The Stones development, which began production in September 2016, is the deepest offshore oil and gas production facility to date, situated in 9500 feet of water.
It is also only the second floating production, storage, and offloading (FPSO) vessel installed in the hurricane-prone US Gulf of Mexico.
The development plan called for steel lazy wave risers joined with a disconnectable buoy. The risers are equipped with additional buoyancy, creating an arched bend in the pipes between the seafloor and the surface.
This bend helps to absorb the motion of the FPSO and boosts production performance at extreme depths. The turret and buoy design allows the vessel to turn with the wind during normal weather conditions — and to disconnect in the event of severe weather conditions.
“We needed to be able to escape from a hurricane,” says J Blake Moore, now of Blake Consulting, who served as Shell’s Stones FPSO project manager.
The Stones project represents the first use of a disconnectable buoy, also known as a buoyant turret mooring.
The Lower Tertiary reservoir, at 17,000 feet below the mudline, is “quite deep and complex”, says Moore, speaking during a Marine Technology Society function in Houston.
“You’ll save time in the long run if you spend the time to plan up front.”
Shell expects to install a subsea pump later, and so installed more power than is needed initially to compensate for the subsea pump. Initially, two subsea production wells were tied back to the FPSO.
Shell, which operates the field with 100% interest, discovered Stones in 2005. In April 2011, the company chose an FPSO as the means of production, and final investment decision followed in 2013.
At the final investment stage, Shell said the first phase of development was expected to have annual peak production of 50,000 barrels of oil equivalent per day from more than 250 MMboe of recoverable resources. Shell estimated the Stones field contains more than 2 billion boe in place.
Shell contracted with SBM Offshore for the FPSO. The double hulled Suez tanker Captain X Kyriakgu Majuro was converted into the Turritella FPSO at the Keppel Shipyard in Singapore, while the Dyna-Mac yard fabricated the topsides.
“The tanker was in very good shape. It wasn’t that old,” Moore says. “We put a lot of steel in it because of the fatigue analysis and how long we wanted it to stay offshore.”
The Asian Hercules II and Asian Hercules III carried heavy-lift operations with the Asian Helping Hand III assisting with module installation on the FPSO.
In October 2014, the lower turret was fitted into the top of the buoy. “The dimensional tolerance for this was within a finger width,” Moore says. “This was a key test. Would it fit?”
It did. There was, however, a quality issue with the upper turret welds, which caused some delays.“The turret was on the critical path,” he says. “We found the issue, fixed it and got it in place.”
Fabricating the buoy required 220 syntactic blocks weighing between two and 20 tons, which were lifted into the buoy “in a very specific order. It was a very big jigsaw puzzle,” he says.
Planning was especially critical with this piece of the project, he says. “We printed a 3D model of the buoy and the foam blocks,” Moore explains.
“We walked through and verified dimensionally that it all fit. We used these models in the yard when we were talking to the workers.”
Heerema installed the 3200-ton buoy at Walker Ridge Block 552. “We had to find a weather window to install within the loop current,” Moore says. “There were delays, but we got it in the water… It was hooked up and ready to go when the FPSO arrived.”
The FPSO left the quayside in October 2015 for sea trials, which revealed an engine problem. That was repaired, and the FPSO departed Singapore in November 2015 for its trip to the Gulf of Mexico.
In January 2016, Shell pulled the buoy into place on the FPSO. “The heave compensation worked fantastic,” Moore says.
Between February and May 2016, the operator pulled in three risers and two umbilicals and integrated them into the system. After commissioning and regulatory approval, Shell carried out the last test: a dynamic disconnect of the turret system.
“The disconnectors were activated, and away it went,” Moore says. The test was predictable and uneventful, which was exactly what the team wanted.
“It was what all of us in project management like to say is, ‘it was boring’. I love boring.”
While the combination of steel lazy wave risers and a disconnectable buoy was “a straightforward concept”, there were some design concerns about the disconnectable turret to be worked out from the inception of the project, he says.
The buoy had to float at the right level in the water column, and it had to descend at the right speed. It needed to tilt no more than 20 degrees as it descended to avoid damaging the stress joints on the risers.
Starting in late 2011, the project team began carrying out basin tests at the Marin facility in the Netherlands.
By January 2013, the team was working with a 1:50 scale model to see how the system worked under metaocean conditions, carry out manoeuvrability tests and gauge the effects of green water on the vessel’s deck.
“There were a lot of design issues we were solving throughout 2013. Eventually we got it on paper,” he says.
Shell provided “shadow engineering” on the buoy and offered its feedback from a “different set of eyes” to SBM to achieve better conclusions and results with the buoy system design.
“We achieved what we wanted to do with the buoy. The most important part of this facility was that this technology that had never been done before worked correctly. We achieved that,” Moore says.
If a hurricane threatens, the FPSO can disconnect in about 24 hours, including the time needed to treat the risers and subsea equipment to prevent hydrate formation.
The FPSO can produce 60,000 barrels per day of oil, 15 million cubic feet per day of gas and 30,000 bpd of water.
When the unit began production, Shell said it anticipated reaching full ramp-up levels of 50,000 barrels of oil equivalent of oil by the end of 2017.
Tankers transport oil from Turritella to US refineries, while a pipeline transports gas.
Moore says the philosophy was to engage early with contractors and work together for the best outcomes. An early focus on safety paid off as well. Of 19 million work hours, he says, only one incident – requiring a few stitches to a finger – was reported.
He attributes some of the success to using a “coaching mindset” at job sites instead of an inspecting mindset.
“It was, ‘we’re here to help you, not write you up’,” Moore says.
In terms of quality, there were three major issues, he says – the upper turret welding, engine failure during sea trials, and leaks in the methanol system on start up.
“We did the safety goal up-front, but we didn’t do that with quality. I think if we had done that, we would have had better results on quality,” he says.
Extensive planning and collaboration lead to better outcomes, he insists.
“I have never heard of a project that they complained they planned too much. Spend the time to plan. You’ll save time in the long run if you spend the time to plan up front.”
Petrobras, which brought the first FPSO in the US Gulf of Mexico online in 2012 at Cascade and Chinook, helped the Stones team understand the regulatory challenges the project faced, Moore says, noting that there are perceived commercial barriers to sharing information and working together for improved outcomes. “I challenge the industry to get past that.”
Source: Blake Moore – Blake Consulting – Upstream
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