Liquid surges from new tie-backs

We just completed an interesting study where a new tie-back was to be started up and whilst all the design data had been validated, no one had considered the liquid surge experienced when increasing the flow rate back up in the large trunkline. I won't talk about that case for now, as it's still a work in progress, but it brought back memories of a similar study I did 15 years ago!

Background

Gas and condensate production from the D development is currently exported via a single pipeline to the P compression platform for onward transfer to shore. The additional production from the DD tie-back, which will add up to 275MMscfd and 2400bpd of additional condensate led to difficulties with liquid handling onshore during start-up and restart.

A large slugcatcher size of 5000bbl that was originally proposed, but due to limited spare onshore it was requested that this be reduced to 2000bbl.

Initial Start-up

The 24" pipeline had been operating at ~80 MMscfd for many months and hence was at it's steady state liquid hold-up of ~30,000bbls. The difference in steady state hold-up between 80 and 275MMscfd would lead to ~28,000bbl of liquid being displaced from the pipeline. An OLGA simulation was run with a 12 hour flow transient, which resulted in a surge volume of nearly 23,000bbl being required onshore.

One option for reducing the surge volume was to increase the gas flow in increments so that the surge volume reduced to a number of smaller more manageable volumes. Ideally, each flow increment would send a pulse of liquid to the slugcatcher, which would be processed before making the next flow increment. To demonstrate this approach, a simulation was run with nine flow increments of 5 MMscfd up to a total flow of 125 MMscfd, followed by two flow increments of 25 MMscfd up to a total flow of 175 MMscfd and a final flow increment of 100 MMscfd to give a total flow of 275MMscfd. The time between each flow increment was set at 15 hours giving a procedure that lasts around 7.5 days (including the final 15 hour processing period). This pipeline simulation predicted a surge volume of less than the desired 1500 bbl, as shown in the figure below.

Normal Restart

Pipeline operation at 80 MMscfd will result in a gradual increase in the pipeline liquid hold-up towards the steady-state value of 30,000bbl. A manageable surge volume onshore would then require the 7-8 days flow ramp-up procedure above when ramping the production to 275MMscfd, which may prove inconvenient.

The production could be returned to 275MMscfd at short notice if the pipeline liquid hold-up is not allowed to reach steady-state at 80MMscfd. This can be achieved by running at 80MMscfd for short periods or by pigging at regular intervals.

Pipeline simulations were run to investigate how long production could be held at 80MMscfd and still give a manageable surge volume for a rapid flow increase to 275MMscfd. The simulation begins with steady state conditions at 275MMscfd, the flow is then stepped down to 80MMscfd for a period of time before being returned to 275MMscfd. The results showed that if the flow is turned down for 5.5 days and then returned to 275MMscfd the surge volume produced is less than 2000bbl.

However, it can not be guaranteed that the DD production will be available after 5.5 days. Hence regular operational sphereing was needed to operate the pipeline at low flowrates and control the liquid hold-up in the pipeline.

Simulations were run from steady state at 275MMscfd, the flow was then turned down to 80MMscfd and the pipeline was sphered. The pipeline was then left for a period before being sphered again. The period between spheres was varied to determine the size of liquid slugs that would be pushed out of the pipeline by the sphere. The table below shows that the pipeline would need to be sphered every 3 days to not flood the slugcatcher.

In Summary, when design a new tie-back the current operation conditions of the existing facility need to be taken into account. The current production rates may be low and whilst there is ample ullage available in the pipeline, liquid handling issues need to be considered and managed.