Subsea trees

Typically, a Subsea Field Development or Subsea Satellite Development would consist of a cluster of special subsea trees positioned on the seabed with produced fluids piped to the host facility. Water injection, as well as lift gas, can be provided from the host facility. Control of subsea facilities is maintained from the host facility via control umbilicals and subsea control modules. 

The most basic subsea satellite is a single Subsea Wellhead with Subsea Tree, connected to a production facility by a series of pipelines and umbilicals. A control module, usually situated on the subsea tree, allows the production platform to remotely operate the subsea facility (i.e. valves, chokes). 

As soon as the subsea tree on the first well has been commissioned, production can commence. The rig will then move to another template slot and start drilling the next well. 

If one or more clusters of single wells are required then an Underwater Manifold System can be deployed and used as a subsea focal point to connect each well. The subsea trees sit on the seabed around the main manifold (compared to the template). 

The manifold is typically a tubular steel structure (similar to a template) which is host to a series of remotely operated valves and chokes. It is common for subsea tree control systems to be mounted on the manifold and not on the individual trees. A complex manifold will generally have its own set of dedicated subsea control modules (for controlling manifold valves and monitoring flowline sensors). 

Modern subsea trees, manifolds, (EH), etc., are commonly controlled via a complex Electro-Hydraulic System. Electricity is used to power the control system and to allow for communication or command signalling between surface and subsea. Signals sent back to surface will include, for example, subsea valve status and pressure/ temperature sensor outputs. Hydraulics are used to operate valves on the subsea facilities (e.g. subsea tree and manifold valves). The majority of the subsea valves are operated by hydraulically powered actuator units mounted on the valve bodies. 

Sensors on the tree allow the control module to transmit data such as tubing head pressure, tubing head temperature, annulus pressure and production choke setting. Data from the downhole gauge is also received by the control module. With current subsea systems more and more data is being recorded and transmitted to the host facility. This allows operations staff to continuously monitor the performance of the subsea system. 

Offshore deep-water oil and gas fields are increasingly developed with subsea production systems. By subsea production systems, we understand production (X-mas) trees, templates, manifolds, pumps, separators, and associated equipment that are placed on the seabed. 

Isolation of subsea well Wells and Christmas trees are like suhsea IPLs. Actions like chemical injections, production master valve operation, etc, are issues here. 

The 700ppm initial diluted concentration was increased to 900 ppm after the injectivity test to obtain the design viscosity of 3.5 cP at top of the riser (corresponding to a 2.9 cP viscosity at reservoir corrditions low shear and 50°C) and take into accoimt some degradation ditring the process and the transit through the choke at the subsea Christmas tree level. 

The drillfloor and the derrick (drilling tower), with pipe handling systems and the flare tower on the top, are located in the centre of the platform. In this area, we find the remaining systems, for drilling and mud treatment. Subsea equipment such as valve assemblies (X-trees) and blow-out preventer (BOP) are stored and handled in the moon-pool area below the drill floor. 

The CCF mechanisms or of mobilization above do not present any difficulty for being taken into account by the PN but could not be modeled correctly by the fault tree. They correspond to the problems we encounter when studying subsea HIPS for which the PN modehng is particularly effective. 

It follows that the fault tree is inappropriate for the probability calculations and a dynamic model becomes necessary. As the Markovian approach is veiy quickly limited, behavioral modeling using stochastic Petri nets and the Monte Carlo simulation are used for modeling and calculations of subsea HIPS. 

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