Numerical Investigation of Thermal Fields Around Subsea Buried Pipelines

Abstract

Subsea pipelines transporting hydrocarbons from very deepwater wells or at arctic sites for long distances present some challenging technical problems. The ambient temperature at the seafloor in deepwater may be about 5°C (∼ 41°F or 278 K) and can be even lower in the arctic regions, while the wellhead hydrocarbon temperatures can be in excess of 149°C (∼ 300°F or 422 K). Insulation of these buried pipelines to mitigate this large temperature gradient can be only part of the solution as temperature losses over long distance may require heating systems to avoid deposition of impurities and clogging of the pipeline. The near field thermal gradient in surrounding soil is investigated using complementary 2-D numerical simulations of finite element and boundary element numerical models. A finite element hydraulic-thermal code designed for porous media was used to investigate the time evolution of the natural convection effects due to pipeline heating of the seawater in soil. For a seabed of clay under these conditions, it was determined that the boundary element model could be directed at steady state heat transfer about the pipeline in layered soil conditions addressing trenching and backfill consistent with the burial of the pipeline by a remotely operated vehicle. Parameters that may affect the thermal field around the subsea buried pipeline such as burial depth, thermal power loss and thermal properties of backfilling soils were investigated. It was shown that the thermal conductivity of the backfill has a critical influence on temperature distribution at the pipe wall, and that the pipe burial depth significantly affects temperature distribution on the seabed right above the pipe in deepwater.