Mechanical analysis of coiled tubing in the process of hydrate and wax plugs removal in subsea pipelines

Abstract

ABSTRACT Hydrate formation and wax deposition in submarine pipelines hinder the transportation of oil and gas. Pigging operation is an important means to ensure pipeline flow assurance. In this paper, the idea of coiled tubing (CT) intervening in hydrate and wax removal of underwater pipeline is put forward, and the feasibility of this method is discussed from the point of view of the ultimate extension capacity of coiled tubing. To this end, theoretical analyses and experimental tests are performed on the mechanical behaviour of coiled tubing in submarine pipelines. The present paper mainly analyses the axial force transfer characteristic of coiled tubing with different sizes and the contact force coefficient values of coiled tubing with fixed and loaded end conditions under different boundary conditions. Besides, the effects of CT size and friction coefficient on the reachable depth of hydrate or wax plugs removal are also studied and the obtained maximum depth of the CT inlet is compared with the total length of the five-segment centreline of the South China Sea submarine pipelines. The results show that CT can further achieve more plug removal as the reduction of friction coefficient, while the depth of plug removal reduces with the decrease of CT size. CTs only with limited sizes can be used to complete the plug removal process in submarine pipelines.