Recent Development of the Empirical Basis for Prediction of Vortex Induced Vibrations

This paper describes the research activity related to VIV that has taken place at NTNU and MARINTEK in Trondheim during the last years. The overall aim of the work has been increased understanding of the VIV phenomenon and to improve the empirical basis for prediction of VIV. The work has included experiments with flexible beams in sheared and uniform flow and forced motions of short, rigid cylinders. Key results in terms of hydrodynamic coefficients and analysis procedures have been implemented in the computer program VIVANA, which has resulted in new analysis options and improved hydrodynamic coefficients. Some examples of results are presented, but the main focus of the paper is to give an overview of the work and point out how the new results can be used in order to improve VIV analyses

Comprehensive Riser VIV Model Tests in Uniform and Sheared Flow

Despite of considerable research activity during the last decades considerable uncertainties still remain in prediction of Vortex Induced Vibrations (VIV) of risers. Model tests of risers subjected to current have been shown to be a useful method for investigation of the VIV behavior of risers with and without suppression devices. In order to get further insight on VIV of risers, an extensive hydrodynamic test program of riser models subjected to vortex-induced vibrations was undertaken during the winter 2010 by Shell Oil Company. The VIV-model test campaign was performed in the MARINTEK Offshore Basin Laboratory. A new test rig was constructed and showed to give good test conditions. Three different 38m long riser models were towed horizontally at different speeds, simulating uniform and linearly varying sheared current. Measurements were made In-Line (IL) and Cross-Flow (CF) of micro bending strains and accelerations along the risers. The test program compromised about 400 tests, which give a rich test material for further studies. In the present paper the test set-up and program are presented and selected results are reported

Dimensjonering av vegoverbygninger: Sammenligning mellom den empiriske og mekanistiskempiriske dimensjoneringsmetoden

This master’s thesis focuses on different methods for designing pavement structures, mainly the empirical and the mechanistic-empirical methods. The first one is used in Norway, and the design is done by following a handbook with empirical data. The other one is up for consideration in Norway and uses, together with empirical data, mechanistic data regarding the properties of the different layers measured by a falling weight deflectometer (FWD). The thesis aims to analyze if the mechanistic-empirical method will reduce the total thickness of the pavement structure compared to the empirical method. Therefore, the research question is as follows: How does the thickness of the pavement structure change when using the mechanistic-empirical method compared to the empirical method? Different methods were used to solve this question. Handbook N200 was used for the empirical design, while PMS Objekt was used for the mechanistic-empirical design. To compare the results between N200 and PMS Objekt, with backcalculated E-moduli. FWD-data was also imported into the program. The FWD-data are backcalculated in Elmod. E6 Soknedal and E39 Lønset-Hjelset were used as case-projects for the design of the pavement structures. When the mechanistic-empirical method is used at E6 Soknedal, the thickness is reduced by 30 cm compared to the empirical method. For E39 Lønset-Hjelset, where the frost depth is almost one meter less than in Soknedal, the thickness is raised by 10 cm. This is because the FWD-data used for this calculation is done on an old pavement structure. That causes low E-modules from the backcalculation for the different layers, resulting in increased layer thicknesses for compensating the reduced moduli. Therefore, as long as it is ensured that a valid E-moduli is used, the mechanisticempirical method will reduce the thickness of the pavement in most cases. In addition to that, the mechanistic-empirical method also assures that the pavement structure is designed so it would last its designated lifetime

Experimental Investigation of Towing of a Semi-submersible Floating Offshore Wind Turbine

Journal of Physics: Conference Series Purpose-Led Publishing, find out more. Paper • The following article isOpen access Experimental Investigation of Towing of a Semi-submersible Floating Offshore Wind Turbine Decao Yin, Robert Indergård, Halvor Lie and Henning Braaten Published under licence by IOP Publishing Ltd Journal of Physics: Conference Series, Volume 2875, EERA DeepWind Conference 2024 17/01/2024 - 19/01/2024 Trondheim, Norway Citation Decao Yin et al 2024 J. Phys.: Conf. Ser. 2875 012026 DOI 10.1088/1742-6596/2875/1/012026 DownloadArticle PDF Authors References Share this article Article information Abstract The offshore floating wind energy sector is poised for exponential growth in the coming decades. A pivotal challenge within this context is the imperative to substantially decrease the levelized cost of energy (LCOE) to enhance competitiveness. A significant proportion of these LCOE costs are attributable to the towing operations entailed in both the installation of assembled floating wind turbines and the maintenance of major components. Semi-submersibles, characterized by multiple columns and pontoons, represent the prevailing conceptual approach in the industry due to their versatility across varying water depths and their cost-effectiveness in terms of mooring and transportation. Towing model tests for the semi-submersible INO WINDMOOR floating wind turbine have been conducted in the Ocean Basin at SINTEF Ocean. The primary objective of these tests is to investigate critical aspects such as towing resistance and dynamic responses, including vortex-induced motion (VIM) and galloping. This paper presents the experimental set-up and initial findings from the model tests. Through comparative data analysis of selected cases, an enhanced and insightful understanding of the intricate fuid-structure interactions is expected to be achieved. Additionally, it highlights the identification of knowledge gaps and research imperatives aimed at expanding the towing operational criteria, with the ultimate goal of reduction of installation and maintenance costs.publishedVersio

An implicit local time-stepping method based on cell reordering for multiphase flow in porous media

We discuss how to introduce local time-step refinements in a sequential implicit method for multiphase flow in porous media. Our approach relies heavily on causality-based optimal ordering, which implies that cells can be ordered according to total fluxes after the pressure field has been computed, leaving the transport problem as a sequence of ordinary differential equations, which can be solved cell-by-cell or block-by-block. The method is suitable for arbitrary local time steps and grids, is mass-conservative, and reduces to the standard implicit upwind finite-volume method in the case of equal time steps in adjacent cells. The method is validated by a series of numerical simulations. We discuss various strategies for selecting local time steps and demonstrate the efficiency of the method and several of these strategies by through a series of numerical examples.publishedVersio

Dominant parameters for vortex-induced vibration of a steel catenary riser under vessel motion

Recent research has confirmed a new type of vortex-induced vibration (VIV) in steel catenary risers (SCRs), purely caused by vessel motion. Vessel motion-induced VIV occurs because the SCR is exposed to the equivalent oscillating current due to its own motions relative to the still water. Preliminary results indicate that vessel motion-induced VIV is quite different from ocean current-induced VIV and is characterized with distinct timevarying features. In the present study, we aim at further summarizing the dominant parameters that govern the general vessel motion-induced VIV responses. Throughout the comparative studies on the instantaneous and statistical VIV responses including strain, displacement, response frequency, fatigue damage and top tension variation, the maximum Keulegan-Carpenter number KCmax and the maximum equivalent current velocity Vn_max are found to be the two dominant parameters that govern the vessel motion-induced VIV responses. Generally speaking, when KCmax is sufficiently large (larger than 39 according to the present study), the general vessel motion-induced VIV response is dominated by Vn max. However, when KCmax is small, the VIV response is less time-varying and shows strong correlation with both KCmax and the local KC number distribution along the SCR. Vessel motion-induced VIV response frequency models are also reviewed and discussed considering different KCmax and Vn max ranges. Hopefully, these results can provide some general guidelines for future vessel motion-induced VIV prediction and for industrial references.acceptedVersion© 2017. This is the authors’ accepted and refereed manuscript to the article. LOCKED until 21.3.2019 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0

Nonlinear domain-decomposition preconditioning for robust and efficient field-scale simulation of subsurface flow

We discuss a nonlinear domain-decomposition preconditioning method for fully implicit simulations of multicomponent porous media flow based on the additive Schwarz preconditioned exact Newton method (ASPEN). The method efficiently accelerates nonlinear convergence by resolving unbalanced nonlinearities in a local stage and long-range interactions in a global stage. ASPEN can improve robustness and significantly reduce the number of global iterations compared with standard Newton, but extra work introduced in the local steps makes each global iteration more expensive. We discuss implementation aspects for the local and global stages. We show how the global-stage Jacobian can be transformed to the same form as the fully implicit system, so that one can use standard linear preconditioners and solvers. We compare the computational performance of ASPEN to standard Newton on a series of test cases, ranging from conceptual cases with simplified geometry or flow physics to cases representative of real assets. Our overall conclusion is that ASPEN is outperformed by Newton when this method works well and converges in a few iterations. On the other hand, ASPEN avoids time-step cuts and has significantly lower runtimes in time steps where Newton struggles. A good approach to computational speedup is therefore to adaptively switch between Newton and ASPEN throughout a simulation. A few examples of switching strategies are outlined.publishedVersio

Forced surge motion of a floating bridge pontoon to evaluate the hydrodynamic damping properties

Dedicated model tests were carried out to investigate the hydrodynamic force coefficients in surge for a pontoon that is designed to support a floating bridge. The experiments were carried out by oscillating the pontoon at various Keulegan–Carpenter (KC) numbers and oscillation periods in forced motion. A pontoon designed for the floating bridge in Bjørnarfjorden, Norway, is investigated herein. Several numerical simulations are performed to supplement the experiments. The estimated damping coefficients in surge are of significant importance at the first horizontal resonant mode of the floating bridge, which may be excited due to swell sea in the fjord. The first horizontal resonant mode of the floating bridge is around 15–16 s. The model tests were conducted with and without turbulence triggers. A finding from the model tests were that turbulence triggers did not prove to be necessary to eliminate model scale effects in oscillatory flow. The KC-dependent added mass and damping coefficient in oscillatory flow is investigated. The added mass and damping coefficients are clearly KC-dependent. The wave radiation damping in surge is small at the tested oscillation periods, and the damping in surge is dominated by the damping due to flow separation. Numerical simulations using a hybrid potential flow-CFD solver with linear free-surface conditions, PVC3D, are carried out with both linear and nonlinear body-boundary conditions. The simulations with nonlinear body-boundary conditions predict the hydrodynamic force coefficients with reasonable accuracy, whilst the simulations with linear body-boundary conditions fail to predict the hydrodynamic coefficients with the desired accuracy — both used to investigate the viscous damping which arises due to flow separation along the pontoon body.publishedVersio

Analysis of Full-scale Riser Responses in Field Conditions Based on Gaussian Mixture Model

Offshore slender marine structures experience complex and combined load conditions from waves, current and vessel motions that may result in both wave frequency and vortex shedding response patterns. Field measurements often consist of records of environmental conditions and riser responses, typically with 30-minute intervals. These data can be represented in a high-dimensional parameter space. However, it is difficult to visualize and understand the structural responses, as they are affected by many of these parameters. It becomes easier to identify trends and key parameters if the measurements with the same characteristics can be grouped together. Cluster analysis is an unsupervised learning method, which groups the data based on their relative distance, density of the data space, intervals, or statistical distributions. In the present study, a Gaussian mixture model guided by domain knowledge has been applied to analyze field measurements. Using the 242 measurement events of the Helland-Hansen riser, it is demonstrated that riser responses can be grouped into 12 clusters by the identification of key environmental parameters. This results in an improved understanding of complex structure responses. Furthermore, the cluster results are valuable for evaluating the riser response prediction accuracy.Comment: Matches accepted versio