search query: @instructor Naar, Hendrik / total: 15
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Author: | Lillemäe, Ingrit |
Title: | Fatigue load assessment procedure for floating production storage and offloading unit |
Publication type: | Master's thesis |
Publication year: | 2009 |
Pages: | 70 Language: eng |
Department/School: | Sovelletun mekaniikan laitos |
Main subject: | Laivanrakennusoppi (Kul-24) |
Supervisor: | Varsta, Petri |
Instructor: | Mikkola, Timo ; Romanoff, Jani ; Naar, Hendrik |
OEVS: | Electronic archive copy is available via Aalto Thesis Database.
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Location: | P1 Ark TKK 5165 | Archive |
Keywords: | spectral (direct) fatigue load assessment FPSO long term response vertical wave bending moment RAO rule based method long crested short crested full stochastic fatigue analysis |
Abstract (eng): | A spectral fatigue load assessment procedure has been developed and implemented for fatigue analysis of FPSO (Floating Production Storage and Offloading unit) hull structures. The advantages/ influences that this time consuming approach may bring to an everyday design practice in comparison with traditional rule based method have been investigated. The procedure is based on linear theory, which means that the response is changing linearly with respect to the excitation (wave height). Wave loads have been calculated with linear 3D panel diffraction/ radiation frequency domain program, which uses source distribution (boundary integration) method. Only hull girder vertical wave bending moment is taken into account. Long-term prediction of vertical wave bending moment has been obtained by weighing several short-term predictions with their occurrence probabilities. Different operational and environmental conditions have been taken into account. Each short-term process has been assumed to be narrow banded stationary Gaussian random process, which follows Rayleigh distribution. Real fatigue damages/ lives are not calculated, the comparison has been made on fatigue load level and also on relative fatigue damage level. Relative fatigue damage is calculated assuming that fatigue loads and fatigue stresses are linearly related. Different effects on the long term load predictions were investigated and main findings are:. Spectral method predicts similar results with the rule based method, when the North Atlantic wave scatter data is used and all wave headings are equally probable. This is expected, since DNV rules also assume uniformly distributed wave headings and the worst possible environment - the North Atlantic. This shows that the procedure can reliably he applied for other conditions as well. Using North Atlantic wave scatter data gives expectedly higher fatigue loads compared with the milder FPSO environment. This results in average 16 times longer fatigue life at the used milder FPSO environment. The highest VWBM comes from the head and following seas and the lowest from the beam seas. FPSOs usually head to the waves, since motions are then smallest, which is very important from the normal operation viewpoint. Using short crested sea model gives more favourable design loads for FPSOs, since it spreads the wave energy more realistically around the main wave direction. Equally distributed wave directions predict lower fatigue loads compared with the case where 95 % of the time waves are coming from the bow. The number of wave headings considered does not influence the results significantly. In addition, the necessary steps towards full stochastic fatigue analysis were taken. Wave loads (pressures and 6 d.o.f. accelerations in COG) were transferred directly from hydrodynamic analysis to structural model including automatically all the local and global load effects and phase information. The accuracy of the transfer process was studied. Stress level full spectral fatigue analysis is, however, out of the thesis scope. |
ED: | 2010-02-05 |
INSSI record number: 38865
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