Reinforced Design of an Unbonded Flexible Flowline for Shallow Water

Abstract

Flexible pipeline has been widely used in offshore engineering. Comparing to steel pipe, flexible subsea pipeline has some merits such as anti-corrosive, easy laying and recycling. So it is suitable to be used as flowline in shallow water. Especially there are lots of marginal oil fields in shallow water of China. Different from deepwater dynamic riser, flexible pipe for shallow water applications has a low requirement of resisting external pressure. On the other hand, Flexible pipe need to satisfy the requirement of tensile property during installation. Therefore, a kind of simple and economical flexible pipe can be developed to meet the requirement of shallow water application. This paper presents an economical unbonded flexible pipeline, which is helically wound by steel wires. In order to guarantee the safety of the economical flexible pipeline under the laying, installation and working loads, a reinforced design of tensile property is necessary. A helically wounded flat-steel layer is introduced to constrain the steel wire radial deformation and strengthen the axial tensile stiffness of the pipeline. A theoretical model is established to analyze the tension behavior of the flexible pipes and some suggestions are proposed to strengthen the flexible pipe design. Additionally, tensile experiments are carried out for the original and reinforced flexible pipes, which verify the theoretical model and the effectiveness of the reinforced design. The results show that the reinforced pipeline not only meets the tensile requirement during installation for shallow-water applications, but also meets requirements of the minimum bend radius as well as hydrostatic collapse (resistance of the external pressure) with low cost.