The landing string is composed of joints of oilfield tubulars (either casing or tubing) which are terminated with premium connections. The purpose of the landing string is the containment of pressurized well fluids during their conveyance between the subsea well and the surface tree at the drill floor. Pup joints of various lengths of the same pipe are to be provided to allow for proper space-out of components and to act as saver subs or crossover subs.
The surface tree provides a barrier at the top of the upper completion string capable of maintaining pressure control of the well during:
- Tubing hanger installation
- Tree cap installation
- Well clean-up and flowback
- Workover operations
The surface tree contains:
- A number of hydraulically-operated valves to isolate the upper completion string
- An interface for providing string top tension via the rig draw works (either with or without a coiled tubing lift frame in place)
- An industry accepted standard interface for the surface wireline or coiled tubing equipment
- Connections to flexible flow lines
The surface tree interfaces with the landing string at the lower end, a wireline BOP/lubricator stack-up or coiled tubing injector head system at the upper end, and flexible flowlines which connect to the side of the surface tree at wing outlets.
The surface tree assembly is to include a swivel on the lower end to allow for make-up of the surface tree to the landing string without rotating the entire surface tree assembly.
A crossover sub between the threadform of the surface tree swivel and the landing string (LS) threadform should be provided.
Crossover subs should be supplied at connections immediately below the surface tree and at a single joint (30 ft to 40 ft) below the surface tree. These two crossover subs should utilize a threadform tolerant of high misalignment, bending, and bearing loads during make-up (such as a tapered thread found in drill pipe and drill collar connections), and should feature redundant seals (either metal to metal or elastomer). These crossovers should have a pin-down design, and provide room for a minimum of two thread re-cuts on the lower LS thread.
The master valve, the production valves, and the kill wing valves should be Fail-Safe-Close (FSC). The Swab valve should be Fail-As-Is. All surface tree valves should be hydraulically actuated.
The structural integrity of the surface tree should be adequate to withstand the combinations of internal and external loading possible under all conditions expected during handling, including both normal and contingency operations. The materials from which the surface tree is manufactured will be suitable for the duty required, and for the fluids with which they may come into contact.