Pipeline Rotation Analysis &amp; Modeling During S-Lay Installation

The safety of offshore pipelines during installation has drawn a great deal of attention due to the combined actions of high external pressure, axial tension and bending moment. Subsea pipelines have the tendency to rotate during installation. This rotation can have multiple causes which are interfering with each other. During S-lay installation, the pipe is exposed to plastic strains when it passes over the stinger, exceeding a certain curvature. That residual curvature causes the pipeline to rotate along its suspended length. Additional causes which contribute to pipe rotation are possible tensioner misalignments, pipeline curves or vessel offsets. Pipeline rotation is also dependent on other factors such as water depth, pipeline characteristics (bending stiffness, submerged weight, etc) and stinger configuration. Pipe rotation is not permissible if inline structures (valves, connections) are installed with the pipeline, it is therefore important to quantify the safety against roll for a given residual strain in the pipe due to plastic deformations over the stinger. The goal of this thesis is to accurately quantify pipeline rotation during installation of inline structures with S-lay method. A sequential model is built based on mechanical principles in order to solve the pipelay and rotation problem simultaneously and identify the effect of the plastic strains and residual curvature on the rotation phenomenon. The model includes also mitigation measures (buoyancy modules) and their effect in the reduction of total rotation as well as the effect of soil friction. The report consists of two main parts. The first part is the analytical mathematical modelling and the numerical solution of the pipe-laying problem, considering the pipeline as tensioned beam and solving the nonlinear bending equation along its suspended length using finite difference method. The second part consists of the rotation problem analysis and solution. Having found the pipeline configuration and its physical quantities along the length, the pipe rotation profile is found based on Hamilton's energy minimization principle using the Lagrangian equation, including soil friction and buoyancy module effect. Finally, a sequential model which simulates the installation of a pipeline including inline structures and buoyancy modules is built in order to investigate the roll profile evolution during real operations. A number of different cases studied based on actual projects were conducted to determine the pipeline configuration and its physical quantities (bending moment, strain, axial tension) along its suspended length. The validity of the pipe-laying model is verified by means of a comparison with results obtained from the commercial finite element software OFFPIPE. Rotation results are verified by results observed in actual projects.

Subject

offshore pipelines
pipeline rotation
S-lay Method
residual curvature
energy minimization

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http://resolver.tudelft.nl/uuid:d93866a3-f1e4-4636-a893-ad113867011d

Embargo date

2020-08-01

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Student theses

Document type

master thesis

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PIPELINE_ROTATION_ANALYSI ... LATION.pdf

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