The EVDT is designed as a slimbore completion system. The outer diameter of the tubing hanger equipment is 13.4 in., small enough to pass through a 13 5/8 in. BOP stack. The bore of the system can accommodate 7-in. tubing completions and handle pressures up to 15,000 psi. (Images courtesy of FMC)

The objective when designing production equipment is to create components that are reliable, rugged, durable, and simple to place and retrieve. When it comes to subsea trees, especially those that are going to be used in deep water, meeting those design objectives is a considerable challenge. Deepwater conditions affect how well the tree can perform and impact considerably the cost of installation and workovers.

The vertical tree

FMC Technologies’ enhanced vertical deepwater tree (EVDT) is changing the concept of vertical completion. “For years, the concept of vertical trees has had a negative connotation with several operators in the industry,” said Marcus Smedley, field development and technical sales manager for subsea systems at FMC Technologies. “The development of this new tree is changing the way people think.”

The EVDT is designed as a slimbore completion system that provides large bore results. The outer diameter of the tubing hanger equipment is 13.4 in., small enough to pass through a 13 5/8-in. blowout preventer (BOP) stack, even though the typical BOP stack measures 18 ¾-in. The bore of the system can accommodate 7-in. tubing completions and handle pressures up to 15,000 psi. This translates to a single product that can be used in a wide range of applications, from typical deep water subsea placement to shallowwater jackup depth and new deep water surface BOP applications.

The EVDT also incorporates a retrievable flow module that houses a subsea flow meter that can be retrieved and replaced in a matter of hours compared to the several days required by the typical meter mounted in a flowline jumper, Smedley said. Reduced downtime can translate to more than US $5 million in production revenue for a single change-out.

The tree comes to life

The EVDT was developed to global standards by a multinational team. While developing the tree, FMC worked with major operators to design a product that would solve production challenges on major projects in different parts of the world. One of the primary design objectives was to create a tree that would minimize production time and reduce costs.

From the outset, Smedley said, a major oil company was anticipating a global solution that addressed all types of projects. Because of that success, this operator has contracted for more than 45 of the trees to be installed in the near future. By aligning common systems across their various global projects, the operator is in a position to minimize production and maintenance costs while increasing local content without added risk.

Installation

The vertical tree system increases installation efficiency. One installation method uses the tubing head to land and orient the tubing hanger, which provides flexibility for offshore installation vessels and sequencing of events. For example, it allows jumpers to be installed before or after tree installation.

“Traditionally, the operator would drill the well with the rig, put the horizontal tree on the wellhead, put the drilling riser back on top of the wellhead, and complete the well. Installation of the flow line jumpers and controls would be done at a later time. With the new vertical tree, you can drill multiple wells and come back later with a lightweight, less expensive vessel to install tubing heads and jumpers on a wire. Completions can be done later through the tubing head using another, less expensive rig. This can yield savings of up to $15 million per well,” Smedley said.

Another unique feature is the tree’s top interface. Typical vertical trees have a smaller hub that would allow a specific tool to run the tree. The EVDT has an 18 3/4-in. H4 mandrel that is commonly used in the subsea industry. It also has a unique set of profiles inside that hub that allow for various items to lock into the inside of the hub. This allows operators to use the subsea BOP stacks already found on the rigs and attach a simple landing string inside. This approach eliminates the need for an open-water riser system, which was one of the reasons operators moved away from using vertical trees in the first place.

Installation is simplified with the new tree design because it can be run on wire, Smedley said. “We run almost all of our trees on wire. What used to take days to run, takes literally minutes. Some trees have been run on wire in 2-3 hours, as opposed to 2-3 days.”

Another installation option is to land the EVDT tubing hanger into the wellhead, which eliminates the tubing head. This method does not require the tooling or any of the common tree interfaces to be changed and allows for a more efficient installation when completion and drilling operations are conducted without retrieving the subsea BOP and drilling riser.

According to Smedley, the tree cap no longer has to run on drillpipe inside a riser. The BOP can be disconnected, and an ROV can be used to pick up and place the tree cap on the top of the tree. “This can save a day or two of rig time in deep water,” Smedley said.

Putting the system to work

The EVDT is a proven success in the offshore industry. Operators have placed orders for use in deep water projects offshore Brazil and the Gulf of Mexico. FMC also has orders from operators to use the EVDT in a deep water project offshore Sabah, Malaysia and Congo, West Africa.

The success of the EVDT in deep water has led FMC to investigate using the product in shallowwater developments from a jackup rig. In combination with a new 13 5/8-in. slimbore wellhead, the system can not only eliminate mudline tieback operations, it can also eliminate divers. “It has the potential to revolutionize the use of subsea systems for shallowwater developments,” Smedley said.

Building for tomorrow

Though the trees will soon be working in a number of fields and are scheduled for installation in several more, FMC is investing in product improvement. Though the present design allows an unprecedented 11 downhole lines, Smedley said, “We’ve got people now starting to look at whether we need more.”

In addition to the increased number of downhole lines, tomorrow’s equipment will be working at greater depths than the trees being deployed today. Challenges at greater depths include the ability to withstand higher pressures.

“There is a possibility that we will need 15,000-psi, or even 20,000-psi systems to contend with some of the Lower Tertiary discoveries in the Gulf of Mexico,” Smedley said. “Even though these fields are going to produce beyond 15,000 psi for only a short time, the MMS will probably require designs to 17,500 psi or 20,000 psi to ensure the wells can safely produce. The future is higher pressure and higher temperature.”

Meeting performance standards under higher temperatures will be difficult because high temperatures cause problems with elastomer and metal-to-metal seals. “The expansion caused by heat allows the seals to leak. At temperatures above 350°F (176°C) the elastomer seals begin to lose their viscosity,” Smedley explained. Pressure also affects the seals. “We can design seals that work to 20k, but H2S affects seal integrity. The greater the pressure, the greater the effect of the H2S. It is more corrosive at higher pressures.” The company is investing in research and development (R&D) to develop materials that can withstand the rigorous high-pressure/high-temperature conditions they will encounter in deepwater operations.

Improving installation methods is also on the R&D agenda. For example, the company is investigating ways to use electronics to orient components on the seafloor and eliminate the need for costly and damage-prone umbilicals, Smedley said.

Some of the advances under development come from the telecommunications industry, Smedley said. Acoustic communications that do not require an umbilical for surface-to-subsea tree transmissions are being investigated, and long-offset communications, which would be advantageous in the arctic regions, are also being evaluated.

No avenue to progress is being overlooked. The search for answers is not confined to internal research. “FMC is also looking outside the oil and gas industry for materials and technologies that can be adapted for our industry,” Smedley said.