Marc Bonissel
Stephanie Abrand
Fabrice Bacat
Saipem
Deepwater fields are characterized by very low minimum ambient temperatures. To limit heat losses, subsea lines are normally insulated. One purpose of the insulation is to ensure a high steady-state temperature in the system to avoid wax deposition. Another is to increase available cool-down time (CDT), i.e. the period during which the system cools down from normal operating temperatures to a hydrate formation temperature at shut-in pressure.
The differing requirements considered essential for flowline design could lead to technical limitations if applied to long subsea tiebacks in deepwater. These issues relate mainly to the performance of the insulation material, plus manufacturing and installation techniques.
To accommodate all insulation and operational demands, the most common solution employed today for developing deepwater fields is through a production loop. During normal operation, production is split between the two separate lines; during shutdown, the pigging loop is open through all connections of the lines. At this juncture, dead oil circulation is also conducted to displace the untreated live oil.
With longer tiebacks, the proportion of friction losses in the system’s overall pressure drop also rises. Increasing the pipe’s inner diameter could be a solution, but this would strongly impact both installation capability and hydraulic stability, at a low flow rate.
Longer tiebacks also generate higher heat losses and therefore impose the need for lower U-values to ensure the same arrival temperature. Very low U-values may become necessary, along with technologies such as pipe-in-pipe.
Another side effect of longer tiebacks is apparent due to increased travel of pigging devices during fluid displacement. As the pipeline increases in length, a longer cool-down period is required. The design is usually driven by CDT for short distances and by U-value (up to the 0.5W/m2K) for long distances.
The key to attaining longer tieback distances varies according to the parameters that drive the design at its maximal length. For a design driven by CDT, the U-value is lower than the requirements. So, if the CDT requirement is decreased, the tieback length could be increased up to the limit where the U-value is the designing criterion. One solution to CDT reduction is to put in a single production line with a subsea pig launcher. The latter allows the pig to be launched from the most distant manifold, thereby reducing the pig’s trip to the surface facility.
The single line architecture brings significant advantages including reduction in the number of pipes installed, and shortened transient operation of tasks such as pigging. On the other hand, drawbacks can arise from the use of large diameter pipes (same flow rate, one pipe instead of two), leading to installation issues in ultra-deepwater and potentially more hydraulic sensitivity to slugging.
A more obvious way to extend the tieback length is to use better insulating materials and to ensure U-values lower than 0.5W/m2K for the whole length. This modification is costly, however, and the gain is not large in terms of tieback length. In addition, ensuring a very low U-value over the length of the flowline can be difficult due to the presence of cold spots.
For U-value driven designs, the main constraint is the fluid arrival temperature at a reduced flow rate. As this criterion relates mainly to low flow rate at low water cut, it should not be a common occurrence during field life. One solution may be to inject solvent during the short period instead of designing too stringent an insulation.
Longer tiebacks can be achieved by modifying the fluid composition to control hydrate and wax deposition by means of subsea processing. One possibility could be subsea water separation to produce an oil or gas stream that would require little or no addition of hydrate inhibitor.
In conclusion, development of long satellite tiebacks via conventional methods (production loop) and procedures can lead to installations with high capex, opex and length limitations. Alternative approaches to attaining longer tiebacks could involve using a single production line with a subsea pig launcher, or relaxing arrival temperature design criteria.
But these options are also a risk, and may require further study and qualification if based on equipment or materials close to maturity. For very long tiebacks, modifications to the fluid will allow such distances to be reached. Subsea separation, for example, for removing hydrate fluids, could allow the oil to be exported directly from the well to the beach.•
