Breaking new frontiers

Audrey Leon

September 1, 2016

Shell’s Stones development will be home to the Gulf of Mexico’s second FPSO, but while it may be No. 2, the FPSO may hold a few firsts itself. Audrey Leon found out more.

Aerial views of the Turritella FPSO while docked at Keppel’s yard. Photos from Shell.

Shell’s Stones project is special in a lot of ways. The supermajor boasts that the floating production, storage and offloading (FPSO) unit engaged on the project, Turritella, will be the deepest production facility in the world at 9500ft (2900m) water depth.

The Turritella FPSO – owned and operated by a joint venture owned by affiliates of SBM Offshore (55%), Mitsubishi (30%) and Nippon Yusen Kabushiki Kaisha (15%) – nudged out the previous record holder, the BW Pioneer (the Gulf of Mexico’s first FPSO), which is installed at Petrobras’ Cascade/Chinook development, also close by the Stones project within Walker Ridge, in approximately 8900ft of water. But, while it is the second FPSO unit in the Gulf of Mexico, the project has enjoyed many firsts (See chart).

The field

Stones was discovered in 2005 in Walker Ridge Block 508, some 200mi off Louisiana. Back then, the operator on the project was BP (59%) along with Shell (26%) and Marathon (15%). The prospect was drilled in 9576ft of water and reached 28,560ft true vertical depth in March 2005.


Three years later, the Stones-3 well would reach 29,400ft and confirm the discovery of multiple oil-bearing sands. The field consists of nine Outer Continental Shelf blocks. Many considerations were taken into account during the concept selection phase. For Shell, Stones is interesting because the reservoir is not particularly well known, says Curtis Lohr – Stones project manager, Shell International E&P.

“We don’t have a lot of production history in this part of the Gulf of Mexico – that’s why it’s considered a frontier,” he says. “If you look at where the FPSO is located, the drill center is in 9500ft of water, but also in this area there is a plateau that’s in 7500ft of water. The sea floor is very rugged, which created some challenges in terms of routing pipelines.”

Other potentials challenges, native to the Gulf, would be extreme weather events such as hurricanes, and in particular, how to safely shut down operations and get people out of harm’s way. This was one factor that led Shell to choose a disconnectable FPSO for the project. Another factor, Lohr said, was the flexibility to disconnect and leave in the event of extreme weather, but to also not have to use an oil export pipeline.

“The advantage of using a FPSO for this frontier project is the flexibility that it gives Shell –to learn more about this reservoir and grow accordingly,” Lohr says. “Stones is a really exciting prospect for Shell. It could turn out that it is an extremely large find, but we won’t know until we start producing.”

According to SBM Offshore’s Turritella factsheet, released this May, the disconnectable capability allows the FPSO to not only quickly and safely sail away in the event of a hurricane, but also quickly resume production once the hurricane has passed the location, which Shell highlighted as an important factor for productivity at the field.

Additionally, SBM Offshore said another highlight of the Turritella FPSO is its ability to readjust each mooring line’s tension without the need to install any device on the FPSO. “It pioneers the use of an in-line mooring connector (ILMC), which gives direct access to the mooring line for re-tension purposes. This feature allows more flexibility when the need arises to adjust the tension of mooring lines, even during the early phase of the system installation,” according to the company’s factsheet.

The development


Shell reached final investment decision on the project in May 2013. Stones will be a phased development, with the first phase comprising two subsea production wells tied back to the Turritella FPSO. In all, there will be eight wells connected back to the FPSO. Shell expects to add six wells, in later phases after first oil, with multiphase pumping. The eight wells will be connected to the FPSO through two drill centers. The reservoir depth is around 26,500ft (8077m) below sea level and 17,000ft (5181m) below the mudline.

In October 2015, Technip was awarded the contract for the development of subsea infrastructure for Stones that includes two subsea production tiebacks to the FPSO, in addition to engineering of the second pipeline end terminations (PLETs); fabrication of the PLETs and piles; and installation of the subsea production system, inclusive of associated project management, engineering and stalk fabrication.

In August last year, OneSubsea landed a contract to supply subsea processing systems, which includes a dual pump station with two 3MW single-phase pumps and two subsea control modules, a topside power and control module, a barrier-fluid hydraulic power unit with associated spares as well as installation and maintenance tools.

Shell indicated multiphase seafloor pumping is planned for a later phase to pump oil and gas from the seabed to the FPSO, potentially increasing recoverable volumes and production rates. Shell estimates peak production could achieve approximately 50,000 boe/d.


While Stones is the first project in the Gulf of Mexico to utilize a FPSO for Shell, it is not their first FPSO project overall. Shell has 11 other FPSOs globally including the Bonga FPSO, 120km offshore Nigeria, in the Gulf of Guinea. Bonga came online in 2005 at 3281ft (1000m) water depth. It’s one of the largest FPSOs in the world, measuring 300m long and 12 stories high; the deck is the size of three football fields. The FPSO, when full with oil, weighs 300,000-tonne.

Lohr has worked with Shell for over 30 years on some of the company’s biggest deepwater frontier projects, such as Auger, Bonga, Perdido, and now Stones. Lohr joined the Stones project in 2010.

“The exciting thing about all the Shell projects I’ve been involved with and what they have in common is that they are all deepwater frontier projects,” Lohr says. “And in many cases these projects opened up new frontiers in different regions.

“Auger, for example, opened up deepwater worldwide – that one is also in the Gulf of Mexico. Bonga did a similar thing for Nigeria, and Perdido is the deepest drilling and production system in the world. And now Stones will be the deepest production system anywhere (in 9500ft of water).”

Lohr says that with Stones, Shell opted to have contractor SBM Offshore design the facility, with the Shell providing the functional specifications.

“For Shell, most of our experience in the deepwater Gulf of Mexico has been with TLP hosts. Perdido being an exception,” Lohr says. “There was a learning curve, but (in terms of working with the US Bureau of Safety and Environmental Enforcement) they had the advantage of working with Petrobras (on Cascade/Chinook).”

In July 2013, Netherlands-based floating production contractor SBM Offshore was chosen to supply and lease the Stones FPSO, Turritella, which is a converted 159,000-dwt Suezmax tanker. The Turritella will be capable of producing 60,000 b/d of oil and 15 MMcf/d of gas. The hull will be able to store 800,000 bbl of oil.

At the time the contract was signed with Shell, SBM Offshore said the Turritella will be moored using buoyant turret mooring (BTM) technology, allowing the vessel to weathervane on location or to disconnect in the event of a hurricane. Steel lazy-wave risers (SLWRs) connecting the subsea facilities to the BTM will be used for the first time with a disconnectable FPSO, the company said at the time. Shell indicated the SLWRs have an arch bend, which absorbs the motion of the FPSO and boosts riser performance at extreme depths.

Lohr says that Shell has used both disconnectable FPSOs and SLWRs before, and for the first time brought these technologies together for the Stones project.

“For the SLWRs, we actually install buoyancy on the risers, so there’s a S-shape to them that helps to take some of the load off the buoy and the FPSO. We have used it before on other projects and we felt comfortable with this technology,” he says.

The Turritella underwent conversion work at Keppel Shipyard in Singapore with teams from SBM Offshore and Shell present.

“When we did the conversion in Singapore, one of the things we communicated to the contractor – Keppel – is even though this is not the largest FPSO in the world, even though it is not the most complicated FPSO in the world – it is still special because it is the deepest,” Lohr says. “We did that because we wanted them to understand that we wanted them to build something special with no harm. We have been very focused on safety – we have an outstanding safety record on the project.” And Lohr says there were over 13 million man hours spent on the construction of the FPSO in Singapore without incident. The Turritella set sail from Singapore to the Gulf of Mexico in November 2015 and arrived at its location in late December last year. “We’re working hard on (first production), and it should be sometime in the coming months,” he says.

In June this year, InterMoor announced it had completed the final tensioning and chain cutting operations on Turritella. InterMoor’s work scope consisted of chain final tension adjustments through the ILMC system, subsequent cut and removal of excess chain, and riser pull-in rope stretching and transfer to the FPSO.

Bruno Amann, project manager, InterMoor, said that work began around Christmas and wrapped in February this year. Amann says that the project was very particular and took about three months. He notes that normally it takes longer.

Amann discussed the work scope and cited weather as a challenge. “It was winter time, and we didn’t have optimal weather conditions,” he says. InterMoor used the Seacor Keith Cowan anchor-handling vessel (AHV) to perform first phase operations, and later used a larger construction vessel on charter and on standby. Amann says that the AHV was used primarily to keep costs down.

Meeting challenges

An aerial view of Stones topsides at the quay in Singapore. Photo from Shell.

In addition to rugged terrain and weather conditions, there are other challenges to meet when working in deepwater frontier areas.

Lohr highlights designing umbilicals that could meet the challenges associated with the extreme water depth.

“It’s easy to say 9500ft of water, but more challenging to execute,” Lohr says. “Because of the water depth, the tension and the load on the umbilicals becomes quite high, and we actually had to work with the contractors to come up with something that works in this water depth.”

Additionally, Lohr highlights the disconnectable technology used on the project. “The way we worked with our contractors to meet the challenges of 9500ft of water is a very special thing.”

The 3D advantage

For Stones, Shell took advantage of 3D printing technology to ensure the disconnectable buoy on the project came together flawlessly during the construction process. The disconnectable buoy features a design that uses syntactic foam, unlike typical disconnectable buoys that are hollow steel. The buoy features 222 pieces of foam that need to fit together in sequence. Lohr says that the use of 3D printing technology to create a model allowed Shell to ensure that there weren’t any safety issues or schedule delays.

Shell produced a video earlier this year discussing how 3D printing came together to ensure the buoy construction went according to plan.

“A fundamental part of the design process is to visualize what the end product will be,” says Robert Patterson, executive vice president, engineering, Shell, in the video. “3D printing allows for very rapid prototyping; allows you to engage with a design, installation sequence, safety risks of putting it together. If you do all of those things early, it leads to far better outcomes.”

Amir Salem, construction engineering, Stones project, SBM Offshore, added: “Having a model like this bridges the gap between design and fabrication.”