One of the main risk factors for subsea pipelines exposed on the seabed is fatigue failure of free spans due to ocean current or wave loading. This paper describes how the structural response of a free span, as input to the fatigue analyses, can be assessed in a simple and still accurate way by using improved beam theory formulations. In connection with the release of the DNV Recommended Practice, DNV-RP-F105 “Free Spanning Pipelines”, the simplified structural response quantities have been improved compared to previous codes. The boundary condition coefficients for the beam theory formulations have been updated based an effective span length concept. This concept is partly based on theoretical studies and partly on a large number of FE analyses. The updated expressions are general and fit all types of soil and pipe dimensions for lower lateral and vertical vibration modes. The present paper focus on estimation of simplified response quantities such as lower natural frequencies and associated mode shapes. Hydrodynamical aspects of Vortex Induced Vibrations (VIV) are outside the scope of this paper.
The present paper describes a reliability-based design procedure against upheaval buckling of rock or soil-covered pipelines. The failure mode considered is “snap-through” buckling. The study is performed using state-of-the-art design methodologies, including an assessment of all known uncertainties related to the load and capacity, measurements, surveys, and confidence in the applied models. A response surface technique is applied within the level III reliability analysis. Target safety levels are discussed for both SLS and ULS conditions, and a case-specific reliability-based calibration study is performed in order to derive a consistent design format.
The Ormen Lange subsea pipeline shall be designed to meet a specified risk acceptance criterion, established by consideration of failure probability and consequences of failure. Traditional design for Vortex Induced Vibrations (VIV) of free spans limits the allowable free span length and implies that interventions work may be required. Through a risk based approach the probability of fatigue failure of free spanning pipelines is quantified, and the governing uncertainties identified. A sensitivity analysis of different risk control options is performed. The outcome facilitates to focus in the design process such that a preferred design solution can be identified and implemented via testing campaign in the design stage, prelaid rectification activities and inspection programs. The aim is to obtain a cost efficient design that comply with the given acceptance criterion. Best practices as reflected in DNV-RP-F105 “Free Spanning Pipelines” and updated field specific design guidelines form the basis for the analysis. A probabilistic module is implemented on top of DNV-RP-F105 methodology, which allows application of a dedicated uncertainty modeling for a specific project. Parameters considered include: Pipeline properties, effective axial force, span length and gap, soil properties, ocean current (distribution, depth and directional variation), multiple mode response analysis (VIV response models, natural frequencies, damping, effect of concrete, static deflection), different SN curves, strakes and monitoring. Both the As Laid phase and the Operational phase are considered for different locations along the pipeline route.
With the rapid development of wind energy technologies and growth of installed wind turbine capacity in the world, the reliability of the wind turbine becomes an important issue for wind turbine manufactures, owners, and operators. The reliability of the wind turbine can be improved by implementing advanced fault detection and isolation schemes. In this paper, an observer-based fault detection and isolation method for the cooling system in a liquid-cooled frequency converter on a wind turbine which is built up in a scalar version in the laboratory is presented. A dynamic model of the scale cooling system is derived based on energy balance equation. A fault analysis is conducted to determine the severity and occurrence rate of possible component faults and their end effects in the cooling system. A method using unknown input observer is developed in order to detect and isolate the faults based on the developed dynamical model. The designed fault detection and isolation algorithm is applied on a set of measured experiment data in which different faults are artificially introduced to the scaled cooling system. The experimental results conclude that the different faults are successfully detected and isolated.
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