Steam-Powered Offshore Oil Producing Platforms

1. ABSTRACT This paper will briefly describe the parallel development of an oil production platform topsides with a conventional gas turbine driver system. This will include waste heat recovery and a topside with steam turbine drivers for large rotating equipment and steam heating system. The base process configuration is outlined and key elements in the steam system design philosophy are described by diagrams, along with brief system explanations. Principles regarding the layout of major steam system equipment are also described. Comparative conclusions are drawn regarding weight and space requirements, as well as operation, maintenance and safety aspects. Generally, the conclusions are promising and indicate potential for topside simplification by utilizing steam for heating and powering purposes. 2. INTRODUCTION The conventional method of producing electric power on a large oil production platform is by gas turbine driven generators. Additionally diesels may be used for auxiliary purposes. Heating for processing or other purposes is accomplished by a heating medium or directly by fired heaters. A heating medium heat source may be gas turbine exhaust. Thermal efficiency has seldom been a major consideration since the local price of gaseous fuel is low. Looking at the landbased pendant, you will often see a different picture, with back-pressure steam turbine driven power generators and the exhaust steam utilized directly as the heating medium. There are many arguments for rechecking the validity of old conclusions, such as fluctuating oil prices, changing on-board value of associated gas, possibility for reduction of operation and maintenance cost, or better tools for production availability analyses, etc. This paper describes the key elements in the development of a topside concept based on both a conventional system, as outlined above, and a topside based on steam turbine drivers for large rotating equipment and the use of steam as the heating medium. None of these concepts are optimized, and since the steam option is new, it will most likely have a greater potential for further savings than the conventional topside. The development is done for the Heidrun field outside mid-Norway where Conoco Norway Inc. is operator. 3. SYSTEM DEFINITIONS 1. Heidrun field. The Heidrun field is planned to be developed with a single concrete TLP platform with a concrete subsea crude storage. The platform will produce approximately 1450 m3/hr of stabilized crude, exported via offshore loading system. Gas will initially be reinjected. No decision has been made regarding later gas disposal. Gas-oil ratio will initially be approximately 100 Sm3/m3. Water depth is approximately 350 m. 2. Steam system basics. The main elements in a steam system will be a high pressure steam boiler, steam turbines as drivers for rotating equipment, and steam operated heaters for various purposes. A simplified flow diagram is shown in fig. 1. In this case, the steam turbine is working against a low pressure (vacuum) condenser, cooled by seawater.