Subsea control systems

Hasvold 0, Henriksen H, Mevar E, Citi G, Johansen B0, Kjpnigsen T, Galetti R. Sea-water battery for subsea control systems. J Power Sources 1997 65 253-261. 

In spite of many similarities between land-based counterparts, subsea control systems have additional complexities that need to be addressed. 

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). 

As subsea production systems are remote from the host production facility there must be some type of system in place which allows personnel on the host facility to control and monitor the operation of the unmanned subsea system. 

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. 

The subsea master control station needs to interface directly with topside control systems, which could be from multiple vendors and protocols for communication. 

Various valve commands and interlocks, emergency shutdown (ESD), choke and kill controls and commands, etc. receive and monitor subsea controls and provide HMI for the operator. So, seamless information flow is very important. Following open system architecture it may be possible to integrate the systems. In fact, as long as the two control logics are separate (subsea items normally supplied by specialized OEM), NORSOK requirements are not really violated. 

Subsea PSD system The purpose of the subsea PSD system is to control abnormal operating conditions by stopping hydrocarbon flow by shutting down processes. Two such processes are ... 

Transocean was responsible for operation of the drilling rig and for the safety of operations, and to maintain well control equipment. Transocean purchased, maintained and operated a range of subsea equipment, including the blowout preventer (BOP) and associated control systems. 

Pre-accident test procedures were incapable of revealing these failures, because the design of the BOP control system did not allow independent functional testing of the redundant systems, neither while the BOP was on the rig nor while it was in subsea service. Withont snch independent functional testing, latent failures had caused three of the four channels to fail. (Appendix 2B of the CSB report.)... 

Cai et al. (2013) discuss the use of BNs to assess the maintenance of subsea Blowout Preventer (BOP) control systems. Neil Marquez (2012) discuss the use of hybrid BNs the estimate the availability of renewable systems. Jones et al. (2010) apply BN modelling for maintenance planning in a manufacturing industry, while Melo Sanchez (2008)... 

Cai, B., Liu, Y., Fan, Q., Zhang, Y, Yu, S., Liu, Z., Don, X. 2013, Performance evaluation of subsea BOP control systems using dynamic Bayesian networks with imperfect repair and preventive maintenance . Engineering Applications of Artificial Intelligence, vol. 26, pp. 2661-2672. 

With the electro-hydraulic system the signals, power and hydraulic supplies are sent from a Master Control Station (or MCS) on the host facility down Control Umbilicals (Fig 10.36) to individual Junction Boxes on the seabed or subsea structure. 

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, well control equipment and associated operations present some differences from that seen and used onshore. In some instances onshore equipment can be employed, but the offshore environment generally dictates a modification of equipment and procedures. There are several different well configurations used offshore, often on the same well at different drilling intervals, and each configuration has specific well control procedures that should be followed. A well may be equipped with a surface blowout preventer stack a subsea blowout preventer stack, riser and diverter system a riser and diverter system with no blowout preventer a diverter only or a riserless system with no well control equipment. 

In harsh operational conditions environmental factors can affect the performance of DP which can lead to loss of the position control abiUty. For a Mobile Offshore Drilling Unit (MODU), loss of position without safe disconnection could result in critical damage not only to the well, but also to the subsea equipment. For an Offshore Support Vessel (OSV), loss of position could lead to colhsion with the nearby installation during operation or to damage to the subsea fadhties. In order to increase the safety and reliabiUty of a DP system, it is important to identify and quantify all influence factors and the way that they can affect the DP performance. For example bad weather condition is an effective factor on the position reference sensor or the local position reference system. Such operational conditions can vary from place to place and may have a dynamic nature in a specific location. Moreover, it is important to investigate safe recovery back to position, safe disconnection and reconnection when the loss of position is outside the safe operational range. 

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