Reeled PIP SCR
Authors: Bell Mike , Daly Roland 
Published: October 2008

The installation and operation of a reeled PIP SCR was studied using general finite element models. A comparison of the analysis results to results from dedicated pipelay software was made as well as a review of the effects from the reeling operation.

The currently available pipelay software does not offer the possibilities of general finite element software in modelling the details of the interaction between the components of a PIP system and between the PIP and the pipelay equipment. Understanding this interaction is important in the assessment of fatigue strength and fracture resistance of a PIP SCR. The paper demonstrates how this can be achieved, accounting for the presence of the pipes and centralisers within a PIP system during reeling, installation and operation. Reference is made to fabrication and installation procedures.

The reeled SCR comprises a cost effective installation method that offers the superior insulation performance of a PIP over conventional coating systems. There exists the additional potential for incorporating active heating cables/tubes within the annulus.

The paper describes an in-place static analysis of a particular SCR configuration. The analysis is used to quantify the influence of pressure and temperature on the axial forces developed within the system.

Introduction

Pipe-in-pipe (PIP) static flowlines are an established product in offshore oil and gas developments. The subject of PIP for use in a dynamic steel catenary riser (SCR) configuration however is relatively new and as yet not well documented. Interaction between the various components makes analysis of a PIP system considerably more complex than analysis of a single flowline, specifically in analysing the reeling process and in achieving a numerically stable analysis of the PIP catenary.

The main features of a PIP SCR are a concentric insulated inner pipe or "flowline" and a protective outer pipe, known as the "carrier". The function of the flowline is to convey fluids and therefore is designed for internal pressure containment. The flowline is insulated, by placing thermal insulation materials within the annulus of the two pipes. The carrier pipe protects the insulation material from external hydrostatic pressure and mechanical damage.

CSO have utilised two systems for maintaining the pipes concentric during reeling of a PIP. The first utilises centralisers typically spaced 1-3 meters apart. However, an alternative system has also been used whereby insulation foam provided continuous support along the pipeline's length.

Bulkheads can either be placed at the pipeline extremities or at discrete positions along the pipeline length, to transfer axial loads. Water stops may also be required in the event of a wet buckle to limit sea water ingress within the annulus.

For deepwater pipelines the use of buckle arrestors is more economical to limit the extent of a buckle than having a thick wall to resist buckle propagation.

The reeled PIP SCR may encompass all of the above features, in addition however, it may also incorporate active heating cables along the reeled PIP SCRs length, and optical fibre for real time temperature assessment.