Bundle Flowline Thermal Analysis
Authors: Zabaras G.J. , Zhang J.J. 
Published: December 2007

Abstract
The bundle flowline involves packaging multiple flowlines, control and injection lines within a single carrier pipe. A bundle is claimed to provide an effective means of insulating flowlines using low-cost materials. However, no rigorous heat transfer analysis of bundled flowline systems has been offered by the literature or the vendors. For this reason, the thermal performance of bundle-insulated flowlines has been studied using a general-purpose, finite-element, partial-differential equation solver.
Steady state and transient cooldown performance were analyzed for six different bundle configurations. Overall heat transfer coefficients for individual flowlines within a bundle have been determined in addition to cooldown performance for different insulation levels and pipe sizes. These results have been compared to results obtained for the simple bundle geometry of a pipe-in-pipe system, and found comparable.
To expedite the calculation of overall heat transfer coefficients for bundle flowlines, simplified heat transfer calculations were developed as an approximation to the finite-element solutions. These approximations are on the average within 5% of the finite-element solution.

Introduction
When a subsea flowline is cooled down to ambient temperature, hydrates can form thus increasing the potential for flowline blockage and production shut-down. The bundle flowline concept involves packaging multiple flowlines, control and injection lines within a single carrier pipe. A cased, nitrogen-filled bundle, consisting of one or more production flowlines, water injection and control lines, insulated with open cell polyurethane foam, all inside an outer steel carrier pipe, has been promoted by vendors and the published literature as a practical method of reducing heat losses during steady-state operation and system cooldown.
The bundle is thus claimed to provide an effective means of insulating flowlines using low cost materials. Additional claims include greater operational flexibility for handling hydrate plugs versus individually-laid lines. For example, hot water can be introduced at the host platform in one of the pipes to warm-up the produced fluids flowing in the production flowlines Finally, the flowline bundles are claimed to result in lower overall flowline/subsea costs compared to systems based on individually-laid, insulated flowlines. Recent bundle installations include GOM (Enserch MC-441, GC-29) and North Sea (Gannet, Osprey, Alba).
Technical limitations of the bundle technology include towing length, size and weight of bundle. Leading issues involve selection of land fabrication site, mid-line connections, damage consequences and repair strategies. The feasibility, operational, and cost incentives for bundled flowlines have not been adequately proven. Field data on the thermal performance of flowline bundles is lacking in the literature. The ability of a bundle system to handle hydrate plugs has not been demonstrated in the field. Cost estimates for bundle flowlines by bundle technology vendors are far lower than recent quotations by flowline fabrication vendors. The present study focused only on the thermal performance of bundle flowlines. No attempt was made to assess other lacking information including mechanical integrity, towing, installation, cost and other issues. The technical objectives of the study were to determine the steady-state thermal flowline performance and the transient flowline cooldown behavior after a system shut-down. The numerical procedures used for this thermal analysis can also be extended to investigate other thermal transient scenarios such as flowline warm-up with or without heat from a hot water pipe, hydrate plug melting etc.

Design Data and Calculation Premises
Six different bundle configurations were considered shown schematically in Figure 1 as Cases 1 through 6. Description of each case is presented in Table 1. Case 1 corresponds to the pipe in pipe insulated flowline Although general calculational procedures have been developed applicable to any flowline size and insulation thickness, the results that are presented in this report refer to the input data listed in Table 2.
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