Subsea: the final frontier

In their continued search for new oil fields developers are looking to deeper and deeper waters. As E&T discovers, Shell's new Perdido platform is pushing the engineering boundaries further than ever before.

As the traditional oil wells begin to run dry, oil companies are looking to ever more inhospitable locations to extract oil, and there can't be many locations more challenging than 8,000ft (2,438m) under the sea. That is the challenge facing Shell engineers at the Perdido oilfield in the Gulf of Mexico

Located about 200 miles from Houston in an isolated sector of the western Gulf of Mexico, Perdido will be the world's deepest oil and gas drilling and production facility when it comes online in early 2010. The facility is capable of handling 100,000 barrels of oil and 200,000 cubic feet of gas per day.

To get the oil and gas to market required installing 77 miles of oil export pipelines and 107 miles of gas export pipelines in a remote part of the Gulf of Mexico over very rugged sea floor terrain to connect to the existing offshore pipeline infrastructure.

The project has already set a world water depth record in drilling and completing a subsea well 9,356ft (1.77 miles) below the surface and it intends to drill an even deeper well at 9,627ft (1.82 miles). Nine polyester mooring lines averaging more than two miles in length now hold the 50,000-ton floating structure in place, which is almost as tall as the Eiffel Tower

"We are on the last leg of a pretty exciting journey," Bill Townsley, Shell Perido ventures leader explains. "We are about to turn on the world's deepest oil and gas and drilling platform. It has taken a lot of innovative engineering and advanced technology."

There can be no doubting that fact. There are a lot of firsts involved in the project, so much so that it has been deemed a high-risk field by Shell. "The subsea boosting system is one and there are several firsts that are being done in the industry," Townsley says. "So when you have got the deepest subsea well in the world and the deepest Spar in the world - and I haven't even talked about the subsea pipeline, which was also a world record water depth- all of those things in accumulative fashion show that we are really pushing deep water boundaries. Again we have done all that we can to mitigate them, but there is some residual risk that we are aware of as we start this thing up."

Wet tree DVA

One of those technologies that Townsley mentions was wet tree DVAs that sit right underneath the Spar platform. In past developments there would have been a pipeline from each well up the platform where what are deemed dry trees would process the oil and gas.

The primary function of a tree is to control the flow into or out of the well, usually oil or gas. A tree often provides numerous additional functions including chemical injection points, well intervention means, pressure relief means, and tree- and well-monitoring points.

When the operator, well and facilities are ready to produce and receive oil or gas, valves are opened and the release of the formation fluids is allowed to flow into and through a pipeline. The pipeline then leads to a processing facility, storage depot and/or other pipeline eventually leading to a refinery. Subsea wells and, thus, trees usually flow through flowlines to a fixed or floating production platform or to a storage vessel (known as a floating storage offloading vessel (FSO), a floating processing unit (FPU), a floating production and offloading vessel (FPSO) or other combination of structures).

A tree may also be used to control the injection of gas or water injection application on a producing or non-producing well in order to sustain economic 'production' volumes of oil from other wells in the area (field).

The set up at Perdido uses the real estate on the sea floor, with 22 wells that are connected to two manifolds, which in turn pump the oil and gas up to the Spar above.

"By putting all the wells into that manifold we only have five risers, what we call production risers, that go to the surface and that allow the footprint of the Spar and facilities to be significantly smaller than it normally would be for a 22-well development," Townsley continues.

"Historically, before Perdido those 22 wells right below the Spar would have needed 22 pipes to the production platform."

The Gulf of Mexico

Townsley is a veteran of the North Sea oil platforms, having worked there for 12 years, and concedes that the day to day conditions in the Gulf are more benign, with warmer water and generally more temperate weather. But the one big challenge is the hurricanes that can attack that region each year during the hurricane season.

"Prior to Katrina we built all topside design to survive what we call 500-year storm specification, but since Katrina that has gone to a 1,000-year storm specification and so that is what the Perdido is built for."

The Spar itself has nine polyester mooring lines that tie it to the seafloor that will hold the Spar in place for any kind of a hurricane. "One of the technologies that we have in order to move on top of those 22 wells is that the Spar can actually move itself around [an area] about the size of a football pitch by pulling on these mooring lines," he adds. "So for a storm there is a storm position in the very centre that they position themselves in, and then our procedure in the Gulf of Mexico is to abandon the platform of all people and we shut down the production and leave the facility there to take on the storm."

Subsea boosting system

One of the key technological developments of the Perdido project is the subsea boosting system that separates the oil and gas at the sea floor and sends it to the surface. The traditional approach is to separate the oil and gas on the platform itself.

"The initial separation is done on the sea floor and, although this is the first time we have done this at this depth, we know that it is going to work because we have piloted it on shore for two years, developing this design and the electrical submersible pump," Townsley explains.

All the hydrocarbons from the well come into the inlet assembly and cause a cyclonic separation where the gas breaks out of the oil and starts to rise up the outer annulus - the liquid falls to a 350-foot hole in the ground that has been drilled just to hold these liquids.

Then a 1,500 horsepower electrical submersible pump pumps that liquid to the surface. "That really does two things," Townsley continues. "Firstly, it provides artificial lift for all 22 of those wells while also reducing the back pressure on the wells so they produce much more prolifically."

The benefit is that reservoir pressure is lifted all the way to the surface. By putting an electrical submersible pump at the sea floor and pumping it to the surface you essentially have that 8,000ft of pressure that is not applied to the wells any longer, so the amount of pressure that the oil needs to push against to get to the surface is that much less.

This part of the western Gulf of Mexico is a new geological area. Normally with offshore developments there is sufficient pressure in the reservoir to push the oil and gas up on its own, but unlike the rest of the Gulf of Mexico that is a geopressure system with high pressure. This area has normal hydro static pressure that means an extra challenge to pull the pressure down in order to get as much of the oil out as possible.

The pipe to the surface is essentially three in one. The gas goes in that outer annulus, the oil then is pumped up the second annulus and the middle one is there to allow oil to be pumped down to prime the pump. "The pump needs at least 10,000 barrels a day to work, so if we ever find ourselves with less than that we can just pump additional fluids from the surface to keep the pump in its operating range," Townsley explains.

Subsea tie-in

Having extracted the oil and processed it, there was then the small matter of getting it to the shore, which is 200 miles away. The cost of building a brand new pipeline would have rendered the project financially non-viable so another solution had to be found. The answer was to tie a pipeline into an existing pipeline that came from the ExxonMobil Hoover Diana field 70 miles away.

"From an engineering viewpoint, this is one of the really neat things that has happened on the project," Townsley says. "Being out here in the western Gulf of Mexico there is very little infrastructure of artery system of pipelines that you can tie into to get your product to shore.

"Again, taking an oil and gas pipeline 200 miles to shore would cost a lot of money. There was one pipeline, 71 miles away, that was the right size and had the ability to take our product, but it didn't have any place to tie into. It wasn't built with any flexibility to have another pipeline tie into, so our engineers worked to develop a way to cut a piece of the pipe out of the existing pipeline."

To achieve this they made an arrangement with ExxonMobil to shut the pipeline for 17 days while they effected a tieback. First of all the pipeline had to be cleaned - this was chieved by using what are called pigs, which are generally spheres that are pushed through the pipeline to take all of the oil and gas out of it and replace it with water.

The whole endevour, 4,500ft under the sea, was carried out with no humans involved on the seabed. "Using robots we cut a piece of pipe out of there and put in more or less a 'y' piece to allow ourselves to tie into that," Tomwsley says. "That was designed completely by Shell engineers. That way we were able to only have a 70-mile pipeline rather than a 200-mile pipeline.

"The end result is that the common pipeline now takes oil and gas from both the Hoover Diana field and the Perido field to a shore-based refinery. All the fiscal metering happens on the Spar, so each oil producer gets paid for what goes onto the platform before it goes into the pipeline."

Safety systems

As with all oil production facilities safety is paramount, and on such an innovative platform this was even more of an issue. "One of the things that you have to think about is that we have made a lot of effort to make the Spars smaller, so as a result you have to deal with the challenge of making sure that your people who are working there are safe," Townsley says.

There are quarters on board that can hold up to 150 people and one of the prime safety devices was to divide the Spar into two - a living half and a working half - separated by a blast wall that protects the people from the process so should there ever be any kind of explosion or fire, the blast wall would protect the people.

In addition, only a very minimal amount of gas is flared to the atmosphere. The Spar features a helipad that can accommodate two helicopters and there is also a fast rescue craft on the platform.

The oil industry is watching the project with great interest because if it proves to be a success then it will open up even deeper waters and oil fields around the globe. "Every step that we take in the deep water opens up some doors," Townsley says. "So this whole concept of wet tree DVA and subsea boosting systems, as we get deeper and deeper, allows us to continue to build the technology on this as we go forward. 

"Each project generally comes with its own set of challenges, some of which we can plug and play from previous developments, other things we have to push the boundaries a bit and get the barrier back to allow for these developments. But there is no doubt that this is an enabler and the whole industry is watching to see how this works."

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