Topside Facilities for Floating Production Systems Require New Engineering Thinking
Authors: Burn A.J. , Graaf Gerard 
Published: June 2014

Since the oil industry moved offshore, great emphasis has been placed on the need to reduce space and weight requirements of topside facilities. For successful design of floating production systems, weight consciousness assumes an even higher level of importance, necessitating systematic attention to detail even at the feasibility study stage of a project.

INTRODUCTION

Floating production units are gaining increasing acceptance by the oil industry as alternatives to fixed platforms for deep water applications. Many floating systems are already in operation, mostly based on converted drilling rigs or tankers. In 1976, Shell/Esso installed a custom built, floating, oil storage and tanker loading system, the Brent Spar, in the Northern North Sea. This has operated satisfactorily ever since, and is still in regular use. Following this experience, studies were started to investigate whether the Spar concept could be extended to include production facilities. In order to incorporate a multi-flowline riser and to accommodate the extra weight and space requirements of production equipment a total reappraisal was necessary. The resultant, patented concept combines the storage and tanker loading features of the Brent Spar with the stability and mobility of a semisubmersible floater, and is called Semi-Spar.

A compact design is essential to combat the external forces from wind, waves and current while also to float, to be intrinsically stable, to have acceptable motion characteristics, and to be economically competitive. It follows that a highly integrated arrangement is necessary with optimum use made of space both above and below the waterline. This requires a careful balance between the traditional conservatism of oilfield engineering and the fine tuning required in naval architecture to preserve buoyancy and stability.

This paper summarises the Semi-Spar concept and describes the engineering philosophy necessary for successful design of floating systems, with particular emphasis on the reduction in topside weight necessary in comparison with fixed platforms. Detailed attention is paid to design aspects of process and utility systems. The riser system and the marine systems are deliberately excluded from detailed coverage, being separate major topics in themselves. The paper also discusses safety features, affects of motion on equipment, and operational downtime, in all of which Semi-Spar has interesting advantages compared to other concepts.

SEMI-SPAR CONCEPT AND STUDY PARAMETERS

An artist's impression of the Semi-Spar is shown in Fig. 1. The main features are a toroidal shaped hull providing buoyancy and storage capacity, six retractable ballast legs, and six columns piercing the water surface to support a circular deck. On top is a cylindrical superstructure accommodating the main process systems, and a turntable structure which incorporates the tanker mooring and offloading carriage, two flares, a central swivel, a helideck and lifeboats. The turntable feature allows weather vaning of the export tanker. It also ensures that the helideck is always upwind, that the flare (when used) is downwind, and that the lifeboats are in optimum positions relative to the waves. The floater is stationed by a twelve point catenary anchorline system.