Composite Armored Flexible Riser System for Oil Export Service
Authors: Kalman M. , Blair T. , Hill M. , Lewicki P. , Mungall C. , Russell B. 
Published: October 2008

This paper discusses the development of a light weight high strength flexible pipe for the conveyance of oil and gas from deep water subsea wells to floating production vessels. Flexible pipe is a multilayer structure of helically wound metallic strips and tapes and extruded thermoplastics. The tensile armor layer in conventional pipe consists of high strength steel rectangular wires wound in opposing directions to provide torque balance. The light weight design incorporates carbon fiber/thermoplastic composite strip to replace the steel armor. A weight reduction of 30% is expected for a pipe designed to meet the same performance requirements. The results of tests conducted to qualify the composite armor material and pipe structure and initial efforts to develop a composite armor layer service life model are presented herein.

Introduction

Non-bonded flexible pipe has been applied in the offshore oil and gas industry for about 20 years. It is used for dynamic risers connecting seabed flowlines to floating production facilities, and for static seabed flowlines where it is more cost effective to install than rigid pipe, such as in harsh environments, or it is desired to recover the flowline for reuse after a short field life. The basic pipe design (Fig. 1) consists of a stainless steel internal carcass for collapse resistance, an extruded polymer fluid barrier, a carbon steel interlocked hoop strength layer, helically wound carbon steel tensile armor for axial strength, and an extruded watertight external sheath. For dynamic applications extruded polymer or tape polymer antiwear layers are applied between adjacent steel armor layers. For extremely high pressure applications, an additional layer of rectangular shaped helical reinforcement over the interlocked hoop strength layer, or a second set of tensile armor layers, may be applied.

A 9.125-inch 207 barg oil export flexible riser system designed for service in 1500 meters water depth at the P36 semisubmersible platform in Petrobras' Roncador field has been developed. The unique attributes of this riser system are:Free hanging catenary consisting of two sections. The top section is designed for high tension capacity and lower collapse resistance. The bottom section is designed for lower tension capacity and high collapse resistance.The top section employs composite material for the tensile armor layer.The bottom section is designed so that the hoop strength layer provides a higher contribution to the pipe collapse resistance.

These design attributes were implemented to meet the design requirements while maximizing weight reduction and minimizing top tension. In a floating production application, the installation vessel and operating platform or vessel as well as the top section of the pipe itself, must support the hanging weight of the remainder of pipe with dynamic loading. It is desirable to reduce the weight of the pipe to minimize stress on the pipe wall and hangoff structure. It is also desirable to reduce the vessel deck loads so that buoyancy requirements are reduced or payload capacity increased.

In addition the composite armor offers the advantages of being highly fatigue resistant, and essentially inert to corrosion, hydrogen induced cracking and sulfide stress cracking, all potential mechanisms for reducing the service life of a pipe.

The use of the different structures for the top and bottom section and the dry hoop strength layer design resulted in a reduced cost when compared to the conventional design. The main cost reduction drivers are the reduced weight of stainless steel required for the internal carcass to achieve the necessary collapse resistance and less tensile