Page 482 - Pipelines and Risers

Pipe-in-Pipe and Bundle Systems 449

Global Buckling Analysis
To simplify the global buckling analysis, the pipe-in-pipe system could be modeled as single
pipe by use of equivalent section concept. In contrast to upheaval buckling, lateral buckling is
accepted if it does not result in unacceptable stresses and strains.

24.2.8 Insulation Considerations
Thermal analysis is fundamental to the design of a pipe-in-pipe system. The main drivers for
an insulated flowline system are:

To ensure that the product arrives at the topsides at a temperature above the wax
appearance temperature.
To ensure that hydrates do not form anywhere in the system
To reduce the rate of cool down in the event of an unplanned shutdown in order to allow
sufficient time to re-establish flow or inject wax and hydrate inhibiting chemicals before
the product reaches the WAT or hydrate formation temperature at any point in the system.
The required cool down duration usually ranges from several hours to a few days.

Some of the typical thermal analyses are briefly described in the following:

Flashing analysis of production fluid to determine hydrate curve. From this data the
critical minimum temperature is established.
Global thermal hydraulic analysis of the flowline system to determine the required overall
heat transfer coefficient (OHTC) at each point in the system and length weighed average
overall heart transfer coefficient for the system as a whole and hence determine if
insulation is required and where.
The required OHTC determine the type and thickness of insulation to be used and hence
determines the required cross-section of the pipe-in-pipe system. At this stage a trade off
between the cost of insulation and the cost of injecting inhibition chemicals during
operation may be feasible.
Local heat transfer analysis to calculate the mechanical heat transfer coefficient for each
component of the pipe-in-pipe system.
Based on the calculated MHTC’s performance a global thermal hydraulic analysis of the
insulated flowline system to determine it is LWAOHTC and check to see if it satisfies the
required value.
Perform local transient heat transfer analysis at strategic points along the system to
develop cool own curves and hence determine cool down times to the critical minimum
allowable temperature at each location.
24.2.9 Fabrication and Field Joints

Dependent upon the installation method chosen, a pipe-in-pipe flowline system may be the
ideal candidate for utilizing onshore fabrication to reduce offshore fabrication time, as any
such offshore operation will be fairly time consuming leading to low production rates in