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Trondheim, Norway

Gaidai O.,MARINTEK AS | Storhaug G.,DNV GL | Naess A.,Norwegian University of Science and Technology
11th International Symposium on Practical Design of Ships and Other Floating Structures, PRADS 2010 | Year: 2010

Paper describes a method for prediction of extreme ship roll angles occurring during sailing in harsh weather. Rolling is coupled with other ship motions and exhibits highly nonlinear behavior. Ship capsizing probability is an extremely important issue in the ship design. Due to nonstationarity and complicated nonlinearities of both waves and ship motions, it is a considerable challenge to model such a phenomenon. In case of extreme motions role of nonlinearities dramatically increases, activating effects second and higher order. Moreover lab tests may also be questioned because of the scaling and the choice of sea state. Therefore data measured on the actual ships during their voyages in harsh weather provides a unique insight into statistics of ship motions. The aim of the work is to develop specific methods which make possible to extract the necessary information about the extreme response from relatively short time history. The method proposed in this paper opens up the possibility to predict simply and efficiently both short-term and long-term extreme response statistics. © 2010 COPPE/UFRJ.


Gaidai O.,MARINTEK AS | Storhaug G.,DNV GL | Naess A.,Norwegian University of Science and Technology
11th International Symposium on Practical Design of Ships and Other Floating Structures, PRADS 2010 | Year: 2010

This paper describes a method for prediction of extreme stresses measured in deck amidships of a container vessel during operation in harsh weather. The nonlinear structural response known as "whipping" is studied. It refers to transient vibration response of the hull girder due to wave impacts mainly in the bow area. For the safety of crew and cargo it is essential to assess all aspects of the wave loading to ensure that ships are built to endure such extreme storm events. Due to non-stationarity and complicated nonlinearities of both the waves and the ship response, there are no reliable numerical simulation tools available for predicting the extreme response including whipping. Moreover, lab tests may not fully reproduce the critical conditions that may occur in reality. Therefore measurements on real ships during their voyages in harsh weather provide unique insight into statistics of the measured responses. The aim of the work is to develop a specific method which makes it possible to extract the necessary information about the extreme response from relatively short time series. The method proposed in this paper opens up for the possibility to predict simply and efficiently both short-term and long-term extreme response statistics. © 2010 COPPE/UFRJ.


Naess A.,Norwegian University of Science and Technology | Stansberg C.T.,MARINTEK AS | Batsevych O.,Kongsberg Maritime
Journal of Offshore Mechanics and Arctic Engineering | Year: 2011

The paper presents a study of the extreme value statistics related to measurements on a scale model of a large tension leg platform (TLP) subjected to random waves in a wave basin. Extensive model tests were carried out in three irregular sea states. Time series of the motion responses and tether tension were recorded for a total of 18 three hour tests (full scale). In this paper we discuss the statistics of the measured tether tension. The focus is on a comparison of two alternative methods for the prediction of extreme tether tension from finite time series records. One method is based on expressing the extreme value distribution in terms of the average upcrossing rate (AUR). The other is a novel method that can account for statistical dependence in the recorded time series by utilizing a cascade of conditioning approximations obtained by defining the average conditional exceedance rates (ACER). Both methods rely on introducing a specific parametric form for the tail part of the extreme value distribution. This is combined with an optimization procedure to determine the parameters involved, which allows prediction of various extreme response levels. © 2012 American Society of Mechanical Engineers.


Gaidai O.,MARINTEK AS | Ye N.,MARINTEK AS | Jin J.,ExxonMobil | Reid D.,ExxonMobil | Maincon P.,Sintef
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2015

Dynamic umbilicals are usually exposed to cyclic environmental loads due to waves, motions of the host vessel, and current induced vibrations. These cyclic loads drive the fatigue design of functional components such as armor wires, steel tubes and conductor wires in an umbilical cross section. Fatigue assessment of umbilical components involves Finite Element Method (FEM) modelling both globally and locally. The "global" model is used to predict dynamic responses of an umbilical with equivalent cross-sectional properties in waves and currents. Stress calculation in umbilical components requires a special "local" FEM tool such as Uflex2d to correctly capture the interaction mechanics among the components within the cross-section. This paper compares three methods for the combination of global and local analysis in order to obtain fatigue estimates. First method can be regarded as exact one and is used to benchmark other two, which are approximate. Copyright © 2015 by the International Society of Offshore and Polar Engineers (ISOPE).


Ushakov S.,Sudan University of Science and Technology | Valland H.,Sudan University of Science and Technology | Nielsen Jo.B.,MARINTEK AS | Hennie E.,MARINTEK AS
Fuel Processing Technology | Year: 2014

Particle emission characteristics were studied from heavy-duty diesel engine operating on fuels with sulfur levels relevant to marine operation, i.e. 0.05% S and 3% S respectively. Effects of primary dilution temperature (PDT) and primary dilution ratio (PDR) were investigated together with effect of filter media and time of filter conditioning. PDT increase was found slowing down nucleation rate due to increase of saturation vapor pressures of volatile species. In turn, increasing PDR reduces partial pressure of exhaust species and hence weakens both homogeneous and heterogeneous nucleation. All these effects are amplified by high sulfur content in marine fuels which increases available amount of nucleation-prone vapor-phase semivolatile compounds. At the same time, water condensation artifact was observed at PDR = 3. No filter type was found to be overwhelmingly superior as certain positive and/or negative measurement artifacts are inherently associated with all filter materials. The filter conditioning time was also found to cause substantial PM mass variation, as control over VOC take up from (or lost to) laboratory air and hydration of sulfuric acid is required. The standard 24 hour conditioning time was found insufficient to reach complete PM mass equilibrium, so longer time is required when measuring from high-sulfur fuels. © 2013 Elsevier B.V.

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