goupee Andrew J. , Wright Alan , Abbas Nikhar
Published: December 2020

Designing a collective blade pitch controller for floating offshore wind turbines (FOWTs) poses unique challenges due to the interaction of the controller with the dynamics of the platform. The controller must also handle the competing objectives of power production performance and fatigue load management. Existing solutions either detune the controller with the result of slowed response, make use of complicated tuning methods, or incorporate a nacelle velocity feedback gain. With the goal of developing a simple control tuning method for the general FOWT researcher that is easily extensible to a wide array of turbine and hull configurations, this last idea is built upon by proposing a simple tuning strategy for the feedback gain. This strategy uses a two degree-of-freedom (DoF) turbine model that considers tower-top fore-aft and rotor angular displacements. For evaluation, the nacelle velocity term is added to an existing gain scheduled proportional-integral controller as a proportional gain. The modified controller is then compared to baseline land-based and detuned controllers on an example system for several load cases. First-pass results are favorable, demonstrating how researchers can use the proposed tuning method to efficiently schedule gains for adequate controller performance as they investigate new FOWT configurations.
New Approach for Offshore Pile Decommissioning With Hydraulic Presses and Floating Panels
Authors: Lehn Patrick , Hinzmann Nils , Gattermann Jörg
Published: December 2020

Renewable Energies become more and more important in industries and society all over the world. In Germany, offshore wind farms generated 49 % of the renewable energies in 2018. Monopiles are the preferred system for the foundation of offshore wind turbines in water depths up to 40 m. They are authorized by the competent authority for 25 years. When reaching the end of lifetime, the structure inclusive the foundation must be decommissioned. The decommissioning of monopiles will be challenging in the future and can lead to unexpected costs and risks for the owners. Removing the monopiles in it’s entirely ensures the opportunity to reuse the space for new offshore wind farms.

The Institute of Geomechanics and Geotechnics of the Technische Universität Braunschweig (IGG-TUBS) obtained the funding for the research program on technical solutions with large-scale tests for decommissioning of offshore monopiles named DeCoMP.

Several decommissioning methods such as vibratory extraction, internal dredging, external jet drilling, decommissioning with overpressure and the use of buoyancy force are investigated. The proposed paper will present technical opportunities and issues for extracting the pile with hydraulic presses in combination with a steel framework. Hydraulic presses brace the steel framework with the monopile. Further hydraulic presses, positioned at a certain distance to the pile on the framework, use the seabed as abutments to push out the monopile. In addition, results of a feasibility study to remove monopiles with floatation panels are presented in this paper. This method is based on floating panels, which are attached to the monopile above the mud line. These panels are inflated with air pressure to reach the required amount of buoyancy to overcome the pullout resistance. The decommissioning solutions are compared to point out possible combinations.
Numerical Wave Tank Including a Fixed Vertical Cylinder Subjected to Waves, Towards the Investigation of Floating Offshore Wind Turbine Hydrodynamics
Authors: Clément Constance , Bozonnet Pauline , Vinay Guillaume , Borras Nadal Adria , Pagnier Philippe , Réveillon Julien
Published: December 2020

Specific engineering tools are used to design Floating Offshore Wind Turbines (FOWT). These so-called aero-hydro-servo-elastic solvers simulate the coupled behaviour of the turbine subjected to wind with the floater motion due to waves, including elasticity of the whole structure. The implemented hydrodynamic forces rely on a strong Oil&Gas background and include potential flow theory and empirical laws, such as Morison forces. The undergoing study aims at re-evaluating the validity range of such theories, when applied to FOWT. To do so, CFD simulations will be run to model wave propagation and interaction with a FOWT floater. Hydrodynamic forces will be extracted from the CFD simulations and compared to current hydrodynamic theories.

A fixed cylinder in regular second order deep water waves (steepness of 0.9) is simulated and results are validated against experiments [1]. This basic first case implemented with Open-FOAM using waves2Foam library allows to master regular wave generation and interaction with a rather simple structure, running multiple simulations. Convergence (mesh refinement, time step) and parameterization (numerical schemes, turbulence models) studies are carried out to ensure controlled wave generation. An accurate Numerical Wave Tank (NWT) is finally obtained. However, the resolution of air/water interface with Volume Of Fluid (VOF) MULES method seems to be responsible for extreme air velocities on crests resulting in wave damping. This phenomena is solved by decreasing time step. Hydrodynamic forces on the cylinder match experiments with an error below 3%. As the flow is turbulent (Re = 105), a turbulence model is included in the simulation giving results rather close to the ones obtained without turbulence model.
Model Tests and Numerical Prediction of the Low Frequency Motions of a Moored Ship in Shallow Water With a Wave Splitting Method
Authors: Fonseca Nuno , Tahchiev Galin , Fouques Sébastien , Stansberg Carl Trygve , Rodrigues José Miguel
Published: December 2020

Prediction of shallow water low frequency (LF) motions of vessels in the context of mooring analysis is challenging. Model tests are often performed to calibrate and validate numerical models and, in this way, reduce the uncertainty. Model tests are part of the positioning system qualifying process. However, model tests also present challenges and uncertainties related to parasitic low frequency wave systems which are unavoidable in shallow water ocean basin conditions.

The paper presents model tests with a ship moored in shallow water (20 m), the analysis and discussion of the test data and comparisons with numerical predictions. The focus is on the low frequency motions and related wave drift forces. The tests have been performed in harmonic waves, bi-harmonic waves and irregular seastates, including conditions with and without current. The first part of the study consists of analysing the wave field measured by a long array of wave sensors distributed along the ocean basin. The analysis provides split wave systems, namely the low frequency components including the bound wave, the incoming free parasitic wave, the reflected component and additional very long waves. The second part proposes a method to calibrate and validate mooring analysis numerical models, based on comparisons with model test data which includes the unavoidable effects from parasitic waves. Simulations of LF motions with the calibrated model show a good agreement with the measurements.
Evaluation of the Ice Load Acting on an Arctic Offshore Structure With Different Ice Drifting Angle
Authors: Kim Young-Shik , Kim Yun-Ho , Song Hyung-Do , Jang Jin-Ho , Han Solyoung , Kim Joosung , Kim Hyunjoe , Cahay Marc
Published: December 2020

In this study, an evaluation method and results for ice load acting on an Arctic offshore structure with various ice drifting angles are discussed. Korea Research Institute of Ships and Ocean Engineering (KRISO) has conducted a research project to develop a hull form design for year-round floating type offshore structures in the Arctic condition with dynamic positioning and mooring system. Six cooperating organizations participated in the project: Samsung Heavy Industry, Korean Register, Pusan National University, Korea Maritime & Ocean University, Dong-Eui University, and Inha Technical College. In the design stage of an Arctic offshore structure, ice load consideration is the key component for the safety and reliability analysis. However, there is no generally used tool for evaluation of ice load acting on an Arctic offshore structures. In this study, ice loads acting on an Arctic FPSO in managed ice conditions with various ice drifting angles are examined by experimental methodology. Dramatic mean value changes in ice load with different ice drifting angles are observed in the model test. This experimental ice load evaluation method can be applied to the other types of offshore structure which might operate in sea ice condition.
Sequential Annular Fluid Expansion Analysis for Wellbore Completion Design
Authors: Liu Zhengchun , Samuel Robello , Gonzales Adolfo , Kang Yongfeng
Published: December 2020

Annular fluid expansion analysis (AFEA) is a vital step when designing an oil/gas well or geothermal well. Traditionally, an AFEA is performed between a drilling operation (initial condition) and a production operation (final condition). However, well design engineers desire to perform an AFEA between any two operations. The objective of this work is to address that challenge.

Sequential AFEA refers to the procedure in which output data (e.g., fluid volume change) of one AFEA are used as input data for another AFEA; the underlying operations are “sequential”. As a result, an AFEA can be configured between any two operations, provided that the “initial condition” operation is before the “final condition” operation.

This two-pass AFEA model has been implemented in computer programs and has been integrated with a commercial software tool, including a wellbore thermal flow simulator and tubular stress analyzer. Offshore well examples were used to validate the model. Case study results indicate that a sequential AFEA is in good agreement with a traditional one-pass AFEA in terms of both forward and backward operation sequences.
Instantaneous Center of Rotation of a Vessel Submitted to Oblique Waves
Authors: de Oliveira Costa Daniel , Sena Sales Junior Joel , Fernandes Antonio Carlos
Published: December 2020

When under influence of an incident wave system, any freely floating body presents a general motion with all six degrees of freedom. The Instantaneous Center of Rotation, as defined in classical mechanics, is a concept that allows the description of a general motion in 6 degrees of freedom as a pure rotation around such point. This approach, although not widely used in ocean engineering, might be an alternative tool that allows fast and precise analysis in many cases. Recent studies have shown that under specific conditions, such as a heading wave condition, the ICR varies in time but it is always located along a line for one wave frequency. Similar results were presented regarding beam waves as well.

The present work continues with the investigation regarding the behavior of ICR under more generic conditions, assuming oblique waves exciting a vessel with typical geometry of a FPSO platform. The study extends the knowledge derived based on 2D approaches from previous works, comparing the results obtained from the different methods. An analytical model is presented, assuming only harmonic motion to all 6 degrees of freedom and showing that, similar to what was observed in the simplified 2D cases, the ICR tends to present dependence on the frequency of motion. Numerical data acquired from commercial codes based on potential theory is also presented.