Finn Gunnar Nielsen

University of Bergen | UiB · Geophysical Institute

Describes how energy is extracted from the wind, the components of a wind turbine, various concepts for use offshore, offshore wind farms and future perspectives for offshore wind energy.

Many countries have plans to expand wind energy to meet CO2 emissions targets. Lack of available land area and the need for good and stable wind conditions have stimulated the development of offshore wind turbines, which allows for the development of larger turbines. The offshore environment, however, involves new challenges related to the design,...

Many countries have plans to expand wind energy to meet CO2 emissions targets. Lack of available land area and the need for good and stable wind conditions have stimulated the development of offshore wind turbines, which allows for the development of larger turbines. The offshore environment, however, involves new challenges related to the design,...

Introduces the concepts of hydrodynamic mass and damping, and shows how the inertia, damping and restoring matrices may be established for a six-degrees-of-freedom floating structure built from vertical columns and horizontal pontoons. Also, the computation of mass and damping matrices and wave loading general body shapes are addressed. How cancell...

Shows how wave loads on fixed bodies can be computed, within a linear framework as well as including viscous effects. The validity of the slender body assumption is addressed and MacCamy and Fuchs’ theory is introduced to account for diameter-to-wavelength ratios which are not very small. The concepts of modal mass and loads are introduced. Nonline...

Discusses issues related to offshore wind farms such as layout of the wind turbines, wind turbine wakes and wake control. The wake behind wind farms is also illustrated. Further, the concept of levelized cost of energy is introduced.

Part one gives a description of the characteristics of the wind field over the ocean, including wind shear, turbulence and coherence. It shows how these parameters are modeled and used as an input to wind turbine analyses. The long-term statistics of the mean wind speed are discussed as well as the most common principles for wind speed measurements...

Covers the classical theory for conversion of wind energy into mechanical energy, including various methods for computing loads on aerofoils. These include momentum disc theory with and without rotation, lifting line and lifting surface approaches. Classical approaches for handling the unsteady effects are discussed and illustrated. Standard contro...

Describes the main components of an offshore wind turbine and discusses various substructures presently in use. For bottom-fixed turbines, the characteristics of monopiles, jackets, tripods and gravity-based substructures are discussed. Similarly, for floating wind turbines, the characteristics of semisubmersibles, tension-leg platforms, spar platf...

Covers weather windows and duration statistics for marine operations as well as some issues related to multibody dynamics for lifting operations from a floating vessel. The impact load during mating operations is illustrated through an example and the statistics for impacts and snatch loads during mating operations are discussed. Mathieu instabilit...

Gives a short review of the linear dynamics of mechanical system, starting with a single degree of system, continuing with multibody systems, and ending with a continuous beam. Both frequency domain solutions and time domain methods are discussed.

Large-eddy simulation (LES) resolves large-scale turbulence directly and parametrizes small-scale turbulence. Resolving micro-scale turbulence, e.g., in wind turbine wakes, requires both a sufficiently small grid spacing and a domain large enough to develop turbulent flow. Refining a grid locally via a nesting interface effectively decreases the re...

Large-eddy simulation (LES) resolves large-scale turbulence directly and parametrizes small-scale turbulence. Resolving the micro-scale turbulence, e.g., in the wind turbine wakes, requires both a sufficiently small grid spacing and a domain large enough to develop the turbulent flow. Refining the grid locally via a nesting interface effectively de...

In the coming decades, energy from offshore wind turbines is expected to be an important energy source in electric power systems [...]

Wake meandering studies require knowledge of the instantaneous wake evolution. Scanning lidar data are used to identify the wind flow behind offshore wind turbines but do not immediately reveal the wake edges and centerline. The precise wake identification helps to build models predicting wake behavior. The conventional Gaussian fit methods are rel...

In the design of offshore wind turbines, it is important to make a realistic estimate of the wind load. This is particularly important for floating wind turbines, having natural frequencies in a frequency range where the wind loads are high and large turbulent structures exist. This study shows that turbulence modelling greatly impacts the response...

Wake meandering studies require knowledge of the instantaneous wake shape and its evolution. Scanning lidar data are used to identify the wake pattern behind offshore wind turbines but do not immediately reveal the wake shape. The precise detection of the wake shape and centerline helps to build models predicting wake behavior. The conventional Gau...

This study compares a wind field recommended by the wind turbine design standards to more realistic wind fields based on measurements. The widely used Mann spectral tensor model with inputs recommended by the standard, is compared to FitMann, the Mann model with inputs fitted to measurements and TIMESR; using measured time series combined with the...

In the design of offshore wind farms the simulated dynamic response of the wind turbine structure includes loading from turbulent wind. The International Electrotechnical Commission (IEC) standard for wind turbine design recommends both the Mann spectral tensor model and the Kaimal spectral model combined with an exponential coherence formulation....

In this paper, the primary objective is to investigate flow structures in the wake of wind turbines based on applying a truncated Proper Orthogonal Decomposition (POD) approach. This scheme decomposes the three-dimensional velocity fields produced by the high-fidelity PArallelized LES Model (PALM) into a number of orthogonal spatial modes and time-...

As the rotor diameter of offshore wind turbines increases, an improved understanding of the structure of the turbulent wind field approaching the rotor is important. Present standards for computing wind loads are resting upon assumptions of neutral atmospheric conditions and a simplistic formulation of the coherence of the turbulence. In the presen...

The calculation of the velocity deficit in the wake of individual wind turbines is a fundamental part of the wind farm analysis. A good approximation of the wake deficit behind a single wind turbine will improve the power estimation for downwind turbines. Large-eddy simulation (LES) is a research tool widely used in studying the velocity deficit an...

As the size of offshore wind turbines increases, a realistic representation of the spatiotemporal distribution of the incident wind field becomes crucial for modeling the dynamic response of the turbine. The International Electrotechnical Commission (IEC) standard for wind turbine design recommends two turbulence models for simulations of the incid...

Quality assured measurements from offshore masts may provide valuable information of the characteristics of the offshore wind field, which is of high relevance for simulations of offshore wind turbines’ dynamic response. In order to obtain these high quality data sets, a processing procedure tailored to offshore wind turbine applications must be fo...

The influence of 3 MW Hywind-II wind turbine wakes from an upstream offshore floating wind turbine on a downstream turbine with a separation distance of seven rotor diameters was studied for a site in the Gulf of Maine. The turbines and the platforms were subjected to atmospheric boundary layer flows. Various sensitivity studies on fatigue loads wi...

To assess the accuracy of the National Renewable Energy Laboratory's (NREL's) FAST simulation tool for modeling the coupled response of floating offshore wind turbines under realistic open-ocean conditions, NREL developed a FAST model of the Statoil Hywind Demo floating offshore wind turbine, and validated simulation results against field measureme...

A team of researchers from the National Renewable Energy Laboratory and Statoil used large-eddy simulations to numerically investigate the merging wakes from upstream offshore wind turbines. Merging wakes are typical phenomena in wind farm flows in which neighboring turbine wakes consolidate to form complex flow patterns that are as yet not well un...

Mounting a wind turbine on a floating foundation introduces more complexity to the aerodynamic loading. The floater motion contains a wide range of frequencies. To study some of the basic dynamic load effect on the blades due to these motions, a two-dimensional cascade approach, combined with a potential vortex method, is used. This is an alternati...

A method of moving a floating wind turbine relative to a body of water, the floating wind turbine having a buoyant body with a nacelle at the upper end thereof, including the steps of floating the floating wind turbine in the body of water, and towing the floating wind turbine while holding the buoyant body in an inclined position, whereby the nace...

A comparative dynamic analysis between full-scale measurements from the floating wind turbine Hywind Demo and corresponding numerical simulations is presented. A description of the main characteristics of the Hywind Demo structure, control system and important measurements are given.A method for estimation of the incident wave elevation on Hywind D...

The present invention relates to a blade pitch controller for a floating wind turbine structure, wherein the floating wind turbine structure may include a support structure supporting a rotor having a number of blades. The controller may include standard blade pitch control means and active damping means. The standard blade pitch control means is a...

A complete mathematical model of a hydraulic transmission concept for use in wind turbines is presented. The hydraulic system transfers the power from the nacelle to ground level. The main focus has been to develop a model that takes into account the most important dynamics affecting the wind turbine and the hydraulic transmission system involved,...

NORCOWE is an industry-led consortium including research institutes and universities which address key challenges for development of offshore wind. These include understanding marine boundary layer properties for improving industry standards and developing control and maintenance procedures which reduce costs.

As the search for oil and gas takes place on increasing water depths, the traditional production systems based on fixed or floating platforms becomes increasingly costly. A good alternative to the traditional production systems are subsea to shore solutions, where pipelines transport the product to a land terminal for processing. One of the main ch...

The committee's mandate was : Concern for structural design of ocean energy utilization devices, such as offshore wind turbines, support structures and fixed or floating wave and tidal energy converters. Attention shall be given to the interaction between the load and the structural response and shall include due consideration of the stochastic nat...

A computer tool for simulation of the dynamic response of floating wind turbines exposed to forces from wind, waves and current has been developed for Hydro Oil & Energy's floating wind turbine concept, HYWIND. Two existing, independent, computer program systems are used as basis for the new tool. The HAWC2 computer program from Risø National Labor...

The general design procedures for free spanning umbilicals are discussed. They may be classified as something between risers, where in-line VIV response normally is disregarded, and free spanning pipelines, where in-line VIV-response controls the VIV fatigue damage. To investigate this free span VIV test a 20m full-scale prototype-section of the Or...

Exploitation of wind energy at deep-waters locations requires floating wind turbine foundations. Several floating wind turbine foundation concepts are reported in the literature, and a common challenge is to make a low cost foundation with acceptable motion characteristics. In order to analyze the fatigue life of floating offshore wind turbines, th...

Particular design parameters for free-spanning high temperature-high pressure (HTHP) pipelines such as the Ormen Lange multiphase lines and a case study of Ormen Lange that can be applied to other HTHP pipelines are examined. Requirements for impact resistance and coating weight may be established by studying potential seabed interference from traw...

Two different simulation models for integrated dynamic analysis of floating offshore wind turbines are described and compared with model scale experiments for the Hywind concept for floating offshore wind turbines. A variety of both environmental conditions and wind turbine control schemes are tested. A maximum power control strategy is applied for...

Uneven seabed and seabed intervention may introduce free-spans in pipelines where interaction between VIV responses of several modes occurs, including longer single spans and moderate length multiple spans. A series of model tests were performed within the Ormen Lange project to study the phenomenon of multi-mode behaviour. While for a beam control...

Pipelines on undulating seabed may be subject to free-span areas where interaction between VIV response of several modes occurs, including both single and multi-spans. Within the project Ormen Lange a model test program, as completed in 2004, was performed to investigate this phenomena. The implementation of the test results into detail design went...

Statistical information about the joint occurrence of metocean parameters is of importance for many offshore activities. For instance, in marine operations, environmental limitations may be brought about by both wind and wave conditions. Thus, knowledge of their joint occurrence is important as the persistence duration (i.e., the duration of the se...

A new strategy for active control in heavy-lift offshore crane operations is suggested, by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimization of variations in the relative vertical velocity between payload and water using a wave amplitude measurement. Wave synchronizat...

A new strategy for active control in heavy-lift offshore crane operations is suggested, by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimization of variations in the relative vertical velocity between payload and water using a wave amplitude measurement. Wave synchronizat...

A new strategy for active control in heavy-lift offshore crane operations is suggested by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimizing variations in the relative vertical velocity between payload and water using a wave-amplitude measurement. Wave synchronization is...

A new strategy for active control in heavy-lift offshore crane operations is suggested, by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimization of variations in the relative vertical velocity between payload and water using a wave amplitude measurement. Wave synchronizat...

Present engineering practice for computing airgap (i.e. the clearance between waves and deck) on floating platforms relies on very simplified approaches. However, recently several new codes for computation of nonlinear wave diffraction have been developed. To obtain a state of the art of the capabilities of these methods, test cases related to airg...

A new strategy for active control in heavy-lift offshore crane operations is suggested, by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimization of variations in the relative vertical velocity between payload and water using a wave amplitude measurement. Wave synchronizat...

Current design practice for free spanning pipelines is to allow free spans as long as the integrity with respect to potential failure modes are checked and found acceptable. The case study for Ormen Lange (OL) pipelines planned in the deep waters of the Norwegian Sea is associated with a large number of very long free spans, which requires signific...

The Ormen Lange subsea pipeline shall be designed to meet a specified risk acceptance criterion, established by consideration of failure probability and consequences of failure. Traditional design for Vortex Induced Vibrations (VIV) of free spans limits the allowable free span length and implies that interventions work may be required. Through a ri...

A new strategy for active control in heavy-lift offshore crane operations is suggested, by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimization of variations in the relative vertical velocity between payload and water using a wave amplitude measurement. Wave synchronizat...

Vortex induced vibration (VIV) of free spanning pipelines in current is considered. In standard VIV estimation, one mode of oscillation is considered only. Increasing the length of the span, several modal shapes may be excited. Further, due to the sag effect of a long free span, the dynamic properties in vertical and horizontal direction of the spa...

In areas with very uneven ocean bottom topography, the floating pipeline concept may be used. An example is the Ormen Lange gas field west of Norway. The floating pipeline has positive buoyancy and is moored to the seafloor at regular intervals by tethers. Before the tethers are installed, drag chains will keep the pipeline in position. To study th...

Statistical information about the joint occurrence of metocean parameters is of importance for many offshore activities. For instance, in marine operations, environmental limitations may be brought about by both wind and wave conditions. Thus, knowledge of their joint occurrence is important as the persistence duration and the seasonal dependence o...

Most present-day programs for engineering prediction of VIV response are based on experimental lift and mass coefficients derived from 2D tests, where the cylinder has been constrained in in-line direction. The present paper describes a preliminary investigation into the coupling between in-line and transverse VIV response. A simple pendulum test s...

A mooring line submerged in water is considered. The equations for the tension in the line and damping due to viscous drag forces on the line is developed under the assumption that the tension in the line is large relative to the submerged weight of the line. Simple approximate expressions for the maximum tension as well as mooring line damping are...

A method for measuring mooring line tension is proposed based on observation of the natural frequencies of the mooring line segment between the winch and the fairlead. The anchor line tension is observed through the string equation where an analytical expression for the line's eigen-frequencies is obtained. The tension is observed on line by utiliz...

One of the main challenges in estimating impact energy in collisions between marine risers is the assessment of the riser mass involved in the collision. Evidently the entire riser mass does not contribute to the collision. Hence, the question is: What is the equivalent riser mass which contributes to the impact energy? This article presents three...

Mooring of floating platforms in deep waters is a challenging effort which calls for new solutions. A deepwater catenary mooring system has a rather low horizontal stiffness resulting large vessel excursions and correspondingly long risers to accommodate the excursions. A taut leg mooring system with polyester fiber ropes reduces the vessel excursi...

Recent investigations have shown that transverse vortex-induced vibrations (VIV) in risers may lead to tension fluctuations at twice the VIV frequency. For deep sea risers the corresponding stresses represent a significant fatigue loading. A theory covering the case where the tension fluctuation excites axial non-resonant vibration in the riser was...

The paper describes a series of tests with 90 m long models of deep sea risers, performed at an open air test site in a Norwegian fjord. The main purpose of the tests was to provide experimental data for the vortex induced vibrations (VIV) of such risers in a shear current. The test results can also be used to study collision criteria, drag coeffic...

For natural reasons, Norwegians have always had strong relations to the ocean and its environment. Therefore the shipbuilding industry and the related disciplines naval architecture and ship hydrodynamics have very long traditions in our country. Despite of our small population we have very active University departments performing research within m...

Damping of jacket structures are discussed on the basis of full-scale measurements. During a submarine impact with a jacket structure, unique transient response time series were recorded. Based on recorded acceleration time series, damping properties are evaluated in terms of the nature and magnitude of the total damping. It is shown that the dampi...

The Oseberg Jacket was installed at the Oseberg Field in the North Sea during summer 1987 and production started on 1st December 1988. On March 6th, 1988 a submarine accidentally impacted with the Oseberg jacket. This paper presents results from the evaluation of the importance of the damage to the overall structural safety. A nonlinear progressive...

Based on the far field momentum formulation, a potential flow analysis is derived for the case of low forward speed in the long wavelength limit. The well known mean drift force at zero forward speed is recovered, together with an expression for the wave drift force damping. Results are obtained showing the significance of this damping effect in co...

An approximate method to compute the drift forces on semisubmersible platforms is presented. The platform is supposed to have vertical, circular columns. It is assumed that the drift forces is governed by the scattering of waves from vertical columns only. The interaction between the columns is treated in a simplified manner; only the effects of ph...

A method for calculating second order forces on a vertical, large diameter circular cylinder is reported.Force spectra at sumfrequencies and difference frequencies as well as the first order force spectrum are calculated as a result of irregular waves generated from a Pierson-Moskowitz wave spectrum. Non-viscous fluid is assumed.Forces at sumfreque...

The expected additional resistance due to the propeller race when towed large structures is discussed. Model test and full scale tests, measuring the velocity distribution behind working propellers are performed. The axial force due to the propeller race on 3 different structures inserted in the propeller race is measured. The results show that in...

The paper presents results of drag measurements on two-dimensional oil barriers of various geometries exposed to a water current. Different oil quantities and qualities are supplied to the water. The drag coefficient of the barrier depends highly on the geometry of the barrier, while the oil affects the drag coefficient less. The geometry and behav...

Concern for structural design of ocean energy utilization devices, such as offshore wind turbines, support structures and fixed or floating wave and tidal energy converters. Attention shall be given to the interaction between the load and the structural response and shall include due consideration of the stochastic nature of the waves, current and...