Pipe in pipe systems allow a range of advanced and highly efficient insulation materials to be used to achieve Overall Heat Transfer Coefficients less than 1 W/m2 K. These systems are important components of subsea developments where untreated well fluids may have to be transported large distances and wax and hydrate problems have to be managed. However, as a result of this efficient insulation, thermal expansion challenges are increased and techniques such as probabilistic analysis, upheaval buckling design, snake lay or cooling spools employed to mitigate high expansion loads.
CHALLENGES
- High Temperature Well Fluids
High temperature well fluids increase the overall levels of energy in the system increasing the risk of uncontrolled buckling of the flowlines and increasing project costs through the use of pipeline anchors, rockdump or trenching, more exotic materials and the concern that the predicted design temperature is outside the range of applicability of existing design codes.
- Wax/Asphaltene/Hydrate formation within product stream
In heavy oils or multiphase pipelines excessive cooling of the product during transportation can result in drop out of high molecular weight waxes and asphaltenes. Increasing the operation increasing the operational pigging requirement of the system, increasing OPEX. In wet gas systems hydrate formation during startup or blow-down conditions can block pipelines.
Long tie back from a subsea facility to the host platform increases the length of time that the production fluid is subjected to heat loss through the pipeline wall, increasing the risk of flow assurance issues, particularly in the event of shutdown condition and extended residence time of thefluid in the pipeline.
- Low OHTC value requirement (<1 W/m2K)
For long tie backs or production fluids with high critical temperatures the Overall heat transfer Coefficient requirements of the system can be onerous. Requiring conventional wet insulation thickness in excess of practical thicknesses for application or installation.
SOLUTIONS
There can be significant cost benefit from cooling the product stream from very high temperature wells to minimise the thermal expansion forces and then maintaining this lower temperature for through efficient pipeline insulation reducing the volume of post lay rock dump or trenching required or enabling more conventional materials and analysis techniques to be employed.
Installing a second pipeline around the product pipeline isolates the carrier pipeline from the seawater surrounding it and creates a dry chamber around the pipeline that can be engineered to accommodate a range of advanced insulation techniques.
Typically pipeline insulation must be able to withstand the stresses imposed as a result of the installation methods and also be strong enough to withstand constant external pressure and function effectively when submerged and saturated. The dry, load-free environment within the annulus of the pipe allows non-typical insulation materials with much lower thermal conductivities to be applied subsea than has historically been possible e.g. rock-wool systems etc.
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