SINTEF Ocean Research Articles

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

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.

Performance of a Cable-Driven Robot Used for Cyber–Physical Testing of Floating Wind Turbines

Cyber–physical testing has been applied for a decade in hydrodynamic laboratories to assess the dynamic performance of floating wind turbines (FWTs) in realistic wind and wave conditions. Aerodynamic loads, computed by a numerical simulator fed with model test measurements, are applied in real time on the physical model using actuators. The present paper proposes a set of short and targeted benchmark tests that aim to quantify the performance of actuators used in cyber–physical FWT testing. They aim at ensuring good load tracking over all frequencies of interest and satisfactory disturbance rejection for large motions to provide a realistic test setup. These benchmark tests are exemplified on two radically different 15 MW FWT models tested at SINTEF Ocean using a cable-driven robot.

Extensive hydro-elastic floating bridge tests: Planning, design, implementation, and numerical comparison

Extensive hydrodynamic and hydro-elastic tests for a long floating bridge have been carried out in the ocean basin of SINTEF Ocean for the Bjørnafjord floating bridge project. The complexity of the floating bridge plus cable-stayed bridge, in addition to its 52.58 m model length, mark this test campaign a major undertaking in ocean basin model testing. The design, execution and analysis of the test program has been an iterative process over 5 years. The tests provide data for validation of numerical analysis, study of hydrodynamics phenomenon that cannot be well simulated by state-of-the-art engineering tools, and verification of the design under controlled environmental loads. Wave-current-structure interaction, hydrodynamic interaction between pontoons, and their impacts on hydro-elastic global responses of floating bridge (with cable stayed bridge) are the main scope of tests. Oscillation tests under different Keulegan–Carpenter (KC) numbers were also carried out for viscous drag coefficient of a pontoon in defined direction. Correlation of test results with numerical analysis is an important part of the work scope, which provides solid basis for a robust design with reduced risk. This article tries to provide an overview of the background, design, execution and correlation work of this extensive tests. Selected examples of correlation work are also provided in the article. The tests have validated the analysis method and software, provided important input to the next phase design, and enhanced the structural reliability of the bridge concept in continuous evolution.

Preface

The 12th International Workshop on Ship and Marine Hydrodynamics 28 August to 1 September 2023 Aalto University, Espoo, Finland Editors Eur.Ing. Dr. Spyros Hirdaris CEng FRINA, FSNAME Aalto University, Finland AND Professor Decheng Wan Shanghai Jiao Tong University, China The 12th International Workshop on Ship and Marine Hydrodynamics (IWSH 2023) was held from the 28th of August to the 1st of September 2023 at Aalto University, Finland. Aalto University was established in 2010 as a merger the Helsinki University of Technology (HUT), the Helsinki School of Economics and the University of Art and Design of Helsinki. Marine and Arctic Technology has been solid part of the well cherished traditions of HUT dating back to the 19th century.IWSH inaugurated by Prof. Zaojian Zou of Wuhan University of Technology in 1999. Early national events were organised by Wuhan University of Technology (2001,2003), Shanghai Jiao Tong University (2005), Jiangshu University of Science and Technology (2007) and Harbin Engineering University (2009).In 2011 Professor Decheng Wan organised the first international version of the workshop at Shanghai Jiao Tong University. Since then, the event has been organised by Seoul National University - South Korea (2013), the University of Strathclyde - UK (2015), the National Taiwan Ocean University (2017) and Hamburg University of Technology - Germany (2019). The 2021 edition of IWSH did not take place because of the COVID-19 pandemic. The “all-inclusive” nature of the 12th sequel of the workshop held in 2023 included scientific and social activities, fostering opportunities for networking.The workshop focused on publications of relevance to ship dynamics, hydrodynamics, wave mechanics, sea loads and responses, fluid structure interactions, ship resistance and propulsion, etc. Notably, emerging multiphysics methods with application on both ships and ocean engineering including renewable energies were evident. Six keynotes were delivered by Dr. Nuno Fonseca of SINTEF (Norway), Dr. Sime Malenica of Bureau Veritas (BV, France), Alessandro Iafrati of Istituto di iNgegneria del Mare (INM, Italy), Dr. Michel Visonneau of Centre National de la Recherche Scientifique (CNRS, France), Professor Stefano Brizzolara of Virginia Technological Institute and State University (USA) and Professor Yonghwan Kim of Seoul National University (South Korea), were invited to foster top level discussion on relevant subjects. As part of the workshop proceedings the main event sponsor, SINTEF Ocean (Norway) hosted a special session of their International Collaboration Network in Marine Technology.The oral technical program was organized in parallel sessions. In these proceedings, the papers are organized by major themes. The themes are: (1) Ship dynamics, (2) Wave mechanics, (3) Fluid Structure Interactions, (4) Ship motions and sea loads, (5) Ocean Engineering, (6) Ship resistance, (7) Marine propulsion.The workshop confirmed that the complexity of the subject as related to conventional and emerging technologies attracts a great deal of interest of relevance to engineering science and technological innovation. The field of ship and marine hydrodynamics remains more than relevant for the safety and sustainability of the ship and ocean engineering sectors. Dr. Spyros Hirdaris CEng FRINA FSNAME Aalto University, Espoo, Finland, September 2023 List of Sponsors, Reviewers acknowledgement, Aalto University’s IWSH 2023 local organizing committee, IWSH 2023 International Standing Committee, IWSH 2023 International Scientific Committee are available in this pdf.

A Calibration Study with CFD Methodology for Self-Propulsion Simulations at Ship Scale

This paper summarises the main findings from the full-scale Computational Fluid Dynamics (CFD) analyses conducted at SINTEF Ocean on the case of MV REGAL, which is one of the benchmark vessels studied in the ongoing joint industry project JoRes. The numerical approach is described in detail, and comparative results are presented regarding the propeller open water characteristics, ship towing resistance, and ship self-propulsion performance. The focus of numerical investigations is on the assessment of the existing simulation best practises applied to a ship-scale case in a blind simulation exercise and the performance thereof with different turbulence modelling methods. The results are compared directly with full-scale performance predictions based on model tests conducted at SINTEF Ocean and sea trials data obtained in the JoRes project.

Profiles of Two JOMAE Associate Editors (The Fifth in a Continuing Series)

Profiles of Two JOMAE Associate Editors (The Fifth in a Continuing Series)

Wave Effects on Vortex-Induced Vibrations of a Top-Tensioned Riser

Abstract Offshore drilling risers and many production risers are top-tensioned risers (TTRs), connecting the vessel and seabed via joints. External loads such as currents, waves, and vessel motions introduce cyclic loads and motions on riser sections and associated components, such as universal joint, flex joints, and well head at the bottom, which may shorten the service life due to accumulated fatigue damage. Dynamic responses under combined currents and waves are more complicated than vortex-induced vibrations (VIVs) caused by pure currents, and it is not fully understood. Several model test campaigns on TTR have been carried out at SINTEF Ocean (former MARINTEK) during the past decades. Currents, waves, and vessel motions were modeled, and the riser model responses were measured. In this study, selected cases from such model tests are analyzed, and used to validate a semi-empirical time domain VIV prediction tool—VIVANA-TD. A better understanding of the dynamic responses of TTR under combined currents and waves has been achieved. By comparing the results from numerical simulation using VIVANA-TD and model test measurements, validity and limitation of the time domain tool have been investigated. Important features that need to be considered are discussed. The experience gained from the present study establishes a good basis for VIV and wave load prediction of full-scale TTRs under combined currents and waves where the uncertainty of combined wave and VIV prediction is further reduced.

A rational model for maneuvering in irregular waves with the effect of waves on the propeller and rudder inflow taken into account

A modular model for maneuvering in regular and irregular waves is presented. A new model is presented for the y-component of the rudder inflow. Moreover, an efficient time-domain method for estimation of slowly varying drift loads in irregular waves is presented. Simulations of turning circles in irregular waves with the time-domain method, are compared to simulations where the difference-frequency loads, with the Newman approximation, is used to estimate the slowly varying drift loads. These two methods show similar accuracy, but the time-domain method is significantly faster. The numerical simulations are compared with new free running experiments of the Duisburg Test Case (DTC). Based on extensive wave documentation in the Ocean basin at SINTEF Ocean, the uncertainty in maneuvering experiments, due to variation of wave height throughout the basin, is investigated. The uncertainty is not insignificant, but it is less than expected. The sensitivity to number of realizations in the same irregular wave spectrum is investigated, both experimental and numerical. For some responses, up to 20 repetitions are needed before the width of the 95% confidence intervals approach convergence.

Reduction of the wave propagation error of a sigma grid based numerical tank using a vertical spacing based on the constant truncation error

Numerical wave models that solve the Laplace equation for the velocity potential with a σ-grid in the vertical direction have been proven to be flexible and efficient. However, it is observed that the dispersion relation is sensitive to the variation of the vertical σ-grid arrangements. So far, there is no recommended practice to optimise the vertical σ-grid for a correct representation of the dispersion relation. In order to reduce the uncertainty of the dispersion relation and optimise the choice of the vertical grid distribution, a new method based on the constant truncation error of the finite difference scheme is presented in this article. The optimised vertical grids estimated by the novel method are applied in the fully nonlinear potential flow model REEF3D::FNPF to provide simulated wave surface elevations, which are compared with the measured time series in the long wave flume at SINTEF Ocean, Trondheim. Several different wave scenarios are considered, including regular waves, bichromatic waves, trichromatic waves as well as irregular waves. The wavemaker motions in the physical flume are used to generate waves in the numerical wave tank of REEF3D::FNPF. The proposed truncation error method is seen to effectively mitigate the dispersion (phase) error in all scenarios and the simulations show high accuracy in comparison with the experimental data.

Bag and floater motions of a fabric membrane cage

We investigate bag and floater motions of a scaled fabric membrane cage model. Model tests with an assumed model scale of 1:27 were performed at SINTEF Ocean’s towing tank, where the cage model consisted of an enclosing fabric (or a bag), an elastic floater, a mooring system, and an internal water mass. The model was moored in a towing tank and subjected to regular waves with different periods and steepnesses. The first harmonic component of the floater and bag motions time series were analyzed, and image analysis was performed for the latter. Several interesting responses of the fabric membrane cage are discussed: cage set-down, wavelike bag motion, and floater motions. The magnitude of the cage set-down is dependent on the incident wave period and steepness; the magnitude increases in response to a longer and steeper wave. The bag also exhibits a wavelike motion that propagates from the floater to the bottom part of the cage and has apparent implications for the dynamics of the bag bottom and the snap loads on the fabric material. Simplified numerical simulations were performed using WAMIT, which assumed a rigid cage model. A floater motions comparison between model tests and WAMIT results shows the importance of taking the bag and floater flexible modes into account when performing hydrodynamic analysis of a fabric membrane cage.

Experimental measurements of propulsive factors in following and head waves

The results from resistance measurements in calm water and load-varying self-propulsion tests in calm water and regular head and following waves are presented. The experimental campaign is conducted in the large towing tank at SINTEF Ocean (formerly MARINTEK). The openly accessible hull of the single screw Duisburg Test Case is selected as the test case. The wave added resistance, ship motions RAOs, axial wake fraction, thrust deduction fraction, relative rotative efficiency, hull efficiency, propeller open-water efficiency, propeller efficiency behind ship, and propulsive efficiency are determined.Regarding the calculation of the thrust deduction fraction, the results of the experiments show the effect of utilizing the bare hull resistance instead of the linearly extrapolated ship resistance at zero propeller thrust. If the former is applied, the thrust deduction fraction will be dependent on the load of the propeller. If the latter is utilized, the thrust deduction fraction will be independent of the propeller loading. As expected, the wave added resistance is lower in following waves than in head waves. The heave and pitch motions are larger in head waves, whereas the surge motion is higher in following waves. The effective wake fraction is affected by both the propeller loading and the ship motions. In the case of the former, the higher the propeller loading, the lower the effective wake fraction. For the latter, a general decrease in effective wake fraction is noticed in head waves compared to calm water. On the contrary, the effective wake fraction is higher in following waves in comparison to calm water. The thrust deduction fraction computed with the extrapolated ship resistance appears to be slightly affected by the ship motions. However, a final conclusion was not drawn because of the large uncertainties in the measurements of this propulsive coefficient. The variation in propeller open-water efficiency is mainly related to the change in propeller loading. The relative rotative efficiency is barely affected by both the propeller loading and the motions of the ship. Except in the case of very large wave amplitudes, the hull efficiency is hardly influenced by the ship motions. The propulsive efficiency is primarily affected by the change in propeller open-water efficiency.Based on the results of the experimental campaign, overload tests in calm water provide a good estimation of the propulsion efficiency in waves for the selected case vessel.

Forced Vibration Tests for In-Line Vortex-Induced Vibration to Assess Partially Strake-Covered Pipeline Spans

AbstractHelical strakes can suppress vortex-induced vibrations (VIVs) in pipelines spans and risers. Pure in-line (IL) VIV is more of a concern for pipelines than for risers. To make it possible to assess the effectiveness of partial strake coverage for this case, an important gap in the hydrodynamic data for strakes is filled by the reported IL forced-vibration tests. Therein, a strake-covered rigid cylinder undergoes harmonic purely IL motion while subject to a uniform “flow” created by towing the test rig along SINTEF Ocean's towing tank. These tests cover a range of frequencies, and amplitudes of the harmonic motion to generate added-mass and excitation functions are derived from the in-phase and 90 deg out-of-phase components of the hydrodynamic force on the pipe, respectively. Using these excitation- and added-mass functions in VIVANA together with those from experiments on bare pipe by Aronsen (2007 “An Experimental Investigation of In-Line and Combined In-Line and Cross-Flow Vortex Induced Vibrations,” Ph.D. thesis, Norwegian University of Science and Technology, Trondheim, Norway.), the IL VIV response of partially strake-covered pipeline spans is calculated. It is found that as little as 10% strake coverage at the optimal location effectively suppresses pure IL VIV.

The fate of hydrocarbon leaks from plugged and abandoned wells by means of natural seepages

When a well reaches the end of its productive life, it is permanently plugged and abandoned. Even though new technology and verification methods are being evaluated, it is evident that operational, barrier material and qualification challenges together contribute to risk of leaks from abandoned wells or wells to be abandoned in future. Most regulatory authorities constitute zero leak acceptance criteria to protect the environment; however, natural hydrocarbon seepages are occurring all over the world on a daily basis. In this study, we conducted a theoretical comparison between leaking wells and natural seeps and suggest conducting a fate analysis is appropriate to provide necessary data for evaluating environmental implications of leaking wells. Two case studies were analyzed using SINTEF Ocean's OSCAR (Oil Spill Contingency And Response) software; one historical gas leak (Field A) and a theoretical oil leak (Field B). It is found that for releases of natural gas, 95–99% dissolve in the ocean, and the fraction of gas reaching the atmosphere is dependent on the initial gas bubble size. Fate of oil is more complex than gas, but evaporation, sedimentation and biodegradation are the main contributing mechanisms in the fate of hydrocarbon analysis.

Coupled vessel and moonpool responses in regular and irregular waves

Hydrodynamic coupling between the ship and moonpool responses is investigated. Dedicated experiments are carried out in the Ocean Basin at Sintef Ocean. Three different moonpool sizes are studied, where the moonpool length is 1/20, 1/10 and 1/2 of the ship length. The two former are square moonpools, while the latter is a rectangular moonpool with a width 1/2 of the ship’s beam. Numerical simulations are carried out using WAMIT. The coupling between the ship and moonpool is seen to be highly dependent on the volume of the moonpool relative to the submerged ship volume. WAMIT greatly over-predicts the moonpool responses in the proximity of resonance, which indicates that wave radiation damping is small, and that damping due to flow separation at sharp edges at the moonpool entrance is dominant. Two main nonlinear effects are observed; Swirling-type sloshing and secondary resonance. These nonlinear effects are excited in a range of wave periods where it is expected that a realistic sea environment will contain significant wave energy. This is most prominent in the largest moonpool. The piston mode shape in freely floating conditions is signficantly different relative to the one in forced heave.

Time Domain Modeling of Propeller Forces due to Ventilation in Static and Dynamic Conditions

This paper presents experimental and theoretical studies on the dynamic effect on the propeller loading due to ventilation by using a simulation model that generates a time domain solution for propeller forces in varying operational conditions. For ventilation modeling, the simulation model applies a formula based on the idea that the change in lift coefficient due to ventilation computes the change in the thrust coefficient. It is discussed how dynamic effects, like hysteresis effects and blade frequency dynamics, can be included in the simulation model. Simulation model validation was completed by comparison with CFD (computational fluid dynamics) calculations and model experiments. Experiments were performed for static and dynamic (heave motion) conditions in the large towing tank at the SINTEF Ocean in Trondheim and in the Marine Cybernetics Laboratories at NTNU (Norwegian University of Science and Technology). The main focus of this paper is to explain and validate the prediction model for thrust loss due to ventilation and out of water effects in static and dynamic heave conditions.

A Comparison of Physical and Numerical Modeling of Homogenous Isotropic Propeller Blades

Results of the fluid-structure co-simulations that were carried out as part of the FleksProp project are presented. The FleksProp project aims to establish better design procedures that take into account the hydroelastic behavior of marine propellers and thrusters. Part of the project is devoted to establishing good validation cases for fluid-structure interaction (FSI) simulations. More specifically, this paper describes the comparison of the numerical computations carried out on three propeller designs that were produced in both a metal and resin variant. The metal version could practically be considered rigid in model scale, while the resin variant would show measurable deformations. Both variants were then tested in open water condition at SINTEF Ocean’s towing tank. The tests were carried out at different propeller rotational speeds, advance coefficients, and pitch settings. The computations were carried out using the commercial software STAR-CCM+ and Abaqus. This paper describes briefly the experimental setup and focuses on the numerical setup and the discussion of the results. The simulations agreed well with the experiments; hence, the computational approach has been validated.

Testing of a new transport and installation method for offshore wind turbines

Transport and installation of offshore wind turbines require considerable efforts and can be time consuming, in particular when jackups are used for lifting operations. This is mainly due to the specific weather window requirements for towing and locating the jackups. A goal is ultimately to finalize all turbine assembly work onshore and at quayside and to transport the completed turbine to the site for easy installation. We suggest that this can be achieved by a transport and installation method we denote “the MINT method” whereby the wind turbine foundation is located on a barge while the tower is resting on a support system, a “MINT” system that is located at the stern of a mid-size standard offshore support vessel. The MINT is specifically designed for the transport and installation operations allowing relative motions and rotations between the tower and the transport vessel. In order to qualify the “MINT installation method”, a numerical study of the feasibility of the transport and installation was carried out by Sintef Ocean using their SIMO software package [1]. The method was found feasible in favourable weather conditions. To further document feasibility, it was decided to carry out a proof of concept test in a wave tank to identify concerns with respect to the suggested method for transport and installation. This paper reports the findings of this test. The test is to be considered to represent an initial activity in the concept qualification process, see [2], and is not a full wave tank test carried out to measure stresses and strains for member sizing. The objective of the paper is therefore, to show how an initial wave tank test can be useful for concept qualification.

Preface

This Special Issue presents peer-reviewed papers presented at the International Maritime and Port Technology and Development Conference (MTEC), and the International Conference on Maritime Autonomous Surface Ships (ICMASS), held jointly as MTEC/ICMASS 2019 in Trondheim, Norway, 13th - 14th November 2019. The International Maritime and Port Technology and Development Conference (MTEC) is arranged every second year and alternating between Singapore and Norway. The first International Conference on Maritime Autonomous Surface Ships (ICMASS) was arranged in 2018 in Busan, Korea.MTEC/ICMASS 2019 was organized by SINTEF Ocean and Department of Marine Technology, NTNU in Norway, and co-organized by Maritime and Port Authority of Singapore (MPA), Singapore Maritime Institute (SMI), and Maritime Singapore. The International Network for Autonomous Ships (INAS) co-arranged the track on autonomous transport systems. The conference was supported by the Research Council of Norway.List of 1-Maritime Digitalisation, 2-Autonomous Systems and Vessels, 3-Green Shipping, MTEC/ICMASS 2019 Editorial Board, MTEC/ICMASS 2019 Scientific Committee, MTEC/ICMASS 2019 Local Organizing Committee are available in this pdf.

Multiple-degree-of-freedom actuation of rotor loads in model testing of floating wind turbines using cable-driven parallel robots

Model testing of offshore structures in ocean basins has been accepted as a necessary step for the validation and calibration of numerical models, as well as for final design checks in extreme environments. While offshore wind power makes no exception, model testing has not shown its full potential due to inherent modeling challenges. Generating highly-controlled wind fields in ocean basins and alleviating aerodynamic modeling errors due to Reynolds number mismatch in Froude-based scaling are prominent examples. To circumvent these issues, the concept of Real-Time Hybrid Model (ReaTHM) testing has been suggested by SINTEF Ocean and NTNU, Norway. Here the wind loads are no longer physically modeled but computed from online-measured motions and a numerical wind field. They are then actuated in real time on the scale model by means of actuators, while subjected to -physical-wave and current loads.This paper aims at presenting design considerations regarding the choice of the actuator(s) and its/their interface with the scale model. The pros and cons of the chosen solution, namely cable-driven parallel robots using industrial servo drives, are presented. The focus is then directed toward the mapping between wind loads to be actuated and tension commands on each cable, called tension allocation. Two layouts corresponding to two ReaTHM testing campaigns performed in SINTEF Ocean’s ocean basin are then compared on various aspects, with emphasis on tension allocation.In addition to proving the feasibility of the chosen technical solution, results show a tradeoff between flexibility (with respect among others to wind direction) and usability on other structures versus minimization of cable tensions. The latter aspect is treated in detail, using theory first adapted from literature on cable-driven parallel robots, then illustrated through relevant examples for the current application.

Experimental validation of a time-domain approach for determining sectional loads in a floating wind turbine hull subjected to moderate waves

To achieve cost-effective and reliable structural design of floating wind turbines, efficient and accurate time domain numerical approaches are required to analyse structural responses in design conditions, e.g. wind and waves. This paper focuses on validation of a time-domain numerical approach for determining forces and moments in structural components of floaters. The approach considers floating wind turbines as a system of several structural components, e.g. blades, rotational shaft, nacelle, tower, mooring lines, columns, pontoons and braces. A finite element model is developed to represent global stiffness of the structural components. The external and inertia loads on the structural components are modelled as distributed loads. Hydrodynamic loads on each structural component are derived from the corresponding hydrodynamic coefficients obtained by solving the first order boundary value problem using WAMIT. A 1:30 scaled braceless semi-submersible model test which implements the ReaTHM® testing approach was done by SINTEF Ocean, formerly MARINTEK, in its ocean basin. Measurements of the global forces and moments at the base of a side column of the model and rigid-body motions of the model are compared to the corresponding simulations. This paper focuses on responses in moderate waves for which linear hydrodynamic loads are applicable. Differences in the corresponding simulations and measurements are found to be small, while possible reasons, e.g. synchronizations, non-linear effects, and uncertainties in the measurements and simulations, for the differences are analysed. Essential information about the model test, descriptions of the numerical models, calibrations and results and discussions of the validation are given in this paper.