Flow Assurance With Water Heated Pipe-in-Pipe in Fields With High Gas Oil Ratio and High Wax Appearance Temperature

Abstract

The development of pre-salt fields in the Santos Basin has presented a plenty of technological challenges, which include the production of fluids with high values of GOR (Gas Oil Ratio) and high values of WAT (Wax Appearance Temperature). To avoid the wax deposition, it is absolutely imperative handling the temperature by insulation and/or heating. If in one hand the production system thermal insulation minimizes the heat losses to the environment, on the other hand the cooling due the thermodynamic effect of gas expansion is inevitable, mainly in fields with high GOR values. To mitigate the problem of temperature drop, mainly in the risers where the expansion effects are pronounced, various heating systems for subsea pipelines have been studied by PETROBRAS. In parallel to these studies, it was noted that the design philosophy of the offshore production units for the pre-salt fields have indicated the use of turbo–generators to supply electrical power to the production unit, leading to the opportunity of using the generated super heated gases, originally discharged into the atmosphere, in order to feed the riser heating systems based on heated fluids circulation. Studies of heat exchange performed indicated a great potential for the risers heating by the flow of heated water through the annular of a Pipe-in-Pipe riser, by using a WHRU (Waste Heat Recovery Unit) to recover the power discharged into the atmosphere from the turbo–generators. It was identified the possibility of heating Pipe-in-Pipe risers by circulating heated fresh water in a closed loop with water return to the production unit, or by coupling the riser heating system with the water injection system, using all the facilities for treatment and pumping of water injection. This paper aims to describe the methodology considered in the development of a conceptual design of a heating system for a Pipe-in-Pipe riser, by the flow of heated water in the annular, in a Free Standing Hybrid Riser configuration. The paper will also present the characteristics associated with the two possibilities of heated water circuit, as well as advantages and disadvantages of each system, and the aspects related to subsea production layout.