Thermal gradients from a heating front travelling down a flowline at start-up can cause a flowline to walk much like a worm creeps by repeated contractions and expansions of its body. To stop this for the Malampaya flowline, pipe-clamping mattresses (PCMs) were invented, developed, and deployed within a period of 12 months. The objective of this paper is to share the knowledge and experience from this novel but effective solution to mitigate pipeline walking.
PCMs provide a cost-effective alternative to rockdump or conventional mattresses to axially restrain a pipeline at a location chosen so that the required restraint capacity is minimized. They are inspired by conventional mattresses and bear some similarity to them, but they are designed so that the weight of the mattress acts to clamp the pipeline with a high leverage. Thus 100% of the weight of the mattress is effective in generating axial friction with the seabed. This solution can be applied at any point along the line (chosen to minimize the required resistance) without requiring flanges or collars on the pipeline.
From the most recent survey results 15 PCMs with a dry weight of around 9 tons per PCM, plus 7 tons for the logmat installed over every PCM appear to be effective to stop the walking of the Malampaya flowline. This performance is as expected from extensive analysis (FE and otherwise) to reproduce the observed walking behavior prior to restraining, to estimate the required restraint capacity, and to estimate the resistance provided by the PCMs.
This paper describes the PCM, the clamping forces they generate by leveraging the weight of the PCM and logmats installed over them, and how the friction generated with the soil is estimated from interface shear tests on samples collected from the site, considering cyclic pore pressure generation and dissipation effects. It also briefly covers FE analyses to reproduce the observed walking behavior, and determine the required restraint capacity, the PCM fabrication, installation, and monitoring of the post-installation performance.