Advanced Electrostatic Internals in the 1st Stage Separator Enhance Oil/Water Separation and Reduce Chemical Consumption on the Troll C Platform
Authors: Wolff Erik A. , Knutsen Trond L. , Piasecki Wojciech A. , Hansson Peder , Nilsen Pål J. 
Published: October 2008

The paper describes the design, testing, installation and operation of new coalescer technology in the 1st stage separator in offshore oil production. This technology is called VIEC ("Vessel Internal Electrostatic Coalescer"). The paper describes the concept as well as the technical challenges experienced. Then the results from the successful installation on Norsk Hydro's Troll C platform are presented, where large reductions in demulsifier injection have been achieved.

Introduction

The development of new coalescing devices has taken place in ABB since 1999. In 2003 the VIEC concept had passed prototype trials and was ready for field testing.

The Troll C platform was commissioned by Norsk Hydro in 1999 to exploit the thin oil layers below the Troll field gas cap (ref. 1). The Troll field shown in figure 1 is currently the largest oil producer on the Norwegian continental shelf with a daily production of over 400,000 BOPD. Oil data for the Troll field are given in Table 1.

The separation process in the 1st stage separator at Troll C was at the time not satisfactory, which gave reduced capacity and poor water level monitoring. The consequences of this challenge were a high content of water in the oil out of the 1st stage separator (above 10%), and possibility for accidental discharge to sea.

Troll Projects, managed by Mr. Trond Knutsen, is the Norsk Hydro department in charge of defining and executing modification projects on Troll B and C platforms. An internal Norsk Hydro report (ref. 2) recommended that internals should be modified and a VIEC installed. ABB received a contract in February 2003 for delivery of VIEC and adjacent replacement baffles by June 1st 2003. The project culminated with installation of the new equipment in June 2003. The results from the first half year are good and details of the project are given in this paper.

Technology description

The application of electrostatic force to help break oil/water emulsions and increase water droplet size is an old and proven technology (for example ref. 3). Oil production facilities all over the world have used electrostatic coalescers for many years to reach the sales specification on the water content of the oil. The settling velocity of water droplets in oil depends on the droplet radius squared, which illustrates the high improvement possible through increased droplet size. The traditional electrostatic coalescer is a liquid filled horizontal vessel with a residence time of 10-45 minutes, depending on the oil density and temperature. It is usually the 3rd treatment step after removal of bulk water by gravity and degassing to a suitable vapour pressure.

Coalescer Performance theory

The main feature of electrostatic coalescers is the effect of the electrostatic field strength on the conductive droplets (water) in an insulating media (oil). The water molecules in oil act as dipoles with a positive and a negative end. The electrostatic field from high voltage between two electrodes exerts a force on the droplets that press them together and helps drain the film between them. This enhances coalescence to larger droplets.