Aker Solutions Develops ROV Tree Cap to Improve Offshore Technology

Aker Solutions has developed a new ROV installable/retrievable Internal Tree Cap for use in the offshore technology industry.

As the challenges of the offshore development are growing more acute, technologies are keeping up the pace with renewed innovations. Aker Solutions is constantly striving to develop new technology and finding solutions to counter the obstacles set by large field developments, by deepwater activities by high pressure high temperature reserves, by long step outs and increased oil recovery demands. It is always challenging to design a product or system that fulfills the industry requirements while keeping in focus all the constraints. Aker Solutions continues to introduce "best in class" technology and seeking good ideas from concept to hardware and processes.


The following list describes some of the advantageous objectives of this innovation.

  • The device should not present a "snag hazard," and should aid in damage control from dropped objects.
  • This device is an environmental pressure barrier and is fitted, within and locked to, the Subsea Tree Spool Body.
  • This device has a service water depth rating of 10,000 FSW and a working pressure of 10KSIG.
  • The device is delivered ready for service, minimizing rig activities, reducing the possibility of safety incidents, and is manipulated Subsea by the ROV.
  • The device, when submerged, has relatively neutral buoyancy, thus minimizing the tasks that the ROV has to perform in its installation, operation and subsequent retrieval.


The design describes herein is for an ROV (Remotely Operated Vehicle) Installable/Retrievable, pressure containing Tree Cap for use on concentric bore "Conventional" or "Vertical" Subsea Tree. A wellhead assembly, such as that employed on the seabed for offshore drilling and production operations use "Conventional" or "Vertical" Subsea Tree to access the well bore. The Tree Cap works as a barrier and keeps the subsea environment clean in case of any leaks above the Annulus and Production swab valves. Tree Cap includes a mechanism that indicates that metal seals have been set, within the seal bore of Subsea Tree, and the design also permits testing of the metal seals after installation. The Vertical Subsea Tree typically has Annulus and Production Bores that are isolated with the insertion of a Tree Cap. The Tree Cap is rated for 10,000 ft water depth, with a working pressure of 10,000 psi and an operating system pressure of 3,000 psi.

The Tree Cap can be installed, tested and recovered by an ROV. Upright orientation and submerged weight reduction of the Tree Cap is accomplished via Syntactic foam. ROV holds the Tree Cap via a foldable handle for transport; the handle profile folds down to decrease the probability of snagging.

Prior Tree Caps generally require specific radial alignment of the Tree Cap in order to be inserted and secured in the Subsea Tree. The alignment of the Tree Cap to a particular radial alignment is difficult when installing a Tree Cap with an ROV. This particular Tree Cap is designed in such a way that an ROV can install it into a Subsea Tree at any rotational orientation. The Figure-1 shows the cross-section of the Tree Cap showing the Annulus and Production Bores, also the Tree Cap being run, landed and locked into a Subsea Tree. A Centralizer is used to align the Tree Cap with the Production Bore.

Tree Cap

Hydraulic pressure is used to actuate the primary and secondary locking mechanism to lock the Tree Cap into the Subsea Tree and to set the metal seals. It is very important to prevent metal seals from damage during installation, for this reason the Tree Cap lands and locks first into the Subsea Tree before setting the metal seals. The Tree Cap initially locks into the Subsea Tree by a primary lock mechanism. Hydraulic pressure is applied into the cavity to move the piston downward, and the downward movement of the piston moves the upper set of locking dogs outward into the locking profile of the Subsea Tree, and selectively locking the Tree Cap in the Subsea Tree. Setting and securing of metal seals is done by a secondary lock mechanism. A cam, rod and spring method is adapted to set the secondary locking mechanism. Pressure continues to force the seal carrier to move downward which allows the cam and rod to move downward which permits the lower sets of dogs to engagement, thus securing the seal carrier with both the metal seals into the designated position. Metal seals isolate the Annulus and the Production Bores of the Subsea Tree. The seal carrier also includes one elastomeric seal adjacent to each of the metal seals.

Fluid can become trapped between the cavity of the Tree Cap and Subsea Tree, which prevents the Tree Cap from properly landing onto the Subsea Tree. This is a problem for the light weight Tree Cap. The design includes a flow path and a valve which permits the removal of water trapped between Subsea Tree and Tree Cap. The Tree Cap allows corrosion inhibitor to be injected into the cavity between Subsea Tree and Tree Cap and into the Bores above the Annulus and Production valves. The ROV releases and folds down the running and retrieval handle, and remove the hot stab. The Tree Cap design has a flow path to test the upper and lower metal seals. Tree Cap vent pressure contained in either bore through vent line prior to its recovery. The upper and lower metal seals used to isolate the Annulus and Production Bores in the Tree Cap are tested and qualified to a working pressure of 15,000 psi and temperature 0 degrees F to 250 degrees F.

It is very important to provide a solution that secures health, safety and environment and should be tested against the principles of protecting the environment and preventing loss to people and assets. Tree Cap provides a complete metal barrier that is reliable, durable and resistant to the most extreme conditions and ensures that completion of offshore operations can be performed as safely as possible.