PSI - Issue 57

Yann Chevalon et al. / Procedia Structural Integrity 57 (2024) 633–641 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Nomenclature FPSO

Floating Production, Storage, and Off-loading unit

SCF

Stress concentration factor

PA TA

Pressure armour

Tensile armour Unbonded flexible pipes are used in the offshore industry for conveying gases and liquids between sub-sea installations and topside floating units. The flexibility of these pipes is brought by the capacity of each layer to slide relatively on their adjacent layers. Design requirements for offshore flexible pipes are found in the API 17J specification and recommended practices are found in API l7B. A brief description of the role of each layer is provided below and an illustration of a typical flexible pipe cross section is given in Figure 1. 1. Carcass : providing radial resistance towards external pressure and resistance to abrasive wear from the fluid. 2. Inner pressure sheath : providing barrier to the transported fluid. 3. Pressure armour : reinforcing the inner pressure sheath when the pipe is internally pressurized and contributing to radial forces resistance from outside the pipe (on top of carcass role) when radial compressive pressure is applied to the pipe. 4. Inner tensile armour : providing tensile capacity. Axial forces are predominantly hang-off and endcap loads. 5. Outer tensile armour : counter wounded to layer 4, this also provides tensile capacity without creating twist in the pipe. 6. Insulation : Providing thermal resistance to the bore content. 7. Outer sheath : Protecting the armour wires against external environmental conditions such as corrosion of the steel armor. Besides layers 2 and 7, which are extruded, all the other layers are wounded helically along the longitudinal axis of the pipe in order to provide flexibility.

Fig 1 Illustration of an unbonded flexible pipe (FAM III based on API classification)

The role of these flexible pipes is to connect the floating production units to the sub-sea installations. An illustration of a system configuration is to be found in Figure 2 to provide an easier understanding.