A Robot for Removing Hydrate and Paraffin Plugs in Offshore Flexible Lines – Development and Experimental Trials

Abstract

Methane Hydrates and Paraffin Plugs in flexible lines are concerns in offshore production. They may stop wells for months, causing high financial losses. Sometimes, operators use depressurization techniques for hydrate removal. Other strategy is using coiled tubing or a similar unit in order to perform local heating or solvent injection. However, frequently these strategies are not successful. In those cases, a rig may perform the operation or the line may be lost. This project developed a robotic system in order to perform a controlled local heating and remove obstructions. The robotic system developed is able to access the line from the production platform. It uses a self-locking system in order to exert high traction forces. An umbilical with neutral buoyancy and low friction coefficient allows significant drag reduction. It allows moving upwards and in pipes with a large number of curves. Coiled tubing and similar units cannot do that. Carbon fiber vessels and compact circuits give flexibility to move inside 4-inch flexible pipes. A novel theoretical model allows the cable traction calculation using an evolution of the Euler-Eytelwein equation. Experimental tests validated this model using curved pipes, both empty and filled with a fluid and using different loads. Experimental tests also validated the external layer traction resistance. Furthermore, the carbon fiber vessels were pressure tested, indicating a collapse resistance of more than 550 bar (8.000 psi). In addition, exhaustive tests of the onboard electronics and of the surface control system guarantee the communication reliability. Additionally, the 25 kN (5.6 kip) traction system was modeled theoretically considering the self-locking system, the contact with the wall and a diameter range. Four prototypes allowed to: a) compare hydraulic and electric drive systems, b) validate the self-locking mechanism up to its limit, c) analyze the hydraulic system for leg opening and translation and d) prove the traction capacity. Finally, a theoretical model for the local heating system was developed. The system experimental validation on a cooled environment demonstrated its capacity of increasing temperature. Furthermore, it allows the obstruction removal in a controlled manner, avoiding damage to the polymeric layer of the flexible line.