Multipurpose barges are used for accommodations, maintenance, equipment transport and installation and pipelay. For all these operations, the correct understanding of the motions of the vessel is essential for safety and to increase the uptime in the desired region. The motions of barges are well evaluated by the use of potential theory, but due to the presence of nonlinearities, asymmetries and devices such as bilge keels and stinger, the determination of the unit motions can be challenging, especially for roll motions. This work presents several results of motions, obtained from model tests and numerical calculations, for different configurations of a barge, with a focus on roll motion, but also considering heave and pitch, with and without the presence of bilge keels and stinger.
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.
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.
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.
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.
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