Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering Transport Technology
R.E. Tjepkema Subsea control system for the Quaestor field development. Masters thesis, Report 90.3.2656.OS, Transport Engineering and Logistics.
Continental Netherlands Oil Company obtained an exploration licence for the Q5 block, which is situateds just off the coast in the Dutch Continental Shelf. A military shooting area extends over this block. Mining installations above the surface and mining activities are only permitted within a maximum
predetermined period of 3 months per year and during all weekends. This restriction means that development of a hypothetical marginal oil or gas field, named "Quaestor", is only possible by the use of subsea installations.
For every development plan a trade-off must be made between costs and income. The costs comprise the capital investments and the operational expenditure. The availability of the system to be designed plays an important role in it, as this is the percentage of time a system will properly function during its life time. In the availability rate the downtime caused by restricted access time and intervention duration have to be considered. The development of the Quaestor field with an assumed life time of 15 years could be possible by a two well completion sytem, producing to a platform outside the restricted military shooting area at an offset distance of approximately 8 kilometres. The subsea installation would be a 'wet' type diver assisted installation and intervention, remote controlled subsea completion system. To reduce height and increase protection a so called caisson type X-mas tree is selected, where the master valves are placed sub-mudline. A dome shaped cover is used to protect the equipment above mudline against shipping and fishing activities.
For the type of intervention a Through Flow Line system is selected, to provide frequent maintenance jobs. The valve arrangements and flowline layout will be such that servicing, killing, testing, chemical injection, optional gas lift and normal shut down procedures are possible. The two satellite wells will each individually produce to a platform, where the oil or gas will initially be treated. The probability of producing 5 years without a failure that would shut-in a well is estimated at approximately 72% per well.
The control system for the Quaestor field development will be a direct hydraulic control system with a subsea boost accumulator to improve valve closing response times. An indication of the closing response time of a subsea gate valve will be 1,5 to 2 minutes from signal initiation to complete valve closure.
Conclusions
The conclusions resulting from this study concerning the Quaestor field development and its control system are:
Governmental regulations: the governmental requirements concerning mining activities in the military shooting areas dictate the use of subsea installations placed sub-mudline or above mudline in such a way that fishing and shipping activities are not obstructed. No specific regulations exist on the type of protection or level of completion height, but in the authors opinion a standard completion height of 6 to 8 metres including the protection structure will probably not be acceptable. A to the government technically demonstrable safe completion system with a height above mudline of the protection structure which is kept to a minimum will be sufficient. The amount of governmental rules concerning subsea installations in general is limited to safety and pollution aspects. This is due to the fact that there has been until now only one recently placed subsea gas completion system in the Dutch Continental Shelf (Placid L10 block).
Quaestor field development layout: The final Quaestor field development layout will consist of two satellite wells, each producing to a platform outside the shooting area at an offset distance of approximately 8 kilometres. The field life will be 15 years at an assumed recoverable reserve of 20 to 70 mmbbls of oil, or 130 to 500 bcf of gas. The subsea X-mas tree will be of the insert tree type (master valves placed sub-mudline) with a dome shaped cover as a protection structure. The total height of the completion will be less than 2 metres above mudline. The type of servicing will be a full Through Flow Line system (TFL). The X-mas tree will consist of two 3 inch production tubing strings each provided with a sub-mudline placed production master valve and a 2 inch annulus access provided with a sub-mudline placed bi-directional annulus master valve. One of the production strings is directly connected to the annulus, above the master valves. The two TFL flowlines are each provided with a production wing valve and a cross-over valve is interconnecting the flowlines to provide a TFL circulation path. The two flowlines will be connected to the top of the tree, in which also a provision is made for the connection of an annulus monitoring line. The flowlines can be disconnected by divers when needed. Each tubing string has a tubing retrievable surface controlled sub-surface safety flapper type valve with a nipple to receive a pumpdown retrievable safety valve. To each tubing string a chemical line is connected just above the master valves, each provided with an injection isolation valve. The two lines are commingled into one line in which a check valve is placed and runs to the platform. One flowline can be used for gasilft provisions (in case of an oil well, and an easily providable gas source), setting gas lift valves into the downhole completion.
The probability of one well functioning properly over 5 years time, without a failure that would shut in the well, is estimated to be 72%.
Control system: The control system for the Quaestor field development will be a direct, closed loop hydraulic control system with a boost accumulator to assist valve closing. Each valve is controlled individually. The supply pressure for the subsea tree valve actuators is 3000 psi. The boost pressure is 1500 psi and the supply pressure for actuating the downhole safety valve will be 5000 psi. The control umbilical will consist of at least 8½ inch control hoses to control the tree valves, 2¼ inch control hoses to control the downhole safety valves (which are not boost pressure assisted), a chemical injection line, a boost line, and some additional ½ inch and ¼ inch lines for monitoring or back-up purposes. The expected response time for valve closure (time between
signal initiation and complete valve closure) will be approximately 2 minutes for the 3 inch gate valves. Common practical values of response times are 2 to 5 minutes.
Quaestor field development alternative: depending on the oil or gas conditions, a wireline servicing type completion system is worth evaluating. This system would consist of one 4 inch production tubing string with a downhole master valve and 2 downhole safety valves, of which one would be a "cold" back-up (standby valve) to increase availability. A flowline with a production wing valve will be connected to the production tubing. An annulus line with an annulus wing valve is connected to the 2 inch annulus string. The annulus string is provided with an annulus master valve placed sub-mudline. An additional connection is made between the production string above the master valve and the annulus line after the wing valve by means of a KIT valve (Kill-Inject-Test and Bleed off valve). A vertical access to the tree is provided by diver replacable plugs and wireline retrievable plugs.
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