Heat loss to the ambience
Thermal insulation in oil and gas production systems are required for the following main reasons:
- Avoid operation below wax appearance temperature (WAT) to avoid wax deposition totally. (When the flowing temperature drops below WAT, wax deposition will occur).
- Minimize wax deposition rate, and thereby minimise wax pigging frequency, in case operation below WAT cannot be avoided. Insulation helps because below WAT the rate of wax deposition is proportional to the heat loss to the ambience.
- Avoid operation below hydrate formation temperature
- Allow shutdowns without cool down into hydrate formation region. (Wax formation during shutdown is a lesser problem, as wax deposited during a now flow situation tends to be very soft and is easily removed during restart).
Other (not so common) reasons are:
- Avoid liquid dropout (water and/or condensate) in wet gas pipelines.
- Avoid excessive pressure drop due to high viscosity at low temperatures
- Avoid separation problems due to more stable emulsions at low temperatures.
Thermal insulation is expensive. For deep water risers it can even be a feasibility problem with sufficient insulation to achieve cool-down times required for hydrate prevention during unplanned shutdowns. To avoid expensive “over-insulation” it is therefore important to make correct temperature calculations. Even worse is it if temperature calculations are wrong on the optimistic side, as this may lead to serious operational problems, e.g. cool-down to hydrate formation before all complete hydrate prevention can be achieved, limited turndown ability, too low heating duty in the processing facilities etc.