Abstract

A number of large deep water oil fields offshore West Africa are developed on the basis of a large spread moored FPSO with a CALM buoy located at a safe distance to allow tankers to moor alongside for oil off-loading operations. For the transport of the stabilized oil from the FPSO to the CALM buoy, the proposed base case for a number of fields has been to use large diameter steel pipe hung between the two floaters. Design experience has however shown that the fatigue life of such an off-loading line is extremely critical and as such negatively influences the reliability of the system.

By adapting the configuration and/or using flexible pipe instead of rigid steel pipe, the fatigue life can be significantly increased. A different configuration can reduce the loads in the pipe and a flexible pipe, by its inherent compliance, is significantly less sensitive to curvature variations and will thus have a longer fatigue life than a rigid steel pipe.

After a general overview of the impact of different design choices on the total solution and the feasibility of flexible pipe oil off-loading systems, this paper presents in details the flexible pipe system that has been chosen for Shell's Bonga field offshore Nigeria. It addresses the main phases of design: namely, the flow aspects that dictate the number of lines and the operating pressures, the mechanical and hydrodynamic aspects that determine the structural integrity and the fatigue life and also the installation operations.

1 INTRODUCTION

For a number of Ultra Deep Water fields West of Africa the export of oil is performed by tanker. However as the FPSO is spread moored it is not practical for the tanker to unload directly from the FPSO barge. Therefore a separate offloading buoy situated approximately 1 nautical mile from the FPSO is used for this purpose. The export of the stabilised oil is typically achieved through a subsurface partially buoyant line. The line is given buoyancy in it's middle section to obtain a wave shape and as such provide a certain amount of flexibility to accommodate relative motions of the off-loading buoy and the FPSO.

Design experience in the industry has shown that fatigue is a real problem for rigid steel pipe oil off-loading systems and that to achieve the specified fatigue life special mitigating measures are often required (double flex-joints, thick steel forgings for offshore welds and VIV suppression strakes) (ref. 1 and 3). These specific items are not required when flexible pipe is used, which is therefore considered a good alternative as is illustrated in this paper.

2 INFLUENCE OF THE MAIN DESIGN PHASES OF AN OIL OFF-LOADING SYSTEM
2.1 Flow Aspects.

To optimise the design of an oil offloading system, thus to maximise the flowrate, the following parameters need to be considered :

  • Pressure drop allowable along the lines (inlet pressure and requested outlet pressure)

  • Diameter and type of line (internal roughness) of the lines

  • Number of lines

This section describes how these parameters influence the design and show how optimisation is possible.

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