Influencing factors analysis on coiled tubing-in-riser systems considering the force excitation of ocean loads

The mechanical behavior of coiled tubing-in-riser systems is critical to success of coiled tubing operations in the development of offshore oil and gas. The purpose of this work is to guide the coiled tubing's marine applications by analyzing influencing factors of the coupled system. This paper describes a mechanical model of micro elements of the coiled tubing-in-riser system under force excitation of ocean loads. Nonlinear equations simplified by finite difference method are solved through the Newton-Raphson iteration method. Then the analytical analysis is compared with a three-dimensional finite element model of the coiled tubing-in-riser systems. To ensure safe operation, lateral displacement and internal bending moment of the riser are calculated. Besides, four influencing factors annular clearance, ocean depth, platform offset, and friction coefficient are investigated. The results show that strength design of the riser needs to consider the critical position, and larger outer diameter of coiled tubing, smaller water depth operation, smaller platform offset, and smaller friction coefficient are preferred to improve the injection efficiency of coiled tubing. This research can provide important theoretical support for ocean applications of coiled tubing.