Round and round

June 13, 2014

Sevan Marine’s Sevan 300-designed circular FPSO, which serves Petrobras’ Piranema field off Brazil. Photo from Sevan Marine.

A new study by RPSEA, Doris-Inc., and Sevan Marine investigates whether circular hull FPSOs can provide a low- cost production solution for remote ultra-deepwater Gulf of Mexico operations. Audrey Leon spoke with the project leads to find out more.

US-based research nonprofit RPSEA (Research Partnership to Secure Energy for America) selected Doris-Inc. to conduct a feasibility study on circular-hull FPSOs for application in ultra-deepwater Gulf of Mexico.

This region poses many technical and economic challenges, including extreme water depth, complex Paleogene reservoirs with high pressure and temperatures, not to mention the distance from shore and existing pipeline infrastructure. The GOM also is prone to hurricanes in the summer months with high wind and high seas.

RPSEA’s members discussed what other options could serve the ultra-deepwater GOM at a lower cost than giant spars and TLPs. FPSOs, mostly due to regulations and the Jones Act (1920, 46 U.S.C.A. § 688), have not been favored. However, for one operator, Brazil’s Petrobras, it’s the only preferred production option.

Currently, Petrobras operates the GOM’s first FPSO at its Cascade/Chinook fields, approximately 250km south of Louisiana. In 2012, the BW Pioneer FPSO, which serves the fields, was forced to shut-in during Hurricane Isaac, a category one storm that made landfall along the coast of Louisiana on 28 August 2012, a mere six months after production first came online at the Cascade field in February 2012.

The BW Pioneer boasts a production capacity of 80,000bo/d and 500,000 cu. m/d of gas, and storage capacity of 500,000 bbl. It also utilizes a disconnectable riser system. In the event of a severe hurricane, the turret riser buoy can be disconnected and submerged to a depth of 46m; the buoy remains at that depth, anchored, until the storm passes and the crew can recover it back to the host vessel.

A moonpool was added to specs for a Sevan-designed circular FPSO to accommodates steel catenary risers.

Nine hours before the storm made landfall, the buoy recorded wave heights of about 5.9m, according to a US Geological Service report entitled, “Hurricane Isaac: Observations and Analysis of Coastal Change.” The FPSO was able to stay moored while shut down with its riser turret remaining in vessel, since the hurricane’s winds were below the vessel’s mooring capacity.

The RPSEA study aims to investigate storage options for the Central GOM. While the BW Pioneer ship-shaped FPSO can store 500,000 bbl, RPSEA aimed to study a design concept that could safely hold 1MMbbl while remaining on anchor during hurricane conditions.

Originally, RPSEA members looked at several other offshore storage options in ultra-deepwater GOM, while also looking at subsea completions concepts, says Bill Head, Project Manager, RPSEA. The members opted to investigate options for a 2MMbbl floating storage facility, which included semisubmersibles, MODUs and circular FPSOs.

Head says RPSEA wanted to start the project by assessing circulars and comparing them to ships, spars, and large semisubmersibles. Houston-based Doris-Inc., a subsidary of France’s Doris Engineering, was selected as a subcontractor for the project. During the project, Doris collaborated with different stakeholders, including government and industry. Success, Head says, depends in large from support at RPSEA and the RPSEA project working group that consists of volunteer subject matter experts from six major operators: Statoil, ConocoPhillips, Chevron, Petrobras, Total and Marathon. “Operators desire a safe, low-cost system, that feature a high level of concept maturity in order to reduce the time-to- market especially when innovative design is involved,” says Jelena Vidic-Perunovic, principle investigator on the project, Doris-Inc. “An FPSO concept offering local oil storage, offloaded by use of DP shuttle tankers has been selected as the main focus in our study.”

Before the study settled on circular FPSOs, Head says that the project partners didn’t believe spars, could give the desired storage capabilities. “Theoretically you could build huge spars,” he says. “They cost a lot more than circular. If you build a spar that size, you’re investing a significant amount of money, more than you need to. A circular is portable; it is shallow draft, and you have more options where you can take it and where you can build it.”

After some consideration, Head says the project partners doubted that US regulators would allow storage of 2MMbo during a hurricane, and the study param- eters were downsized to 1MMbbl. “We had to get to a point where regulators wouldn’t worry about what was in the container,” Head says.

Vidic-Perunovic says for the study, the full life cycle of the system needed to be considered. Thus, an important element, she says, is design flexibility.

“We are applying it in terms of topside production facility flexibility with regard to uncertainties in reservoir and fluid characteristics, and the possibility for an expanded production rate of the topside facility,” she says. “That way, doing a conceptual study based on a hypothetical field, Doris’ team can offer a design with a wide range of application in the Gulf, which gains importance if production is expanded, in case a platform should be moved to a new location, or change in market conditions."

Vidic-Perunovic says the hull and riser play a dominant role in the production concept selection and may result in total cost reduction. Norway’s Sevan Marine was brought into the project as it has sev- eral circular FPSOs in operation, two in the North Sea, one serving Petrobras’ Piranema field off Brazil, and one (under construc- tion at HHI in South Korea) that will even- tually serve Eni Norge’s Goliat field.

Vidic-Perunovic says out of many circular designs considered, Sevan’s hull was selected because it featured the high- est technology readiness levels.

“The technological readiness level of other Sevan’s units in operation is the highest, seven according to API,” she says. “They have experience with harsh environment in the North Sea, although not hurricanes. The North Sea is quite a severe environment, and the experience counts a lot.”

Sevan Marine’s participation included providing knowledge gained from wave tank experiments on their circular-hull FPSOs. Doris, in turn, incorporated hull hydrodynamic data for the FPSO design with steel catenary risers (SCRs), which would serve a hypothetical field in ultra-deepwater Lower Tertiary play with HPHT reservoirs and central GOM metocean conditions.

Design parameters also included a 2500m water depth, 60,000b/d initial topside production capacity, 1MMbbl storage capacity, 20-year service life, 20,000psig reservoir pressure, 13,000psig wellhead shut-in tubing pressure, and 240°F reservoir temperature.

Sevan’s circular FPSOs traditionally use flexible risers in the North Sea, but SCR was selected for this study because it was the simplest and most economic riser configuration for the GOM, Vidic-Perunovic says. “SCRs are suitable for HPHT fluid conditions, which we encounter in the Lower Tertiary GOM reservoirs,” she says.

The BW Pioneer at the Keppel Shipyard in Singapore before it became the first FPSO to serve the US Gulf of Mexico. Photo from Keppel Corp.

The hull was designed with a central moonpool with a diameter of 30m for hanging SCRs. The hull measured 93m in diameter with a bilge box of 124m in diameter, and main deck that is 103m in diameter. The draft, ballast/loaded came in at 22m/31m.

For the study, the FPSO’s hull needed to be optimized to accommodate the SCRs. The size of the bilge box was increased to temper the vertical motions, Vidic-Perunovic says. Another big differ- ence was the introduction of a moonpool in the hull’s center.

In a paper to be presented at OTC in May entitled, “Steel Catenary Riser Design for Cylindrical FPSO Application in Ultra-Deep GOM,” Vidic-Perunovic and her co-authors, which includes Head, states that due to the riser hang-off location being close to the hull’s center, the effect of hull angular motions on riser response is minimized. For the excitation wave periods of interest, heave motion response ampli- tude operators (RAO) are smaller than that of the conventional ship-shaped FPSO, in either head- or beam-sea condi- tion. This will result in an improved riser wave fatigue response. RAOs were calcu- lated by Sevan Marine using the WAMIT/ WADAM 3D diffraction/radiation panel method, according to the OTC paper.

“Results indicate that both strength and wave fatigue design criteria are satisfied,” Vidic-Perunovic says. “In addition, the minimum effective tension in the riser touch down region remains positive in extreme 100-year hurricane condition. The study demonstrates that cylindrical FPSO in combination with a SCR is a technically feasible solution for developing ultra-deepwater GOM fields.”

Because of this study, Head believes when hurricanes are involved circular trumps ship- shaped FPSOs. “There are no angles so wind and waves go around it,” he says. “We have good wave tank data that shows that.

Minimum and maximum tensions along riser arc length, 100-year hurricane (load case No. 4). Sensitivities on C. 
Fatigue life along riser arc lenght. Source: Doris-inc.

“In terms of storms and hurricanes, circular design has significant advantages over very large tankers,” he says. “They have a much higher survivability than ships and MODUs, similar to spars.

“The intent of a circular FPSO is to stay on during a storm. You would have longer production, you could shut-in during the most intense part, and then come back online.”

The Future

The project is ready to enter phase two study, but due to issues with the latest US Congressional budget changes to the 2005 Energy Policy Act where RPSEA receives most, but not all, of its funding, the fate of phase two is unknown.

“If DOE efforts to secure OMB approvals to release funds allocated by Congress before 9 January 2014 result in no forthcoming energy funding, we will try to pursue outside resources,” Head says. Currently, Sevan Marine is cost-sharing in the project. Vidic-Perunovic is optimistic. “I hope for the best because phase two would validate the applicability of this very interesting novel production concept in ultra-deep Gulf.”