A Gas Turbine Power Generator For Improving Autonomous Subsea Control Systems
Authors: Maneschi Giuseppe , Menegazzo Fabrizio , Chimisso Claudio , Mattei Lucilla 
Published: October 2008

ABSTRACT
A subsea autonomous control system has been designed for the Deepwater Autonomous Multiwell Production System (DAMPS). Its most outstanding features are: a power generator based on a gas turbine, a hydraulic closed loop circuit and signal communication via acoustic links. In particular a full scale prototype of the power generation system has been designed, fabricated and successfully tested in the laboratory. This paper outlines the most outstanding features of the power generator and its appropriate scenarios of application together with subsea autonomous control systems.

INTRODUCTION
Deep water Autonomous Multiwell Production System- DAMPS is a research project which began in February 1990 and was completed in August 1992. The project developed a guide lineless and diver less subsea production system, made up of a 6 slot drilling/production template with a lattice protection structure (fig.1), production modules and dedicated tools for their installation/removal and maintenance, deployed by a dedicated installation vehicle (fig. 2). Each module is equipped with electro-hydraulic connectors performing all process and electric couplings. The main features of each production module are depicted in tab.1. Control requirements have been complied with an autonomous systems in which the signal communication is via an acoustic link and the required hydraulic and electric power is generated in loco. The technology for the subsea hydraulic power unit and the relevant hydraulic closed loop has been derived from the Subsea Well Autonomous Control System-SWACS, operating since 1987 on Agip's Luna field in the lonian Sea. As for the electric supply, a generator based on a gas turbine has been designed. Tests carried out on a full scale prototype, equipped with pressure compensated lubrication and turbine alternator magnetic coupling, have demonstrated the technical feasibility of this concept.

GENERAL
The first generation of subsea autonomous control systems was based on primary Lithium batteries, which have demonstrated to be a suitable solution, but they require periodical costly interventions for the replacement of the discharged batteries. In the DAMPS project, various alternative power systems have been considered, among which:Sea water batteries (SWB), utilising a controlled corrosion reaction through sea water, between two electrodes.Closed Organic Rankine Cycles (ORC) using as a heat source the fluid flowing through the production system, either gas or oil.Thermo-Electric systems (TES), which utilises the same primary energy as the above, but with static components.Turbo-generators (TG),' utilising the energy of the high pressure fluid.
Considering the state of the art of the respective technologies, sea water batteries seem to be the most suitable for oil production, whereas turbo generators are a promising alternative with gaseous fluids (see comparison in tab.2). In particular, it should be pointed out that closed organic Rankine cycles and thermoelectric systems are very sensitive to production fluid flow rates and temperature.