This paper is based on the experience made during the execution of a project recently completed in West Africa, where a number of production and injection lines (10” and 6”, in 650m water depth) were found susceptible to pipe walking.
To stop pipe walking, measures have been taken, consisting in connection points installed in line during the laying of the pipelines (J-laying) and then anchoring structures, post-installed.
The Pipe Walking Mitigation Structures (PWMS) are skirted structures, sized to provide anchoring forces up to 95mT, with a max 32m × 17m footprint, and a dry weight up to 150mT.
This paper presents the engineering, fabrication and installation of these PWMSs (8off).
The engineering was driven by layout and schedule constraints. The layout, that at the start of the detailed engineering was almost frozen and was already quite congested, did not allow the use of solutions based on anchoring the pipelines at their ends. Therefore, the chosen solution was the placement of anchors along the pipeline.
In not more than 6 months, the detailed engineering had to go through the screening between alternative concepts, the validation of the chosen solution and the engineering for the procurement, fabrication and installation activities.
The tight schedule required to maximize the post-installation of the structures. Only the connection points were laid at the same time of the installation of the pipeline, and were installed 4 months ahead of the installation of the anchoring structures. The schedule has benefited of a standardization of the structures that made the fabrication and the installation engineering easier.
To give additional flexibility to the schedule, the latest time by when, during the operation, a PWMS was to be installed, was defined, considering the expected anticipated number of shutdown/restart cycles and the maximum displacements that could be accommodated by the tie-in structures. Only 2 out of 8 PWMS were required to be installed before the start-up, whilst the installation of the others could be postponed, to a maximum of 4 years from the start of the operations. Furthermore, on a number of locations, only the connection points were installed, where the anchoring structures can be retro fitted, should they be required based on monitoring of the pipeline behavior during operation.
Pipe walking is a well understood phenomenon — However, there are uncertainties in the key parameters impacting the results. This paper discusses the main contributing factors, and how the uncertainties have been tackled.
A monitoring plan has also been laid down — The purpose is to verify the design of the mitigations and also to gather in service information that may allow to defer further the installation of the PWMS or even to avoid their installation.
The post installed anchoring structures are quite large and heavy structures. A close interfacing between design engineering and installation engineering — since the early inception phase — allowed to achieve a safe installation and an accurate positioning of the structures, with the tight installation tolerances that were a key for the PWMS to provide the intended function.
The paper presents the lessons learnt, from the engineering, the fabrication and the installation of the PWMS.
The paper provides also some recommendations for further optimizations of the proposed solution, which will allow savings in costs and schedule in future projects.
Pipe walking is a ratcheting phenomenon — As such, the foreseen time by when the accumulation of the pipe walking could become excessive can be assessed, and this time can be used to calibrate the response models and gather more reliable data about the operation of the system.
The solution proposed allows to minimize the initial investments as only the connection points are to be installed together with the pipelines, while it maximizes the postinstallation of the mitigations, so not impacting the schedule for pipeline installation. A modular design of the structures would allow the use of lighter support vessels/construction vessels which will give more flexibility for a deferred installation of the mitigation structures, as retrofitting.