Moving into the unknown

Petrobras holds a working interest in four Lower Tertiary discoveries: Cascade (operator), Chinook (operator), Stones, and St. Malo. Cascade and Chinook are expected to be the first to produce in mid-2010. Map courtesy of Petrobras.

That same year, Minerals Management Service (MMS) announced it would begin to accept applications for the use of FPSOs in the GoM.

Petrobras, with more than 25 years’ experience in operating FPSOs and shuttle tankers in deepwater, is poised to be the first company to tap the hydrocarbons in the remote Lower Tertiary, with work under way to develop the Cascade and Chinook discoveries in the Walker Ridge area. These discoveries lie along the same trend as Jack.

“Petrobras by far is the major oil company with the most experience in floating production and oil offloading operations,” says Cesar Palagi, Walker Ridge production asset manager for Petrobras America Inc. “In the Campos basin offshore Brazil, with an FPSO portfolio of almost 30 units (in operation or under construction), Petrobras conducts more than 600 oil offloading events every year with no significant accidents or spills. Based on this, we can say the development of ultra deepwater reserves is within the comfort zone of Petrobras.”

Reservoir, well challenges

The Cascade and Chinook reservoirs are in the Lower Tertiary depositional system; also known as the Wilcox formation. The Wilcox reservoirs are a sequence of stacked, fine-grained sandstones deposited by large-scale unconfined turbidite flows. The reservoirs are of Eocene and Paleocene age at about 26,000 ft (7,925 m) deep.

There is plenty of compartmentalization and folds in Cascade, Palagi says. This is based on the existing seismic information only. Palagi anticipates that updated interpretations will identify even more complexities in the reservoir.

But one of the biggest challenges in this project, Palagi explains, is reservoir characterization. And it will take a couple of years before we understand that, he says.

The Chevron-operated Jack and St. Malo discoveries, Shell-operated Stones, and Petrobras-operated Cascade and Chinook discoveries all lie along the same trend in the Lower Tertiary play. These fields are analog accumulations with similar intrinsic rock properties. All are flow-rate closure oil-bearing sands. Some are subsalt; others may have associated aquifers. If one of these discoveries is successful, it is likely that the others will follow, Palagi says. Cascade is expected to be the first onstream in mid-2010. The well depth at Cascade is about 26,000-27,000 ft (7,925-8,230 m). The reservoir pressure is 20,000 psi and the temperature is about 250° F (120° C). Some of the wells in the Lower Tertiary will be drilled through big salt intervals. Thus, casing design is a major issue, Palagi says. Well completion is also a challenge. “We are talking about extensive multi-fractures in the reservoir, which will require complex and extensive operation,” he says. Drilling and completing one well in the Lower Tertiary is estimated to cost around $250 million.

Three semisubmersible drilling rigs will be allocated to Phase 1 of Cascade and Chinook: Diamond Offshore’sOcean Endeavor, Seadrill’s West Sirius, and Larsen Oil and Gas’ PetroRig 1, which is under construction. The first Cascade well was scheduled to spud in 3Q 2008, and the first Chinook well is slated to spud in mid-2009. “In today’s market, these rigs are very expensive pieces of equipment, so design and planning of well operation is a critical factor for the success of the project,” Palagi says.

Development plan

The Cascade and Chinook fields are in the Walker Ridge quadrant close to the US/Mexico border. Cascade is 160 mi (257 km) south of the Louisiana coast and Chinook is 15 mi (24 km) south of Cascade. Both are in about 8,500 ft (2,591 m) of water.

Cascade was discovered in 2002, Chinook was discovered in 2003, and two appraisal wells were drilled on Cascade in 2005. There have been several recent discoveries in the Lower Tertiary trend, but to date, there are no production analogs. “And this is one major thing that we had to take into consideration when designing our project,” Palagi says. The only flow information available is from a well test by Chevron on the Jack field about 60 mi (97 km) southwest of Cascade and Chinook in Walker Ridge block 758.

The discoveries shown in this image lie along the same trend in the Lower Tertiary, about 26,000 ft (7,925 m) deep. Petrobras believes that if one field is successful, the rest will follow. Image courtesy of Petrobras.

The information from these wells and the existing seismic data provided Petrobras with enough information to generate a field development plan. However, this data set was determined insufficient to support a full field development plan. “If you look at where we were two years ago, we had limited geological and well data, lack of field analogs, harsh environmental conditions, ultra deep waters, lack of pipeline infrastructure, FPSO regulations under development, tight suspension of production deadlines with the MMS, and the willingness of all partners to produce as soon as possible,” Palagi says.

Given these conditions, there were plenty of alternatives, he explains. “We could go with additional wells or seismic, a short- or long-term well test, an early production system, or full field development.” Petrobras, Devon, and Total determined an early production system with short-term expandable capabilities would be the best way forward, in a typical phased development approach, which Petrobras has used many times offshore Brazil. Devon holds a 50% working interest in Cascade, and Total holds a 33% working interest in Chinook.

Phase 1 of the development comprises the connection of two subsea wells from Cascade and one subsea well from Chinook to an FPSO located over Cascade. Petrobras selected an FPSO for the development as there is no oil pipeline infrastructure near Cascade and Chinook. Processed oil will be offloaded from the FPSO to shuttle vessels and transported to shore.

“The GoM is a hurricane-prone environment, so the development facilities needed to be designed to either resist hurricanes or to move away from them,” Palagi says. “Also, Cascade and Chinook, like the entire Walker Ridge quadrant, are far from existing oil pipelines. As such, you can understand why oil transportation through shuttle vessels is an alternative for developments in this area.”

The produced gas will flow through new pipelines to the existing pipeline infrastructure. The rest of the gas will be consumed for energy on the FPSO. “The Cascade and Chinook gas-to-oil ratio is not very large – we are talking about 200 cf/bbl,” Palagi says. “We will not flare any gas, not even in the early production phase.”

With no production analogs and limited geological data, Petrobras is taking a phased approach to develop the Cascade and Chinook discoveries. The infrastructure (shown in the rendering) is being designed with enough flexibility for possible future phases. Rendering courtesy of Petrobras.

The objective of Phase 1 is to obtain information about the reservoirs from the wells in order to optimize future phases, Palagi explains. “What we need to optimize is the number and type of wells, completion design, production system type and capacity, subsea layout and boosting system, and secondary recovery potential. We’ll need to understand the oil rate than can be obtained from these wells, and if the flow rate will sustain for a given period of time.

“We are looking at how compartmentalized the reservoirs are to see if there are isolated blocks in the reservoir or if they behave as connected blocks. And we need to study the aquifer behavior. By putting all this information together, we will be able to design future wells and determine if a recovery mechanism by water injection is needed.”

New technology

Petrobras is using five technologies new to the US GoM for Phase 1 of Cascade and Chinook development. The new technologies include an FPSO with a disconnectable turret, crude oil transportation via shuttle vessels, subsea electric submersible pumps, free-standing hybrid risers (FSHR), and a polyester mooring system.

Subsea system

The development plan calls for subsea architecture (trees, manifolds, booster pumps, PLETs, umbilicals, flying leads, jumpers, etc.) consistent with existing subsea facilities in the GoM, with the exception of subsea booster pumps which will be used to enhance production when reservoir pressures decrease.

Petrobras has selected electrical submersible booster pumps over other artificial lifting techniques for Phase 1 due to the fields’ water depth, wellbore depth, and low bubble point of the production fluids. The pumps will be mounted on skids on the seafloor near the wells.

“One of the main challenges of the boosting system is the qualification of the specific components of the equipment, like the high-voltage penetrator, for instance, which is an ongoing process and is something that always requires some attention,” Palagi says.

Subsea boosting still is very limited in the GoM, but Petrobras has been using this technology offshore Brazil. Currently, it is in development for its Jubarte and Golfinho fields, similar to the system being developed for Cascade and Chinook.

Hydrocarbons from the subsea wells will be redirected through manifolds, which are common practice in the industry. “But we’re talking about 15,000 psi,” Palagi says. The idea behind the use of manifolds in this case is to have flexibility for additional wells in future phases, in the event that the first wells succeed, Palagi explains. The manifolds will have four slots each. Initially, just one well will be connected to each manifold. Later, up to three more wells may be added to each manifold.

One subsea well in Cascade East, one in Cascade West, and one in Chinook will be tied back to the field’s FPSO using dual, piggable 9 5/8-in. (24.4-cm) flowlines and free-standing hybrid risers. The subsea infrastructure will be operated and controlled by an electro-hydraulic system with power and control umbilicals running from the FPSO to the drill centers.

Free-standing hybrid risers

Another technology new to the GoM is free-standing hybrid risers. This technology has been used on several projects outside the GoM, including Girassol and Kizomba A and B offshore West Africa.

Two FSHRs will move production from the seafloor upward to the disconnectable turret buoy at the FPSO. Processed oil will be transported ashore by shuttle vessels. Produced gas will be sent from the FPSO via FSHRs and a 6-in. (25-cm) export pipeline to the BP-operated Cleopatra pipeline system in Green Canyon block 829 or the Enterprise-operated Anaconda pipeline system in Green Canyon block 606. Cleopatra is 43 mi (69 km) from the FPSO and Anaconda is 66 mi (106 km) from the FPSO. There will be two production risers for Cascade, two for Chinook, and one gas export riser. Each will be approximately 1,200 ft (366 m) from the FPSO.

The FSHR design consists of a section of vertical line pipe anchored to the seabed and tensioned at the top by a buoyancy can. The riser is connected to the FPSO turret buoy by a flexible jumper.

Petrobras says it has selected FSHRs for the following reasons:

• Using flexible jumpers effectively isolates the riser from the fatigue inducing motions of the FPSO<
• The FPSO needs to support only the load of the flexible jumper from the vertical riser to the turret, compared with supporting the entire load of a steel catenary riser in 8,200 ft (2,499 m) water depth
• An FSHR can be installed by a number of vessel types, providing construction flexibility
• If required, an FSHR can be isolated and retrieved after installation without impacting production through the other FSHRs
• FSHRs are a good fit with a single point mooring system with a disconnectable turret.

FPSO

Petrobras is deploying the FPSOBW Pioneer for Phase 1 as the host production facility. It will be converted from the 1992-built ship-shaped double-hull vessel, M/T Sarasota. It will be equipped with capacity to process 80,000 b/d of oil, 16 MMcf/d of natural gas, and 16,000 b/d of water. It also will have storage for 500,000 bbl of oil.

BW Offshore will deliver the converted FPSO,BW Pioneer, to Petrobras under a lease-term agreement for a fixed period of five years with optional periods up to three years. The FPSO will have a single-point mooring and disconnectable turret buoy system. It will be the first FPSO to operate in the US GoM. Rendering courtesy of Petrobras.

The FPSO is being converted in China and Singapore. It sailed from China to Singapore in mid-August 2008. The vessel is expected to arrive in the GoM ready to produce in early 2010. It will be moored in 8,200 ft (2,500 m) of water on Walker Ridge block 249. This will mark a word-record for the deepest water depth of an FPSO. It also will be the first FPSO in the US GoM.

Mooring system

The FPSO is scheduled to be fitted with an internal single point mooring system with a disconnectable turret buoy for full weathervaning capability. It will be connected to the seafloor via polyester fiber rope in a taut leg configuration and secured with torpedo piles. The mooring system will be designed to hold the FPSO and shuttle vessel on location in a 100-year storm.

The turret buoy can disconnect from its moorings to allow the self-propelled FPSO to move to safety in the event of a hurricane and then to return after the storm. After disconnection, the turret buoy (which is attached to the mooring lines, FSHRs, and umbilicals), will be left behind submerged 656 ft (200 m) deep – out of harm’s way.

Single-point mooring with a disconnectable turret buoy configured in an FPSO is new to the GoM, but is common offshore Western Australia where harsh weather conditions, similar to that in the GoM, exist.

Polyester mooring has been selected for the Phase 1 FPSO as the technology has been proven safe and reliable while being more flexible and cost-effective relative to steel mooring systems, says Petrobras. Furthermore, the company says use of a polyester mooring system will allow the use of a significantly smaller turret buoy for enhanced safety and project viability.

“We are very confident with the mooring system for the FPSO, but we understand that the unique metocean conditions in the GoM provide for a very specific case,” Palagi explains. “We will have sensors in the mooring system to identify any corrosion, but the mooring system will be there just to support the FPSO itself, and not most of the production lines.”

Shuttle vessels

Processed oil from the FPSO will be transported ashore by shuttle vessels. Two double-hull shuttle vessels, each with capacity for 330,000 bbl of oil, have been contracted for Cascade and Chinook. Both vessels will be built in the US and operated under Jones Act regulations.

The shuttle vessels will be similar to those used for lightering operations in the GoM. The difference is they will be outfitted with a bow loading system to support offloading from the FPSO as well as controllable pitch propellers and internal bow thrusters to enhance maneuverability for offloading operations.

During offloading, the shuttle vessels will connect to the FPSO via a hawser system (i.e. mooring line) and offloading hose between the bow of the shuttle vessel and the stern of the FPSO in a tandem offloading configuration. The shuttle tanker will weathervane in a similar manner as the FPSO. The first vessel is scheduled to be on location in mid-2010.

Regulatory environment

The regulatory framework is a major challenge for Petrobras with so many new technologies proposed for the development, Palagi explains. “This adds one more degree of complexity and uncertainty to the project. One of the major challenges is to satisfy the MMS and US Coast Guard requirements for a production platform, which also stores oil and sails like a tanker,” Palagi says. “In other words, it’s regulated as an FPSO and a shuttle tanker at the same time.”

Petrobras must comply with a number of regulations outlined by MMS and the US Coast Guard. The conceptual plan for the development of Cascade and Chinook was approved by MMS on Nov. 29, 2006; the development operations coordination document (DOCD) was approved on April 29, 2008; and, at print, the deepwater water operations plan (DWOP) was in MMS’ hands pending approval. Petrobras expects to have this and all other major documents approved by the end of 2009.

Palagi points out that the entire set of regulations for FPSOs, shuttle tankers, and other technologies that are new to the US GoM are being developed as the company progresses with the Cascade and Chinook project. “So in some sense, what the industry is doing now is going to form the background for the regulatory framework for the GoM,” Palagi says.

Future phases

Future phases of the Cascade and Chinook development will be designed based on the performance of the reservoirs in Phase 1. A potential Phase 2 will consist of drilling, completing, and connecting the manifolds with up to six more wells in Cascade and seven more wells in Chinook, he says. The subsea and FPSO infrastructure will allow each field to produce up to 40,000 b/d of oil. One field can produce 50,000 b/d of oil, but both fields cannot produce a total of more than 80,000 b/d of oil. The Phase 1 FPSO is designed to stay on location five to eight years.

The Phase 2 design will depend very much on the performance of the Phase 1 wells, Palagi says. Phase 3 also is contingent on future reservoir interpretation from Cascade and Chinook, and other analog fields. This phase may consist of replacing the Phase 1 FPSO with a new platform to be designed to stay on location for several decades.

A number of scenarios are being studied by an integrated project team of geoscientists and engineers in Houston and in other parts of the world. Phase 3 is expected to be on stream not before 2015.

“We are very confident that the Cascade and Chinook projects will pave the way for future ultra deepwater Lower Tertiary fields in the US GoM,” Palagi says. “There are several discoveries in this trend that are similar, and the results we get from one also should be very much valid for the others.”

Editor’s note: For more information on the Cascade and Chinook development, visit the archive of Offshore Magazine’s exclusive webcast with Cesar Palagi athttp://www.offshore-mag.com/webcast/display_webcast.cfm?id=721.