Optimized Design of Pipe-in-Pipe Systems
Authors: Hausner M. , Dixon M. 
Published: October 2008

Deepwater subsea developments must address the flow assurance issues and increasingly these are forming a more critical part of the design. Pipe-in-pipe systems are one of the options available in the designers' toolbox for overcoming these problems and are recognized as a thermally efficient, reliable and proven technology for insulated subsea transportation of well bore fluids. Although extremely low U-values are achievable pipe-in-pipe systems come at a cost and have their increased weight as a penalty for use in deepwater developments.

By applying an ‘inside-out’ optimization process for the design of pipe-in-pipe systems the top tension loading on the surface vessel (installation or production) can be significantly reduced whilst also minimizing procurement expenditure on raw materials. Specifically the design optimization of each component reduces steel volumes and the overall outer diameter of the system.

This paper presents the methodology for optimized design of pipe-in-pipe systems and illustrates the potential cost savings in terms of raw materials and installation cost through a case study for a typical large West African field. In addition commercial savings relating to surface platform hull costs are presented for a case where the development employs pipe-in-pipe risers.

Introduction

At present the pipe-in-pipe (PIP) market is dynamic with numerous projects requiring pipe-in-pipe solutions and many more examining pipe-in-pipe as a development option. The objective of this paper is to present an optimization design process for pipe-in-pipe systems for deepwater applications, specifically 1000m or deeper. The focus is on establishing the actual required pipe diameters for flowline and carrier, rather than employing API standard sizes, by performing the thermal and mechanical design in an integrated manner. In this way the design meets the project requirements for production rate and steady state thermal performance whilst minimizing as-installed system cost. Cooldown considerations have not been included in the designs generated here.

The ‘inside-out’ design methodology is presented along with the ‘as-installed’ costing, which has been used as the ultimate comparison condition. The following parameters are investigated with pipe-in-pipe designs and costs generated for each variable combination:U-values of 1.0, 1.5 and 2.0 W/m2KFlowline lengths of 5, 10, 20, 40 & 60 kmWater depths of 1000, 1500, 2000, 2500 & 3000m2 types of insulation material-polyuethane foam (PUF) and microporous material (MP)

In addition to presentation of the results for the parametric matrix detailed above, a project with typical characteristics for a large West Arican development are discussed including the cost and top tension implications on the host platform when employing pipe-in-pipe steel catenary risers (SCRs).

With an increasing number of pipe-in-pipe systems on offer it is increasingly difficult to rapidly evaluate the options to determine or identify the most appropriate options on a technical and economic basis.

What follows is a brief definition and classification of pipein-pipe systems. There are two specific criteria that can be used to describe any particular pipe-in-pipe system:Insulation type (material dependent)Structural compliance (configuration dependent)

Associated with each of the above are compatible types of field joints and installation methods.