Offloading Systems for Deepwater Developments: Unbonded Flexible Pipe Technology is a Viable Solution
Authors: Tuohy Justin , Loper Cobie , Wang Don 
Published: October 2008

Offshore exploration for oil and gas is being performed in even more challenging waters, with fields being developed in water depths of 2000 meters and greater. To recover hydrocarbons from these depths, a number of technical challenges are presented to the designers of riser and offloading systems. Metocean characteristics and relatively low reservoir temperatures compound the challenges.

This paper discusses how unbonded flexible pipe technology has overcome the challenges of floating production systems in 2000 meters water depth and how its continued development enables solutions for ultra deep water riser systems. During the discussion of the status of flexible pipe technology, an overview is given to the advancements in flow assurance technology and integrity monitoring, both of which can evolve as integral parts of the flexible pipe.

These achievements in technology are applied to the design of large diameter offloading and export systems that are being considered for large field developments where crude oil is being offloaded to other floating structures and tankers.

The paper details the design and detailed analysis of the unbonded flexible pipe solutions developed by Wellstream to overcome the deepwater challenges associated with large diameter export riser systems. Design comparisons are developed with the alternative products currently proposed.

Finally, the paper concludes with an economic appraisal of why flexible pipe systems should be considered a cost effective solution in the overall CAPEX and OPEX of deepwater fielddevelopments.

Introduction

The exploration of oil and gas offshore is accelerating to even greater depths with each new discovery. Deepwater drilling in the US GOM reached a record high[1] in the final week of 2000. A total of 40 rigs were drilling in the region compared to 26 a year previous. Activity in the ultra-deep is also growing with seven rigs working at the end of the year. The deepest well was drilled by BHP Petroleum at a water depth of 2,545 meters. The recovery of hydrocarbons at such depths presents challenges to the designers and installers of riser and off-loading systems.

Since unbonded flexible risers are an enabling solution for floating production and storage systems, being a proven technology spanning three decades, these challenges have already been overcome for the smaller diameter pipe structures in water depths up to 2000 meters. Continued developments will have much larger internal diameters qualified to these water depths in the near future.

Unbonded flexible pipe is a multi-layer structure of helically wound metallic wires, tapes and extruded thermoplastics. Each layer has a unique role in the flexible pipe. Refer to Figure 1. The inner metallic layer, which is interlocked, provides the collapse resistance. An extruded thermoplastic layer provides the seal for the fluid to be transmitted through the pipe. An interlocked steel layer wrapped over this thermoplastic layer provides resistance to internal pressure and radial compression. A dual layer of counter-wound tensile armors provides the tensile strength to the pipe structure.