Dewatering facility provides production optimization, water management solutions

The 135-D platform, on location offshore Brazil.

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The four platforms had been constructed with limited or no produced water treatment facilities. As a result, produced water had to be sent onshore for treatment by either dedicated shuttle tankers or via a 24-in. crude oil export line. This export line fed into a terminal at Cabiunas, just north of Macae. The overall water cut for the area averaged about 25%, but was expected to increase to 45% during the next 10 years.

To ensure maximum crude oil export volumes and avoid high water transport costs, it was determined that a centralized, offshore plan was needed for fluid treatment. The targeted quality improvement for the crude oil (25°-28° API gravity) prior to export was less than 1% basic sediment and water (BS&W), and for the produced water, less than 20 ppm total oil and grease (TOG) before its disposal overboard. The forecast total fluid, crude, and water production volumes from the four platforms are listed in Table 1.

Existing equipment limitations on the central and satellite platforms drove the need for a separate facility to treat the crude oil and water mixture to meet the stringent specifications. After 14 months of technical discussions and commercial negotiations, Petrobras contracted Schlumberger to convert an existing semisubmersible into a floating dewatering facility.

The build, own, operate, and transfer contract was signed in December 1999 for a minimum of eight years, with provisions for possible extensions. The contract was based on conventional charter and services and included a performance risk-reward incentive with a focus on performance from both qualitative (quality of water disposed to sea) and quantitative (oil export quality and full liquid process capacity) standpoints.

Dewatering facility construction

The rig selected for upgrade and conversion was Transocean's Sedco 135D, also known to Petrobras as the SS-06, a 35-year-old semisubmersible that had been operating in Brazil as one of the first offshore drilling units and later as a floating production facility. Selecting a local rig eliminated the high capital cost of a new platform purchase and the expense and schedule of either a wet or dry tow from Europe or the Gulf of Mexico

The 135D rig upgrade and conversion involved opening the mothballed Fels Setal shipyard in Angra, Brazil, located 150 km south of Rio de Janeiro. This was one of the major Brazilian offshore construction projects undertaken locally in recent years, and had been much sought after by the state of Rio de Janeiro. Moreover, the construction work fell in line with the Schlumberger Cie strategy to develop local solutions.

A new process plant was planned, designed, and installed to treat the oil and water combination to specified quality standards. For maximum operational flexibility, the process plant consists of two parallel trains of 85,000 b/d each, for a 170,000 b/d total fluid capacity, with a two-stage separator (degasser) mounted on top of an electrostatic coalescer.

The dry oil is then directed to the high-pressure export pumps and sent to Cabiunas, located 90 km away. The separated oily water is directed to a water surge drum, from which it is pumped through a deoiling hydro cyclone to reduce the TOG to less than 40 ppm. This water is then sent to an induced gas floatation unit, which uses gas sparging technology to achieve the targeted less than 20 ppm TOG.

A 10-year life enhancement upgrade was planned for the rig structure, which included its associated facilities and support systems. The upgrade required that the engine room be completely stripped and rebuilt with four new 2.2 mw diesel-driven Wartsillas. The 135D rig upgrade also included complete ballast and bilge systems, a new seawater lift and firefighting system, living quarters for 74 personnel, and new port and starboard cranes. Inspection and repair of the entire substructure was also required. In addition, it was necessary to strengthen the K-nodes for increasing the deck load capacity to withstand the 2,000 tons of newly installed equipment.

Phase 1 of the project entailed the detailed design, equipment procurement, and construction of the facility. The completed facility had to be ready for tow to the field by Petrobras within 16 months. This was an extremely tight schedule, since the long lead-time for equipment delivery was 8-10 months. The equipment, which included new engines, cranes, export pumps, TEG fired heaters, and other equipment had to be shipped to Brazil and then passed through customs.

In addition, the construction sequence re-quired careful planning because each equipment item had to be available at its designated time. For example, the new cranes had to be installed early. These were needed to lift other new equipment on board because the existing cranes did not have sufficient reach. Also, the separators had to be installed before the export pumps, the engines needed to be in place before the engine room could be outfitted, and power was required for pre-commissioning activities.

The success of the project was owed largely to a dedicated taskforce and an excellent work relationship with Fels Setal shipyard. The Phase 1 project taskforce consisted of key personnel from Schlumberger and Transocean as well as Fels Setal and Brazilian Engineering Cia Projemar in Rio de Janeiro. Schlumberger performed the preliminary design of the process plant, but subcontracted its engineering detail to AGA in New Orleans, Louisiana. The detailed design work for the rig upgrade itself was contracted to Projemar under the supervision of Transocean Engineering.

After issuing an invitation to tender, the construction contract to upgrade the rig and install the process plant was awarded to Fels Setal in early August 2000. The rig was delivered to the yard on Sept. 10, 2000, and eight months later, was completed and shipped out on time and within budget. The shipyard workforce peaked at more than 600 people. By completion, more than 2,000 tons of new equipment and 55,000 meters of cable had been installed.

More than 1,000 tons of scaffolding was used to inspect, repair and paint over 20,000 cubic meters of steel substructure. Following sea trials, the rig was towed and installed in the field by end of June 2001. The schedule for the facility construction project from conception to completion and installation is shown in Table 2.

Dewatering facility operation

As early as October 2000, Schlumberger and Transocean started forming the onshore support base and offshore operating team. Totaling over 100 employees, the project team hired 25 new technicians and trained the operating personnel in the specifics of the new processing plant. The operators were brought to the shipyard to observe the installation of the process equipment and instrumentation and supervise the pre-commissioning activities and tests.

During this time, a preventive maintenance system was established. Spare parts were ordered and stored in warehouses both onshore and offshore on the rig.

From July 2001 to April 2002, additional training was performed on the dewatering facility. Experienced mentors from the IPM-PSE group empowered the project team members by exposing them to valuable experience in operating the process plant safely and efficiently. During this period, the operation team obtained ISO 14001, OHSAS 18001, ISM Code, and ABS certifications. Additionally, a complete set of operating procedures was developed specifically for the 135D facility.

When the first oil and water volumes came through the plant in April 2002, the operation team was faced with unexpected challenges related to large flow rates and fluctuations as well as substantial water content and quality variations. Through lessons learned, the integrated team developed long-term solutions for handling and treating the higher than expected water rate changes and variations.

With support from Schlumberger Production Services & Engineering, Clariant and Oil Plus, the PSE chemical partners, as well as a dedicated operation team, a series of modifications to optimize plant efficiency successfully brought the plant to full quantitative specification by March 2003. During this period, the 135D team members became specialists in oil drops size particle detection, gas floatation mechanism, coalescing techniques, and polishing filtering.

Completed on schedule in 16 months and 10% below budget, the 135D dewatering facility has been considered a huge success. Since the facility has been in operation, effective water treatment and efficient disposal has reduced transport costs by eliminating two shuttle tankers from Calm Buoys. Also, by freeing the 24-in. export line from produced water, its oil rate capacity has been increased by 16%.

Outlook

Upon reaching plant performance and efficiency targets, severe water breakthrough problems and resulting high water cuts, now averaging 45%, have led to discussions for a Phase 2 plan. Under Phase 2, planned action items include increasing water treatment capacity from 60,000 b/d to 96,000 b/d and the total fluid handling capacity from 170,000 b/d to 220,000 b/d. This must be carried out offshore for minimal disruption to the current operations, presenting an ideal opportunity to apply the new Light Water Treatment Unit technology from Schlumberger.

This novel water treating technology will create a major breakthrough in Brazil, both at the 135D facility as well as at other Petrobras-owned fields where similar water situations are expected. The LWTU-based water treatment systems technology, coupled with Schlum-berger experience of managing a large treatment plant, provides a combination that will enable Petrobras and other operators to optimize oil production and effectively manage large associated produced water volumes as oil fields around the world mature and present new technical challenges in their continued economic operations.

Acknowledgment

Schlumberger wishes to thank Petrobras for its permission to publish this article.