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.

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.

Application of Structural Reliability Analysis to UHB Pipeline Design

2007 ◽  
Author(s):  
Terry Griffiths ◽  
Dermot O’Brien ◽  
Richard Johnson
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Coefficient Of Thermal Expansion ◽  
Design Sensitivity ◽  
Potential Cost ◽  
Upheaval Buckling ◽  
Structural Reliability Analysis ◽  
Rock Cover ◽  
Wide Range ◽  
Pipeline Design

The detailed design of pipelines susceptible to upheaval buckling (UHB) has been characterized by a wide range of industry approaches, especially in relation to selecting safety factors. DNV recently published the draft of their HT/HP design code (RP-F110), which aims to codify and unify approaches to this aspect of design. Sensitivity analysis using Structural Reliability Analysis (SRA) methods highlights the sensitivity of rock cover requirements to a small number of key parameters. Several of these, including: Young’s Modulus; coefficient of thermal expansion; rock cover height; and rock uplift resistance factor have very poor published public domain statistically measured information on mean and variance. It is clear that the early identification in a project of the potential cost benefits associated with an SRA approach maximizes the potential to collect good project-specific statistical data. The clear benefit is an improvement in overall safety by ensuring a more uniform probability of failure along the pipeline which meets target reliability levels.

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.

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.

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