Principles of structural safety studies

This chapter addresses principles of advanced structural safety studies in association with various types of extreme and accidental events. The structural consequences of extreme conditions and accidents are inevitably volatile, uncertain, complex, and ambiguous (VUCA). Methods to model random parameters affecting such extreme conditions and accidents are presented. The importance of limit states- and risk-based approaches is emphasized to manage VUCA environments. Future trends toward advanced structural safety studies are addressed

Ultimate compressive strength of deteriorated steel web plate with pitting and uniform corrosion wastage

Steel structural members are likely exposed to corrosive environments, and thus corrosion is one of the dominant life-limiting factors of steel structures. Extensive studies on the effectsof pitting and uniform corrosion on the strength performance of steel structural members under a wide variety of loading conditions have been undertaken to assess the relationship between pitting corrosion intensity and residual strength. The aim of this study is to investigate the ultimate compressive strength characteristics of steel web plate elements with pit and uniform corrosion wastage. A series of ABAQUSnonlinear elastic-plastic large deformation finite element analyses are carried out on I-shapedsection steel girder models with varying pittingcorrosion intensities. Artificial pitting of different intensities is considered on the web plates and a uniform loading applied vertically on the upper flange section. The ultimate load-carrying capacity of deteriorated models with different levels of uniform thickness loss is also studied. The results are applied to assessing the ultimate compressive strength of web plates with different pitting corrosion intensities and a uniform loss thicknessby developing design formulae that represent the average loss thickness versus the ultimate load-carrying capacity

Numerical study of erosion in critical components of subsea pipeline: tees vs bends

Elbows are a vulnerable part of piping systems in erosive environments. Traditionally, plugged tees are used instead of elbows when the erosion rate is high. However, the advantage of plugged tees over elbows in large-scale pipelines is unclear. A comprehensive computational fluid dynamics study was carried out to predict the erosion rate in plugged tees and elbows. A numerical method was first used for aluminium elbows and tees with available experimental data through which the accuracy of the numerical solution was verified. After validating the model, numerical modelling was used to compare the erosion rates of plugged tees and elbows in varying geometrical conditions, ranging from 0.0254 to 0.6 m diameter carbon steel pipes transmitting multiphase gas/sand flow. The effects of internal flow velocity and sand particle size on erosion rates were also investigated. The numerical results revealed that the erosion ratio between plugged tees and elbows strongly depends on the internal diameter of the pipe, the flow velocity and particle size. Hence, the influence of these parameters should be considered for proper selection of the fittings to be used. Finally, numerical modelling of erosion in two subsea jumpers outfitted with standard elbows and plugged tees was presented

A numerical study on water wetting associated with the internal corrosion of oil pipelines

Long distance pipelines are considered as the vein of the oil and gas industry on land and offshore. A well often produces water along with crude oil. The presence of water as well as dissolved gases such as CO2 and H2S introduces a serious menace of internal corrosion. It is well known that the distribution of water and oil inside the pipeline has a great influence on the corrosion rate. As a matter of fact, internal corrosion occurs when a free layer of water comes in contact with the pipe. Hence, predicting the distribution of water inside the pipe and identifying the continuous phase that directly wet the wall is of foremost importance when dealing with internal corrosion of oil pipelines. The accurate prediction of the distribution of water significantly increases the accuracy of corrosion prediction as well as the confidence regarding the integrity of the pipelines. In spite of all the great efforts toward studying different influential factors associated with the internal corrosion of steel pipelines, a large gap of knowledge is observed in predicting the water wetting. The objective of the present study is to employ a tuned two-fluid model by taking advantage of computational fluid dynamics, that is capable of predicting the distribution of water and the type of wetting (water wetting/oil wetting) at the bottom of the pipe. Furthermore, the effect of different parameters such as pipe diameter, oil density, oil viscosity and interfacial tension on the transition from water wetting to oil wetting is studied

Proposed formulas for evaluation of the equivalent material properties of a multiholed structure

It is widely known that the development of fine mesh in the immediate vicinity of the holes in a multiholed plate is a challenging issue. In practice, due to the amount of time consumed and the quality of the modelling, it is not desirable to model the structural characteristics of a multiholed plate in detail. In this regard, an effective means by which to avoid the unnecessary work of simulating a multiholed plate is to replace it with an equivalent solid plate while considering the decrease in stiffness associated with the increasing area of the holes. The objective of this study is to numerically and experimentally investigate the equivalent material properties of a multiholed plate of stainless steel 316L with respect to ligament efficiencies. Simple design formulas are proposed to determine the equivalent material properties of a multiholed plate that is completely perforated with closely spaced circular holes in a square or diagonal pattern by means of nonlinear finite-element method computations. It is concluded that the proposed formulas are accurate for prediction of the equivalent material properties of multiholed structures for their design and engineering

Ultimate Compressive Strength Computational Modeling for Stiffened Plate Panels with Nonuniform Thickness

© 2020, The Author(s). The aim of this paper is to develop computational models for the ultimate compressive strength analysis of stiffened plate panels with nonuniform thickness. Modeling welding-induced initial deformations and residual stresses was presented with the measured data. Three methods, i.e., ANSYS finite element method, ALPS/SPINE incremental Galerkin method, and ALPS/ULSAP analytical method, were employed together with existing test database obtained from a full-scale collapse testing of steel-stiffened plate structures. Sensitivity study was conducted with varying the difference in plate thickness to define a representative (equivalent) thickness for plate panels with nonuniform thickness. Guidelines are provided for structural modeling to compute the ultimate compressive strength of plate panels with variable thickness

Quantitative collision risk assessment of a fixed-type offshore platform with an offshore supply vessel

Offshore installations are designed to withstand against potential collisions from offshore supply vessels (OSV). Quantitative risk assessment (QRA) provides an overall picture of expected collision frequencies and consequences for the design life of a platform and subsequently estimates the damage repair cost. However, the main challenge of the QRA study is how well various uncertainties are implemented into the model. This study aimed to introduce and demonstrate an advanced and efficient QRA model for the collision between an OSV and a jacket type offshore platform. A set of fifty collision scenarios were selected using probabilistic sampling techniques, and vessel motion analyses were performed to determine collision load characteristics. Extensive nonlinear structural crashworthiness analyses were conducted using advanced computational modelling techniques, and the repair cost of the damaged brace and column members were calculated. Probability exceedance diagrams were established for different consequence parameters, and asset risks were calculated. A comparison study of the design values of damage parameters and repair costs were carried out in association with the NORSOK and HSE risk acceptance criteria. A sensitivity study was carried out to study the effects of various collision load parameters on the structural consequences. The methods and insights developed in this study could be applied to both new and existing offshore platforms and practically useful for the platform owners to aid in their decision-making process towards the risk-based safety analysis of offshore platforms

An improved procedure for generating standardised load-time histories for marine structures

The load sequence effect has been proved from laboratory tests to be an influencing factor that cannot be neglected in the fatigue analysis of marine structures. To take account of this significant factor, fatigue life prediction should be based on fatigue crack propagation theory rather than the currently used cumulative fatigue damage theory. Accordingly, fatigue loading needs to be provided as the load-time history in the time domain rather than the load spectrum in the frequency domain. A general procedure for generating the standardised load-time history for marine structures based on a short-term load measurement has been proposed by the authors. This article seeks to further improve on this procedure and explain how to apply the determined standardised load-time history in the fatigue life prediction method based on the fatigue crack propagation theory. Finally, generation and application of a standardised load-time history are given for a tubular T-joint of an offshore platform, which demonstrates the practical and effective use of the proposed approach