Horizontal Trees Phillips using horizontal subsea trees in Joanne development

An FMC horizontal subsea tree being run from the Santa Fe jackup Magellan on the Phillips Joanne Field in the North Sea (Photo courtesy Santa Fe Drilling -- North Sea).

Horizontal subsea trees are attracting global attention as the flexible, economic alternative to conventional completions in a wide range of applications, including natural drive and artificial lift, shallow water and deepwater, satellite and template.

As the name implies, the horizontal subsea tree diverts the flow of wellbore fluids horizontally through the tree body. Unlike conventional subsea trees, the system provides well control and vertical access through a standard marine riser and subsea BOP. This unique feature eliminates the need for expensive completion and workover risers and allows retrieval of the downhole completion without having to retrieve the tree.

With the successful installation of the first completion in the Joanne Development, Phillips Petroleum Company United Kingdom became the first operator to install an 18-3/-in. horizontal subsea tree from a jackup rig. This revolutionary configuration is helping Phillips realize significant cost savings and operational advantages over conventional subsea completions while enhancing safety, reliability, and service.

The Joanne Development consists of five clustered satellite wells in approximately 70 meters of water in the northern sector of the North Sea. The trees are located on the sea floor within a 45-meter radius and connected to the production manifold using flexible flowline jumpers. Production control pods and chokes are located on the manifold.

When Phillips released the Joanne Development project for bid, they removed the obstacles associated with traditional detailed prescriptive specifications. The functional specification permitted selection of either a conventional or horizontal subsea tree provided the equipment satisfied the operational requirements, including installation from either a semisubmersible or jackup rig.

The diminutive configuration of the horizontal subsea trees enabled integration of an interlocking overtrawlable protective structure (Photo courtesy Phillips Petroleum).

Although the horizontal subsea tree was still considered to be a novel concept at the time, FMC's Subsea Systems design team decided the Joanne project represented the ideal opportunity for the new technology. Phillips provided substantial operational input during the early design phase of the Joanne project. This spirit of cooperation enabled the FMC design team to optimize the equipment design and to verify its safety barrier philosophy.

The key benefits of FMC's horizontal subsea tree configuration, which resulted in its selection by Phillips for the Joanne project, are as follows:

• Significant decrease in blowout exposure due to ability to run or pull completion without ever nippling down the tree and BOP.

• Permits tubing workovers without removing the tree or flowlines.

• Eliminates the need for complex dual riser systems and associated tools.

• Streamlines the workover control system and reduces workover umbilical size.

• Reduces total number of valves required; valve size is independent of tubing string size.

• Minimizes overall tree height allowing integral overtrawlable structures.

• Concentric design permits installation over a wider weather envelope.

• Greatly simplifies tooling package and installation procedures (reduces trouble shooting downtime).

Phillips preferred that the final design of its 18 3/4-in. tree permit installation and workover from a semisubmersible or a jackup rig. The diminutive size of the horizontal tree became a real advantage in this situation. The stack-up height of a horizontal tree is determined by the internal arrangement required to permit installation of the concentric tubing hanger, wireline plug and internal tree cap.

To minimize the impact of the wireline plug on the overall height of the tree, FMC worked closely with the Halliburton organization. These efforts combined with the packaging work carried out on the tubing hanger and internal tree cap resulted in a tree stack-up height from the wellhead of just 62 in.

The low profile of the FMC horizontal subsea tree enabled integration of an interlocking overtrawlable protective structure with no complex riser package (EDP, LRP, etc.) required. The entire assembly was able to be installed from a jackup rig.

Production and annulus master valves integral to the tree valve block were selected to reduce the number of leak paths and to enhance the safety and reliability of the completion. Metal-to-metal seals were used as the primary seals on the tubing hanger, wireline plug, and internal tree cap. Each seal was extensively analyzed using finite element analysis to confirm the geometry prior to building and conducting gas, hydraulic and thermal cycle testing on each seal type.

Life cycle testing was conducted by FMC on various proprietary HNBR compounds to identify the optimum compound for potential exposure to product and control fluids and to verify seal design at operating conditions.

Use of a mechanically operated running tool for the concentric tubing hanger eliminated the costly umbilical and reel associated with hydraulically operated systems. A further advantage of the design was the slimline nature of the tubing hanger which allowed passage through the 13-5/8-inch surface BOP and 13-3/8-inch riser system used on shallow water jackup rigs. This feature was of keen interest to Phillips since they are active both in the northern (deep water) and southern (shallow water) sectors of the North Sea.

A design feature aimed at further standardization was increasing the pressure rating of the tubing hanger, wireline plug, and internal tree cap from the 7,500 psi required by Phillips to 10,000 psi anticipated by Phillips and other operators future requirements.

Verification of the design benefits of the horizontal subsea tree and confirmation of significantly reduced operating costs came with the successful installation of the first tree by Phillips in the third quarter of 1994. Additional advantages confirmed during installation included major reductions in the footprint area required on the rig, testing and preparation time, and complexity of the equipment being installed, The ability to retrieve the completion tubing through the tree proved to be a considerable advantage when remedying downhole problems.

Although horizontal subsea tree technology is gaining acceptance for both natural drive and artificial lift applications, the advantages of the horizontal tree can easily be lost without careful design and functional requirement reviews. Because the horizontal tree is relatively new in design and fundamentally different in approach, operators and suppliers must work together in establishing standards commensurate with emerging technology.

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