The Application of Strain B ased Design Methods to Achieve Cost Effective Subsea Pipelines
Authors: Walker A.C. , Williams K.J. , Wickham A.H.S. 
Published: October 2008

Abstract
This paper is concerned with a Joint Industry Study which has prepared criteria and analysis method for the strain based design and assessment of subsea pipelines. The study has been sponsored by six oil companies, two regulatory authorities, a design contractor and an installation contractor. ?The main study was preceded by a pilot investigation performed on behalf of the UK Health and Safety Executive. The work in the project encompasses full scale testing of a range of pipeline diameters and materials, development of finite element models and simplified formulations to evaluate the limit states of pipes. The results from the tests have been used to validate the numerical analysis method.
The procedure for the tests and details of the test specimens are described in (he paper. Some example results from the tests and analysis are presented to illustrate the approach followed in the project.

Introduction
Pipeline engineering is a rapidly evolving area of technology in which new materials and installation techniques are being developed to cope with requirements for the pipelines to operate in deep water transporting sometimes very corrosive contents at high temperatures and pressures. Allied to these technical challenges are the ever-present requirements that engineers face of designing sub-sea systems which will operate at appropriate evels of safety during a specified lifetime and can be installed and maintained at acceptable costs. Much of current design of pipelines is performed according to stress-based codes. That is, the loads applied 10 the pipe must not cause stresses, longitudinal, circumferential and combined, which exceed a specified proportion of the material yield stress. ?This approach has resulted in pipeline systems which have been successfully installed and safely operated.
In some circumstances the requirement to restrict (he stress levels means that the pipeline may incur unnecessary economic penalties during installation or operation and in some circumstances, may not be feasible. These circumstances correspond to states in which the deformation of the pipeline is completely controlled. Examples of this arebending over a stingerbending on a reel and straightener systeml bending in a J-tubeconforming to the seabed topographyoperating at high temperatures
In these cases when the pipeline deformations are controlled experience has shown that the pipe can sustain strains greater than those corresponding to the limiting stresses without loss of capacity to operate safely, Similarly, pipelines may experience damage during operation, or may have requirements for alterations of the operational conditions. In these circumstances if the pipe is assessed using the method with which they were originally designed, i.e. the stress based approach, it may show the pipe to be unacceptable for continued use. On the often hand, if the deformations are controlled the application of limiting strain criteria may confirm that in fact the pipe could be tit for purpose.