General Reinsurance

Pipe in pipe system in Oil and Gas Industry (Part I)

The increasing amount of High Pressure High Temperature (HP/HT) reservoir, challenge oil and gas industry practitioner to maintain the flowlines temperature. One method to tackle this problem is adding thermal insulation layer to pipeline. Furthermore, engineer found a good method to apply thermal insulation to the pipeline called Pipe in Pipe.
 
As it’s named, Pipe in Pipe (PIP) basically consist of rigid inner pipe and rigid outer pipe (also known as carrier pipe). Inner pipe is the main pipe that located inside the outer pipe and the function of this pipe is as hydrocarbon exposed media to transfer it. On the other hand, outer pipe is the part of the PIP system that exposed to environment. Consequently, this part need corrosion coating or sacrificial anode to prevent itself from corrosion threat.
 
Between outer pipe and inner pipe usually insulation material is applied. Beside insulation layer, there is also air gap between this two pipe acts as additional thermal insulation and consider as dry insulation, which better than wet insulation. This dry annulus U-values (overall heat transfer capacity) is as low as 0.5 W/m2K. Because of these reason, pipe in pipe is proven to maintain the heat transfer along the pipeline on HP/HT reservoirs. This system can assure hydrocarbon at temperature above 120oC and pressure above 10000 psi [3]. To maintain the position between all of this pipe and insulation, centralizer is used in the middle of the pipeline and bulkhead is used on the edge of pipeline.
 

Figure 8 Pipe in Pipe configuration [7]
 
In general, there are several advantages in using pipe in pipe system as hydrocarbon transporter, such as:
 

1. Excellent thermal insulation

PIP system is fit for HP/HT circumstances because it can obstruct the hydrate formation. In addition, this system also make the outlet temperature still high [7].

2. Outer pipe give extra protection to the inner pipe

Mechanically, if something is hit the pipeline, it does not directly hit the hydrocarbon carrying pipeline. It will damage the outer pipe first [4]

3. More stable

This PIP system is heavy and as consequence, not need additional concrete coating, and rock dump [3]

4. Not need to trench or burried the pipe

PIP system use large diameter pipeline. As we know, large diameter pipelines means that it does not need to trench and buried it. As a result, upheaval buckling is rare to happen because the pipeline is not restrained axially [3].

5. PIP can be installed by numerous method

Almost all of the installation method such as S-Lay, J-Lay, Reel-Lay, and tow method can be used [4].
 
Compare to ordinary pipeline (singe wall pipeline), PIP system has several disadvantages, such as:
 

1. Relatively expensive

Because two steel pipes is used in transporting one hydrocarbon lane system. So in other words the total cost of steel will be much higher than the single wall pipe. Moreover, with the increasing amount of steel, it means more welding should be done. Furthermore, the size of PIP system is much larger compare to single pipe. As a result, special vessel is needed because this type of system require giant lay barge [7]. These three factors are the main reason why this method consider as expensive method.

2. Increase the difficulty of engineering design

Installation design for this PIP system is quite different, more things need to analyze carefully. Also, welding process while install the PIP will become harder and longer due to increasing the steel pipe amount. Further discussion about installation will explain later.
 
After the discussion of the advantages and the disadvantages of pipe in pipe system, we will discuss about how the structural design of this type of pipes, how it can give more insulation compare to ordinary pipe system, and the method of installation for PIP in next article. So, stay tuned for the part II!