Saturday, November 27, 2010

Insights into Subsea deepwater operation

Floaters in deep water normally use dynamic positioning system to keep the position above the well, and make use of a guideline-less drilling system. Drilling from a floater also means vertical floater movement (heave) due to sea waves. Introduction of a heave compensation system reduces the relative movement between the drill string and the sea floor. This heave system improves the drillbit performance and reduces wear between the drill string and the well. Most of the time the drill string is free to move relative to the well. That may however not be the case during well testing and landing of the BOP, meaning that the system is vulnerable to heave compensation failures.

Drilling can start when the bit located at the bottom of the drill string reaches the seabed or the bottom of the hole. Drilling requires weight on bit (WOB), bit rotation and mud circulated through the drill string and out through nozzles in the bit. The mud controls the bit temperature, provides bottom hole cleaning and transports cuttings to surface through the annulus. The driller and mud logger must have full volum control of the mud (volume going in and volume coming out of the well) for early detection of kick or lost circulation.

Running the subsea BOP and marine riser consist of :-
The upper part of the subsea BOP is called the lower marine riser package (LMRP) and contains (from the top):
Flex joint
Annular preventer
Hydraulic connector
The lower part of the subsea BOP is called the BOP stack and contains (from the top):
Annular preventer
Blind shear ram
Pipe rams
Wellhead connector

The BOP stack also have a more kill and choke line 'outlets' controlled by choke and kill valves.
Choke and kill valves

Typical diagram of a subsea BOP system :-

The LRMP also includes two redundant subsea control pods (one is back-up) with hydraulic pilot valves which are connected to a common power fluid supply at the floater. The hydraulic pilot valves are activated by a signal from the floater (hydraulically or electrically) to open and close and thus open and close the LMRP and BOP valves. The pods are normally retrievable. Subsea accumulators are located on the LMRP and the BOP and applied to reduce the closing time for the valves. The hydraulic oil return from the valves are normally vented to the sea.

The subsea BOP's are also frequently equipped with back-up control systems like:

Acoustic system
Auto disconnect system
Autoshear system
Deadman system
Emergency disconnect system (EDS)
ROV intervention system

The back-up control system is is typically used when the primary control system has failed during an emergency sistuation. Several scenarios, like accidental riser disconnect, means that the hydrostatic wellhead pressure will drop and that formation the fluid may starts to flow from the reservoir. The flow of formation fluids often starts slowly but increases rapidly. Fast response from the back-up system is therefore highly important. It is also important that the back-up system is independent of the primary system. The auto disconnect, autoshear, deadman and emergency disconnect systems are automatically actuated back-up systems. Note that the auto disconnect, autoshear and deadman need to be armed to be operational. Any back-up system should be reliable and not too complex so that the rig crew does not fear using the system.

The LMRP is connected on the top of the subsea BOP at the cellar deck, which is the lower deck on the drilling rig. The LRMP/BOP stack assembly is functional tested and skidded on rail-beams to the well center of the cellar deck where there is a large hole in the deck called the moon pool. The LMRP is connected to the marine riser coming down from the rotary table in the drill floor above. The LMRP/BOP stack assembly and the marine riser are then run through the moon pool, the splash zone, further through the sea, landed and hydraulically latched on the wellhead by the wellhead connector. The BOP is then overpull- and pressure tested.

A floater will always move up and down and sideways due to waves and sea currents. The floater will thus also move up and down and sideways relative to the marine riser, since the marine riser is fixed to the wellhead at the seabed. After being tested, the marine riser is disconnected from the topdrive and from now on kept in tension and heave compensated through a dedicated riser heave and tension system. The marine riser will have an upper assembly which is fixed to the floater, consisting of (from the top):

Flex joint
Male part of the slip joint

This upper assembly will move with the rig and relative to the rest of the riser. The slip joint is a telescopic joint that allows movement up and down while maintaining a hydraulic seal. The flex joint allows sideways movements. There is also a flex joint on the riser bottom just above the BOP. The diverter has an annular preventer that can be used to control returning fluids that have escaped the BOP and entered the riser. E.g., it may be difficult to detect gas escaping the BOP when drilling in deep waters with a high hydrostatic pressure at the BOP, since the expansion of gas is restricted. The gas may however expand in the riser due to reduced pressure. This will be detected by increased mud return at the surface. This gas will be stopped by the annular preventer and diverted to flare. A simple illustration of the floater after the marine riser is in place is given in figure 1.

One of the annular preventers is closed if a kick is detected. The well must be killed by circulating heavy mud into the well. To make this possible there is a kill and choke line from the BOP and up to the platform. These lines are connected outside the riser. Kill mud is circulated into the well through the drillpipe if possible and the return through the choke line with 'outlet' below the annular preventer. If not, kill mud has to be pumped into the well through the kill line. The return is directed to the choke and kill manifold. The gas may be separated in the poorboy degasser or burned without separation. The diverter on the top of the riser may also be used in a kick situation.

Subsea operation1

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