Bend Stiffeners for Flexible Risers
Authors: Lane M. , McNamara J.F. , Gibson R. , Tyrer A. 
Published: October 2008

ABSTRACT
As the offshore hydrocarbon industry advances into deeper waters and more hostile environments, the use of bend stiffeners with flexible riser systems becomes increasingly essential. This paper reviews the state of the art in bend stiffener materials and manufacture. Customized software for the iterative design of bend stiffeners based on an equivalentbeam bending model is described and illustrated. Detailed finite element models in two- and three-dimensions are used to validate the operation of this software. Excellentcorrelation is reported between these detailed models and the equivalent beam design approach.

INTRODUCTION
The end connections of a flexible dynamic riser can be subject to large cyclic loadings. In order to prevent damage to the pipe structure from over bending the termination is fitted with a device known as a bend stiffener. Typical applications are shown in Fig. 1.
The stiffener is a conically shaped compliant component hose purpose is to ensure that the pipe bending radius does not fall below a certain limiting value.

BEND STIFFENER TECHNOLOGY

General Arrangement
A typical bend stiffener arrangement is shown in Fig. 2. The stiffener is assembled from the following components.

Stiffener Body
The stiffener body is a flexible structure configured as an extended cone surmounted by a shorter cylindrical section. The body has a cylindrical axial opening to accommodate the flexible pipe. There is normally a nominal radial gap between stiffener and pipe - it is not common practice for the stiffener o be bonded to the pipe.
The stiffener body deflects when the pipe rotates relative to the end fitting. This sets up restoring forces which support the pipe structure and limit its curvature. The body effectively accepts loading from the pipe and provides an alternative path for transmission to the end fitting.
This alternative route includes a number of ancillary components as follows.

Sleeve
The sleeve is a cylindrical structure of variable diameter. Its external surface is bonded directly to the stiffener body. The internal surface is free to slide along the pipe end fitting with the minimum of radial clearance.
The sleeve provides a bearing surface between the end fitting and the stiffener body to assist with the transmission of shearforces.

Flange
The flange forms the base of the stiffener and provides a rigid surface which bears directly against the reaction collar.

Internal Structure
To assist with the transfer of forces to the end fitting the stiffener body may be fitted with an internal structure. This will differ in geometry from case to case but it is normallylocated within the cylindrical portion of the body.
External connection to the internal structure is made by a set f axial rods which pass through the end flange. The rods are rigidly connected to the flange and are threaded at their extremity. The presence of the threads allows direct connection to adjacent structures.

Reaction Collar
The reaction collar is a ring which fits closely over the end fitting body. It is prevented from moving axially by either bearing on a circumferential projection.