UHB Model Uncertainty for Structural Reliability Analysis of Pipeline OOS Design

2018 ◽  
Author(s):  
M. Liu ◽  
C. Cross
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Universal Design ◽  
Limit State ◽  
Load Factor ◽  
Design Curve ◽  
Design Load ◽  
Structural Reliability Analysis ◽  
Load Factors ◽  
Curve Method

Design load factor structural reliability analysis is critical for pipeline postlay OOS design to mitigate global UHB for a trenched and buried subsea pipeline configuration operating at elevated temperature and pressure. During the detailed engineering phase it is necessary to evaluate and define any measure available to be finalised for UHB mitigation such as deep trenching selection, enhanced blanket or spot rockdumping. In order account for inherent uncertainties in the design variables, a pre-emptive SRA is normally performed for the probabilistic UHB design load factors prior to pipeline installation according to the typical trench imperfection statistics and some specified survey accuracy. As per the current practice the semi-analytical universal design curve method is used in the limit state for design load factor predictions. The SRA results will be updated once the OOS survey data become available. A rockdump schedule can then be established by FEA incorporating appropriate safety or load factors to address uncertainties in the design parameters and as-built pipeline OOS survey measurement accuracy. This paper examines the UHB model uncertainties in the load factor and backfill cover assessment with a view to improving the SRA OOS analysis. Sources of uncertainties and variability in the UHB design are discussed first. Some disparity and inconsistency arising between the SRA and FEA models for the limit state are considered. Alternative UHB models are investigated by taking Timoshenko shear stiffness and associated deformation with pipe-soil interactions into consideration. A comparison is made with the conventional universal design curve method, the improved model and FE modelling to demonstrate the findings and conclusions. Of these, the pipe-soil interaction and its representation in the SRA limit state assessment are identified as a significant factor.

Subsea Pipeline UHB OOS Design: Structural Reliability Analysis

2017 ◽  
Author(s):  
M. Liu ◽  
C. Cross
Keyword(s):  
Reliability Analysis ◽  
Survey Data ◽  
Structural Reliability ◽  
Design Parameters ◽  
Load Factor ◽  
Data Sets ◽  
Subsea Pipeline ◽  
Upheaval Buckling ◽  
Structural Reliability Analysis ◽  
Load Factors

Upheaval buckling (UHB) is a major design concern for a trenched and buried subsea pipeline operating at high temperature and pressure. A predictive assessment is necessary during the detailed engineering design and optimisation to evaluate and define any measure that may be utilised for UHB mitigation such as deep trenching, backfilling, blanket or spot rockdumping. A pre-emptive UHB structural reliability analysis (SRA) has to be performed prior to pipeline installation based on the typical trench imperfection out of straightness (OOS) statistics. The SRA results are updated once survey data is made available. A rockdump schedule can be established by incorporating appropriate safety or load factors to address uncertainties in the design parameters and as-built OOS survey measurement accuracy. This paper examines the basis for processing the OOS features from survey data and stochastic distributions assumed for SRA with a view to improving the SRA OOS analysis. A number of OOS issues are considered. To cut conservatism an alternative distribution and interpretation is proposed for the key SRA input parameters with regards to imperfections and survey resolution. The random imperfection height assumption used in the current SRA practice for UHB is thus challenged — the rationale and argument for an alternative approach are constructed through a review of stochastic process theory, additional integrity criteria, a parametric analysis and evaluation of multiple OOS survey data sets. To add to the strength of the argument, a range of engineering issues are discussed in the context of stochastic distributions of imperfections. A worked example and case study is presented leading to a rationally reduced load factor and rockdump volume requirement for OOS UHB mitigation and protection.

First-Order Reliability Method for Structural Reliability Analysis in the Presence of Random and Interval Variables

2015 ◽  
Vol 1 (4) ◽  
Author(s):  
Umberto Alibrandi ◽  
C. G. Koh
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Limit State ◽  
Computational Cost ◽  
Computational Effort ◽  
Stochastic Dynamic ◽  
First Order Reliability Method ◽  
First Order ◽  
Structural Reliability Analysis ◽  
Reliability Method

This paper presents a novel procedure based on first-order reliability method (FORM) for structural reliability analysis in the presence of random parameters and interval uncertain parameters. In the proposed formulation, the hybrid problem is reduced to standard reliability problems, where the limit state functions are defined only in terms of the random variables. Monte Carlo simulation (MCS) for hybrid reliability analysis (HRA) is presented, and it is shown that it requires a tremendous computational effort; FORM for HRA is more efficient but still demanding. The computational cost is significantly reduced through a simplified procedure, which gives good approximations of the design points, by requiring only three classical FORMs and one interval analysis (IA), developed herein through an optimization procedure. FORM for HRA and its simplified formulation achieve a much improved efficiency than MCS by several orders of magnitude, and it can thus be applied to real-world engineering problems. Representative examples of stochastic dynamic analysis and performance-based engineering are presented.

A New Holistic Approach for Subsea Pipeline Upheaval Buckling Design

2017 ◽  
Vol 139 (5) ◽  
Author(s):  
M. Liu ◽  
C. Cross
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
A Priori ◽  
Holistic Approach ◽  
Load Factor ◽  
Fe Model ◽  
Potential Cost ◽  
Upheaval Buckling ◽  
Structural Reliability Analysis ◽  
Uplift Resistance

For a trenched and buried pipeline, the propensity to upheaval buckling (UHB) is a major design concern. Predictive UHB design is typically required at the outset to determine both trenching and backfilling requirements. Additional rockdump schedule can be established by analyzing post pipelay out of straightness (OOS) survey data incorporating appropriate safety factors based on a structural reliability analysis (SRA). The normal approach is to examine the as-laid pipeline imperfection survey statistics and data accuracy. The structural reliability analysis and load factor calculation are typically performed a priori based on the assumed initial imperfections using the universal design curve methodology. A new pseudo-energy method for UHB and OOS is proposed and discussed in this paper based on the variational principle and modal analysis. The approach takes into account the effects of varying effective axial force, trench imperfections, and vertical uplift resistance, by combining both axial friction and lateral resistance methods into a unified model. A new concept, effective uplift resistance and associated load, is also introduced to deal with nonuniform backfill cover. Adjacent imperfections and backfill profiles are considered in detail. A finite element (FE) model is developed to consist of three-noded quadratic pipe elements using abaqus Ver 6.12, and iterations of FE analyses are performed to demonstrate the tangible benefits of the approach specifically for UHB OOS design in relation to target trenching and backfilling, leading to improved reliability and potential cost saving in UHB OOS design and rockdump installation.

On the Application of Structural Reliability Analysis

2017 ◽  
Author(s):  
Torfinn Hørte ◽  
Gudfinnur Sigurdsson
Keyword(s):  
Reliability Analysis ◽  
Structural Engineering ◽  
Structural Reliability ◽  
Limit State ◽  
Design Analysis ◽  
Ultimate Limit State ◽  
Hull Girder ◽  
Structural Reliability Analysis ◽  
Code Calibration ◽  
Calculated Probability

Structural Reliability Analysis (SRA) is a useful tool in structural engineering. Uncertainty in input parameters and model uncertainties in the analysis predictions are explicitly modelled by random variables. With this methodology, the uncertainties involved are handled in a consistent and transparent way. Compared to a deterministic analysis, SRA provides improved insight in how the various uncertainties involved influence the results. The main results from SRA is the calculated probability of structural failure, but other useful results such as uncertainty importance factors and design points being the most likely combination of all variables at failure represent helpful information. The present paper illustrates some the features using SRA for two different types of application. The first application is the use of SRA as a tool for code calibration and the second shows the application of SRA to a problem where common practice is likely to be rather conservative and therefore leading to unacceptable results, but where the degree of conservatism is not known. Two examples are chosen to illustrate code calibration; i.e. hull girder ultimate limit state (ULS) for tankers and ULS for mooring design in the ULS for floating offshore vessels. Code calibration involves both SRA and design analysis following the code. It is shown how the design analysis can be modified in order to better reflect a chosen target reliability level across a selected set of test cases representative for what the code should cover. Fatigue of subsea wellhead systems is selected as an example of a special case when application of existing rules may lead to unsatisfactory results which are likely to be rather conservative. It is shown how results can be presented in terms of the accumulated probability of fatigue failure as a function of time. This may be a more suitable basis for decision making than a calculated fatigue life from a standard analysis. It is also illustrated how importance factors from the SRA can be used as guidance on how to prioritize effort in order to improve prediction of the fatigue damage. The present paper is not intended to be detailed in all input and analysis methodology, but draw the attention towards the possibilities and benefits of applying SRA in structural engineering, where the examples are used to illustrate this potential.

scholarly journals Structural Reliability Analysis of Limit State Functions With Multiple Design Points Using Evolutionary Strategies

2009 ◽  
Vol 10 (2) ◽  
pp. 87-97 ◽  
Author(s):  
Federico Barranco-Cicilia ◽  
◽  
Edison Castro-Prates de Lima ◽  
Luís Volnei Sudati-Sagrilo ◽  
◽  
...  
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Limit State ◽  
Evolutionary Strategies ◽  
Structural Reliability Analysis ◽  
State Functions

scholarly journals Structural Reliability Analysis of Ship Hulls Accounting for Collision or Grounding Damage

2020 ◽  
Author(s):  
Branka Bužančić Primorac ◽  
Joško Parunov ◽  
C. Guedes Soares
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Limit State ◽  
Random Variables ◽  
Bending Moment ◽  
State Equation ◽  
Limit States ◽  
Moment Capacity ◽  
Ship Hulls ◽  
Structural Reliability Analysis

AbstractClassical structural reliability analysis of intact ship hulls is extended to the case of ships with collision or grounding damages. Still water load distribution and residual bending moment capacity are included as random variables in the limit state equation. The probability density functions of these random variables are defined based on random damage parameters given by the Marine Environment Protection Committee of the International Maritime Organization, while the proposed reliability formulation is consistent with international recommendations and thus may be valuable in the development of rules for accidental limit states. The methodology is applied on an example of an Aframax oil tanker. The proposed approach captures in a rational way complex interaction of different pertinent variables influencing safety of damaged ship structure.

Subsea Pipeline UHB Design: An Improved Analysis Approach

2016 ◽  
Author(s):  
M. Liu ◽  
C. Cross
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
A Priori ◽  
Load Factor ◽  
Fe Model ◽  
Potential Cost ◽  
Subsea Pipeline ◽  
Upheaval Buckling ◽  
Structural Reliability Analysis ◽  
Uplift Resistance

A subsea pipeline operating at temperature and pressure may buckle both vertically and laterally. For a trenched and buried pipeline, the propensity to upheaval buckling (UHB) is a major design concern. Predictive UHB design is typically required at the outset to determine both trenching and backfilling requirements. Additional rockdump schedule can be established by analysing post pipelay OOS survey data incorporating appropriate safety factors based on a structural reliability analysis. The normal approach is to examine the pipeline imperfection survey statistics and data accuracy. The structural reliability analysis and load factor calculation is typically performed a priori based on the assumed imperfections using the methodology outlined in ref [1]. The additional rockdump schedule is derived from the crown of the pipeline imperfections regardless of adjacent profiles and overall backfill data. A new pseudo energy method for UHB and OOS is proposed and discussed in this paper based on the variational principle and modal analysis. The approach takes into account the effects of varying effective axial force, trench imperfections and vertical uplift resistance, by combining both axial friction and lateral resistance methods into a unified model. A new concept, effective uplift resistance and associated load is also introduced to deal with non-uniform backfill cover. Adjacent imperfections and backfill profiles are considered in detail. An FE model is developed to consist of 3-noded quadratic pipe elements using ABAQUS Ver 6.12 and iterations of FE analyses are performed to demonstrate the tangible benefits of the approach specifically for UHB OOS design in relation to target trenching and backfilling, leading to improved reliability and potential cost saving in UHB OOS design and rockdump installation.

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