Flow Assurance and Optimal Design of Thermal Characteristics in Flexible Pipes
Authors: Nielsen Karsten Gorm , Kristiansen Mikael , Bendiksen Erik 
Published: October 2008

The optimal design of thermal characteristics in flexible pipes for offshore pipes is increasingly moving into focus, as the application of flexible pipes faces deeper water, greater distances, more adverse operating conditions, and associated aspects of economic feasibility.

To prevent hydrate formation and to ensure adequate arrival temperature, the fluid temperature must be maintained above critical limits throughout the pipeline system, during normal operation and shut-in situations, implying that substantial thermal insulation may be required for the pipes. Such insulation is today obtained by application of layers of syntactic polypropylene tape, spiraling around the flexible pipe core, applied in between steel tensile armour layer and thermoplastic outer sheath.

It is of paramount importance to take all relevant physical phenomena into account to obtain an optimized and reliable thermal design methodology, including layer to layer thermal resistance, true geometry and properties, temperature distribution in annulus, performance of dry/wet layers in pipe annulus, effects of pressure and temperature variation etc.

In order to further document and develop the thermal design methodology for flexible pipes, thermal testing for a range of representative operating conditions was carried out at NKT Flexibles on a 10 m long full-scale instrumented pipe section. The pipe was encased in a pressure test tank, sealed by end fitting assemblies. By controlling and accounting for the heating effect supplied in the pipe bore, in relation to the precisely controlled pipe internal and external temperature, the thermal performance of the pipe was monitored.

In supplement, a dedicated small scale test program was carried out on the insulating tape, yielding material specific issues such as end-of-life water absorption, creep, and associated thermal conductivity and specific heat capacity.

The test program and test setup are described, together with the valuable results and improved experience gained, including pipe thermal conductivity and cool-down performance throughout the test program. Further, the continuous implementation of the test results in the improved design methodology for optimum thermal design of flexible pipelines is covered, including the long term prediction of the pipe full thermal behavior.

Introduction

The ever increasing water depth of new subsea installations for oil and gas production means a growing focus on flow assurance issues, in order to bring the flow safely to the sea surface, and beyond. Thereby, new challenges for the optimal design of thermal characteristics in flexible pipes have arisen.

Multiphase flow from subsea completions, in combination with the long length of pipe required to carry the flow to the surface, external cooling by seawater, and internal cooling by gas expansion, in conjunction with a possibly low well outlet temperature, warrants that design considerations must include aspects of wax and hydrate formation. Also, a certain minimum arrival temperature of the flow products at the surface installation may be required.