Sea Water Batteries Application to the Luna 27 Autonomous Well
Authors: Galletti Robin , Citi Gianfederico , Battaia Claudio 
Published: December 2007

The paper describes the application of Sea Water Batteries (SWB) on subsea wells with autonomous control systems. The concept is illustrated via the description of the installation of a SWB pack on the LUNA 27 well, performed in January of 1996, to supply power to the Subsea Wells Autonomous Control System (SWACS).

SWB's are applicable to subsea production systems which require local power generation. Principally these are systems that utilise an autonomous control system, i.e. one that does not have any umbilical connection to the surface. The advantages of autonomous control systems are tied to the elimination of the umbilical, although one of the main limitations to date has been the local power generation system. Past experiences with Lithium batteries and investigations into turbines and thermoelectric solutions have highlighted their limitations, which for satellite well applications have been resolved with the SWB.

The use of SWB has seen very limited use to date, with only a prototype cell having been instilled in Norway. This paper demonstrates the application of this innovative technology on a live well and illustrates both the technical and economical advantages of the concept for the Luna 27 well.

Finally the paper analyses the general applicability of this technology in particular as regards the potential capital and operative cost savings.

Introduction

On the 20th of January 1996 the first application of Sea Water Batteries (SWB) on a live subsea well was successfully performed on the LUNA 27 well, in 180 m w.d., in the Ionian Sea. The SWB pack is composed of 6 cells each 1 m in diameter by 2 m high and is designed to provide all the electrical energy required by the autonomous control system that controls the well. The only operations that shall be required in the future shall be the periodic substitution of the anodes via ROV every 3 to 5 years.

This application of the SWB's is a part of the continuing research by Agip within the area of autonomous control that began with the SWACS (Subsea Wells Autonomous Control System) project (Fig. 1). This project commenced in 1982 and culminated in the installation of a SWACS prototype on the Luna 27 gas well, located offshore Crotone at a distance of 4 km from the Luna A platform, in December of 1987. Notwithstanding its 5 year foreseen life the system is still operating today without any noticeable problems.

These eight years of operational experience have highlighted the critical areas within this technology, that are: the range for the communication system using hydroacoustics, which at present is around 10 km; and the local power supply. Studies into the former are ongoing while for the latter the potentially most suitable system identified for single satellite wells was the use of Sea Water Batteries, leading to this system being installed on Luna 27 to provide power for the SWACS.

SWB description

The seawater battery system developed for application on the Luna 27 well consisting of 6 cell system based on metal anodes which use sea water not only as electrolyte, but also as oxidant with an inert cathode of titanium. Such cells are nominally capable of supplying 40 watts continuously based on the results of a prototype instilled previously in Norway. This nominal power supply was calculated on the assumption that the SWACS control system had an average continuous consumption of 20 watts.

The 6 cells of the SWB are located in a lattice structure which is designed to ensure that the cells are located 2 m from the seabed and 1 metre from each other in order to ensure a reasonable sea water velocity through the system. Also located within the lattice structure are the electronics canister and the monitoring equipment. Each element is briefly described below:

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