New Generation of Platform Retrofitted Riser System
Authors: Brackenridge Alan 
Published: October 2008

The development of optimal solutions to satisfy the riser requirements for new and existing platforms involves the consideration of matters of major financial and safety significance.

Recently developed technology offers substantial cost, safety and technical benefits over traditional retrofit riser solutions. Todays design draws on practical experience gained from over seven years development and installation of new generation riser systems. In this paper the new generation riser systems for new and existing platform applications are compared with traditional riser systems.

INTRODUCTION

Platform risers are key components of all offshore production facilities having major economic and safety significance. The riser provides access from the sea bed to the platform facilities for subsea satellite developments, (produced oil and gas, test lines, water injection and control umbilicals), product import and export pipelines and platform utilities (control and power lines).

A riser is usually a pipe which connects between a subsea pipeline and the platform topside. The riser pipe can be made from rigid or flexible pipe in the size range from 2" DIA to 36" DIA depending 011 requirements.

The riser design accommodates substantial environmental loading, particularly ill the splash zone. To accommodate the environmental load risers of smaller diameters are usually located inside a carrier pipe called a caisson which acts as a structural member to provide protection. The need for a caisson depends on the number size and location of the risers. Generally risers above 20" OD do not requirea caisson, risers less than 12" OD require caisson protection and those 12" OD to 20" OD may require a degree of additional protection. Most risers systems consist of several risers of different diameters' and materials. Typically, a satellite field development will require production, test and water injection lines together with control umbilicals.

Risers are of key importance to platform safety as their failure can compromise the complete production facility. For this reason many safety consideration are incorporated into riser designs.

These may include:Accidental Vessel Impact Protection.Increased Requirements ForCorrosion Protection.Structural Strengthening.Subsea Safety Valves.Passive Fire Protection.

Riser design is covered by numerous industry specifications and government legislation included in which are recent recommendations by the UK Department Of Energy which requires risers to be protected from accidental vessel impact damage.

The new generation of riser systems meet 01' exceed all existing design requirements and have additional technical features which improve the flexibility of the system at substantially reduced cost. Several of the new generation of risers are now in operation, confirming the feasibility of the system. Two variations of the system have been developed:Retrofit Riser System On Existing Structures.Retrofit Riser System On New Structures.

RETROFIT RISER SYSTEMS ON EXISTING JACKETS

The conventional method of retrofitting a caisson riser system on existing platforms is to fabricate and assemble the riser system onshore. These are typically 150 m long, and are shipped offshore, and installed using a heavy lift vessel.