Optimising Reeled Pipe Design Through Improved Knowledge of Reeling Mechanics
Authors: Denniel Sylvain 
Published: July 2009

Pipeline reeling is a well established method for offshore installation of rigid steel pipelines and catenary risers. The basic process of loading stalks of pipe that have been fabricated onshore onto large reels before transit to the worksite, un-spooling, straightening and lay is well known. However, despite the wide use of this installation method, the actual mechanics of reeling are generally not well understood. This occasionally leads to concerns that a reeled pipeline has somehow been degraded by the process and must be down-rated in comparison to a similar line installed by another method. There is, therefore, a risk of unnecessary over-design of the pipeline.

The expanding ultra deepwater market drives the industry towards streamlining its designs to reduce submerged weight and costs. Over-conservatism related to misconception is certainly not beneficial.

This paper intends to demonstrate that there is no general reduction in pipeline performance as a result of the reeled installation method, provided that certain items are considered during design, procurement and fabrication process.

The argument is constructed by assessing each ‘limit state’, meaning the fundamental ways that a pipeline can fail (burst, collapse, fracture), and considering the possible effect that reeling may have on each during service, in comparison to other installation methods. The paper makes use of a wide range of supporting data from extensive in-house engineering and testing programme, reference to relevant Joint Industry Projects and the largest reel-lay installation track record in the industry.

Introduction

The first application of the reeling technology occurred in 1944, when a 3??OD gas line was installed between England and Northern France to supply the allied forces, as part of the PLUTO (Pipe Line Under The Ocean) project. Commercial application of this installation method for the oil and gas industry was only implemented later in the seventies. Since then, reel-lay has become one of the most established methods to install rigid pipelines offshore, for very good reasons; it is fast, reliable, cost effective and less dependant to offshore weather conditions. The majority of the pipe length is welded onshore into stalks, using high quality procedures in factory-controlled conditions, away from vessel the critical path. The duration of the offshore installation campaign is relatively short with a pipe-lay speed that can readily reach two kilometres per hour.

As the leading reeling contractor, Technip has built a significant track record over the years with Apache and Deep Blue, including the first ever fully reeled Steel Catenary Riser (SCR) [1], the first ever reeled pipe-in-pipe (PiP) SCR, which is also the deepest to date [2], as well as the largest and best insulated reeled PiP [3].

The storage of the pipeline onto the reel-lay vessel is achieved by plastically bending the rigid pipe onto the reel hub in a displacement controlled manner. During pipe-lay, the pipe is wound off the reel and straightened, by application of a reverse curvature, before entering the water. Reeling mechanics essentially focuses on the effects of these plastic deformation cycles on the pipeline integrity during reeling and on its in-service behaviour. This remains a specialist subject and the lack of detailed understanding occasionally leads to concerns that a reeled pipeline has somehow been degraded by the reeling process and therefore must be down-rated in comparison to a similar line, which has been installed by another method. Such concerns can be detrimental to the commercial benefits of installation by reeling as they may sometimes drive towards an unnecessary increase in line pipe wall thickness.