Reeled Pipe for Dynamic Riser Applications
Authors: Harland M.O. 
Published: October 2008

A comprehensive test program has been running within the CSO organisation to establish the feasibility of using reeled pipe technology as an installation method for dynamic rigid riser systems. A scope of work was developed by CSO in conjunction with The Welding Institute of Cambridge, England to design and test line pipe girth welds under fatigue conditions. The object of the test program was firstly to provide a comparison mechanism between girth welds in the virgin condition and girth welds that had been subjected to a simulated reeling process. Secondly, to establish data points for an S-N curve, thereby providing a basis for the development of design criteria that could be applied to rigid riser systems installed by the reeled pipe technique.

The test results have illustrated that no significant detrimental effect on the fatigue performance of line pipe girth welds can been detected providing the prescriptive weld procedures are adhered to.

Introduction

The installation of rigid pipe dynamic riser systems using the reeled pipelay method, for appropriate pipeline diameters, potentially offers significant adventages over other available installation techniques. The use of reeled pipe as an installation method for subsea pipelines has a well established worldwide track record. However the distinct differences in operational conditions between subsea flowlines and dynamic riser systems, demands the application of alternative design and acceptance criteria.

Pipeline reeling subjects the material to a strain cycle that extends into the plastic domain of the Stress Strain curve (Fig 1). Under certain conditions the level of strain imposed on pipeline and weld material may have an effect on the fracture toughness or may cause an extension of weld flaws, initially induced during welding.

Given that these effects are understood and controllable for subsea pipeline applications, the suitability of the reeling technique for rigid dynamic isers will be governed, in part, by the ability of reeled pipe material and weld to withstand the operational conditions imposed on a dynamic riser.

A major contributor to the successful design of a riser system is the prediction and demonstration of adequate fatigue resistance during operation. Therefore to establish the basic feasibility of using reeling as a riser installation method, the fatigue performance of reeled pipeline material and weld has had to be assessed.

The following sections within this paper will expand on the potential advantages of reeling as a riser installation method and present and discuss the work that has been undertaken to assess the fatigue performance of reeled pipeline material.

The Benefits of Reeled Installation

General Principles.

The installation of subsea pipelines using the reeled pipe method has been employed by numerous companies for a number of decades. From a practical viewpoint the technique relies on the availability of a one to two kilometre strip of land with sufficient water depth at the seaward end to accept the draft of the installation vessel. Fig. 2 illustrates the typical layout of a reeled pipe fabrication site. Currently, pipes of upto 16" diameter can be welded into long strings and stored on racks ready to be spooled onto the installation vessel.

From a mechanical viewpoint, the technique relies on the ductility of the pipe and weld material to withstand the strains that are imposed on the pipe string as it is spooled onto the hub of the horizontally or vertically mounted reel of the installation vessel. Adequate back tension coupled with the controlled cuivature applied by the reel ensure that the pipe string can be spooled without inducing collapse th