BOP CONTROL SYSTEMS
Table of contents
BOP CONTROL SYSTEMS 1
Table of contents ............................................................................................
.........1 INTRODUCTION.....................................................................................................2 MULTIPLEX BOP CONTROL SYSTEMS ...........................................................4 POWER FLUID SUPPLY...................................................................................5 SHEAR RAM POWER PACKAGE ...................................................................6 ACOUSTIC BACKUP CONTROL SYSTEMS ................................................7 FAST RESPONSE HYDRAULIC CONTROL SYSTEMS .............................8 RECOMMENDATIONS FOR DEEP WATER APPLICATIONS .................... 10 Configuring the BOP Control System for Deep Water ............................... 10 Emergency Disconnect System for Deep Water Applications .................. 11 Preliminary Disconnect Sequence................................................................. 12 Final Disconnect Sequence ............................................................................ 13
Subsea blowout preventer (BOP) stacks used in deep water drilling are hydraulically actuated as are BOP stacks used in conventional shallow water operations (Figure 1). The primary element of the hydraulic control system is the subsea control pod mounted on the BOP stack. The control pod contains hydraulic control valves which on command from the surface direct the flow of hydraulic power fluid to and from the blowout preventers, hydraulic connectors and valves, etc. of the BOP (Figure 2). The control valves are two-position, three-way valves or three-position, four -way valves actuated by hydraulic pilot pressure. In conventional, shallow water hydraulic control systems, pilot pressure to actuate the pod control valves is supplied directly from the surface through individual pilot hoses contained in a flexible hose umbilical (Figure 3). In most deep water control systems, pilot command signals are transmitted electrically through a multi-conductor cable (Figure 3) to the subsea pod in order to achieve quicker response times. These deep water control systems are termed electro-hydraulic or E/H control systems. In both hydraulic and E/H systems, hydraulic power fluid to actuate BOP stack components is supplied to the pod from a surface pump/actuator unit either through a control hose umbilical, a separate hose, or a rigid conduit (usually a corrosion resistant alloy) integral with the riser joints. Generally, subsea control components are duplicated to provide redundancy. Rapid response is of particular importance when drilling in deep water. Rapid response is important because of the possibility of a DP system drive-off necessitating an emergency disconnect from the BOP stack. The response time constitutes the two main elements; signal time and hydraulic execution (flow) time. According to NPD, the response time for closing of blowout preventers, when located at seabed, will be up to 45 seconds. API RP 16E, section 16E.3.1, states the control system for a subsea BOP stack should be capable of closing of each ram BOP in 45 seconds or less. Closing response time should not exceed 60 seconds for annular BOPs. Operating time response time for choke and kill valves (either open or close) should not exceed the minimum observed ram close response time. Time to unlatch the LMRP should not exceed 45 seconds. The above should be regarded as a minimum requirement. Stricter requirements with regards to unlatching the LMRP may apply if a possible drive-off situation of a drilling unit demands this. Electro-hydraulic control systems were developed to provide rapid signal time. In 3000 ft water depth, an E/H control system will function substantially faster than a conventional hydraulic system. Pilot signal transmission time with an E/H system (3000 ft depth) is a fraction of a second whereas hydraulic pilot signal time through 3000 ft of pilot hose
can be as much as 30 seconds and more depending on the type of system and the type of pilot hose. A hydraulic system called quick response system is able to transfer a pilot signal through a pilot hose in about 4.5 seconds in 3000 ft water depth. This system is briefly described later. The basic difference between E/H and hydraulic control systems is pilot signal transmission. The E/H system achieves its rapid response by transmitting an electrical pilot command signal to a solenoid operated pilot valve in the subsea pod which in return directs the pilot pressure to the designated hydraulic control valve. Early electro-hydraulic BOP control systems utilised DC circuits to actuate each solenoid pilot valve in the control pod. This required a large number of conductors resulting in a large, stiff control cable (Figure 3). Considerable difficulty with system reliability was experienced with the early DC circuit E/H control systems due, in part, to problems with the large, multi-conductor control cable and its terminal fittings. The E/H cable terminal fittings were subject to progressive sea water flooding which resulted in short circuiting the command functions. Additionally, the early E/H circuitry did not have sufficient safeguards against spurious commands. In one instance, a BOP stack was dropped to the sea floor when sea water created a short circuit across the face of a cable connector causing the upper hydraulic connector to open. These problems with the early E/H control systems prompted the development of the multiplex E/H control systems which are now in use on most specially designed deep water DP drilling vessels. Hydraulic execution time is in the time it takes for the hydraulic fluid to move the hydraulic actuator from the one extreme position to the other, for example a set of rams from fully open to fully closed position. This time depends mainly on operating pressure, line size, line length, valve and fittings restriction factors and hydraulic fluid viscosity. Large size lines, valves and fittings are the main parameters that can be varied to improve on the hydraulic execution time. Execution times of about 15 seconds for 18?” ram BOPs and 30 seconds for annular BOPs can be obtained with proper selection of hydraulic components. In addition to the dual main control system, back up systems are also often installed. These are normally an acoustic control system as described later, and a ROV operated hot line system. The hot line system allows a hydraulic line to be connected to the desired function on the BOP stack (such as the LMRP) connector, the wellhead connector, etc.) by means of a ROV.
MULTIPLEX BOP CONTROL SYSTEMS
The primary difference between multiplex E/H systems and the earlier individual DC circuit E/H systems is in the mode of signal transmission between the surface control units and the subsea pods. A typical multiplex control systems is illustrated schematically in Figure 4. Utilising electronic transceivers in the surface control unit and the subsea control pods, the multiplex system transmits coded command and data signals similar to FM radio transmission over a small multi-conductor cable (Figure 3). Command signals are received at the control pod, decoded, verified by reciprocal transmission to the surface and then executed within a fraction of a second. In addition to command/data signal transmission, the multiple x control cable transmits television signals and power for solenoid valve actuation. The principal advantages of multiplex over earlier E/H systems are a much smaller, lighter weight control cable and less susceptibility to spurious command signals that could be generated by short circuits or other component failures since coded signals are used. A typical deep water multiplex BOP stack with TV cameras can be controlled through a 16 conductor, 1.4-inch OD. armoured cable. Earlier standard E/H control systems required a 2.5 in OD. cable with in excess of 100 conductors to control a similar BOP stack. The incident mentioned earlier of a BOP stack being dropped because of a short circuit would not have occurred with a multiplex control system. Multiplex E/H BOP control stack systems currently in use on DP vessels are those manufactured by Shaffer (formerly Koomey), Cooper Oil Tools (formerly Cameron Iron Works) and Tri-Tech Systems. All of the systems have performed quite successfully.
POWER FLUID SUPPLY
As mentioned earlier, hydraulic power fluid to actuate BOP stack components is supplied either through a control hose umbilical (hydraulic system), a separate power fluid hose or through a rigid conduit that is integral with the riser joints similar to choke/kill lines. The rigid conduit can supply high pressure power fluid with less lag time than hoses because it does not exhibit the “ballooning” effect that occurs with pressure surges in hoses. Some of the early individual circuit E/H systems incorporated a 0.5-inch hydraulic supply hose in the electrical cable (Figure 3). This design was abandoned when it was determined that ballooning of the hydraulic hose due to pressure surges was contributing to cable flooding problems. To minimise actuation time for BOP stack components, most deep water BOP stacks have been equipped with accumulators mounted directly on the BOP stacks. Subsea accumulators would provide an immediate source of high pressure power fluid which could actuate BOP stack components faster than surface accumulators alone supplying power fluid through several thousand feet of hose or conduit. However, by introducing an acoustic control system according to the NPD rules, a substantial accumulator bank will be required just for this. If one wants to stay within a normal BOP stack envelope size, it will be difficult or impossible to fit enough accumulators to cater for both needs, especially for a large size BOP (18 ? ” – 10 or 15000 psi wp). In addition subsea accumulators becomes less efficient with increasing water depth (see discussions in section 5). It should therefore be considered to use large size conduit lines (2-2?” ID) in combination with a few surge accumulators only.
SHEAR RAM POWER PACKAGE
According to NPD, the shear rams shall be capable of shearing the drillpipe in use after a certain number of BOP operations have been performed and with the hydraulic pumps not running. As shearing may require hydraulic operating pressures close to the maximum hydraulic accumulator pressure a vailable, one of the following arrangements will be required: A very large accumulator bank (at surface) Higher accumulator system pressure (5000 psi system) Dedicated accumulator bank for the shear rams (subsea)
The first option is normally unwanted because of large space requirements and high cost, the second option may be unwanted because of high cost and that such a system is not normal standard. In case of system conversions, high cost and additional space requirement will be an important consideration. The third option may therefore be a practical solution, and especially the solution described below that requires only a relatively small accumulator bank which is more important for a BOP stack for deep water as discussed in the next chapter. Hydril Company has designed a Shear Ram Power Package expressly to facilitate simple and economical compliance with the NPD’s regulations for shear ram operation (Figure 6). This power package provides dedicated power fluid just when it is needed the most – when shearing pipe. The emergency hydraulic supply is triggered to action by contact of the shear rams when closing on pipe in the bore. The system senses shear ram closing pressure at the BOP. The reserve accumulators feed power fluid to the shear rams to sever the pipe and seal the bore.
ACOUSTIC BACKUP CONTROL SYSTEMS
Most deep water drilling vessels are now equipped with an acoustic control system to serve as an emergency backup in case the primary control system becomes inoperative because of cable failure or hydraulic supply line failure, etc. A simplified schematic of an acoustic backup system is shown in Figure 5. These systems function independently of the primary control system and are usually configured to operate three or four critical BOP stack functions such as shear rams close, pipe rams close, ram locks close, and lower riser connector open (including possible kill and choke line connectors and disconnect assist (lift-off) system). NPD requires the use of an acoustic back-up system or a n alternative control system when drilling on the Norwegian Continental Shelf. This includes the Voering Plateau. This system shall as a minimum be capable of operating the pipe ram preventers, shear ram preventer and the connection for the marine riser. The dedicated accumulators for this system shall have sufficient capacity for closing of two (2) pipe ram preventers and one (1) shear ram preventer, as well as opening of the riser connection plus 50%, The accumulators shall have sufficient pressure for cutting of the drill string, after having closed a pipe ram preventer. In addition, the pressure shall be sufficient to carry out disconnection of the riser package (LMRP) after cutting of the drill string has been completed. The acoustic control system operates basically like a multiplex E/H system except that coded command signals are transmitted acoustically through the water rather than through an electrical cable. The surface control unit transmits coded acoustic signals from a transponder beneath the vessel hull or from a portable unit that can be operated from a work-boat. The signals are received by a hydrophone and processed by electronics in an acoustic “mini-pod” on the BOP stack. The acoustic control pad contains solenoid pilot valves and pilot operated control valves for directing power fluid to the designated BOP stack functions. Electric power for the acoustic pod is supplied by a battery pack. Hydraulic power fluid if supplied by a dedicated bank of accumulators that is charged through a check valve by the primary control system supply line. Acoustic control systems can operate in the water depths at the Voring Plateau. Compared with systems for the shallower water, a more narrow acoustic beam angle may be used to achieve sufficient acoustic signal strength and thereby reduce power requirements and the size of the battery package. Also, the system container will be of a heavier design to withstand the increased external pressure. Acoustic control systems have a longer response time than the main hydraulic or E/H systems and is therefore not suited for emergency disconnect situations.
Subsea accumulators are less efficient with increasing water depth. The reason is because of the higher nitrogen precharge pressure that is required (surface accumulator precharge pressure plus hydrostatic pressure of the sea). This causes less expansion of the gas (smaller relative difference between maximum and minimum gas pressure). For that reason operations in 2000m water depth will require about 30%-50% more accumulator volume than in the more normal water depths. The accumulator bank required for the acoustic system on a deep water BOP stack will therefore occupy most of the available space inside a normal BOP stack envelope. Increasing this envelope size may have significant impact on the handling and storage systems on board the drilling unit. It should be noted that the maximum absolute pressure in subsea accumulators is the sum of the surface accumulator pressure plus the hydrostatic pressure between the surface and the subsea accumulators. This pressure (about 5900 psi in 2000m water depth) would be trapped in the accumulators when pulling the BOP stack to surface. Therefore either a system (or procedure) for bleeding down the pressure must be provided or the pressure rating of the system must be sufficient. Acoustic back-up systems are manufactured by Shaffer (formerly Koomey), Cooper Oil Tools (formerly Cameron Iron Works), Tri-Tech Systems and Simrad.
FAST RESPONSE HYDRAULIC CONTROL SYSTEMS
Hydril Company and Shaffer has developed hydraulic fast response systems that extend the operational water depth for straight hydraulic systems considerably. The fast response systems offers the simplicity and reliability of hydraulics with an operating response comparable to multiplex at water depths up to 5000 feet. As an example, a standard hydraulic system has a signal time of 15 seconds at 3000’ water depth; a fast response system has a signal response time of only 4.5 seconds at 3000’. The difference in response time widens as depth increases. A fast response hydraulic control system is shown in Figure 7. The increase in operating speed is accomplished by keeping the signal hoses at about 1000 psig minimum pressure at all times. First, this eliminates most of the hose expansion due to pressure. Second, signal pressures are varied from 1000 psig to 2000 psig, rather than from 0 psig to 3000 psig. A complete fast response system includes a special surface manifold and a subsea fast response manifold, together with the remainder of a conventional hydraulic subsea system.
Such a system can be normally retrofitted to an existing hydraulic subsea system with a minimum of complications. The main consideration when selecting between a multiplex and a hydraulic fast response system will normally be the time required for an emergency disconnect. The potential of getting additional 4-10 seconds available in an emergency disconnect situation may be a determining factor, especially for a dynamically positioned drilling unit. This has to be evaluated for the individual unit based on its marine characteristics (drift-off and drive -off characteristics for a dynamic positioned unit and transient movements for an anchored unit).
RECOMMENDATIONS FOR DEEP WATER APPLICATIONS
Configuring the BOP Control System for Deep Water
When designing new or retrofitting existing drilling units for deep water operations, the following is recommended to consider: Select and install a multiplex control system or a quick response hydraulic control system to obtain sufficient short signal time. This should be based on the available time for emergency disconnect and on the NPD/API requirements. Select and install large size lines (including conduit lines along the marine riser), valves and fittings for the critical hydraulic control functions to obtain sufficient short hydraulic execution time. This should be evaluated in connection with the signal time and based on the available time for emergency disconnect and on the NPD/API requirements. Install an emergency disconnect sequence system as discussed below. Install an acoustic control system that is designed for the water depth and the BOP stack in question and that has a minimum has the functions required by NPD. Install sufficient number of hydraulic accumulators for the acoustic control system. Install a dedicated accumulator package for the shear rams. This system may be actuated by both the main and the acoustic control system. Install hydraulic “hot line” connectors for ROV operations. The function to consider ROV operated are: LMRP disconnect, shear rams close, pipe rams close, wellhead connector open, and wellhead connector gasket release.
Emergency Disconnect System for Deep Water Applications
The follo wing should be considered when planning an emergency disconnect system: Install an emergency disconnect activated mini-panel. This panel should be installed where the Driller can reach it without leaving the brake. The panel should contain three buttons to activate a pre-programmed disconnect sequence: Preliminary Disconnect Final Disconnect Push and Hold to Operate Configure the Riser Connector Unlatch buttons on both Driller’s and Toolpusher’s panels to invoke the final disconnect sequence if Riser Connector Unlatch is depressed while the stack is on the wellhead.
Preliminary Disconnect Sequence
The preliminary disconnect sequence would be initiated on a “yellow warning”, with the drill pipe in hang -off position. The sequence may be as follows : (T=time in seconds from T=0 to the time the described function or set of functions is initiated). Close pipe rams for hang off (#2 from T=0 1 bottom) T= T= T= T= T= T= T= T= T= 2 3 4 5 6 7 8 9 10 Block pipe rams #3 and 4 Block acoustic charge Block extension system for hydrophones (if fitted) Block wellhead connector Block wellhead connector gasket release (if fitted) Block all failsafes Close all failsafes Disconnect and retract kill and choke line connectors (if fitted)
Final Disconnect Sequence
The final disconnect sequence would supersede all previous commands. The final disconnect sequence would be initiated on a “red warning”. This sequence may be as follows: T=0 T= T= T= T= T= T= T= 1 2 3 4 5 6 7 8 Close pipe rams for hang off (#2 from bottom) Close all failsafes Close shear rams Block shear rams Block all failsafes and pipe rams