Innovative Optimization of a Large-ID Sour-Service Flexible Riser for the Baobab Project in 1000m Water Depth, West of Africa
Authors: Decoret L.C.A. , Mullot D. , Paterson J. , Taylor T. 
Published: October 2008

The vast majority of flexible pipe dynamic risers installed in deep water are for sweet service application, with no or a very limited level of H2S present in the produced fluid. For the Baobab development in 960m water depth, the 11" ID flexible production risers required to be fully sour service rated to safely accommodate potential future reservoir souring. The design of such riser structure considering the standard approach proved challenging and leading to significant installation constraints. The optimization of a one length sour service flexible pipe structure to satisfactorily accommodate all loadings associated with the deep water environment (high tension at the top, high hydrostatic collapse pressure and reverse end-cap effect loading at the bottom, high installation tension, etc ..) is indeed difficult and ultimately results in a compromise solution.

Through detailed engineering work, the technically optimum and most cost effective solution was identified to be a two sections riser system. Each flexible riser structure can then be optimized for the design loading pertinent to the top or bottom area respectively. This approach leads to a significant weight reduction over a one section design, bringing the top tension and installation loading to a similar level as for a sweet service riser structure.

Optimization of the riser configuration, using a Deep Lazy Wave where buoyancy modules are incorporated at the touch down point area, also proved very effective to maintain large bending radius at that location in extreme storm conditions. The installation considerations are shown to be an integral part of the riser system design optimization. In particular the use of partial flooding during the critical installation stages contributed in the optimization of the riser structure design and the reduction of the risk of armour lateral buckling. The paper demonstrates that innovative design and optimization of riser configuration and installation can enable the use of large diameter flexible pipe risers for sour service production application in deep water.

Introduction

Deep water developments increasingly require larger diameter production risers suitable for sour service application. For flexible pipe, compatibility with sour service production fluid means using lower yield strength carbon steel wires resistant to H2S induced corrosion. This makes the design of the riser structure much more challenging to accommodate the different loading associated with deep water application, particularly so for larger diameter.

Figure 1: Typical flexible riser construction (available in full paper)

The paper describes the design challenges associated with such risers and the solutions selected to overcome them for the Baobab development.

The benefit of splitting the riser into two separate sections is demonstrated, each section being optimized for the pertinent loading that it needs to accommodate. The optimization of the riser system configuration along with the installation measures taken are also shown to be an integral part in achieving a technically suitable and cost effective solution.

Baobab field description

The Baobab field is owned and operated by Canadian National Resources Limited (CNR) and is located offshore Ivory Coast, in Block CI-40, approximately 65km South West of Abidjan, in water depth ranging from 915m to 1200m.