Subsea Control Pods 1
The subsea control pods contain the equipment that provides the actual fluid transfer from the hose bundle to the subsea stack. A typical pod assembly (Fig 9.14) consists of three sections -
- a retrievable valve block
- an upper female receptacle block permanently attached to the lower marine riser package
- a lower female receptacle permanently attached to the BOP stack
Control fluid enters the pod at the junction box and is routed either direct to an SPM valve or to one of the two regulators (one for the BOP rams and one for the annular preventers) from where it is sent to the appropriate SPM. When a SPM pilot valve is actuated it allows the control fluid to pass through it to one of the exit ports on the lower part of the male stab and into the upper female receptacle attached to the lower marine riser package.
For those functions which are part of the lower marine riser package the fluid is then routed out of the upper female receptacle and directed via a shuttle valve to the functions operating piston. For those functions which are part of the main BOP stack, the fluid is routed through the upper female receptacle and into the lower female receptacle from where it goes via a shuttle valve to the appropriate operating piston.
Not all the functions on the BOP stack are controlled through pod mounted pilot valves. Low volume functions such as ball joint pressure are actuated directly from surface through 1/4" lines. These are generally referred to as straight through functions.
SECTION 9 : SUBSEA BOP CONTROL SYSTEMS & MARINE RISER SYSTEMS
- Figure 9.14 KOOMEY
SECTION 9 : SUBSEA BOP CONTROL SYSTEMS & MARINE RISER SYSTEMS
The integrity of each fluid route between the different sections is achieved by using a compression seal that is installed in the retrievable valve block section of the pod. Compression between the three sections is achieved by hydraulically locking the pod into the lower receptacle (which is spring mounted on the BOP stack in order to facilitate easier engagement).
Locking is accomplished by hydraulically extending two dogs that locate under the bottom of the upper female receptacle. A helical groove on the outside of the lower skirt of the pod ensures correct alignment of the fluid ports. To retrieve the pod independently of the lower marine riser package, the locking pressure is bled off and the dogs are retracted mechanically when an overpull is taken on the retrieving wire.
A more recent design utilises the same concept but consists of a cube shaped retrievable valve block which latches over two tapered blocks mounted on a base plate permanently attached to the lower marine riser package. A single tapered block mounted on a spring base is permanently attached to the BOP stack. The packer seals on the retrievable valve block are pressure balanced in a breakaway condition so that there is no tendency for it to be blown out of the pocket if the pod has to be released under pressure.
Besides the latching system, packer seals and piping, the principal components of the retrievable valve blocks are the SPM pilot valves and regulators.
SPM VALVES
As described above these valves direct the regulated power fluid to the desired side of the preventer, valve or connector operating piston and vent the fluid from the other side of the piston to the sea. The annular preventers typically use large 1 1/2" SPM valves in order to provide sufficient fluid flow, the ram preventers use 1" valves and the other functions such as failsafe valves and connectors use 3/4" valves. Fig 9.15 shows a NL Shaffer 1 in SPM valve.
The valve is a poppet type in which a sliding piston seals at the top and bottom of its travel on nylon seats. In the normally closed position a spring attached to the top of the piston shaft keeps the piston on the bottom seat and prevents the power fluid from passing through the valve to the exit port. Power fluid pressure, which is permanently present, also assists in keeping the valve closed by acting on a small piston area on the spindle. In this position fluid from the valve's associated operating piston is vented through the sliding piston at ambient conditions.
When pilot pressure is applied to the valve the sliding piston moves up and seals against the upper seat which blocks the vent ports and allows regulated power fluid to flow through the bottom section of the valve to function the BOP. Note that the pilot fluid therefore operates in a closed system whilst the hydraulic power or control fluid is an 'open' circuit with all used fluid being vented to the sea.
As illustrated in Fig 9.3 two SPM pilot valves are required to operate a BOP function.
SECTION 9 : SUBSEA BOP CONTROL SYSTEMS & MARINE RISER SYSTEMS
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