Thermal Performances of the Flexible Bundled Risers
Authors: Secher Ph. , Felix A. , Secher Henry 
Published: October 2008

In order to address the flow assurance challenges associated with the Ultra Deep Water developments, a specific flexible pipe structure termed IPB (Integrated Production Bundle) has been developed. This paper describes the main features of the IPB thermal qualification programs and in particular the latest developments with the electrical heating technology. It includes the experimental validation of the thermal performances in passive mode as well as in active heating mode. The correlations and validations with numerical simulations are presented in both steady state and transient conditions.

The results of these investigations are presented and discussed with an emphasis on the comparison between water heated and electrically heated concepts. The possible applications are then illustrated over relevant study cases mainly from West African developments.

Introduction

The development of flexible solutions in ultra deep water has already a long history with fields as MARLIM or more recently RONCADOR6. The growing evolution is towards more and more demanding application in terms of thermal performances to answer the wax and hydrate formation risks.

Besides the mechanical challenges related to the weight load, and the hydrostatic pressure, the thermal challenges are well recognised by now to be an inherent aspect of the design of Riser Systems in Ultra Deep Water (UDW) development.

Indeed, cooling down of the crude along the long vertical span of the riser can bring the temperatures below the critical values of hydrate or wax formation. This is all the more true, that several identified reservoirs in UDW are associated with rather low well temperatures. Also, along the lifetime of the field, temperatures and flowrates of crude can vary, so that flow assurance issues can be encountered over some periods of the life cycle. In these conditions, even with infinite passive insulation, the arrival temperatures can not be kept over the wax formation temperature in some cases, which means active heating is required.

Also, during shut-down of production, the crude will experience cooling down, where the temperatures will reach the hydrate formation limits after more or less short periods of times (hours). These are the "cool-down" and the "start-up" aspects of the Flow Assurance.

In order to address the various aspects of the flow assurance, Coflexip has developed a flexible bundled riser structure termed IPB (Integrated Production Bundle). However some of these components can be dedicated to other functions like gas lift (which is expected to be required in UDW), or any kind of umbilicals function. Also, a part of the assembly elements, can possibly be used for mechanical functions : transmission of radial forces.

Therefore the construction of the bundle will be highly dependant of the application specificity ranging from a single armour wire type layer to a complex mix of cables hoses and insulation layers. The limits of complexity will be dictated by the economics and the capability to assemble around a single pipe a great number of components preferably in one section.