Flexible Riser and Mooring System Develops Small Oil Fields in the North Sea
Authors: Thompson A.M. , Holmes G.A.A.H. , Benstead P.J. 
Published: October 2008

The Flexible Riser and Mooring System (FRAMS) provides a technically feasible and commercially attractive method of unlocking small U. K. North Sea oil fields. A moored 60,000 DWT oil tanker, fitted with deck mounted production equipment is restrained by a mid-water buoy, while the swivel-less riser bundle conveys well fluid from, and injection water to, the subsea wellheads. In severe weather, oil production is stopped, and the tanker disconnects from the mooring, re-connecting when weather allows.

INTRODUCTION

In a typical FRAMS field development, a small North Sea oil field would be developed by a moored 60,000 OWT tanker restrained against environmental forces by a 5" diameter spiral strand wire to an anchor base via a mid-water buoy. Three 5" 0.0. flexible pipes are held off the wire the wire with spiders, one conveying well fluid, one injection water, and the third is an electro-hydraulic umbilical cable. The tanker is free to rotate about the mooring, using the inherent flexibility of the lower riser hoses, up to a design limit of ± 300° rotation. The mooring acts like an inverted pendulum being displaced when environmental forces cause the tanker to move or weathervane, and providing a restoring force to the tanker. Figure 1 provides an over-view of the FRAMS concept. The work carried out shows that a suitable field for development by FRAMS would be one with an oil reservoir of 10-20 million barrels, that could be recovered over 5 years using a total of up to 5 wells. The gas/oil ratio should not exceed 500, and the water depth should not be less than about 80m. The field should be in the central North Sea area.

The study work described in this paper outlines the sequence of events from conceptual design, analytical work, model testing and voyage simulation, supporting the overall hypothesis that a buoy/tanker system can be engineered to provide an attractive method of developing small oil fields.

DESIGN PHILOSOPHY

The overall design aim was to develop the FRAMS conceptual design towards reducing capital and operating costs by simplifying the design requirements and using straightforward engineering. Consequently FRAMS has avoided the use of fluid swivels, and dynamic or even active positioning. No provision is made for the ship to remain connected to the mooring in all weathers, thus avoiding the need to size components for the most severe conditions. A tanker, approximately 5 years old, would be converted and equipped with above deck, palletised units, and will use its attendant anchor handler as an occasional tug. The tanker returns to port to discharge its cargo, providing opportunities to carry out inspection, and dry-docking if needed.

PRELIMINARY DESIGN

Analytical work, using the AQWA suite of programs was carried out to determine the sizes of the mooring components using a typical Base Case field. A detailed diffraction wave loading analysis was carried out, using a 60,000 DWT tanker hull, for four wave directions and ten wave periods. Loading co-efficients for wind, current, and yaw mooring drag were calculated. This analysis provided top and bottom line loadings.