Papers by Finn Gunnar Nielsen
Volume 1: Offshore Technology; Offshore Wind Energy; Ocean Research Technology; LNG Specialty Symposium, 2006
A computer tool for simulation of the dynamic response of floating wind turbines exposed to force... more 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 Laboratory is a state-of-the-art aero-elastic code for analysis of the response of fixedfoundation wind turbines, while the SIMO/RIFLEX computer program system from MARINTEK is a tool for simulating the dynamic response of marine structures.
21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, 2002
Vortex induced vibration (VIV) of free spanning pipelines in current is considered. In standard V... more 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 span are different. This causes a much more complex VIV response pattern for long free spans than for short spans. The observed VIV response of long free spans during model testing is discussed. Hypotheses that may explain the observed behaviour are presented. Also a format of new design principles for long free spans is outlined.
Offshore Technology Conference, 1998
21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 4, 2002
ABSTRACT In areas with very uneven ocean bottom topography, the floating pipeline concept may be ... more ABSTRACT 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 the effect of the drag chains on the pipe installation, its static configuration and its dynamic behavior, it would be very inefficient to model each drag anchor using cable elements. This would require a very high number of elements and small time-steps, while the details of the forces in the chain are not critical in the pipeline design. Instead, a non-linear element has been developed that precisely represents the quasi-static behavior of a complete drag chain. To determine reaction forces at the top end of the chain given displacements of the top, the length of dragging chain necessary to balance the forces acting on the top is found by iteration. Proof is provided that the iteration is unconditionally stable. This iteration makes use of the classical catenary solution for the hanging part of the chain, combined with a model of the configuration of the fraction of the chain resting on the seabed. This configuration can be arbitrary, so that any combination of straight chain, heaps and coils resulting from the history of the displacement chain is accounted for. The element behaves in a coherent way also if the whole length of chain is being dragged on the seabed, lifted clear of the seabed, or landed back on the seabed. The incremental stiffness matrix is found by numerical derivation of the reaction forces with respect to the displacements. The element is designed for use in non-linear static or non-linear time domain analyses. It has been implemented in the riser and pipeline analysis program RIFLEX and successfully tested.
Wind Energy, 2014
ABSTRACT A comparative dynamic analysis between full-scale measurements from the floating wind tu... more ABSTRACT 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 Demo is outlined. Comparative dynamic simulations are carried out with the estimated wave elevation time series as input, together measured and further refined statistical parameters of the wind field.Roll, pitch and yaw motions, tower bending moments, mooring line tensions, power production, rotor speed and blade pitch angle are compared for one case below and one case above the rated wind speed. Overall, good agreement is seen between the measured and simulated responses, and it is believed that the considered analysis tool is well suited for confirming the design of a future optimized floating wind turbine based on the Hywind concept. Copyright © 2014 John Wiley & Sons, Ltd.
21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, 2002
: Experimental protection cover.
21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 2, 2002
ABSTRACT Statistical information about the joint occurrence of metocean parameters is of importan... more ABSTRACT 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 of wind and waves appear to be of large interest. However, such a modelling becomes difficult as the number of considered variables increases, especially when utilizing a common parameterization of some conditional distributions. This paper proposes a general methodology that aims at modelling seasonal joint distributions of n such parameters from their correlation structure and the n marginal distributions fitted by generalized gamma ones. Two methods are proposed in order to derive an approximate joint distribution from the modelled margins. The first one matches the correlation matrix only, whereas the second one, which is based on a multivariate Hermite polynomials expansion of the multinormal distribution, is able to match joint moments of order higher than two. However, more restrictive conditions are shown by the latter. An application to the simple example of the joint occurrence of significant wave height and the mean wind velocity at the 10m elevation is used to illustrate the methods. Eventually, examples of applications like simultaneous persistence of wind and wave conditions as well as seastate forecasting from statistics are given.
2012 Oceans - Yeosu, 2012
ABSTRACT NORCOWE is an industry-led consortium including research institutes and universities whi... more ABSTRACT 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.
Offshore Technology Conference, 1999
Marine Structures, 2003
Present engineering practice for computing airgap (i.e. the clearance between waves and deck) on ... more 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 airgap under a floating platform and run-up along platform columns have been defined. Several organizations have been invited to apply their numerical tools to compute airgap and run-up for the defined test cases. The results from the comparisons are summarized and compared to experimental results. The study has been part of the work of ISSC2000 Committee 1.2. r
Journal of Offshore Mechanics and Arctic Engineering, 2004
ABSTRACT Statistical information about the joint occurrence of metocean parameters is of importan... more ABSTRACT 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 sea state persistence above or beneath a given level) and the seasonal dependence of wind and waves appear to be of large interest. However, such a modeling becomes difficult as the number of considered variables increases, especially when utilizing a common parameterization of some conditional distributions. This paper proposes a general methodology that aims at modeling seasonal joint distributions of n such parameters from their correlation structure and the n marginal distributions fitted by generalized gamma ones. Two methods are proposed in order to derive an approximate joint distribution from the modeled margins. The first one matches the correlation matrix only, whereas the second one, which is based on a multivariate Hermite polynomials expansion of the multinormal distribution, is able to match joint moments of order higher than two. However more restrictive conditions are shown by the latter An application to the simple example of the joint occurrence of significant wave height and the mean wind velocity at the 10m elevation is used to illustrate the methods. Eventually, examples of applications like simultaneous persistence of wind and wave conditions as well as seastate forecasting from statistics are given.
Journal of Marine Science and Technology, 1999
Semisubmersible platforms are used for drilling and production of oil and gas. The design of the ... more Semisubmersible platforms are used for drilling and production of oil and gas. The design of the mooring system is an increasing challenge as the operations are moving toward deeper water. For operational reasons, as well as for design verification, it is important to have control of the tension in the mooring lines. A standard way of measuring tension is by use of force transducers mounted at the foundation of the mooring line winches. This is a robust way of measuring, but it has some drawbacks. The mean load can be difficult to measure accurately due to drift in the zero level. Experience has also proven that it is difficult to calibrate these cells accurately. A third important drawback of this approach is that it is expensive and difficult to repair these load cells offshore. The design lifetime of an oilproducing platform may exceed 20 years without going to dock. For very accurate measurements of line tension, instrumented links are introduced in the mooring chain. This solution, however, has serious practical drawbacks, first and foremost lack of reliability over a long time span.
Journal of Marine Science and Technology, 1999
One of the main challenges in estimating impact energy in collisions between marine risers is the... more 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 different ways of estimating the riser mass participating in the collision energy. The first method is strictly experimental. The second method uses a numerical experiment together with system identification techniques. The third method is a strictly analytical method, which results in an asymptotically upper bounded estimate of the participating mass. Two risers are examined as case studies. The first riser is a 1 : 100 model scale riser used in collision experiments carried out at Marintek's towing tank in Trondheim, Norway. The second case uses a real world riser in use on the Troll B oil production platform operating in the North Sea. The proposed methods yield consistent and comparable results.
IEEE Journal of Oceanic Engineering, 2003
A new strategy for active control in heavy-lift offshore crane operations is suggested, by introd... more 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 synchronization is combined with conventional active heave compensation to obtain accurate control. Experimental results using a scale model of a semi-submerged vessel with a moonpool shows that wave synchronization leads to significant improvements in performance. Depending on the sea state and payload, the results indicate that the reduction in the standard deviation of the wire tension may be up to 50 %.
Journal of Physics: Conference Series, 2014
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Papers by Finn Gunnar Nielsen