Structural and Thermal Optimization of Cased Insulated Flowlines
Authors: Chin Yun D. , Bomba John G. 
Published: October 2008

Abstract
Flow assurance and associated thermal insulation of pipelines is an important part of exploitation of deepwater reservoirs. Insulated pipelines and bundles offer a reliable and cost effective solution for deepwater flow assurance. Flexibility of pipe-in-pipe configuration in selection of geometrical parameters, pipe materials, and insulation materials allows optimizing thermal and structural properties of pipelines for specific water depth and operation requirements.
The model proposed herein addressed the synergy between the design issues related to the structural performance and the thermal performance of the pipelines. The structural design of pipe-in-pipe is governed by buckling and collapse resistance of both inner and outer pipes, as well as the weight and overall stiffness of the pipeline. The stress state in each layer needs to be calculated during installation, operation, and maintenance conditions to address the structural design issues. The thermal insulation characteristics of the bundles are primarily governed by the geometric parameters and thermal conductivity of pipe materials, especially that of the insulation layer.

INTRODUCTION
Flow assurance is a key technology for deepwater field development. As oil and gas field production shifts into deeper water, seawater temperature at the wellhead decreases nearly to freezing point, generating a variety of problems, such as hydrate formation and wax, and paraffin accumulation in the pipelines. One current strategy to mitigate these problems is to provide thermal insulation to subsea pipelines and risers to minimize heat loss from the system.
Cased insulated flowlines insulated flowline and bundle technology is capable of assuring the required delivery temperature of a deepwater pipeline up to 10,000 ft, and could prove to be more cost effective than other proposed solutions such as electric heating and methanol injection. Pipe-in-pipe flowlines and bundles enable unprocessed well production from deepwater subsea wells to FPSOs or shallow water process facilities, with the delivery temperature kept above the cloud point of paraffins/waxes and hydrate formation temperature.
A cased insulated flowline consists of a single production flowline concentrically located inside a protective casing pipe or jacket (Fig.1). The annulus is filled with insulation material. There are various candidate materials available as the insulation layer, such as polyurethane foam, syntactic foam, or ceramic microsphere cement. The installation of insulation could be pre-molded, injected, poured-in-place, or multiple coating. The conventional cased insulated flowline configuration uses steel as the casing material, and is usually accepted as "Pipe-in-Pipe" configuration (PIP).
PIPs have been a popular flow assurance method in the offshore flowlines in the Gulf of Mexico. Numerous studies have been reported on various aspects of PIP and bundled flowlines. Tucker, et al [1] presented the investigation of insulated flowline technology for DeepStar joint industry program, discussed the advantages and disadvantages of PIP and integrated towed flowline bundles. Delafkaran, et al. [2] conducted analysis of high temperature thermally insulated PIP risers, provided a design methodology. Nock and Bomba [3] discussed the advantages of cased insulated flowline bundles and the possibility of deepwater application. Nelson, et al [4] reported a new insulation method for controlled depth tow method bundle using foamed polyurethane and silica sphere slurry. Palle and Ror [5] investigated the performance of flowlines insulated with polyurethane foam. Raborn [6] summarized the use of multi-function pipeline bundles for deepwater applications.