Abstract

This paper will present a number of new, novel and cost effective pipeline systems for the export of oil from floating production units in deep water. In very deep water transferring the oil to an offloading system through a system of risers and a sea bed pipeline is both very expensive and inefficient from a liquid flow point of view. A number of different configurations for pipeline systems, which are suspended between the production vessel and the offloading system, have been developed. This paper will highlight the critical elements associated with these export systems from the design, fabrication and installation point of view, based upon experience gained from applying these developments to projects in West Africa.

Introduction

Oil offloading systems are an essential element of deep-water developments to allow safe offloading operations. Rigid offloading steel lines are a cost-effective solution used in several recent West Africa developments.

The purpose of this article is to highlight constraints and identify possible improvements associated to the different components of the system, based on actual industry knowledge and Stolt Offshore experience.

Generic Parameters

Existing Offshore Loading systems for deepwater developments have a certain number of similar characteristics:

  • 2 or more offloading lines suspended between a FPSO and a Single Point Offloading Buoy,

  • Distance of approximately one Nautical Mile between the FPSO and the buoy.

  • An offloading rate of approximately 42,000 bbl/hour in order to allow 1-million barrel transfer in about 1 day.

  • Conventional type offloading buoy capable to moor of tanker of opportunity up to 350,000 tons. Only the dimensions have been increased to provide enough buoyancy to support the weight and tension of the mooring lines and offloading lines,

Capability to withstand accidental situations (collision with tanker, 10 year storm with tanker connected, 100 year storm without tanker)

What is possible to change and to optimize?

  • Number / diameter of offloading lines,

  • Material (steel vs. flexible line): this article focuses on a steel line solution,

  • Length and Shape of offloading lines,

  • Buoy dimension

  • Buoy type if acceptable for operators,

  • Orientation of the lines compare to the main environmental directions if no constraints from the field layout.

This optimization should first focus on costly equipment. The overall cost of the system includes:

  • the offshore loading lines,

  • the pumping system to be installed onboard the FPSO,

  • the offloading buoy with its mooring system.

This overall cost is about ?65 million (Euros) including:

  • buoy : ?30 millions (Euros)

  • Mooring system : ?10 millions (Euros)

  • OLL : ?15 millions (Euros)

  • Equipment : ?10 millions (Euros)

These values are estimations and have to be further refined when taking into account the final characteristics of the system. For example the cost of equipment including offloading pumps and generators is highly dependent of the selected offloading pressure and may vary between ?4 million (Euros) (10 bars offloading pressure) and ?16 million (Euros) (50 bars offloading pressure).

Keywords:
variation, tanker, upstream oil & gas, subsea system, offshore technology conference, thickness, configuration, technology conference, stiffness, new cost-effective solution
Subjects:
Offshore Facilities and Subsea Systems, Floating production systems, Mooring systems
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2005. Offshore Technology Conference
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