Gap Resonance Analyzed by a Domain-Decomposition Method

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Mooring Loads of a Circular Net Cage with Elastic Floater in Waves and Current

The mooring loads on an aquaculture net cage in current and waves are investigated by dedicated model tests and numerical simulations. The main purpose is to investigate which physical effects are dominant for mooring loads, and in this respect, to investigate the validity of different rational hydrodynamic load models. Also structural and numerical aspects are investigated. The model tests are performed to provide benchmark data, while the numerical model is used to study the effect and sensitivity of different load models and parameters. Compared to a realistic aquaculture plant, the total system is simplified to reduce the complexity. The system does, however, include all the four main components of an aquaculture plant: net cage, floater, sinker weights and moorings. The net cage is bottomless, flexible and circular. It is attached to a circular, elastic floater at the top and has 16 sinker weights at the bottom. The system is nearly linearly moored with four crow feet mooring lines. The loads are measured in the four mooring lines. A systematic variation of current only, wave only as well as combined current and wave conditions is carried out. The numerical simulation results are first benchmarked towards the experimental data. The mean loads in general dominate over the dynamic part of the loads in combined current and waves, and they significantly increase in long and steep waves, relative to current only. Next, a sensitivity study is carried out. A rigid floater significantly alters the loads in the mooring lines compared to a realistic, elastic floater. The theoretical model for the wave matters. The mooring loads are rather insensitive to a majority of the parameters and models, in particular: frequency dependent added mass of the floater and nonlinear restoring loads. It seems not to be necessary to represent the net cage with a very fine numerical mesh.acceptedVersio

Hydrodynamic Coefficients of Generic Subsea Modules in Forced Oscillation Tests – Importance of Structure Parts

A systematic study of the hydrodynamic coefficients for simplified subsea modules has been performed, to support the estimation of the coefficients needed for planning of subsea installation operations. The coefficients are assessed for a nearly two-dimensional test setup. The tests are performed as forced oscillations at various amplitudes and periods, representing the forces on the module lowered through the water column. The importance of each of the main components of the subsea modules — mudmat, protection roof and process equipment of different shapes inside the modules are studied at fully submerged condition. Results for the module elements, generic contents and different combinations of these elements are presented. For the tested modules, damping is generally the dominating hydrodynamic force. However, the presence of the content inside the modules will generally increase the importance of added mass. Estimation of the hydrodynamic coefficients by summation of the coefficients for the individual structure elements generally overestimates the damping, compared to the coefficients measured for the complete modules. For added mass, estimation based on summation gives generally good results.acceptedVersio

Experimental and numerical investigations of monopile ringing in irregular finite-depth water waves

In storm conditions, nonlinear wave loads on monopile offshore wind turbines can induce resonantringing-type responses. Efficient, validated methods which capture such events in irregular waves inintermediate or shallow water depth conditions are needed for design. Dedicated experiments andnumerical studies were performed toward this goal. The extensive experimental campaign at 1:48 scalewas carried out for Statoil related to the development of the Dudgeon wind farm, and included botha rigid model and a flexible, pitching-type, single degree-of-freedom model. Twenty 3-hour durationrealizations for 4 sea states and 2 water depths were tested for each model. A high level of repeatabilityin ringing events was observed. Uncertainties in the experimental results were critically examined. Thestochastic variation in the 3-hour maximum bending moment at the sea bed was significantly larger thanthe random variation in repetition tests, and highlighted the need for a good statistical basis in design.Numerical simulations using a beam element model with a modified Morison wave load model and sec-ond order wave kinematics gave reasonable prediction of the ringing response of the flexible model, andof the measured excitation forces on the rigid model in the absence of slamming. The numerical modelwas also used to investigate the sensitivity of the responses with respect to damping and natural period.A simple single degree-of-freedom model was shown to behave similarly to a fully flexible model whenconsidering changes in natural frequency and damping.acceptedVersio

Kjønnsforskjeller på holdninger, atferd og roller hos ansatte i norske barnehager. En studie av atferd som kan påvirke utvikling av kjønnsstereotypier hos barn

I dette studiet undersøkte vi holdninger, atferd og roller hos mannlige og kvinnelige ansatte i barnehage. Bakgrunnen for dette er tanken om at barnehageansattes vil kunne ha stor påvirkning for utvikling av kjønnsstereotypier hos barn og at mannlige og kvinnelige ansatte kan påvirke ulikt. Flere studier har undersøkt holdninger, atferd og roller hos barnehageansatte, i tillegg til dette ser vi på ansattes bruk av kjønnsspesifikke merkelapper med Developmental Intergroup Theory (Bilger & Liben, 2007) som bakteppe. Studiet er basert på et spørreskjema svart på av totalt 127 mannlige (N = 24) og kvinnelige (N = 103) barnehageansatte, med et aldersspenn fra 19 til 60 år (M = 37.72, SD = 10.03). Vi fant få forskjeller generelt, hvilket stemmer godt med andre studier på området. De tydeligste funnene var på ansattes bruk av kjønnsspesifikke merkelapper, som også er et område andre studier ikke har sett direkte på. Forskjell på mannlige og kvinnelige ansattes bruk av kjønnsspesifikke merkelapper kan bety at de bidrar ulikt til utviklingen av kjønnsstereotypier hos barn. Vi gjennomført også en eksplorerende analyse av data, hvor vi undersøkte påvirkningen av ansattes alder og tid jobbet på arbeidsplass i forhold til deres holdninger, atferd og roller. Vi ser også nærmere på Norges politiske mål om å øke andelen menn i barnehage, og hvilke implikasjoner det kan få for utvikling av kjønnsstereotypier, med bakgrunn i våre funn

A Physical-Based Damping Model of Gap and Moonpool Resonance in WAMIT

An engineering model to estimate and incorporate quadratic damping of the piston-mode moonpool responses in the proximity of the piston mode period is proposed. The model provides a physical-based equivalent linearized damping coefficient. The method is not limited to forced motion, but applicable to freely floating moonpool vessels. Further, it is not limited to moonopools, but can be generalized to gap resonance problems, such as side-by-side operations. The soundness of the proposed physical-based method is demonstrated using the panel code WAMIT with a linear damping term in the free-surface boundary condition inside the moonpool using two existing moonpool experiments as case studies; (1) a two-dimensional rectangular box with a moonpool subject to forced heave, and (2) a freely floating offshore vessel in incident waves. The WAMIT computations using the proposed method reconstructs the experimentally obtained piston-mode and vessel responses well. We suggest that the proposed method can be used with fair degree of confidence in an early design or operational analysis phase, in the (often) case that the quadratic damping is not known from either experiments or CFD. To our knowledge, this is the first general, physical-based piston-mode damping model that does not require any tuning from experiments.acceptedVersio

An upright bottomless vertical cylinder with baffles floating in waves

Damping of the surge and pitch motions, as well as the first lateral sloshing mode in a rigid free-floating upright circular dock with bilge boxes and open bottom is investigated. Model tests are carried out on a 0.80 m diameter model in regular waves with wave periods near the highest natural sloshing period, and the internal free-surface elevation and model’s rigid body motions are measured. Perforated and solid annular baffles of relatively small widths are also installed inside the dock at various submergences. The experimental results are compared to a semi-analytical approach, where a three-dimensional domain decomposition method based on linear potential flow theory is adopted to calculate the hydrodynamic coefficients and exciting forces in heave, surge and pitch. A reduced natural sloshing frequency, as well as a damping ratio estimated from the energy dissipated due to flow separation from the baffles, are introduced in the free-surface boundary conditions to model the effects of the baffle. It shows good agreement with experimental data when the ratio between the draft of the baffle and the internal radius of the cylinder is dB/α = 0.27, and tends to under-predict the damping ratio for shallower drafts, most likely due to free-surface interactions. The solid baffle damps the sloshing response most efficiently, reducing the amplitude at the resonant peak by more than 56%.publishedVersio

Sloshing-induced motions of a spar inside a cylindrical dock with baffles in waves

The motions of a free floating offshore wind turbine’s (OWT) spar-type platform inside a bottom-less moored cylindrical dock are investigated for incident wave frequencies near the first lateral sloshing resonance, focusing on surge on pitch motions. The radiation and diffraction problems of the two-body system are first solved through a domain decomposition (DD) approach under linear potential flow assumptions. This semi-analytical model is extended to include the effects of solid and perforated baffles in the annular domain between the dock and the spar, adapting the method developed in our previous paper for the dock alone. Results are compared with those obtained with model tests, performed at scale 1:100 for both regular waves with low steepnesses and irregular sea states. The resonant peak amplitudes of the spar’s surge and pitch motions are reduced by almost half when a solid baffle is installed, with a strong dependency on the incident wave height due to viscous dissipation caused by the flow separation at the sharp edge of the baffle.publishedVersio

Practice variation in surgical procedures and IUD-insertions among general practitioners in Norway – a longitudinal study

Published version. Source at http://doi.org/10.1186/s12875-017-0581-9. License CC BY-NC-SA 4.0.Background: Studies of Primary Health Care (PHC) reveal considerable practice variations in terms of the range of services provided. In Norway, general practitioners (GPs) are traditionally expected to perform IUD-insertions and several surgical procedures as a part of comprehensive PHC. We aimed to investigate variation in the provision of surgical procedures and IUD-insertions across GPs and over time and explore determinants of such variation. Methods: Retrospective registry study of Norwegian GPs. From a comprehensive database of GPs’ reimbursement claims, we obtained procedure codes and GP characteristics such as age, gender, list size and municipality characteristics from 2006 through 2013. Multivariable logistic regression models were fitted to explore determinants of practice variation. Results: We extracted data from 4,828 GPs. In 2013, 91.0, 76.1 and 74.8% were reimbursed at least once for minor and major surgical procedures and IUD-insertion, respectively. Female GPs had lower odds for performing major surgical procedures (OR 0.38, 95% CI 0.32–0.45) and higher odds for performing IUD-insertions (OR 6.28, 95% CI 4. 47–8.82) than male GPs. Older GPs and GPs with shorter patient lists were less likely to perform surgical procedures. GPs with longer patient lists had higher odds for performing IUD-insertions. The proportion of GPs performing surgical procedures increased over time, while the proportion decreased for IUD-insertions. The number of IUD-insertions in specialist care increased from 12,575 in 2011 to 15 216 (+21.0%) in 2014. Conclusion: We observed a large variation in the provision of surgical procedures and IUD-insertions amongst GPs in Norway. The GPs’ age, gender, list size and size of municipality were associated with performing the procedures. Our findings suggest a shift of IUD-insertions from primary to specialist care

Experimental study on the effect of second order wavemaker theory on the response of a flexible large diameter monopile in irregular sea

Motivated by the need for larger offshore wind turbines, large diameter monopile foundations are being developed. To ensure safe design, there is a need for model testing and validation of hydrodynamic load models. Scaled model tests with a piston-type wavemaker commonly apply first order wavemaker theory for irregular waves. This approach results in the generation of second (and higher) order spurious (also known as parasitic) free waves in the tank. In this study, the effect of superharmonic spurious waves on the response of a monopile with eigenfrequency close to three times the wave peak frequency is examined experimentally. The bending moment response statistics are not found to be significantly affected by the wavemaker correction. Different wave breaking patterns are observed for individual events, but our results do not indicate any clear relation between breaking waves and the wave generation technique.publishedVersio