| 1. |
- Bahaj, A. S., et al.
(författare)
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The Effect of Boundary Proximity Upon the Wake Structure of Horizontal Axis Marine Current Turbines
- 2011
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Ingår i: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme. - : ASME International. - 0892-7219 .- 1528-896X. ; 134:2, s. 021104-1-021104-8
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Tidskriftsartikel (refereegranskat)abstract
- An experimental and theoretical investigation of the flow field around small-scale mesh disk rotor simulators is presented. The downstream wake flow field of the rotor simulators has been observed and measured in the 21m tilting flume at the Chilworth hydraulics laboratory, University of Southampton. The focus of this work is the proximity of flow boundaries (sea bed and surface) to the rotor disks and the constrained nature of the flow. A three-dimensional Eddy-viscosity numerical model based on an established wind turbine wake model has been modified to account for the change in fluid and the presence of a bounding free surface. This work has shown that previous axi-symmetric modeling approaches may not hold for marine current energy technology and a novel approach is required for simulation of the downstream flow field. Such modeling solutions are discussed and resultant simulation results are given. In addition, the presented work has been conducted as part of a UK Government funded project to develop validated numerical modeling tools which can predict the flow onto a marine current turbine within an array. The work feeds into the marine energy program at Southampton to assist developers with layout designs of arrays which are optimally spaced and arranged to achieve the maximum possible energy yield at a given tidal energy site.
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| 2. |
- Boström, Cecilia, et al.
(författare)
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Experimental Results From an Offshore Wave Energy Converter
- 2010
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Ingår i: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme. - : ASME International. - 0892-7219 .- 1528-896X. ; 132:4, s. 041103-
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Tidskriftsartikel (refereegranskat)abstract
- An offshore wave energy converter (WEC) was successfully launched at the Swedish west coast in the middle of March 2006. The WEC is based on a permanent magnet linear generator located on the sea floor driven by a point absorber. A measuring station has been installed on a nearby island where all measurements and experiments on the WEC have been carried out. The output voltage from the generator fluctuates both in amplitude and frequency and must therefore be converted to enable grid connection. In order to study the voltage conversion, the measuring station was fitted with a six pulse diode rectifier and a capacitive filter during the autumn of 2006. The object of this paper is to present a detailed description of the Lysekil research site. Special attention will be given to the power absorption by the generator when it is connected to a nonlinear load.
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| 3. |
- Boström, Cecilia, et al.
(författare)
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Temperature measurements in a linear generator and marine substation for wave power
- 2012
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Ingår i: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme. - : ASME International. - 0892-7219 .- 1528-896X. ; 134:2, s. 021901-
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Tidskriftsartikel (refereegranskat)abstract
- This paper analyzes temperature measurements acquired in the offshore operation of a wave energy converter array. The three directly driven wave energy converters have linear generators and are connected to a marine substation placed on the seabed. The highly irregular individual linear generator voltages are rectified and added on a common dc-link and inverted to 50 Hz to facilitate future grid-connection. The electrical power is transmitted to shore and converted to heat in a measuring station. The first results of temperature measurements on substation components and on the stator of one of the linear generators are presented based on operation in linear and in nonlinear damping. The results indicate that there might be some convective heat transfer in the substation vessel. If high power levels are extracted from the waves, this has to be considered when placing components in the substation vessel in order to avoid heating from neighboring components. The results also indicate that the temperature increase in the linear generator stator is very small. Failure due to excessive heating of the stator winding polyvinyl chloride cable insulation is unlikely to occur even in very energetic sea states. Should this conclusion be incorrect, the thermal conductivity between the stator and the hull of the wave energy converter could be enhanced. Another suggested alteration is to lower the resistive losses by reducing the linear generator current density.
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| 4. |
- Jonsson, Mikael, et al.
(författare)
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Experimentally determined deformations and stresses in narrow-gap and single-U multi-pass butt-welded pipes
- 1993
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Ingår i: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme. - : ASME International. - 0892-7219 .- 1528-896X. ; 115:2, s. 116-122
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a study of the multi-pass butt-welding of thick-walled pipes. Two different groove shapes were used: single-U and narrow-gap grooves. Strains were measured on both the pipe outer and inner surfaces during and after welding. Residual stresses and radial deflections were measured on the outer surface after welding. Measured parameters of the two different groove shapes are compared and discussed. Residual stresses at the weld center and radial deflections in the narrow-gap pipe were somewhat larger than those in the single-U pipe. The residual stress fields of both types of groove were found to be rotationally nonsymmetrical. The experimentally obtained residual stresses and radial deflections are compared with corresponding analytical and empirical results based on methods described in the literature and with numerical results for the same pipe from a separate study. Deviations between the measurements presented in this study and published results are discussed.
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| 5. |
- Josefson, B.L., et al.
(författare)
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Welding Residual Distortions in Ring-Stiffened Pipes
- 1996
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Ingår i: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme. - : ASME International. - 0892-7219 .- 1528-896X. ; 118:2, s. 121-126
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Tidskriftsartikel (refereegranskat)abstract
- A fast and simple method for the determination of the residual deformation for a class of welding problems, ring-stiffened pipes, is proposed. The method can predictradial as well as angular distortion of the thin-walled pipe-ring-stiffener/flange assembly. The pipe and stiffener material is elasto-plastic. In particular, the accumulation of deformation in multipass welding is incorporated in the model. Each weld pass is treated separately. This facilitates the assessment of the influence of the sequence in which the weld passes are deposited on the residual deformation state. The method will be included in a conversational knowledge-based "expert" system for the production of a welded ring-stiffened pipe
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| 6. |
- Lindgren, Georg, et al.
(författare)
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First Order Stochastic Lagrange Model for Asymmetric Ocean Waves
- 2009
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Ingår i: Journal of Offshore Mechanics and Arctic Engineering. - : ASME International. - 0892-7219 .- 1528-896X. ; 131:3, s. 1-031602
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Tidskriftsartikel (refereegranskat)abstract
- The Gaussian linear wave model, which has been successfully used in ocean engineering for more than half a century, is well understood, and there exist both exact theory and efficient numerical algorithms for calculation of the statistical distribution of wave characteristics. It is well suited for moderate seastates and deep water conditions. One drawback, however, is its lack of realism under extreme or shallow water conditions, in particular, its symmetry. It produces waves, which are stochastically symmetric, both in the vertical and in the horizontal direction. From that point of view, the Lagrangian wave model, which describes the horizontal and vertical movements of individual water particles, is more realistic. Its stochastic properties are much less known and have not been studied until quite recently. This paper presents a version of the first order stochastic Lagrange model that is able to generate irregular waves with both crest-trough and front-back asymmetries.
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| 7. |
- Lindström, Per, 1967-
(författare)
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Heat Transfer Prediction of In-Service Welding in a Forced Flow of Fluid
- 2009
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Ingår i: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme. - : ASME International. - 0892-7219 .- 1528-896X. ; 131:3, s. 1-6
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Tidskriftsartikel (refereegranskat)abstract
- An algorithm for heat transfer prediction of in-service welding operations in a forcedflow of fluid is presented. The algorithm presented is derived from Rosenthal’s 3D heatflow equation and boundary layer approximations. This was possible by the introductionof an apparent thermal conductivity kPL, which is a function of the boundary layer’s heattransfer coefficient f and the base material’s thickness . This implies that a weldcooling time tT1 /T2 in a forced flow of fluid can now be calculated by an ordinaryengineering calculator and thus enabling suitable welding parameters to be determined.The magnitude of kPLf , was established by regression analysis of results from aparametric finite element analysis series of a total number of 112 numerical simulations.Furthermore, the result of the regression analysis was validated and verified by a weldingexperiment series accomplished on an in-house designed and constructed in-servicewelding rig. The principle design of the welding rig as well as its instrumentation, a PCbased Data Acquisition system, is described. In addition, a method to measure the weldmetals cooling time tT1 /T2 by means of thermocouple elements is described. Finally,the algorithm presented in this study proved feasible for industrial in-service weldingoperations of fine-grained Carbon and Carbon–Manganese steels with a maximum CarbonEquivalent (IIW) (CE) of 0.32.
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| 8. |
- Mao, Wengang, 1980, et al.
(författare)
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Notes on the Prediction of Extreme Ship Response
- 2013
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Ingår i: Journal of Offshore Mechanics and Arctic Engineering. - : ASME International. - 0892-7219 .- 1528-896X. ; 135:2
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Tidskriftsartikel (refereegranskat)abstract
- In this note the relation between two simple approaches to estimate the extreme ship response used when no, or a limited, amount of data are available is discussed. The first one employs the long term distribution of the local maxima of ship response while the second one uses the expected number of upcrossings of a level by the response. It is mathematically demonstrated that the two approaches are equivalent. However, the upcrossing method is more straightforward and convenient for practical applications, particularly for non-Gaussian responses. The full-scale measurements of a 2800 TEU container ship during the first six months of 2008 are used in the comparisons.
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| 9. |
- Ringsberg, Jonas, 1971, et al.
(författare)
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Analysis of impact loads on a self-elevating unit during jacking operation
- 2017
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Ingår i: Journal of Offshore Mechanics and Arctic Engineering. - : ASME International. - 0892-7219 .- 1528-896X. ; 139:3, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- A method is presented that enables the analysis of weather window assessments for the installation and retrieval phases of a self-elevating unit (SEU). The method takes site-specific parameters, defined as soil type and water depth, into account in addition to vessel-specific and environmental parameters. The inclusion of site-specific parameters is the novel contribution compared to assessment methodologies used today. A simulation model is presented that incorporates a coupled non-linear time-domain analysis of vessel motion and soil-structure interaction. Soil deformation behaviour during impact is described by resistance curves based on a bearing capacity theory. A structural evaluation criterion against which impact forces are compared is used for weather window assessments. The simulation model is applied on a case study utilizing different soil types to study impact forces and the capacity of the structure for withstanding such impacts and eventually performing a weather window assessment. The results show that the jacking operation can be divided into two phases when it comes to loads on the spudcan: a phase dominated by vertical forces followed by a phase dominated by horizontal forces. It is found that including soil deformation behaviour is of paramount importance to the magnitude of the resulting impact forces and that class-recommended practice does indeed produce rather large force estimates. Thus, assessments where site-specific parameters are incorporated could definitely increase the operable weather window for SEUs, and, consequently, increase the economic competitiveness of, for example, the offshore wind industry.
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| 10. |
- Ringsberg, Jonas, 1971, et al.
(författare)
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Design of mooring solutions and array systems for point absorbing wave energy devices - Methodology and application
- 2020
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Ingår i: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme. - : American Society of Mechanical Engineers (ASME). - 0892-7219 .- 1528-896X. ; 142:3
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Tidskriftsartikel (refereegranskat)abstract
- Most of the ocean energy technologies are considered to be in a pre-commercial phase and need technical development. This study focuses on the design of mooring solutions and compares array systems of a specific floating point-absorbing wave energy converter (WEC) developed by the company Waves4Power. A full-scale prototype of the WEC is installed in Runde (Norway) where it is moored with three polyester mooring lines, each having one floater and one gravity anchor. Based on this reference installation, the method of systems engineering was used to propose 22 conceptual mooring solutions for different array systems. They were compared and reduced to four top concepts in a systematic elimination procedure using Pugh and Kesselring matrices. The top concepts were assessed in detail by means of levelized cost of energy (LCOE), life cycle analysis (LCA), and risk analyses. The fatigue life of the mooring lines and the energy capture were calculated using results obtained from coupled hydrodynamic and structure response analyses in the dnv-gl deepc software. Two final concepts were proposed for the water depths 75 and 200 m.
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