Abstract

The Ormen Lange project is being developed by Hydro ASA off the Norwegian west coast. It will be a long tie back, with all processing facilities on shore.

The offset is 120 to 140 km, and the water depth ranges from 850 to 1100 meters. Maximum field configuration is 28 wells.

To meet Norwegian environmental legislation, a closed hydraulic circuit was selected for the control system.

The return system includes an active receiver as temporary storing device for hydraulic return fluid. This device is enabling technology for the closed hydraulic circuit for Ormen Lange.

The significance of the active receiver is that it allows for fast response time and increased capacity combined with reduced investment in the umbilical, even with very long offsets. The active receiver also enables the use of reliability enhancing synthetic hydrocarbon fluid combined with an environmentally friendly closed hydraulic circuit.

Introduction

While open hydraulic control circuits which use water-glycol based control fluids are dominating the scene, the use of closed hydraulic circuits is being considered as a real alternative.

The closed hydraulic circuit is characterized by its return line back the HPU. Closed hydraulic circuits do not exhaust control fluid into the subsea environment.

Several arguments can be found in favor of closed hydraulic circuits, and obviously; as the open hydraulic circuit is dominating the scene; arguments which favor open circuits can be identified.

Hydro ASA has been more or less standardized on closed hydraulic circuit since their entry into the subsea oil and gas business. Informal comparisons of reliability data for control systems in operation show significantly better performance for control systems with closed hydraulic circuits using synthetic hydrocarbon based control fluids than the more traditional open hydraulic circuits using water-glycol based fluids.

The other important factor in favor of closed hydraulic circuits, which is becoming ever more important, is the environmental requirements. Hydro ASA operate mainly in the Norwegian sector of the North Sea and have to relate to the very strict emission requirements as laid down by the Norwegian regulations. Not only are the requirements strict, but with improved knowledge of the environmental challenges, these regulations are continuously being upgraded and improved. This leads to an ongoing challenge for the suppliers of water-glycol based control fluids; their products have to be in a continuous development process. What is approved today may be unacceptable tomorrow.

The alternative is the closed hydraulic circuit with no exhaust of control fluid into the subsea environment. There are, however, a number of challenges related to the closed hydraulic circuit.

One obvious negative aspect is the added cost of the umbilical return line(s). Not only is there an added cost, but that cost may be significant as the return line(s) are typically oversized compared with the supply lines.

Another potential negative side of closed circuits is slow response caused by resistance to flow in the return line. This is, however, compensated with a temporary subsea storage device.

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