In response to the UNCCC held in Paris in 2015 the need to reduce the global warming, due to CO2 release in atmosphere, led to a new business for the capture and storage of CO2 in dedicated deep water reservoir. In this sense the transport of the CO2 at low temperature, necessary to condensate the gas, through offshore pipeline is a commercial and technical valid strategy.

One of the issues related to the transport of a condensate gas is the thermal exchange between the transport system, in this case offshore pipelines, and the environment. The gas is usually carried by ships in a liquid phase at very low temperatures, for example −30 °C in case of CO2. The fluid is introduced into the pipeline at the same temperature to not further consume energy for warming up.

The design of the offshore pipeline subject to these operating conditions, very cold fluid internally and a water temperature slightly over 0°C at external side, can be affected by the ice formation around the pipe. The ice thickness formation is primarily governed by the external convection coefficient.

For the offshore pipelines located in deep waters where the sea currents are negligible, only the natural convection phenomena can occur on the external surface of the pipeline. Considering steady state scenario the heat transfer from the internal fluid to the external environmental is governed by the thermal resistance of each component of the system like fluid, steel, anticorrosion coating, thermal insulation if any and external convection due to the seawater. The low temperatures of both seawater and ice formation, approximately at −2°C, allow to be close to the maximum value of the seawater density: usually this occurs at a slightly colder temperatures depending on salinity and water depth (for the fresh water the maximum is at 4°C).

The natural convection is driven by the buoyancy effect due to fluid density variation with temperature: the scenario described above lead to minimizes these effects and consequently the heat transfer due to the natural convection (increasing the thermal resistance).

Most of the correlations in literature are related to different temperature ranges, far away from this particular situation: a numerical investigation using computational fluid dynamics technique has been performed.

The analysis is executed by means of commercial CFD software FLUENT: the model is based on a two dimensional grid of a pipe submerged in water.

In this paper:

• The state-of-the-art about the natural convection coefficient estimate for submerged cylinders proposed by different authors through Nusselt number assessment;

• A description of the proposed numerical approach is given highlighting the different approaches based on the boundary layer behavior;

• A typical application is shown.

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