Advances in Flexible Riser Technology: Thermal Performance of Flexible Pipes for Deep Water Use: Full Scale Test of New Insulation Material at High External Pressure
Authors: Nielsen Karsten Gorm , Bendiksen Erik 
Published: May 2011

The technological and economical challenges of application of flexible flowlines and risers at very deep water creates the incentive of developing insulation materials that meet deep water challenges in an optimized manner, including the establishment of a thermal design methodology valid for the full pipe service life, to avoid temperature profile related flow problems such as hydrate formation in pipe.

In connection with the development and qualification of a new type of syntactic polypropylene insulating material, for application in the form of helically wound tape on deepwater flexible pipelines and risers, a project was carried out at NKT Flexibles I/S to document the thermal performance by full scale thermal test at high external pressure. The deep water in-field conditions were replicated, including circulation of ambient water in the test setup. A 10 meter long instrumented pipe sample was encased in a pressure tank, sealed by end fitting assemblies, and subjected to various combinations of temperature, external pressure, and dry respectively flooded annulus conditions. The heat losses were monitored for subsequent detailed analysis. In supplement, a small scale laboratory test program was undertaken, enabling extrapolation of physical behavior and thermal properties to cover full pipe service life, by considering issues such as end-of-life water absorption, creep, and associated thermal conductivity and specific heat capacity.

The new insulation material was tested and verified for use in water depths down to 2000 meter. For reference and for comparison, an existing traditional type of insulation material was tested through a parallel test program. The full scale test setup incorporated alternative methods for assessing the heat loss through the pipe wall, enabling an assessment of relationship to other methods for thermal testing.

Analysis was undertaken to implement the results in the thermal design methodology, in function of governing design parameters, and including all prototype physical phenomena.

The thermal test program results are described, together with the implementation of these results in the design methodology. A highlight is furthermore given on the novel features of the applied thermal test methodology to represent prototype pipe conditions.

Introduction

The application of flexible flowlines and risers for oil and gas production at very deep water creates the incentive of developing and qualifying insulation materials that meet the deep water challenge in a technologically and economically optimized manner to bring the flow safely to surface facilities and beyond. These efforts also include the development of a documented optimized thermal design approach for such pipes, to avoid costly or even detrimental flow problems such as wax or hydrate formation in pipe, or downstream product arrival temperature at facility below specified value. Often requirements to a specific Thermal Exchange Coefficient (TEC) are specified for flexibles, derived from flow assurance requirements, including cooling effects from ambient seawater and cooling associated with gas expansion in the flow.