CFD Investigation on Secondary Flow Characteristics in Double-Curved Subsea Pipelines with Different Spatial Structures

Abstract

Double-curved pipes are widely employed as essential components of subsea pipeline systems. Considering the layout flexibility and application diversity, there are various spatial structures for the double-curved combinations. However, few studies have compared the flow characteristics in different double-curved pipes. The dissipations of the corresponding downstream flow have not been thoroughly investigated, which are crucial for the measurement accuracy and flow assurance. In this paper, the turbulent flow in double-curved pipes with different spatial structures (i.e., Z-, U-, and spatial Z- type) was numerically studied by employing the ω-Reynolds stress model. The major purpose was to develop an in-depth knowledge on the secondary flow characteristics in different double-curved pipes and quantify the dissipations of the downstream flow. The effects of the spatial angle and interval distance of the two curves on the flow fields are taken into consideration, and the swirl intensity Si is introduced to evaluate the secondary flow dissipation. It is found that the secondary flows in the Z- and U-type structures are in opposite directions when the interval distance is short (3D), and the secondary flow in the spatial Z-type exhibits an oblique symmetric form. Only in the Z-type pipe with a short interval distance the secondary flow exhibits an exponential dissipation, and the fully developed flow is easier to achieve than the other cases. However, as the interval distance increases, the directions of the secondary flow in the U- and Z-type structures are the same, and the flow dissipations in all the structures return to the exponential types. The obtained dissipation rates for the secondary flow downstream of Z-, U-, and spatial Z-pipes with the 9D interval distance were 0.40, 0.25, and 0.20, respectively. The results are expected to guide the design of pipeline layouts and provide a reference for the arrangements of flowmeters in a complex subsea pipeline system.