Frictional characteristics and drag-reduction optimization of flexible drill strings in offshore ultrashort-radius sidetracking wells

Offshore ultrashort-radius sidetracking is an important re-entry and infill-drilling technique for exploiting remaining reserves in mature offshore fields and improving platform-based field redevelopment efficiency. However, in offshore sidetracking wells with large local curvature and short build sections, flexible drill strings are strongly constrained by the wellbore, which leads to severe wall contact, frictional drag, and attenuation of the WOB delivered to the bit. Sidetrack coring is considered as a representative implementation scenario, in which the flexible coring assembly is retrieved after each coring run and the hinge angular limits can be preset at the surface for the subsequent run. In this study, a dynamic model was established by considering hinge constraints, drill-string–wellbore contact and friction, end thrust, and soft angular limits at the joints. The effects of hinge angular limit, segment length, wellbore diameter, radius of curvature, and applied input load on the contact-force distribution and WOB-transfer performance were systematically investigated. The results show that contact friction is mainly concentrated in the curved section and the curved-to-horizontal transition section. Wellbore contact and frictional dissipation along the drill string are the main causes of weight-on-bit attenuation. A smaller hinge angular limit is more favorable for reducing contact loads and improving WOB transfer. When the total drill-string length is held constant, segment length exhibits a coupled and non-monotonic influence on contact-force distribution and WOB transfer. Within the investigated range, a wellbore diameter of 135 mm yields the highest WOB-transfer efficiency. On this basis, separate dual neural-network surrogate models were developed for horizontal-section lengths of 2–5 m to predict segment-wise contact forces and output WOB. A seven-angle periodic unit was optimized under a fixed input load of 20 kN using a multi-objective genetic algorithm, and representative candidates were subsequently re-evaluated using the full dynamic model. For the 5 m horizontal section, the recommended configuration reduced the dominant contact-force peaks by approximately one half and increased the output WOB from 15.841 to 17.937 kN relative to the uniform 3 ∘ baseline.