Subsea Cooling; Passive, Active or Sea Current Controlled?

Winge Rudh, Mattias Gillis (FMC Technologies) | Fritz Jahnsen, Ove (FMC Technologies) | Hasan, Zahidul (FMC Technologies)



Subsea cooling in oil and gas production might seem to be opposite to the usual flow assurance challenge, maintaining a high enough flowing temperature of the produced stream in order to ensure problem free transport of the crude from well to the host. During FEED and detailed design, particular focus is aimed at maintaining a required temperature with insulation and even electrical heating are employed in order to achieve this. Hydrate formation, wax and asphaltene deposits are challenges that are connected with too low temperature, and considerable effort is spent in quantifying acceptable temperature, and cool down times of subsea equipment. So one might ask why and where is the need for subsea cooling? It turns out that there are situations where the well fluids are very warm and reduction in the temperature is required for profitable development of a field. For example, where an expensive flow line material would render the installation too costly, a reduction in temperature might make the investment evaluations look attractive, or where heat is generated subsea by for instance a subsea gas compressor. The temperature greatly affects the corrosion rate, and by changing the temperature, chemical dosage can be optimized, which further strengthens the financial analysis of a field development. This paper focuses on active subsea coolers, i.e. subsea cooling systems that are equipped with adjustable means, and attempts to analyze and benchmark four different subsea cooler types using a generic wet gas production case. A recent development involving a sea current controlled active cooler is introduced and compared with three other active cooler types and how they operate with a given set of operation and turndown conditions are presented. A comparison of weight, size, auxillary equipment and required topside scope is also included.