COBRA: An Uncoupled Riser System for Ultradeep Water in Harsh Environment
Authors: Karunakaran Daniel N. , Baarholm Rolf 
Published: May 2013

Steel Catenary Risers (SCR) and hybrid riser towers have been an attractive choice for recent deep-water field developments. However, design of SCRs for harsh environments or from large motion host platforms remains a significant challenge. The key issues for the design of SCRs in harsh environments are the fatigue near the hang-off and at the touchdown point. Hybrid riser towers have their own challenges and need special bottom assemblies with heavy foundation and complicated spools.

A new un-coupled riser concept is presented called the Catenary Offset Buoyant Riser Assembly (COBRA). COBRA consists of a catenary riser section with a long, slender buoyancy module on top which is tethered down to the sea bed. The top of the catenary riser section is connected to the host platform by a flexible jumper. COBRA is an efficient riser arrangement for host platforms with large motions, e.g. FPSO or Semi submersibles. The flexible jumpers in this riser system effectively absorbs the platform motions, and consequently the steel catenary riser section has almost no dynamic motions, which improves both strength and fatigue performance.

The riser system is developed for water depths ranging from 750 m to 3000 m in harsh Northern Norwegian environments. The results clearly indicate that it is possible to have a robust design of COBRA risers from large motion host platforms in harsh environments using presently qualified material and technology. The first order wave fatigue response of the steel riser section is negligible and the fatigue is purely controlled by VIV and can be mitigated by the use of VIV strakes. The preliminary work also showed that this riser system can easily be installed in harsh environments. The riser components used in this riser system are all field proven as they are used in other riser systems.

This concept is also applicable in less demanding environments, such as in Brazil. Furthermore, due to reduced dynamics in the SCR part of the risers, cost effective CRA materials like mechanically lined pipes can be used in the SCR section, thereby reducing the costs considerably.

The COBRA riser concept will make the applicability of catenary risers a credible alternative option for use in increasingly harsh environments or from host platforms with large motions. Since the components in this riser system are all fully qualified material they are cost effective and ready for project application.

Introduction

Steel Catenary Risers (SCR) and hybrid riser towers have been an attractive choice for recent deep-water field developments. However, design of SCRs for harsh environments or from large motion host platforms remains a significant challenge. The key issues for the design of SCRs in harsh environments are the fatigue near the hang-off and at the touchdown point. Hybrid riser towers have their own challenges and need special bottom assemblies with heavy foundation. These bottom assembly and spools become a great challenge in deep waters and in harsh environment. Hybrid riser towers have successfully been employed in Offshore Africa, Gulf of Mexico and Brazil.

A new un-coupled riser concept is developed called the Catenary Offset Buoyant Riser Assembly (COBRA). COBRA consists of a catenary riser section with a long, slender buoyancy module on top which is tethered down to the sea bed. The top of the catenary riser section is connected to the host platform by a flexible jumper.

This concept combines the advantages of SCR and the hybrid riser tower and eliminates the fatigue challenges at touch down point of SCR and also avoids the use of complicated bottom assembly and spools of hybrid riser tower. Since the platform motions are un-coupled in this riser system, the fatigue in the SCR part is very small. The riser system can be designed for pipeline class welds, e.g., F1 class; hence exotic material such as mechanically lined clad pipe (BuBi pipe) can be used for the SCR part.