A typical landing string configuration for deepwater well testing.
Undertaking production in deepwater and ultra-deepwater provides a number of technical challenges, not least of which is the cost-effective design and provision of control systems for seabed-deployed equipment. The need for built-in reliability is a key feature, more so than in conventional water depths due to the high costs associated with equipment recovery and replacement. Also critical is the need to limit cost increases, compared to conventional equipment. Oil from deepwater prospects carries no intrinsic price premium.
Subsea technology and equipment design is still classed as immature, in the sense that considerable scope remains for step-changes in cost effectiveness. These are accessed through design and re-application of existing technologies, as well as improved project and supply chain management. Contracting strategies employed by asset owners must provide the opportunity to access step-change improvements by permitting innovation.
In order to contrast concepts and highlight the cost reduction potential, this article uses the example of landing string controls during subsea well intervention, such as completion installation.
In many instances, the completion installation program involves the flowing of the well via the landing string. Thus, in deepwater, the string must incorporate a primary well control barrier and an emergency disconnection function.
Operation of these functions must be provided in a swift manner, particularly when operations are being conducted from a dynamically positioned (DP) vessel, as the speed of response will determine the safe watch circle that the vessel must operate within. A smaller watch circle would reduce the weather window for operations and potentially increase project costs due to vessel downtime. Reliable operation of such functions is paramount, since a failure would necessitate a time-consuming and costly recovery to surface, repair or replacement, and re-deployment. The provision of a secondary control system should be considered in order to minimize total project costs.
Finally, the system must incorporate a logic that takes full account of the need to provide emergency and planned shut-in and disconnection functions, at least one contingency control method, event sequencing, and the option for surface examination of data acquired at the mudline during operations.
All this must be incorporated within a control pod that can be deployed within a 19-in. ID marine riser without compromising the integrity of the production conduit to surface. There may also be advantages in providing an interface with the subsea blowout preventer (BOP) system controls, and ensuring the control logic is integrated within a wider control philosophy.
Although not permanently installed packages, landing string controls are nevertheless a good example of the complex nature of control system design where a number of functional requirements must be incorporated, without conflict, while minimizing cost and technical risk.
Direct hydraulic control
A typical monobore landing string configuration for completion installation.
Direct hydraulic control has provided a simple, inexpensive means of landing string control. Simplicity ensures the control system can be highly reliable, and that the cost of manufacture and maintenance is low. High reliability usually negates the need for a full secondary control system.
However, secondary control is often provided to permit safe and efficient recovery to surface for repair and replacement. This usually employs a one-off shut-in and disconnect function via a BOP choke/kill line. In addition, specialist operating personnel, with knowledge of both hydraulics and electronics, are not required.
Direct hydraulics is the preferred method of control in conventional water depths. However, as water depth increases, the size of umbilical reel quickly becomes prohibitive due to the number of hydraulic cores required to control the multitude of functions the landing string can incorporate at the mudline and below.
In addition, speed of response is proportional to umbilical length, and therefore water depth. Once operations are to be carried out from a moored semisubmersible or a DP vessel, the speed of response for the critical emergency shut-in and disconnection sequence must be examined. It is therefore rare that direct hydraulics is considered suitable much beyond 2,000 ft water depths.
Piloted hydraulic control
Typical emergency response times for direct hydraulic, piloted hydraulic, and electro-hydraulic control systems.
The concept of piloted hydraulics utilizes accumulated hydraulic pressure held at the mudline in a pressurized reservoir. This is directed to tool functions by control valves, which are operated by hydraulic signals from surface.
Response times are much improved over direct hydraulics because the volumes of control fluid and the pressure differentials between surface and mudline are much reduced. However, response is still not completely independent of water depth. Nevertheless, such systems have been in use successfully, particularly for DST-type well intervention, for some 10 years, in water depths up to 5,500 ft.
As the system continues to rely on mechanical and hydraulic componentrys, construction and maintenance costs are minimized and multi-skilled operational personnel are not required. Performance repeatability and reliability can be expected to be high and similar secondary control mechanisms employed for direct hydraulics are usually considered sufficient.
Some systems (Expo Group has one) utilize metal-to-metal sealing and multi-port shuttle valves as control valves. These provide repeatable operation and the ability to plumb-in sequencing logic between the various tool functions. For example, the routing of the control signal via the test tree and retainer valve control shuttles prevents unplanned latch disconnection. If these shuttles are not in the closed position, no unlatch control signal can reach the tool.
However, a pilot hydraulic signal is required for each control valve, meaning that a large diameter multi-core umbilical is required. In addition, response times are unlikely to be sufficiently swift for ultra-deepwater operations from DP vessels. This concept therefore has a limited window of operation and may not be applicable to all types of well intervention.
Direct E-H control
Electro-hydraulic control pod for DST operations.
The obvious method of ensuring swift response times is to undertake surface-to-mudline communications with electrical rather than hydraulic signals. This effectively eliminates the depth dependency for response times. The simplest method of such electro-hydraulic control is the direct or simplex control. This concept employs a separate electrical signal path for each control valve. Expro recently took this approach to provide fast-response emergency shut-in and disconnection on DST-type operations.
A multi-core electrical cable provides a number of signal paths, with the cable armor acting as a common earth/return. This cable is incorporated within a multi-core umbilical to simplify rig-up and running procedures. In this system, each signal acts on a solenoid which controls the delivery of signal pressure to a shuttle control valve, which in turn delivers the control signal to the tool function.
The system now can incorporate several control stations or emergency initiation points on the rig, through the use of electrical communications. This can be PC-based if necessary, but simple push-button functions, complete with tool condition displays, provide sufficient options while minimizing costs.
The use of direct electrical signaling eliminates the need for electronics within the mudline control pod, which in turn minimizes costs and concerns over component reliability. The use of an intermediary solenoid enables the fluid flowing through the solenoids to be kept separate from that which functions the tools. This minimizes the risk of loss of fluid cleanliness, affecting the solenoid performance and reliability. The continued use of shuttle valve control via hydraulics has enabled piloted hydraulics as a secondary control mechanism to be retained.
These features may not be necessary or desirable in specific instances and this demonstrates the need to understand particular project drivers and conditions before recomm- ending a particular control concept and design. Taking this approach to completion operations may provide sufficient response times, but will require a very large umbilical reel for ultra-deepwater.
Finally, it should be noted that this system, in isolation, provides no opportunity for data feedback to surface. Although this replicates the situation with direct and piloted hydraulics, it may be considered that information, such as temperature and pressure and tool function confirmation, become more desirable during operations in deepwater. This will be particularly true when hydrate management is a major factor. Inte-grated control and data acquisition, or an additional data acquisition pod utilizing multiplex signaling technology should then be considered as options.
Multiplex E-Hydraulic controls
The technology to permit addressed signals to be sent to a number of control valves via a single electrical communication cable, multiplexing, has been established for some time. This clearly offers a simplification in umbilical design by reducing the number of electrical cables, with the attendant savings in weight, deck space and cost. However, in addition, the introduction of electronics at the mudline, principally to direct the control signals to the correct functions, adds a number of extra opportunities for sophistication:
- Data can now be fed in two directions providing the opportunity for an integrated control and data acquisition system. The data collected can confirm the effective operation of all components in the system including the accumulator, control valves, and the landing string tooling.
- A duplicate secondary control system can be added at little extra cost and the two systems can run diagnostic programs to check for faults and failures throughout the system. Automatic de-selection of a failing or failed system maximizes the uptime and effectiveness of the total system with minimal operator intervention.
- The integration of the landing string control system with host platform, or rig systems such as the subsea BOP, can be achieved relatively simply, now that electronic control has been introduced.
- The introduction of PC-based surface control facilities can assist in the logging and analysis of recorded data.
This sophistication and extra functionality does come at a price. The electronic componentary adds an extra cost and the complexity requires operators to have a broad range of skills covering hydraulics and electronics. Despite the relatively easy introduction of a secondary system, a question can be raised with respect to system reliability, now that finite life componentary is employed.
Flexibility of control logic and system configuration has also been reduced, although surface software and mudline firmware can be reconfigured if necessary.
On the whole multiplexing offers the opportunity to incorporate much more functionality into a system, and where this is considered necessary, the multiplexing concept clearly has an application.
Schematic of a multiplex control system layout.
In order to maximize the benefits of employing a multiplex control system while minimizing the costs and technical risks, Expro and others have investigated the option of using a distributed electronics architecture. Such architecture has already been developed for use as data acquisition, and potentially control systems, as part of intelligent well systems.
In essence, the majority of the mudline or downhole firmware is physically sited with each sensor or control valve. Thus, the failure of any one of these components does not immediately result in total system failure. In addition, electronics hardware and firmware designed and qualified for downhole applications can be transferred to the mudline environment.
In addition, the ease of customizing a system to specific project requirements, such as the selective incorporation of control of downhole and/or tubing hanger running tool functions, and downhole and mudline data acquisition sensors can be achieved relatively easily. This can be undertaken on a modular basis as required.
An Expro Group system, for example, uses a communications architecture that allows data transfer frequencies to be set and modified for both control and data modes during operation, so that critical data, such as build up pressures or control fluid flow rates and volumes, can be collected in an effective manner during a variety of different operational steps.
This approach should provide access to the advantages of multiplex systems without the need for costly development of firmware in order to meet specific requirements.