State of the Art Analytical Tools Improve Optimization of Unbonded Flexible Pipes for Deepwater Environments
Authors: Zhang Y. , Chen B. , Qiu L. , Hill T. , Case M. 
Published: October 2008

Deepwater environments present a number of design challenges. For unbonded flexible pipe, the primary challenges include fatigue life, collapse, and axial compression loading of the tensile armor wires. This paper presents state of the art analytical tools used to predict the performance of flexible pipe under these load conditions. The analytical tools discussed in this paper have enhanced the ability to optimize flexible pipe designs offering operators a viable solution for deepwater field developments.

Introduction

Unbonded flexible pipe has been used in the offshore oil and gas industry for over 20 years. It is most commonly used for dynamic risers connecting seabed flowlines to floating production facilities, for fluid transfer lines (FTLs) connecting two floating production facilities, and for static seabed flowlines. The basic pipe design consists of a stainless steel internal carcass for collapse resistance, an extruded polymer fluid barrier, a carbon steel interlocked circumferential layer for internal pressure loads (pressure armor), helically wound carbon steel tensile armor layers for axial strength, and an extruded watertight external sheath (Fig. 1). For extremely high pressure applications, an additional layer of helical reinforcement over the pressure armor, or a second set of tensile armor layers, may be applied. For deepwater designs, additional reinforcement is applied over the tensile armor layers to increase resistance to birdcaging [1].

Fig. 1 Basic Flexible Riser Design(Available in full paper)

Some of the key analytical tools used to design deepwater flexible pipes include the fatigue life model to predict the expected life of the primary structural layers, the rough bore collapse model to predict the lower bound collapse capacity, and the birdcage model to predict the axial compression capacity for the tensile armor wires. A practical methodology for the fatigue life estimation in the armor layers of unbonded flexible risers with regular wave and irregular wave simulation results is explored. In most cases, the fatigue life of the tensile and pressure armor layers dictates the life of the flexible pipe. Additionally, a 2D finite element (FE) model for predicting the collapse strength of rough bore flexible pipes is discussed herein. The model is calibrated with test data from over one hundred collapse tests performed on pipe samples ranging in size from a 4- to 16-inch inside diameter (ID). Finally, a 3D finite element model for predicting the buckling capacity of the tensile armor wires is discussed. The tensile armor buckling model, or birdcage model, is a new development introduced with the 2,000m water depth Roncador field development in the Campos Basin, offshore Brazil.

Fatigue Life

In offshore applications of unbonded flexible pipe, the service life of the flexible is typically limited by the tensile armor or the pressure armor layer. Therefore, the fatigue life analysis of flexible pipe is normally focused on these layers. For a flexible riser, especially for a free-hanging riser in deep water, the maximum fatigue damage occurs at the top connection or at the outlet of the guide tube (I-tube) on the floating unit.