Testing on floating drilling drilling rigs
The basic differences that exist between fixed drilling rigs and floating drilling units are:
- Motion or heave of the drilling rig
- Production casing protection stops at the seabed and remotely controlled
3. BOP equipment is located at the seabed and remotely controlled
To solve these problems, the subsurface test tree (SSTT) was developed. The SSTT is landed in the wellhead and thus is a fixed point below which no rig movement is transmitted. It also provides both well control and unlatching capability in the event of severe weather or dynamic positioning problems. Following emergency closure of the SSTT and disconnection of the hydraulic operator, the blind rams can be closed above the master valve during temporary abandonment.
The SSTT assembly consists of a fluted hanger, slick joint, valve section and latch section. The fluted hanger is designed to land on the wear bushing in the wellhead. Above it is the slick joint spaced out so the lower pipe rams can close around it to seal off the annulus. The valve section contains two fail-safe valves, either a ball and flapper valve, or two ball valves. On top of the SSTT is the hydraulic latch section. This contains both the operating mandrels to open the valves and also the latching mechanism that releases this part of the tree from the valve section, in the event that disconnection is necessary.
Control of the SSTT is achieved by three control lines in an umbilical hose connecting the hydraulic latch section with the surface control manifold. These three hoses provide hydraulic pressure to:
- Hold the master valve open - control line
- Assist the valve in closing - balance line
3, Unlatch the hydraulic operator from the valve and test string
A fourth hose is usually contained in the umbilical for use as a chemical injection line, such as methanol injection, for hydrate inhibition when testing gas wells. The complete hose is contained on a reel unit which automatically controls pay-out or take-up via a self-contained air driver motor. The hose bundle is connected from the reel to the surface hydraulic control manifold. This is a complete, unitised assembly, usually containing a hydraulic reservoir, pressure-controlled regulator, gauges, gas powered pump and relief valve. The unit is designed to maintain a predetermined pressure on the normally closed SSTT safety valves, to hold them open. It also supplies control pressure to activate the release mechanism to unlatch the tree. A separate chemical injection pump would be provided if chemical injection at the SSTT was required.
6.4.1 Deep water testing operations
The use of anchored semisubmersibles and more particularly, dynamically positioned semisubmersibles and drill ships has led to testing operations being performed in deep water. The testing of wells in deep water requires very long hydraulic lines to control subsea test trees. The response time required for valve closure and disconnect, previously adequate in relatively shallow water depths, would have proved far too slow in deeper water. This is because the time required to bleed down these long lines of hydraulic pressure is considerable and would delay the closing and unlatching of the SSTT in an emergency.
The result has been the development of an accessory package, mounted on the SSTT to operate in water depths greater than 1000 ft. The system, when properly installed, is designed to act as a 'quick exhaust valve' by discharging into the riser pipe and not returning to surface. This allows the SSTT safety valves to close in seconds as compared with the time to bleed off thousands of feet of control line in the conventional system. It is also designed to provide immediate high pressure at the SSTT location, allowing the latch to disconnect in seconds.
This system is controlled from the drilling vessel and the package consists of a deep water actuator, surface hydraulic control manifold complete with high pressure accumulators, a reel unit and control lines. The deep water actuator is mounted on the subsea test tree in the BOP at the ocean floor. It contains a bank of accumulates, two servo valves and piping. One servo valve is designed to bleed off pressure from the SSTT.
Safety valves and the second valve are designed to release high pressure from the accumulator to unlatch the tree. The accumulators are continually charged from surface through a high pressure line.
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