1,721,124 research outputs found
Numerical simulations of the flow around single and tandem circular cylinders close to a plane wall
Full text linkIn order to model the turbulent flow around cylindrical structures in various configurations, Large Eddy Simulations (LES) are used in the present work. LES utilize a highly accurate model at reasonable time-consumption, offering an insight into the details of a complex flow. Standard Smagorinsky subgrid scale model is used to include the effects of the subgrid scale motions.
Three configurations of cylindrical structures are chosen in this study, inspired by the arrangements of the subsea pipelines in the marine environment. The simplest and most extensive studied arrangement is the flow around a single, smooth circular cylinder in a uniform current. Hence, this configuration is used for the assessment of the numerical tools’ performance.
The second configuration, a circular cylinder in the vicinity of a plane wall is chosen to represent the flow around free-spanning subsea pipelines. Performed for various gap to diameter ratios (G/D), the simulations successfully capture the details of the flow in the cylinder wake and the interaction with the seabed boundary layer. The influence of the less explored parameters, the thickness and the shape of the inflow profile, is explored. The three-dimensional (3D) LES results show clear improvements over the previously published two-dimensional (2D) simulations.
Two circular cylinders, placed one behind the other, relative to the incoming current, are called the tandem arrangement. The third configuration, tandem cylinders in the vicinity of a plane wall correspond to double pipelines or flowlines in the subsea systems. This configuration has received little research attention. The flow around tandem cylinders close to the wall is therefore compared to the simpler and more thoroughly understood configurations. It is concluded that the existing classification of the flow around one cylinder close to the wall describes well the flow around the upstream cylinder at large horizontal span ratios (L/D). Behaviour of the flow around tandem cylinders at large G/D can be described through the flow classification for the tandem cylinders in the infinite fluid. At intermediate G/D (and various L/D), the presence of the wall causes stronger influence than recorded for the flow around one cylinder at the same G/D
Global analysis of floating offshore wind turbines with shared mooring system.
Full text linkWind energy harvesting has increased considerably in recent decades across the world. Over the years, wind farms have gradually extended further into the sea to make use of the greater potential of wind resources in deeper water. Floating wind turbines (FWTs) might be a viable option for deep-water applications. The three most researched floating structures so far are spar, semi-submersible, and tension leg platform (TLP). Cost-effective solutions are desired for further viability and commercialization of these concepts. FWTs with a shared mooring system have the potential to be one of the most cost-efficient ways to minimize the cost of mooring. The 5-MW Conceptual Semi-Submersible Concept (CSC) floating structure with a shared mooring line is considered in the present study due to scarce research on this topic.
This study is inspired by the successful research experience of FWTs’ feasibility with a shared mooring system presented in Munir et al. (2021) which the first author is the author of this thesis. The present thesis is the continuation work which aims to further investigate the feasibility of FWTs with a shared mooring line. To achieve this, two different methodologies are adopted in Munir et al. (2021) and in the present study. In Munir et al. (2021), two single CSC FWTs are connected with a shared line by removing one anchor fixed mooring line from each. Two different configurations of FWTs are studied with 6 and 8 rotors diameter distance between the FWTs, placed horizontally. Along with the length of the shared line, static equilibrium position of FWTs is also varied. The floaters of a dual CSC model are rotated to accommodate the shared mooring line and wind turbines are rotated to experience the maximum thrust. Global analysis revealed higher maximum surge and sway offsets because of the reduced mooring restoring stiffness as compared to a single CSC model. The reduced restoring stiffness can be attributed to the reduction of one anchor fixed mooring line from each FWTs.
In the present study, three different configurations of the dual CSC model are studied with different shared line and single mooring configurations. This includes Model 1 with a medium-long shared line, Model 2 with a longer shared line, and Model 3 with a relatively shorter shared line. As an alternation of the methodology presented in Munir et al. (2021), the static equilibrium position of FWTs is kept constant at 9 rotors diameter distance and only the length of the shared line varies. The anchor positions are changed to keep the FWTs at their mean static equilibrium position in all the configurations. The orientation of the floaters and wind
turbines is same as in Munir et al. (2021). Dynamic responses of all the configurations are analyzed and compared, including the natural periods, restoring stiffness of the structures, mooring restoring force, and platform motions. The analysis revealed that the surge and sway DOFs are more influenced by the shared mooring configuration with higher offsets and natural periods as compared to a single CSC model. This happens because of the reduced mooring restoring stiffness of the structure in surge and sway directions and no shared mooring restoring force in horizontal DOFs. To avoid the snap loads and increased tension on the fairleads in extreme environmental conditions, Model 1 is recommended for further study in future as compared to Model 2 and Model 3 with a longer and shorter shared line. The results indicate that the dual CSC model with a shared line is a feasible concept and could become an alternative concept in deep water
CFD investigation of flow through an orifice inside a pipe
Full text linkFlow measurement is found in a large variety of industries and for different types of fluids. Orifice flow meters are widely used for measuring the flow rate in pipelines due to its small size in the piping system and ability to output the flow rate with satisfying accuracy. The main problems with the orifice flow meter are the unrecoverable pressure loss and due to the shear forces exerted from the orifice walls onto the flow, inaccurate flow rate measurement as well as the vortex shedding behind the orifice.
Numerical studies have been performed using the open source CFD code OpenFOAM to investigate the flow through a circular square edged orifice with various thicknesses and diameters inside a pipe. The orifice thickness to pipe diameter ratio (t) varies between 0.125 ≤ t ≤ 2 and the orifice diameter to pipe diameter (�) varies between 0.25 ≤ � ≤ 0.75. The discharge coefficients for the different orifice characteristics at different Reynolds numbers have been determined. Simulations are performed in two different regimes, a laminar flow regime at 0 ≤ �� ≤ 400 and a turbulent flow regime at 2 500 ≤ �� ≤ 40 000. The effects of different ��, � and � on the flow characteristics are discussed by presenting the streamwise velocity contours, streamlines and �-criterion contours.
The results of the present study have been compared with the previous published numerical and experimental results as the validation study. The discharge coefficients are in satisfying agreement with the previous published numerical and experimental data
Numerical Investigations on the Layout Design of Open Cage Fish Farm
Full text linkAquaculture, one of the fastest-growing food production method, has become Norway's second-largest export industry in the past decades. In Norway, the high-value fish like Atlantic Salmon are usually raised in multi-cage fish farms. In the design of these fish farms, the unknown flow velocities within the farm make it difficult to estimate the drag forces on individual cages, thereafter the loads on mooring system. Additionally, as fish farms move to open seas for better environments, fish cages face increased loads from larger waves and faster currents, leading to higher tension in mooring lines and reduced cultivation volume. Addressing these challenges is crucial for optimizing aquaculture system design.
This thesis mainly focuses on the two key issues: the environmental load on downstream fish cages and the structural responses of two different layout designs of fish farms.
The first part of the thesis investigates wake effects on drag forces on fish cages using CFD and a porous medium model. Different cage placements and solidities are considered in the investigation. One interesting finding is that downstream fish cages can experience higher drag forces than upstream cages, particularly when positioned between α = 30° and α = 70°. Additionally, higher solidity will increase drag forces on fish cages. These findings can improve the understanding of the wake effects inside of multi-cage fish farms.
The second part of this thesis presents investigates the fluid-structure interaction in fish farms using a coupling algorithm that combines OpenFOAM and Code_Aster. This part mainly focuses on the structural responses and flow fields of a 2 × 3 Array layout and a new Honeycomb layout design. The study compares flow characteristics, anchor line tensions, drag forces, and cultivation volumes under different flow angles. Results suggest that the Honeycomb layout offers a smaller covered area in the sea, reduced environmental loads, and improved operational efficiency, making it an advantageous approach for fish farming
CFD Simulations of Vortex-Induced Vibrations of a Subsea Pipeline Near a Horizontal Plane Wall
Full text linkMaster's thesis in Offshore TechnologySubsea pipelines, when exposed to free spans, can experience vortex-induced vibrations. This phenomenon was described as a resonance condition occurring when the vortex shedding frequency and the natural frequency of a structure approach common oscillation frequency. Fatigue life of the pipeline can be adversely affected by a high amplitude oscillations attributed to the vortex-induced vibrations. A numerical study has been performed on the effects of wall proximity on the vortex shedding of an elastically mounted circular cylinder. In addition, the study was extended to investigate the influence of a second cylinder with a smaller diameter rigidly coupled with the large cylinder. Such configurations can be regarded as a model of a subsea pipeline or, in case of coupled cylinders, a subsea piggyback pipeline in a free span situation. A series of two dimensional, numerical studies using open source CFD code OPENFOAM has been performed. Simulations were performed in two flow regimes, a laminar vortex street regime at Reynolds number Re = 200 and an upper transition regime at Re = 3.6×10e6. A range of reduced velocities covering a frequency lock-in phenomenon was investigated. Hydrodynamic forces and response amplitudes were mapped with respect to the reduced velocity. Furthermore, a study of the phase differences between the hydrodynamic forces and cylinder displacements was conducted. The motions of the cylinder were recorded and presented on the trajectory plots. The frequency components of hydrodynamic forces and displacements were analyzed with the FFT algorithm in the frequency domain. In order to gain insight into the effects of the shear layers interaction in the area around the oscillating cylinder, flow visualizations of the numerical simulations were analyzed
Numerical Simulations of Turbulent Flow through Piping Systems
Full text linkPiping systems have a wide range of applications across many different industries. Pipelines are essential infrastructure in the offshore oil & gas installations. One of the key challenges when operating complicated piping systems is accurate measurement of the flow characteristics required for example to optimize the process or maintain the flow assurance. The pipe fittings such as elbows, bends, and reducers introduce disturbance in the flow and distort the velocity profile downstream in the pipe which adversely affects the accuracy of flowmeter devices. To counteract these undesired effects, various types of flow conditioners can be installed between the source of flow disturbance and the measuring device. In this thesis, the behavior of turbulent flow passing through three different components of the piping systems is investigated: an orifice plate, a 90-degree pipe bend and thereafter a honeycomb straightener. The Reynolds numbers in present analyses range from 1×104 to ≤ 2×105. First, a validation study of eight different Reynolds-Averaged Navier–Stokes (RANS) turbulence models is performed to choose the model that gives the best prediction of the fluid flow through an orifice flowmeter. Among the benchmarked models, the Explicit Algebraic Reynolds Stress Model (EARSM) shows the best agreement with the experimental validation data. In the second part, the validated turbulence model is used to study the effects of different pipe bend geometries and Reynolds number (Re) on the flow behavior. Based on further numerical simulations of the flow through the pipe bend, it is found that for small curvature radiuses (Rc/D<\ 2), the velocity profiles are highly deformed and separation bubble develops behind the bend. Finally, the influence of a new honeycomb straightener design on the flow quality is investigated downstream of the 90-degree pipe bend. The case of pipe bend with Rc/D=2 is employed to that end. It is concluded that the optimum effectiveness in eliminating the swirl and improving the velocity profile pattern is reached for the honeycomb straightener located at the minimum distance from the bend outlet of Lb=5 and thickness of t=0.5. Furthermore, a detail description of the investigated flow fields are presented in terms of axial velocity profiles, turbulence intensity, velocity perturbation, pressure, vortex formations, secondary flow regions, streamline patterns and swirl intensity
Numerical Simulation of Flow around Subsea Covers at High Reynolds Numbers
Full text linkMaster's thesis in Engineering Structures and MaterialsGlass Reinforced Plastic (GRP) covers are widely used to protect the subsea equipment from external damage. In this work, numerical simulations of a fully developed turbulent boundary layer flow over square and trapezoidal wall-mounted GRP covers are performed using a open source tool: OpenFOAM. The aim of this master thesis is to evaluate the effect on flow characteristics from the shape of the structure, the boundary layer thickness and the Reynolds number. The trapezoidal obstacles with different slope angles are under investigation. Hydrodynamic quantities of turbulent boundary layer flow with various boundary layer thicknesses (���/���=0.73, 1.96 and 2.52) are examined. The structures are subjected into turbulent flows at Reynolds numbers of 0.5 × 106, 1 × 106 and 2 × 106 according to the free stream velocity and dimension of structures.
Three classes of mesh sets were conducted in OpenFOAM supported mesh tool, GMSH. Two- dimensional Reynolds-averaged Navier-Stokes (RANS) equations are solved by ��� − ��� Shear Stress Transport ( ��� − ��� SST) turbulence model. The square cylinder geometry with characteristic length D has been investigated to validate the capability of RANS ��� − ��� SST model. The obtained results in present study show good agreements with previously published experiment data. The results of the streamlines, pressure and velocity distributions were also analyzed for different geometries
Numerical Study on Offshore Lifting Operations of a Subsea Spool
Full text linkMaster's thesis in Offshore technology: Marine and Subsea TechnologyWithin the scope of marine operations, offshore lifting operations present significant challenges due to the instability and harshness of environmental conditions. Therefore, lifting operations become expensive and risky. Lifting operations call for an early planning in order to assess operative procedures, carry out logistical tasks and to determine workable weather windows. In order to contribute to the solution, accurate numerical models and methods have become essential tools for predicting the response of the lifting systems.
This thesis addresses a numerical study on offshore lifting operations of a subsea spool. The two operations studied within the subsea lift process are: the lift-off of the spool from the deck of a transportation barge (the lift-off phase), and the lowering of the spool through the wave zone (the lowering phase). Moreover, accurate numerical models of the lifting systems were developed. The process also included the hydrodynamic analysis of the construction vessel and the transportation barge supporting the operations.
The use of numerical models and methods, along with the conduction of time-domain analyses, bring relevant benefits and advantages to the assessment of allowable sea states. In this thesis work, a systematic methodology was followed for the assessment of the allowable sea states governing the lifting operations. Firstly, the corresponding critical events and limiting parameters were defined. The calculation of the characteristic responses was then carried out by means of time-domain simulations. Finally, the allowable sea states were obtained by comparing the characteristic responses and the corresponding allowable limits.
Particularly, the assessment of the allowable sea states was a comprehensive task in this thesis. The process in itself required the statistical uncertainty to be reduced, which demanded numerous time-domain simulations. In addition, a methodology was suggested to filter seeds that present a suitable scenario for the lift-off phase. The assessment delivered that the allowable sea states obtained for the lift-off phase are lower than those for the lowering phase.
Since the operability of marine operations plays an important role during the planning phase, an operability analysis was also conducted in this thesis. The methodology principally considered weather window analysis and two methods devised for the installation of a given number of spools. One of the differences between the methods resides in the allowable sea states that govern the lift-off of the spool. In the first installation method, the entire subsea lift process is governed by the allowable sea states obtained for the lowering through the wave zone. In the second method, the lift-off occurs from the deck of a transportation barge and thus the respective allowable sea states apply.
The main objective of the operability analysis was to determine which of the methods, in terms of time, provides the most efficient option for the installation of a given number of spools. It was observed that low allowable sea states governing even one single activity may virtually make a marine operation unfeasible.
Lastly, a sensitivity study to the navigation time of the construction vessel was conducted. The outcome suggests that this parameter may become decisive in determining whether transportation barges should be integrated in order to reduce the overall installation time
Finite Element Models for Integrity Assessment of Flexible Riser with Damaged Tensile Wires
No full textMaster's thesis in Offshore structural engineeringMany of the flexible risers in service do not meet their documented service life. A common failure mode is the rupture of the abrasive external layer which, in itself, is not critical, but seawater ingress may corrode tensile wires over time and, in worst case, cause breakage. Damaged tensile armour could significantly reduce the load capacity of the riser. Integrity assessment of flexible riser with damaged tensile armour based on stress concentration factors (SCF) could predict the remaining fatigue life of the structure. SCF could be based on experimental tests, but they are expensive. Finite element analyses can be used instead at a much lower cost.
In this thesis, three finite element models are created in Abaqus to replicate a 2.5 inch flexible riser. In the first model, the tensile wires are represented by beam elements and the other layers by shell elements. In the second and third model, the tensile wires are represented by solid elements and the other layers are represented by shell and solid elements, respectively. However, only the two last models are used in the analyses. To reduce simulation time, the carcass and pressure armour are modelled in a simplified manner with equivalent material properties.
A convergence study is performed to find the minimum element size needed to yield good results. The equivalent material properties used in the simplified layers are evaluated by comparing the hoop stress to analytical values which are in good agreement. Two load cases are considered in the stress analyses, where one is tension and the other is bending. The models are verified by comparing the axial stiffness to the physical one and axial wire stress due to bending to analytical values found in the literature. Both are in good agreement.
A simulation is performed for the second model with two ruptured wires in the outer tensile layer. A tress concentration factor is established based on the current stress results for the mean axial wire stress. The stress concentration factor is compared to results in the literature and show good agreement
Analysis of ROV thrusters and small marine propellers at specific rotational speeds
Full text linkMaster's thesis in Offshore Technology: Marine and Subsea TechnologyThrusters are vital for the functionalities of remotely operated vehicles (ROVs). The development of thruster design is a trade-off between cost, thrust force, physical weight and size. Furthermore, it is known that problems with thrusters are a common fault in ROVs. As a result, this study is performed on different thruster configurations to highlight important aspects in the design of thruster systems. This includes the analysis of different marine propellers with 4 to 6 blades and with a diameter of 300 to 400 mm. A six degree of freedom model is created in OpenModelica to investigate vibrations and bearing responses in thruster systems. The model consists of a marine propeller, a shaft, and two bearings, and is applicable for simulating various steady-state cases. The results of the simulations return displacements in the axial, horizontal, and vertical directions, which are further used to investigate vibration amplitudes and bearing life. The marine propeller contributions to the OpenModelica model are based on propellers from the Wageningen B-screw propeller series and open water tests of this series. The hydrodynamic added mass and damping elements are calculated from different sets of regression equations depending on the number of blades on the propeller, blade area ratio, and pitch ratio. Meanwhile, the mean thrust and torque are obtained from open water test data of the relevant propeller. The mean thrust and torque are then further used to calculate the dynamic forces and moments from the marine propeller.
The bearing life of the bearings in the thruster is highly dependent on the axial load acting on the bearing, i.e., the thrust force. Moreover, if the propeller is not balanced then high centrifugal forces can occur, resulting in severe forces in the radial direction that can be of concern regarding the bearing life. Furthermore, the thruster and bearing design should be related to the maximum thrust force desired from the thruster. It is possible to use different propellers with the same design, to change the RPM-thrust force configuration, or to change the vibratory properties of the thruster system. However, the maximum thrust force for which the original thruster was designed should not be exceeded. Furthermore, the vibrations in the thruster system depend on the bearing configuration, stiffness, unbalance, and the propeller type
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