Dolphin Energy employed pipeline scraping—a common pipeline maintenance practice in today's complex production systems—for its Ras Laffan plant in Qatar. Dolphin was producing natural gas from the Qatar North Field for processing at the Ras Laffan plant before export to the UAE. The production was three-phase fluid (gas, condensate, and water), which was being transported approximately 80 km to Ras Laffan via two 36-in diameter sea lines. Each sea line was designed to transport up to 1,310 MMscf/d of raw wellhead fluids.

A slug catcher at the Ras Laffan plant was used to handle liquid slugs during transient conditions. The slug catcher was essentially divided into two halves, with each half dedicated to an individual sea line.

To ensure flow, this multiphase transport system required the following: 

• Monitoring of liquid accumulation in the sea lines (i.e., hydrocarbon condensate and free water).
• Control and reception of liquid slugs generated during transient operations, such as production ramp-up or scraping operations.
• Hydrate inhibition management (required during winter).

Pipeline management system delivers tracking data online

The company needed to better understand the fluid flow conditions and improve the monitoring of scraper operations. Working with Schlumberger, a pipeline management solution was developed using the OLGA simulator that enabled Dolphin to perform online tracking and visualization of the scraper as it progressed through the sea lines.

Using this system, Dolphin is now able to track the location of the scraper, distance traveled, current velocity, average velocity, estimated arrival time, and amount of liquid in front of the scraper.

Consistent workflow and managed services provide long-term solution

Schlumberger and the Dolphin operations group developed a workflow to ensure consistent, effective operations, from preparation for a scraping operation to evaluation of the operation. Prejob offline simulations enabled estimation of factors such as gas flow rates for scraper velocity, levels of water and condensate, pressure at the slug catcher after scraper arrival, rate of condensate processing, and estimated time of arrival of the slug and scraper.

“What-if” scenarios can be compared against real-time operations for better decisions during the operation. Throughout the operation, the scraper settings were updated and conditions were monitored using dedicated user interfaces. Information from post-operation evaluation and a performance report on the pipeline management system can be used to optimize future operations.

As part of a long-term managed services agreement, Schlumberger coordinated system support with Dolphin to ensure the solution's continued effectiveness. Dolphin was able to optimize production and reduce its operating expenses due to the following:

• Always-available support to ensure optimum system performance.
• Proactive approach and fast response to ensure high availability.
• Expert guidance and user training on how to best leverage the system.
• Remote and onsite assistance from Schlumberger specialists during field operation.
• Increased field knowledge and collaboration among experts, leading to safer operation.

Scraper tracking streamlines liquid transport operations

Knowledge gained during this project led to more efficient operations. Calibrated simulation models were used to match field measurements before the scraper campaign to ensure accurate results. The pipeline management system matched the pipeline pressure drops and onshore gas flow rate within 3%.

A scraper-type cleaner weighing 397 kg was launched at an offshore rate of 1,250 MMscf/d (normal flow). The system predicted a scraper travel time of 4 hours 2 minutes versus the actual field travel time of 4 hours 5 minutes. Based on the scraper estimates, operators can start reducing the feed rate 1 hour before the estimated arrival time to control the scraper velocity. Operators can also start flowing proactively from the slug catcher to reduce the level of increase due to scraping.