Development and Testing of a Non-interlocked Hoop Strength Layer for Unbonded Flexible Pipe

Abstract

Abstract Conventional flexible pipe for high pressure applications generally employs a high lay angle, interlocked steel hoop strength layer to prevent blow through of the internal pressure sheath through gaps between adjacent wraps. The interlocked hoop strength layer is referred to as "pressure armour" in ISO 13628-2/API 17J [1]. The interlocking is designed to prevent large gaps from opening between adjacent wraps. The internal pressure sheath must span the gap under the internal pressure loading, and meet the ISO 13628-2/API 17J requirement of no greater than 30% thickness reduction at the gap location with the interlocking wraps in the fully extended position. To simplify the design of the hoop strength layer so that composite materials can be employed, a means of controlling the gap and preventing internal pressure sheath blow through between non-interlocked rectangular cross section hoop strength layer helical wraps has been developed. The non-interlocked design comprises an anti-extrusion and gap control layer applied between the internal pressure sheath and hoop strength layer. This paper presents the design of this new layer, the requirements to qualify the layer, tests which were conducted to verify qualification, and a validated model based on the test results for use in design of unbonded flexible pipe employing non-interlocked composite hoop reinforcement. The advantages of a non-interlocked composite hoop strength layer in unbonded flexible pipe include lighter weight structures, reducing risk and cost for installation and operation, and corrosion resistance making it ideal for deepwater and ultra-deepwater applications. Introduction Unbonded Flexible Fiber Reinforced Pipe (FFRP®) is a new generation of flexible pipe designed for offshore oil and gas transportation. It is constructed from extruded polymer pressure barrier layers and fiberglass/epoxy reinforcement layers. The unique design of bonding multiple thin reinforcement tapes into reinforcement stacks with interlaminar adhesive allows the reinforcement material to work in both tension and compression. Because of the high strength-to-weight ratio of the composite reinforcement material, FFRP is ideal for deepwater and ultra-deepwater service. Figure 1 provides an isometric view of the unbonded FFRP pipe structure. A brief functional description of each layer is provided in Table 1.