A technique for mitigating or removing a plug of solid-state material such as wax or hydrate coalesced from hydrocarbon fluids in a subsea flowline pipe. In a normal flow mode, a flow of hot production fluid is directed from a subsea well into a production flowline pipe that extends toward a processing or storage location. Simultaneously, a contra-flow of service fluid such as monoethylene glycol (MEG) is transported along a service fluid pipe that extends along and adjacent to the production flowline pipe. The service fluid is injected into the well or into the flow of production fluid upstream of the flowline pipe. On determining plugging or a risk of plugging of the flowline pipe, a heating mode is activated in which at least some of the flow of the production fluid is diverted into the service fluid pipe. This heats the service fluid pipe which, in turn, heats the flowline pipe to disperse the solid-state material.