Bundle Hybrid Offset Riser (BHOR): An Advanced Solution for Improved Riser Tower Systems Installability and Operability in Deepwater West of Africa
Authors: Chiesa Giovanni , Casola Floriano , Pionetti François Regis 
Published: October 2008

Abstract
Several riser system configurations have been developed to assist the recovery of hydrocarbon reserves from deepwater reservoirs. Vertical hybrid riser systems present a number of attractive features and are currently proposed for various West of Africa deepwater developments. The paper describes the Bundle Hybrid Offset Riser (BHOR) concept of a vertical hybrid riser which incorporates bundle arrangements of multiple conduits conceived to enhance the flow assurance performance of the system to allow for the delivery of the production stream to the surface treatment facilities under the production conditions which are anticipated during the field life. Gas lift capability is also incorporated in the bundle arrangement. The passive thermal insulation properties of the system are then addressed by presenting the ability of the concept to accommodate both syntactic and gel materials. Further to the above, the BHOR concept fabrication and installation scenario is discussed in relation to the specific features and constraints which characterize the West of Africa deepwater conditions. Moreover, the paper addresses both the mechanical and structural design issues associated to the proposed system configuration with specific focus on the anticipated improvements in the dynamic and fatigue behavior both under in-place conditions and during installation by tow. Finally, the paper describes the operational advantages offered by the Active Circulation System (ACS) based on hot fluid circulation from the top of the riser down to the gas lift injection point which is incorporated in the BHOR concept to provide hydrate blockage removal capability following either accidental conditions or prolonged shut-downs exceeding the cool down system capacity.

Introduction
The development of hydrocarbons in deep and ultra deep waters requires the identification of suitable gathering and transportation systems able to dispatch the production stream to the surface treatment facilities from remote subsea wells.
Flow assurance management is getting more and more importance as reservoir exploitation moves towards deepwater. The riser system tends to represent one of the most critical elements for the flow assurance management due to the following main aspects:significant temperature drop following depressurization along the riser conduits that may affect the arrival temperature of the produced fluids at the surface receiving facilitiesgas lift injection capability to assist the produced fluids arrival to surface in particular when the gas content in the production fluid is reduced; the gas lift stream has to be delivered to the bottom of the riser conduits under certain pressure and flowrate conditions minimizing the temperature drop along the riser length.production conduits filled with wet gas following shut down leading to potential hydrate formation upon restart; possibility of the production fluids to re-enter the gas lift line through the injection orifice with potential blockage formation in case of low temperature conditionsmethanol injection capability to eliminate hydrate on restartcapability for full depressurization in case of flowlines blockage. The proposed BHOR concept which is described in the following has been developed moving from the experience and lessons learned through the execution of deepwater projects and in addressing. Operators specific requirements to fit their flow assurance management strategy and their system operability objectives.