Specification of Electrical, Optical and Hydraulic Swivels for FPSOS
Authors: Dwyer T.M. , Smith G.A.J. 
Published: October 2008

This paper details the main considerations to be addressed when specifying a swivel for a Floating Production, Storage and Off-loading (FPSO) system or other offshore terminal requiring rotational freedom, The following types of swivels are discussed: electrical swivels for power and control, optical swivels for control/data, hydraulic/pneumatic swivels for control or actuation, and combination units. The detailed design of product swivels is not within the scope of this paper. From the design perspective, typical specifications are discussed for each swivel type, including considerations of capacity, materials, terminations, sealing, weight, torque and position in the swivel stack. Testing and certification for hazardous locations is addressed.

The information presented in this paper can be used primarily by designers and systems engineers in specifying swivels, which are used in various FPSO moorings allowing the vessel to weathervane about the mooring point. Several existing swivels are described to demonstrate the functionality available. These include hollow-bore units for systems with product swivels above the electrical or hydraulic swivel.

There are a number of tradeoffs to be made when specifying a swivel for FPSO applications. This paper highlights these, enabling the buyer to specify a configuration completely, thus minim program changes and cost.

Introduction

Floating Production, Storage and Off-loading (FPSO) systems are in increasing use worldwide. This growth will continue, as many of the more easily-developed fields, served by large production platforms, become depleted. New oil and gas discoveries are found increasingly in smaller deposits, in deeper waters and farther from established infrastructure. FPSOS, in existence for more than three decades, frequently offer the most economical solution, especially for smaller fields.

FPSO mooring systems are versatile and may come in any of several configurations, including:Catenary Anchor Leg Mooring (CALM)Submerged Buoy Turret MooringPermanent Buoy Turret MooringDisconnectable Turret Mooring

The vessel employed with theses mooring terminals may be a modified standard shuttle tanker or a purpose-built vessel, possibly equipped with an internal moonpool into which the buoy is connected. In some applications, a shuttle tanker ties up to a permanently moored production vessel. The choice depends on the size of the field, flow rates, distance from shore and many other factors.

Always, however, the vessel weathervanes about the mooring point, in response to wind and current conditions. Therefore it needs a swivel assembly to allow rotation about the mooring, which is fixed relative to the seabed, The swivel assembly must service the product lines, whether they be condensate, gas or multiphase product. In addition, it usually accommodates electrical slip rings for the delivery of power, generated on board the vessel, to submersible pumps, as well as hydraulic or pneumatic fluid circuits for controls and electrical or fiber optic channels for signals and data.

The purpose of this paper is to examine the designtradeoffs in building up this complex swivel assembly, with emphasis on the electrical, hydraulic/pneumatic and fiber optic components.