Deepwater Installation Techniques for Pipe-in-Pipe Systems Incorporating Plastic Strains
Authors: Dixon Mark , Jackson Dan , El Chayeb Abed 
Published: October 2008

Abstract
This paper presents a numerical and analytical study of pipein- pipe installation in deepwater by the J-lay and Steep S-lay methods. In particular it examines the amount and location on plasticity that may be allowed to occur during the installation phase of construction. The influence of the residual strain to operational performance of the line is discussed. The information in the paper provides guidelines for the installation of pipe-in-pipe systems in deepwater as well as indications of the critical areas that require detailed investigation.
A general limit of 1% strain during installation has industry acceptance although few projects have put this to the test. The results presented in the paper show this to be acceptable under certain circumstances and perhaps not so conservative in others. It also highlights the importance of understanding the numerical modeling techniques as well as the influence of assumptions in the loading and materials modeling.

Introduction
With the flow assurance demands placed on flowline and riser systems in deepwater developments pipe-in-pipe is a technology widely used today and planned in the future due to the exceptional levels of thermal insulation they provide. As production progresses deeper though, pipe-in-pipe systems face the problem of becoming too heavy to install in a conventional manner. They therefore require thorough understanding of their structural response during installation to permit extension of the acceptable loading regime. In addition to plasticity the non-uniform geometries of typical deepwater field joints increases the complexity of the issue.
Flowline installation in deepwater is an area of considerable research at present, not so much in the form of joint industry projects but more internally within the installation contractors. This R & D focus falls into two categories:Incremental improvements to installation layrates and reliability with existing vessels and equipmentInvestigate and develop new methods for installation.
These two areas are not mutually exclusive and in a way the ideal extension to installation practice is one that offers a significant step forward for a small change in equipment.
In recent years pipeline and riser design codes have developed significantly to the point where limit states for the pipe are used to ensure safe installation and operation. This increased understanding within the design codes permits the extension of the installation envelope for traditional methods whilst providing a vehicle to assess new methods within an approved and acknowledged framework. It does however have the drawback of only being directly applicable to a single pipe with simple geometry.
Although the codes now provide enhanced assessment of the various pipe loading scenarios there is an increased requirement to understand and account for the effects of tolerances on the design. Additionally more complex geometries such as pipe-in-pipe systems require investigation through other means, typically finite element analysis, to supplement code assessment.
This paper examines in particular the assessment of the pipein- pipe field joint region during installation in both J-lay and S-lay including the Steep S-lay approach. The process makes use of the DnV 2000 pipelines design code(1) and ABAQUS non-linear finite element analysis.