Active control of a dynamically positioned vessel for the installation of subsea structures

This article presents an active control dedicated to a re-entry problem found in the offshore oil industry. The re-entry operation consists of connecting the bottom end of a very long pipeline to the wellhead, by dynamically modifying the pipeline top-end position, which is linked to a dynamically positioned vessel (DPV). Such long pipelines are usually called risers, because they are used to make the drilling mud or the hydrocarbons rise from the wellhead to the platform. Nowadays, the re-entry operation is often done manually. The use of an active control intends to reduce the operation time and to make it possible even under bad weather conditions. The considered subsea structure can be viewed as a cable submerged in a flow and modelled by the Bernoulli's cable equation, completed with a damping factor, that linearly depends on the structure speed. After some simplifications that are justified in our context, the corresponding model turns out to be differentially flat, a useful property for control design, providing an extension of previous works.