Design, Manufacture, And Tests Of An Electrical Power And Communications Swivel For A Single Point Mooring Terminal
Authors: Cole S.R. , Crowl D.M. , Walker H.C. 
Published: October 2008

ABSTRACT
An electrical swivel for an offshore single point mooring (SPM) terminal was constructed using sliding electrical contact concepts developed for slip ring assemblies. The swivel unit contained its own bearings, seals, terminals and connectors and was designed to be suspended below a fluid swivel in an integrated assembly. The designs of signal and power circuits are discussed along with environmental and mechanical concerns. Qualification test results indicate that the performance of the swivel exceeded the system requirements in all respects. In addition to discussing the specifics of the swivel assembly that is being used in the SPM terminal, general guidelines are given for writing electrical swivel (slip ring) specifications.

INTRODUCTION
Electrical swivels are becoming increasingly important in offshore technology. They are used to carry electrical currents, both power and signal, between rotating and stationary bodies. The usual terminology in other industries for this type of assembly is a slip ring capsule in this paper, the terms electrical swivel and slip ring capsule will be used interchangeably.
Two principal objectives of this paper are (1) to describe the specific design requirements and test results of an electrical swivel that is being used in an offshore single point mooring terminal process tower and (2) to give someone who is unfamiliar with electrical swivels the information needed to write a specification in terms used by the slip ring industry.

DESCRIPTION OF THE SYSTEM
This paper is specifically concerned with an electrical swivel that carries both power (600 V, 3 phase, l50A/phase) and signal currents (24 V DC, 1A). Figure I is a photograph of the complete electrical swivel assembly. The electric swivel is part of an integrated electrical and fluid swivel assembly mounted in an articulated tower anchored to the sea bed. Permanently moored to the tower via a rigid yoke assembly is a storage/processing vessel free to rotate 360° around the tower through the action of the swivel assembly. The electrical power and signal currents are carried between equipment located on the sea bed and the vessel via a cable through the articulating tower, swivel and the yoke.
This type of assembly can be used in other single point mooring configurations and in cable reel assemblies. Figure 2 is a sketch of a typical tower type single point mooring terminal with vessel. Figure 3 shows how the electrical swivel mounts at the bottom of the fluid swivel to make the integrated swivel assembly.

DESCRIPTION OF THE ELECTRICAL SWIVEL
The housing of the electrical swivel was constructed of heavy AISI 316 stainless steel plate. Connections to the slip ring brushes are made through the connectors on the side of the unit. Leads to the ring contacts pass through a bore to terminals in a junction box at the bottom of the assembly. There are ball bearings between the slip ring assembly (Figure 4) which rotates and the frame assembly which is stationary and holds the brush assemblies.