A Variable U Value Flowline Solution for Life-of-Field Thermal Management of Conventional and HPHT Fields

摘要

Thermal management of subsea field developments takes a variety of approaches depending of the nature of the reservoir fluid and the distance over which it is being transported. Current solutions, often relatively high in CAPEX and/or OPEX, are a compromise to cater for the extreme ends of field life. They include passive coatings (wet and dry insulation), active heating systems (e.g. hot water circulation or electrical heating), cooling or warming spools, and chemical injection. Once installed their performance is generally fixed and are thus specified conservatively. This paper outlines the details of a flowline thermal management solution in which the overall heat transfer coefficient (U-value) can be varied as and when required by the field operator to meet the changing field conditions. By changing the pressure in a pipe-in-pipe (PiP) annulus, the rate of heat transfer to the ambient seawater can be varied. Increasing the pressure can significantly increase the heat transfer, thereby cooling the transported fluid and removing the need for large and expensive cooling spools. Reducing the pressure reduces the heat transfer, enabling the fluid to be kept above the hydrate formation and wax appearance temperatures at lower turndown rates, over longer distances. Reducing the pressure below atmospheric provides a thermal management solution that requires minimal or indeed no physical insulation, with the associated savings in CAPEX. This variable solution is ideally suited for HPHT fields where the fluid temperature, pressure and flowrates will vary significantly over the field life. It is also likely to be cost-effective for all field developments requiring thermal management, by maximising the operating envelope and removing system design constraints. The temperature control allows for the tie-in of future fields where the flow conditions may otherwise require a different and more costly development strategy. There are both initial and operational cost savings and benefits that can be realised through the variation of just a single pressure parameter: 1. Lower turndown rates. 2. Extended field life. 3. Extended no-touch times. 4. Reduced insulation system cost. 5. Reduced chemical injection requirements. 6. No requirement for expensive cooling or warming spools. 7. No compromise on production flowrates due temperature limitations of the host facility. In summary, the whole thermal performance of the system can be managed by changing only the annulus pressure, which can be performed from the topside, putting thermal control in the hands of the operator. Full scale testing has been carried out to verify the subsea system and the associated design models for the market.