Autonomous Buoy for Offshore Well Control and Monitoring
Authors: de Pinho O.J. , Euphemio M.L.L. , Correia O.B. 
Published: October 2008

Conventional subsea control systems require expensive eletrohydraulic umbilicals that economically limit the maximum distance between the platform and the well. State of the art umbicalless subsea control systems are limited by the communication media and problems related to locally generate enough power to run the subsea equipment hydraulic valves and the control system itself. The proposed control buoy concept is a hybrid well control system that brings together the reliability of conventional umbilical based systems with the long distance cost advantages of the umbilical free systems. The control buoy prototype installed in a live offshore well has confirmed our premises and field operational feedback has improved the system design and made a solid basis for new developments.

Introduction

The 4-ALS-39 is a marginal (wild cat) gas well offshore the state of Alagoas, northeast Brazil, 22 km off coast, at 25 meters water depth. Its production development became economically feasible by using an autonomous control buoy for well control and monitoring. The project is based on a buoy moored near to the well. A control and data acquisition system installed on the buoy isresponsible for monitoring the wellhead pressure and temperature, controlling the wet Xmas tree valves and production choke, through a conventional eletrohydraulic umbilical. A UHF radio link between the buoy and the shore ensures permanent supervision and interlock with the onshore pipeline valves and metering station. As the onshore plant is normally unmanned, a dialed cellular telephone line allows farther remote well monitoring and operation. In spite of being a small buoy (3.5 meters diameter and weighing about 3.0 ton), it is completely autonomous in terms of energy. All the energy required to power the system is locally generated by solar panels and a low power electrically driven hydraulic power unit. Only six months were needed from conceptual design to installation. The buoy was successfully installed in September 96 and during the first seven months some interventions for maintenance were required, including a retrieval for an overall improvement in the original project. The buoy was reinstalled in march 97, and has been operating continuously since then.

Autonomous system considerations

This control system basic concept is to keep a small buoy moored near the subsea equipment (X-mas tree, manifold, template, etc.) to be controlled. The buoy is in permanent connection with the control station through a communication link and supports the control and monitoring system responsible for the equipment integrity and environmental safety. The short distance connection between the buoy and the subsea equipment is made through a conventional eletrohydraulic umbilical. The long distance link between the buoy and the control room is completely wireless and needs no umbilicals (fig. 1), and so, the cost of the control system is not influenced by the distance. The main point when designing an umbicalless control system is that the non physical communication link required between the control room and the controlled equipment may be unavailable in some cases due to many reasons that will differ depending on the chosen communication technology (radio, satellite, acoustic, etc.).