Bruno Reis Antunes

Analytical Formulation of Distributed Buoyancy Sections to Control Lateral Buckling of Subsea Pipelines

Subsea pipelines designed to operate under high pressure and high temperature (HP/HT) conditions tend to relieve their axial stress by forming buckles. Uncontrolled buckles can cause pipeline failure due to collapse, low cycle fatigue or fracture at girth welds. In order to control the lateral buckling phenomenon, a methodology was recently developed which consists of ensuring regular buckle formation along the pipeline. Distributed buoyancy is one of the most reliable initiation techniques utilized for this purpose which have been recently applied in some projects. The behavior of pipeline sections with distributed buoyancy is normally evaluated by Finite Element Analyses (FEA) even during preliminary design when analytical models could be adopted. FEA are also utilized in order to support reliability calculations applied within buckle formation problems. However, the referred analyses are usually time-consuming and require some experience to provide good results. This paper presents an analytical formulation for a pipeline section with distributed buoyancy, which can be utilized during preliminary design in order to evaluate the influence of buoyancy sections over buckle shape, feed-in length, tolerable Virtual Anchor Spacing (VAS), etc. Regarding buckle formation, this paper also presents a methodology to determine an expression for the critical buckling force to be used as part of the limit state function in reliability analyses, which combines the results obtained from the referred analytical formulation with Hobbs infinite mode.© 2010 ASME

Assessment of Lateral Buckles in a HP/HT Pipeline Using Sidescan Sonar Data

During design stage of high pressure/high temperature pipelines, some conservative parameters are adopted along with sensitivity analyses to assure safe operation in the presence of uncertainties that influence buckle formation, e.g. pipe-soil interaction, as-laid out-of-straightness and initial heat-up. After operation starts and lateral buckles appeared along the line, a survey may provide valuable information regarding confirmation of the design assumptions, evaluation of actual behaviour and the possibility of increase the operating conditions. This work presents the methodology applied to analyse the configuration of the P-53/PRA-1 12″ oil export pipeline in operation using data from a sidescan sonar survey. The aim of such analyses was to gather information for an FE model calibration as well as to obtain preliminary estimates for the bending strains at lateral buckling locations. Special attention was dedicated to smoothing and interpolation of the pipeline coordinates extracted from sonar imagery in order to avoid unrealistic strains estimates.Copyright

Evaluation of the Operational Behaviour of a Deepwater Oil Export Pipeline Designed on the Basis of Controlled Lateral Buckling

An account is given of the methods used to evaluate the operating structural performance of a reel laid deepwater oil HP/HT pipeline which had been designed based on the controlled lateral buckling principle. The objective was to develop a finite element (FE) model of the line based on its operating status and to use the model to confirm its present and future structural integrity. The line is surface laid on a fairly undulating soft clay seabed at its deep end and sand at the shallower end. It incorporates three different man-made buckle triggering mechanisms of buoyancy modules, dual sleepers and locally increased lateral curvature along its entire length. The steps involved in the inclusion of the in-situ operating condition of the pipeline, provided through various surveys made of the as-built and operating line and historical records of operating temperatures and pressures and flow rates made at inlet and outlet of the line, into the FE model, is discussed. Several key considerations essential for the successful development and validation of such an operation-based FE model, and for completion of the evaluation task, are highlighted in the paper. Also, a specific challenge encountered as a result of changes in regulatory guidelines on engineering critical assessments, from initial design to current evaluation stage, is discussed. The evaluation has demonstrated that it is feasible to carry out in-situ assessments of laterally buckling subsea lines, and that such assessments can provide not only reliable information regarding current and future structural integrity of the lines, but also invaluable confirmation of initial design data and rationale. This comparison between initial design and the actual operating behavior of the line is not included in this paper but will be described in detail in a future separate paper.Copyright

Initial Approach to Assess Lateral Buckling Behavior: Comparison Between Design and Operational Condition of Offshore Pipeline

Recently Petrobras has been developing a production module of Roncador field through the P-52 platform in the Campos Basin, offshore Brazil. This platform is a floating production facility located in deep water and was tied back to the PRA-1 platform in shallow water by an 18-inch pipeline in order to export the oil production. This pipeline operates under high pressure and high temperature (HP/HT) conditions and was laid on the seabed. As a result of the extreme operating conditions, this pipeline is highly susceptible to lateral buckling and a buckle initiation strategy based on triggers to control the buckling behavior was designed. Thus sleepers and distributed buoyancies were designed and installed along the pipeline route. In addition to the buckles at the triggers, some additional, on-bottom, buckles were assessed in order not to compromise the design strategy. In recent geophysical data surveys carried out along the route length with the pipeline in operation, both engineered and on-bottom buckles were identified. This paper aims to present the thermo-mechanical design of the P-52 oil export pipeline, performing a comparison between some results obtained in design and observed during operation. Thus this paper intends to evaluate the pipeline as-built plus the operational pipeline configurations, and to assess the robustness of the design strategy applied regarding lateral buckling behavior.Copyright

Corrosion Protection of a Pipeline Section Inside an HDD Coastal Landfall

Directional boring commonly known as Horizontal Directional Drilling (HDD) is a technique widely employed in landfall areas. HDD provides advantages of crossing considerable lengths with local soil disturbance and low environmental impact, which has contributed to turn it into one of the preferred techniques between various authorities.In despite of these advantages, the corrosion protection of the HDD is complex due to the unfeasibility of installation of sacrificial anodes distributed uniformly along the HDD, which would require a remote protection engineering solution as an alternative. The remote solution adopted, the level of coating damage on the pipeline after the installation, the corrosiveness and resistivity of the surrounding environment, in particular when very different environments encounter, i.e. sea-sand-rock, are major uncertainties that must be considered on the design. In addition, the lack of recommendation on international standards and the difficulties involved with the direct inspection of the pipeline section inside the HDD increases the robustness required for the design and its verification.This paper presents the corrosion protection system of a gas line section inside a HDD in a landfall area, describing the combination of pipeline coating and cathodic protection systems both specified to resist the service life of the pipeline. After a brief review of published works related to this subject, this paper will focus on the coating and cathodic protection design presenting not only the premises, design data and methodology adopted, but also associated results and installation issues. Computing modeling and recommended practices are used to identify the best and the most feasible solution. Finally, conclusions and recommendations will be presented in order to support future projects with similar challenges.Copyright

Lateral Buckling and Walking Design of a Pipeline Subjected to a High Number of Operational Cycles on Very Uneven Seabed

Global buckling is a behavior observed on subsea pipelines operating under high pressure and high temperature conditions which can jeopardize its structural integrity if not properly controlled. The thermo-mechanical design of such pipelines shall be robust in order to manage some uncertainties, such as: out-of-straightness and pipe-soil interaction. Pipeline walking is another phenomenon observed in those pipelines which can lead to accumulated displacement and overstress on jumpers and spools. In addition, global buckling and pipeline walking can have strong interaction along the route of a pipeline on uneven and sloped seabed, increasing the challenges of thermo-mechanical design.The P-55 oil export pipeline has approximately 42km length and was designed to work under severe high pressure and high temperature conditions, on a very uneven seabed, including different soil types and wall thicknesses along the length and a significant number of crossings. Additionally, the pipeline is expected to have a high amount of partial and full shutdowns during operation, resulting in an increase in design complexity. During design, many challenges arose in order to “control” the lateral buckling behavior and excessive walking displacements, and finite element analysis was used to understand and assess the pipeline behavior in detail.This paper aims to provide an overview of the lateral buckling and walking design of the P-55 oil export pipeline and to present the solutions related to technical challenges faced during design due to high number of operational cycles. Long pipelines are usually characterized as having a low tendency to walking; however in this case, due to the seabed slope and the buckle sites interaction, a strong walking tendency has been identified. Thus, the main items of the design are discussed in this paper, as follows: lateral buckling triggering and “control” approach, walking in long pipelines and mitigate anchoring system, span correction and its impact on thermo-mechanical behavior.Copyright

Safety Factors Calibration for Wall Thickness Design of Ultra Deepwater Pipelines

Wall thickness often presents a considerable influence in offshore pipeline capital expenditure (CAPEX). This influence is enhanced in design of ultra deepwater trunk lines of large diameter, where any wall thickness increase provides a huge impact on project costs. In ultra deepwater scenarios, thicker pipelines may eventually implicate not only in higher costs, but may also compromise the project feasibility due to installation load constraints related to laying vessels availability.One potential way to reduce the pipeline wall thickness is to calibrate fitness-for-purpose safety factors through application of structural reliability methods, instead of utilizing the standardized safety factors presented in international codes. Since mid-nineties, several offshore pipeline design codes have been allowing the calibration of safety factors by structural reliability analysis. The purpose of such an allowance is that structural reliability methods would eliminate some eventual conservatism presented in the safety factors proposed by codes. Although this enables the achievement of optimized safety factors, more than fifteen years have passed and only few pipeline projects have taken advantage of the benefits of safety factor calibration.This paper evaluates which potential benefits are available through safety factor calibration, particularly for wall thickness reduction purposes in ultra deepwater pipeline design. Calibrated safety factors are presented for some scenarios related to ultra deepwater export pipelines, considering “system collapse criteria” limit state. The calibrated safety factors are compared with the standardized safety factors presented by international pipeline design codes. The potential for safety factor reduction by the utilization of linepipes with more stringent manufacturing tolerances and the consideration of the thermal ageing imposed by coating application are also discussed.Copyright

Comparison of Design and Operational Behaviour of an Offshore Pipeline With Controlled Lateral Buckling

It is imperative to adopt some conservative premises in the engineering calculations undertaken during the design stage of an offshore pipeline susceptible to lateral buckling, in order to achieve a design with adequate levels of robustness and integrity throughout the pipeline’s design life. The conservatism can be attached to many uncertainties such as the pipe-soil interaction — interpreted as-soil friction factors — the seabed stiffness and profile and even the as laid lateral out-ofstraightness. Once in operation, these effects will come into play and the pipeline may behave slightly differently to that anticipated in design, depending on the relative strength of the natural uncertainties compared to the design features such as engineered buckling triggers. The over-riding intention in design is, of course, to enable the pipeline to withstand, with sufficient safety margins, the maximum stresses and strains anticipated to occur by realistic predictions in the design stage.In recent years, many kilometres of deepwater pipelines have been designed and installed along the Brazilian coast using the principle of controlled lateral buckling, in which engineered buckle triggers, such as sleepers and distributed buoyancy sections, are deployed at regular intervals along the pipeline. The purpose of these triggers it to initiate a sufficient number of benign buckles along the pipeline and thereby relax the compressive forces set up as a result of thermal expansion without violating safe limits on stress and strain in the pipelines. In addition to the engineered buckling sites, however, the natural seabed features and associated uncertainties will interact with the pipeline’s behaviour and create additional natural buckle sites. To anticipate these sites and discover their importance at the design stage is recognised as a real challenge, particularly as precise post-installed and in-operation surveys are not normally carried out with the intention of confirming such buckle sites and design assumptions.The work reported in this paper is a detailed comparison between the initial design and observed operational behaviour of an offshore HP/HT pipeline, mainly in terms of the engineered and natural buckles actually formed along the pipeline, the severity of these buckles and some conclusions concerning the effects of initial imperfections and pipe-soil interaction characteristics considered in detailed design. It is hoped that this rare feedback from real operating conditions on installed pipelines, will be of great interest to pipeline designers and lead to more efficient and better understood design processes and encourage Operators to undertake more regular and sophisticated surveys of operating and installed pipelines for the benefit of future projects.Copyright