The Expro Group has deployed a hybrid subsea test tree (SSTT) system offshore Angola on behalf of FMC and BP. The unique system allows the dual-bore completions to be flowed back to the rig prior to installing the production trees. While the system deployed off Angola is
in water depths of around 4,000 ft (1,219.5 m), it is qualified for use in up to 10,000 ft (3,048 m).

For live well operations from either an anchored or dynamic positioning drilling rig, a means

Figure 1. A hybrid test tree allows operators to measure well performance before installing a permanent production tree. (All graphics courtesy of The Expro Group)
for isolating the well and disconnecting at the seabed is required. Closing in the well may be done for routine operational requirements or due to a loss of vessel position as a result of bad weather or some other operational difficulty. The disconnect facility allows the rig to move off location and then reconnect and resume operations in a controlled manner. These requirements are met by what are known as subsea test trees (SSTTs).

The typical configuration is shown in Figure 1. Whether for well testing, completions or intervention operations, the basic systems configuration is similar.

The drilling riser is shown connected to the subsea blowout preventer (BOP) at the sea floor. Within this riser system is the tubing that allows flow from the well. The subsea test tree system provides the means to close in the tubing string at the seabed and, if required, disconnect and then reconnect to it. The subsea test tree is spaced within the BOP stack, enabling pipe rams to be closed with the valves below the BOP shear ram.

Predominantly, SSTTs are used on horizontal or “spool” tree systems. These are relatively straightforward as the SSTT is interfacing to a monobore tubing hanger profile.

Filling needs
Increasingly, however, there are applications where the operator requires flow-back or “clean-up” from a conventional or vertical subsea tree completion to the rig. The key drivers for needing an SSTT system for the conventional trees are:
• Well clean-up debris are flowed to temporary well clean-up facilities on the rig (rather than going through the production valves, into the flowlines and the permanent production facilities);
• Well performance can be confirmed prior to the rig pulling the riser and running the permanent subsea production tree;
• The time from well completion to flow-back is typically shorter (from a week to several months if the production tree is delayed for any reason); and
• The wells can be flow-tested prior to delivery of the production trees, flowlines and topsides facilities.

In order to enable these benefits, the SSTT system crucially must interface with a conventional tree dual-bore tubing hanger profile as shown in Figure 2 (instead of a simple concentric monobore system as used for spool trees) and also orient and control the landing out of the completion.

Such an application for hybrid SSTTs was BP’s Block-18 subsea development offshore Angola. Expro engineered, manufactured, tested and delivered the Block-18 SSTT system during 2005 and 2006, and the system is currently in use in the field.

Functions
Functionally, the hybrid SSTT system is required to meet quite an extensive list of requirements. The key functions it must perform are:
• Providing a means for well isolation and disconnection in the event of drilling riser disconnection (rig positioning or other situation);
• Interfacing to the profile of the dual-bore tubing hanger;
• Converting the offset production bore to a concentric monobore system (to allow monobore production riser);
• Being self-orienting using the rig BOP pin (achieved through the orientation helix on the OD of the SSTT);
• Providing sufficient through-bore diameter and pressure rating for through-tubing completion activites (plug setting, etc.);
• Fitting geometrically within the 19–in. drilling riser and within the space-out envelope within the rig BOP stack;
• Being able to cut coiled tubing and wireline as required;
• Providing hydraulic, electrical and optical conduits through the tool
to control the tubing hanger running tool and completion equipment (surface controlled subsurface safety valves, gauges, inflow control valves, etc.);
• Being a robust service tool compatible with use in remote locations.

Translating these functional requirements into an engineered SSTT system required quite
Figure 2. Shown from the lower profile, the SSTT must line up with a conventional tree’s dual bore tubing hanger.
extensive design effort. Fortunately, the service company had prior experience with dual-bore SSTT systems, first pioneered in the mid-1990s for BP on its Foinaven and Schiehallion fields, and was able to leverage this experience.

The hybrid SSTT system is made up of several modules, these being the upper valve section with latch, the integral slick joint and the lower valve section, complete with external orientation helix and adapter to the tubing hanger running tool. The hybrid SSTT design is shown in Figure 3.

The upper SSTT module is an Expro ELSA ball valve, proven in the field through hundreds of deployments over the last 6 years. ELSA (meaning Enhanced Landing String Assembly) valves were developed in 2000 to handle the exceptionally harsh flow-back conditions prevailing in Statoil’s Åsgard field offshore Norway. The ELSA valves are designed to handle flows with high solids content such as the types of solids carried in fracturing fluids. In more benign well conditions, the ELSA valves offer superior performance and minimal maintenance between operations.

Design challenge
The design challenge with the upper section was fitting the multitude of through-portings in the housing (for hydraulic, electrical and even optical conduits) as required for the Block-18 completions. This was accomplished through the use of 3-D design packages and very high-quality machining processes to deliver the finished components.

The lower SSTT valve section contains a second ball valve, an offset bore transition and the external orientation helix. This section of the SSTT was the most technically challenging to engineer and deliver, as accommodation of the external helix required the inner valve section
Figure 3. Several modules make up the SSTT valve, allowing it to perform in harsh flow conditions.
be “slimmer” than for standard applications. As for the upper section, the housing also has to accommodate the hydraulic, electrical and optical through-porting for the tubing hanger running tool and completion equipment in the well. The external profile is shown in Figure 3.

The helix system is configurable for a range of different rig BOP systems and allows the entire completion to be correctly oriented with respect to the wellhead (and ultimately to the production tree which will be installed on top of the tubing hanger). Within the body of the lower section is the bore transition, converting the dual-bore tubing hanger profile back to a concentric monobore system.

Fluid access to the completion annulus is via the tree vendor’s side access valve in the tubing hanger. Mechanical access to the dual-bore annulus is not required with this system.

The specification of the system is shown in Table 1. The system is fully compatible with either direct hydraulic or electro-hydraulic controls systems, making it suitable for deepwater operations. The Block-18 application is in water depths around 4,000 ft and is controlled using an electro-hydraulic control system.

The system, which has been in operation for more than a year, has completed seven wells to date with support and maintenance being provided locally out of the service company’s Luanda operations base.