Proposal and Experimental Trials on a Robot for Hydrate and Paraffin Removal in Submarine Flexible Lines

Abstract

Two major concerns in offshore production are Methane Hydrates and Paraffin Plugs. They may stop wells for months, causing high financial losses. Sometimes, depressurization techniques allow hydrate removal. Another strategy is using coiled tubing or a similar unit to perform local heating or solvent injection. However, frequently these strategies are not successful. In those cases, a rig may be a suitable but expensive solution, or the line may be lost. The present project aimed to develop a robotic system capable of performing controlled local heating for removing Paraffin and Methane Hydrates. The robotic system accesses the line from the production platform. It uses a peristaltic self-locking traction system to exert high traction forces. An umbilical with quasi-neutral buoyancy and low friction coefficient reduces the cable traction. It also allows moving upwards and in pipes with a large number of curves, something that coiled tubing and similar units cannot. Carbon fiber vessels and compact circuits allowed downsizing it to move inside 4-inch flexible pipes. Initially, a theoretical model for the local heating system allowed the evaluation of this strategy. A prototype allowed testing the system in a cooled environment. This heating system removes the obstruction in a controlled manner, avoiding damages to the polymeric layer of the flexible line. Simultaneously, a modified Euler-Eytelwein equation allowed the development of a theoretical model for cable traction. Experimental tests validated this model. Those tests used straight and curved pipes, both empty and filled with fluid and using different loads. Also, the 20 kN (4.3 kip) traction system was modeled theoretically considering the self-locking system, the contact with the wall, and a diameter range. Prototypes allowed the comparison between electric and hydraulic systems. Those prototypes also validated the traction capacity. Besides, force transmission from the traction system to the umbilical occurs through an external aramid layer. A Universal Testing Machine validated the traction resistance of the external layer. Furthermore, carbon fiber vessels protect the electronic circuits from oil and external pressure. The power electronics designed can provide up to 4kW for the motors to operate the hydraulic system. The onboard computer runs with a real-time operational system and, together with a sensors network, is responsible for monitoring the pressures, temperatures, currents and tensions throughout the entire robot. The fail-safe design allows the robot to operate without risks of catastrophic accidents and guarantees that it can be pulled out at any time. A pressure vessel validated the collapse resistance, reaching more than 700 bar (10.000 psi). In addition, exhaustive integration tests validated the onboard electronics and the surface control system. Finally, factory tests validated the umbilical design.