Browsing by Author "Leppänen, Timo"

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  • Asymmetry and interactions in rotordynamics
    (1999) Leppänen, Timo
     Helsinki University of Technology | Master's thesis
  • Bayesian network model to evaluate the effectiveness of continuous positive airway pressure treatment of sleep apnea
    (2018-10-01) Ryynänen, Olli Pekka; Leppänen, Timo; Kekolahti, Pekka; Mervaala, Esa; Töyräs, Juha
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Objectives: The association between obstructive sleep apnea (OSA) and mortality or serious cardiovascular events over a long period of time is not clearly understood. The aim of this observational study was to estimate the clinical effectiveness of continuous positive airway pressure (CPAP) treatment on an outcome variable combining mortality, acute myocardial infarction (AMI), and cerebrovascular insult (CVI) during a follow-up period of 15.5 years (186 ± 58 months). Methods: The data set consisted of 978 patients with an apnea-hypopnea index (AHI) ≥5.0. One-third had used CPAP treatment. For the first time, a data-driven causal Bayesian network (DDBN) and a hypothesis-driven causal Bayesian network (HDBN) were used to investigate the effectiveness of CPAP. Results: In the DDBN, coronary heart disease (CHD), congestive heart failure (CHF), and diuretic use were directly associated with the outcome variable. Sleep apnea parameters and CPAP treatment had no direct association with the outcome variable. In the HDBN, CPAP treatment showed an average improvement of 5.3 percentage points in the outcome. The greatest improvement was seen in patients aged ≤55 years. The effect of CPAP treatment was weaker in older patients (>55 years) and in patients with CHD. In CHF patients, CPAP treatment was associated with an increased risk of mortality, AMI, or CVI. Conclusions: The effectiveness of CPAP is modest in younger patients. Long-term effectiveness is limited in older patients and in patients with heart disease (CHD or CHF).
  • Probabilistic Seismic Hazard Analysis for Nuclear Power Plant Sites in Finland
    (2024-05-21) Vehmas, Juhana
     Perustieteiden korkeakoulu | Master's thesis
    Probabilistic seismic hazard analysis (PSHA) is a method used to the estimate the likelihood of earthquake induced ground motions at a particular location. These estimates are then used to evaluate the risk to a structure such as a nuclear power plant (NPP). The latest PSHA for a Finnish NPP site located in Loviisa was made in 2021. Some significant sources of uncertainties and areas of development related to the modelling decisions made in the PSHA have been later identified. This thesis seeks to study and develop the latest PSHA for the Loviisa NPP site. The PSHA model is also used to evaluate the seismic hazard in another Finnish NPP site located in Olkiluoto. The primary focus is on the Gutenberg-Richter (GR) parameters, which are used to describe the seisimic activity of a given region. A new declustered seismic catalog is adopted for the analysis, and the completeness of it is assessed in different parts of the study region. The GR parameters are estimated with a maximum likelihood method (MLE) method and with a least squares (LS) method, which was used in the PSHA from 2021. Compared to the PSHA from 2021, a significantly larger amount of earthquake observations were utilized in this work. The GR parameters and the resulting hazard estimates show a high sensitivity to the completeness evaluation of the catalog and to the estimation method used for the GR parameters. The hazard estimates show a larger variation for the Loviisa site than for the Olkiluoto site. A suggestion is made to factor in these uncertainties into the analysis by extending the PSHA logic tree.
  • Sensitivity analysis of a seismic hazard assessment for a Finnish nuclear power plant
    (2022-06-14) Visakorpi, Vili
     Perustieteiden korkeakoulu | Master's thesis
    Probabilistic seismic hazard analysis (PSHA) seeks to determine the frequency of occurrence of seismic events where ground movement exceeds a given threshold. It is a common first step in calculating seismic risk and engineering earthquake-resistant structures. Due to many uncertain parameters and modelling options, an estimate for seismic hazard can only be attained with significant uncertainty. Assessment of the uncertainties is an important part of the analysis. One of the necessary subjects to model is the magnitude-recurrence relationship of eartquake events. The relationship is usually expressed by Gutenberg-Richter law, which approximates the earthquake magnitudes to follow an exponential distribution. Its two parameters are determined empirically from a historic earthquake catalog. Operating a nuclear power plant (NPP) requires that the seismic risk to the plant is assessed. A new PSHA was developed for Loviisa NPP in 2021, taking various different modelling decisions compared to the last PSHA from 2018. In addition to the Loviisa NPP, the model created for the 2021 PSHA was used to produce a hazard estimate for Olkiluoto NPP. The aim of this thesis is to assess how much the new decisions contribute to the estimated seismic hazards of the Loviisa and Olkiluoto NPPs. The general procedure of PSHA is reviewed and a picture of the methods and decisions involved with Loviisa 2021 PSHA is formed. The recurrence estimation methods are discussed with focus on maximum likelihood estimation of the Gutenberg-Richter parameters. An open source hazard calculation software HAZ is adopted for the analysis. Sensitivity is assessed through the primary output of PSHA, the annual frequency of exceedance.
  • Verification and Sensitivity Analysis of Maximum Likelihood Estimation for Loviisa NPP Seismic Hazard
    (2023) Heikkilä, Laura
     School of Science | S harjoitus- ja seminaarityöt
    Probabilistic seismic hazard analysis (PSHA) is used for estimating the risk of earthquakes to nuclear power plants. To achieve a required level of safety, the recurrence of earthquakes of different magnitudes has to be known. This is done by estimating the parameters a and b in the Gutenberg-Richter equation. The purpose of this work is to verify the use of maximum likelihood estimation (MLE) method in the estimation of earthquake recurrence in Finland and perform a sensitivity analysis for this method. The verification and the sensitivity analysis are performed by comparing the estimated values of parameters a and b. The least squares (LS) method is included for comparison. The verification gives results similar to those from previous studies for both of the used methods. For the sensitivity analysis, changing completeness times, minimum and maximum magnitude and the width of the magnitude bins is tested. Both of the methods seem to be the most sensitive to changing the minimum magnitude, but the lack of high-magnitude events and the incompleteness of the data from the smallest earthquakes increase the uncertainty of the estimation. At the end of this paper, parameters giving the most reliable results are suggested.