Abstract

The mechanism of steady-state heat transfer from deeply buried pipes has beenrigorously modeled for a long time. Detailed analytical formulae have beenproposed recently for the calculation of the overall heat transfer coefficientacross the entire range of burial depths. This paper presents an evaluation ofthese formulae and suggests some improvements on the basis of numericalsimulations performed with high-fidelity Computational Fluid Dynamics (CFD)models.

Explicit formulae can quickly be implemented and used for generating profilesof overall heat transfer coefficient along pipelines. The effect ofuncertainties in input data on steady-state heat transfer can easily beassessed for any amount of burial. Four explicit, continuous formulae arepresented and compared to three independent sources of CFD analyses. A relativedifference of 10% or less with respect to CFD can be achieved with analyticalformulae for a comprehensive range of offshore pipeline systems, ambientconditions, soil thermal conductivities, and burial depths. The applicabilityof these formulae to onshore systems is also evaluated.

Introduction

Pipeline embedment into the seabed is a key consideration for offshore oil& gas developments with high temperature fluids. Inaccurate calculation ofthe overall heat transfer coefficient at the design stage of a project maysignificantly affect the integrity of pipelines and ancillary systems afterstart-up. For example, the overall heat transfer coefficient will beoverpredicted if the effect of pipeline embedment into the seabed is ignored. In the case of high temperature fluids this will result in hotter temperatureprofiles for longer. Such errors may cause issues such as uncontrolled pipelinebuckling; under design of cooling spools; accelerated degradation of externalcoatings; excessive corrosion rates; and unanticipated/excessive top of linecorrosion in wet gas pipelines

Two explicit, continuous formulae have been proposed recently to improve thedetermination of the overall heat transfer coefficient for partially and fullyburied pipes (Morud, 2007) (Ovuworie, 2010). This paper presents a detailedanalysis of these formulae with the support of high-fidelity CFD simulation. Two other analytical formulae are introduced and evaluated.

Theory

This section defines the overall heat transfer coefficient (OHTC) and presentsformulae for its calculation.

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