Design considerations for floating facilities for the future

The Tri-phase Rotating Separator Turbine provides for removal of water at the first stage of separation.

The Tri-phase Rotating Separator Turbine is an example of such a new technology, which has been tested through a pilot project. The Tri-phase Rotating Separator Turbine is a rotating separator able to separate well stream except from crude vapor pressure.

The unit consists of a separator rotating at high speed (up to 7,200 rpm), which separates solids, gas, crude, and water in a high G-field. The fluid enters a rotating separator through a number of multiphase nozzles, mixing the inlet fluid in a homogeneous flow pattern. The impulse by the liquid entering the separator gives the rotational speed, but for a low-pressure drop unit, shaft power may be added to maintain the required rotating speed.

The unit is able to separate solids in a dedicated outlet. Water and oil is separated and discharged in separate outlet flow paths and nozzles. Gas flow in the middle of the separator and small particles of dispersed liquid are separated to the liquid phase due to high G-forces.

Advantages of compact process technology are:

• Compact equipment, which is significantly lighter and reduces area requirements

• A significantly lower hydrocarbon inventory in compact equipment, making for an improved safety design

• Compact equipment allows for a modular configuration, which provides added flexibility for the designers, reduces cost and increases maintenance flexibility

• Standard modules can be developed andexchanged offshore, allowing maintenance to be performed on specialized onshore bases

•A modular design will allow the operator to tailor the processing capacity of equipment to varying requirements throughout a field’s life. Early life of a field will give higher priority to crude capacity and crude treatment, while later life may shift priority to produced water capacity.

E-field technology

E-field technology is one of the biggest mysteries of our decade and there are probably as many definitions as there are people. One common belief in the E&P community is that it represents a major change in data processing and information transfer over significant distances.

The Floating Facility for the Future program has focused on facility design and how to operate and maintain it; hence the important parts of E-field technology for this program are related to remote operation, condition-monitoring, and preventive maintenance.

Remotely operated wellhead and small process platforms exist today, but the Floating Facility for the Future program has focused on complex production facilities.

As the study progressed, it became more evident that to achieve the program goals a step change was necessary in the design of the process area itself. Bearing in mind that the key cost and HSE driver is human attendance, it was decided to introduce a permanent unmanned area (PUA). E-field technology is key in introducing unmanned areas, as the technology will allow for remote operation and conditioning monitoring.

The main advantages of a PUA is that in addition to a significant reduction in the number of offshore personnel required, it can be tailored to suit process and equipment requirements rather than design requirements for human protection and safety. The layout is made more compact and less space consuming. All the hydrocarbon process systems can be placed on one deck level only.

This PUA should be segregated from personnel entrances by a wall protecting from excessive noise, thermal radiation or other potential hazards. The PUA area shall not be entered until the processing has stopped and system is depressurized.

The PUA area will be fully operated from an onshore operation center (OOC) supporting one or more operation with a dedicated team and potential to draw on shared expert teams.

The process control, process performance monitoring, and equipment monitoring are all controlled and supervised from shore with only intermittent visits to the facility. Cameras, detectors, and flexible manipulators/remote assisted tools will be installed in the PUA to provide assistance with simple operations when required. Information required for process control, performance and equipment monitoring are all transmitted to OOC.

Additional instrumentation may be installed to support operating availability if required. To allow maintenance during operation, a remotely assisted tool (RAT) has been introduced, which is a well-known tool in the space and nuclear industries. Access via the RAT reduces requirement for stairs, ladders, and walkways for manual inspection and servicing. Furthermore, the area can be configured for “plug and play” to allow standard modules to be removed for onshore maintenance or for “upgrades” to enhance production. Finally, the acceptance limits for passive fire protection, noise, heat flux, explosion pressure, application of fire water, and wind shielding are all significantly altered or made redundant when designing for the “safety” of the equipment rather than safety of humans.

Design philosophies and principles

To ensure a successful execution of the study, success criteria were developed to guarantee correct focus throughout the program. The main success criteria were to establish a Floating Facility for the Future with a 30% reduction in life cycle costs, and a step change performance in HSE performance. In addition, the facility should increase recovery from existing and marginal reservoirs.

To achieve the success criteria inside the study duration, a set of drivers was determined.

Main HSE drivers:

•Number of people onboard the platform
• Hydrocarbon inventory
• Power requirement
• Layout
Main cost drivers:
• Weight and area requirement
• HSE requirement
• Number of people onboard
• Complexity of facility

With the success criteria, drivers and set of enabling technologies in mind a set of philosophies and design principles for the Floating Facility for the Future were established.

General philosophy

Our vision is that a floating facility can be designed to operate without a permanent offshore staff, even for platforms requiring complex process facilities. The HSE design of the platform will be based on a no permanent manning principle and a corresponding simplified design could be developed. Critical incidents or major breakdown which requires immediate action must be covered by a dedicated maintenance team from an onshore base or from a manned platform in the area.

To reach this goal, the whole facility must be designed for minimum maintenance, which has to be concentrated at bi-yearly or every third year during planned shutdowns at revision stops. The planned maintenance would include heavy rotating equipment like gas turbines, compressors, etc., having main overhauls at such intervals.

Rather than having full size living quarters, workshops and storage areas, the platform should be supported from a multi-purpose vessel fully furnished to provide the necessary assistance.

The facility will apply new compact technology and process concepts, which is simpler to operate, more robust and space saving. An advanced simulation and trend-based control philosophy will be applied, using condition-based monitoring on heavy rotating equipment as well as selected stationary equipment.

Process philosophy

The philosophy for the proposed process design is to reduce the size and hydrocarbon inventories of the components in the process systems. This will result in a reduced area requirement and weight savings for equipment and structural steel. One main objective is to reduce the hydrocarbon content in the process in order to reduce the flaring flow rates, the firewater demand and the extent of fire protection. The following items will contribute to an optimized process:

• Implement compact equipment
• Review requirements for building blocks depending on field life assessment and potential use of infrastructure. Consider flexibility requirements for future potential tie-ins
• Consider flexible manifold solutions to cover for variations of wellhead pressure during field life
• Design the systems for remote operation and an unmanned platform
• Establish robust processing solutions for phase changes, slug handling and changing in well composition

The hydrocarbon content in the process has been substantially reduced by introducing the compact equipment. This will have a significant knock-on impact on the overall HSE figures and will result in a reduced demand for firewater and passive fire protection. The introduction of less hydrocarbon volumes topside will also result in a smaller blow down flow rates compared to a traditional process. By simultaneous introduction of HIPPS systems on the production flowlines, the height of the flare tower and the blow down period can be reduced.

Energy optimization has been included by utilizing the heat that is produced by the compressors, and other utility systems have been reviewed for implementation of the most recent technologies.

Instrumentation, control

With no human presence onboard, all activities must be controlled and monitored from afar, necessitating a variety of automatic condition/performance monitoring systems.

Intelligent instrumentation with self-verifying capabilities will be used. A need may arise to make critical instrumentation dual redundant in order to meet processing availability requirements. To prevent shutdowns while maximizing production, advanced control logic will be implemented.

One of the main challenges related to control for the Floating Facility for the Future is that the compact process with reduced gas and fluid inventories will, because of the reduced time it takes for levels and pressures to change by a given amount as a result of a given flow rate, impose stricter response time requirements on the control and process shutdown systems.

However, through discussion with several suppliers of control systems it is believed that these requirements will not be prohibitive, although they may result in a minor cost increase because of additional control/shutdown system hardware (PLCs/nodes) needed to meet the stricter response time.

Operation, maintenance

Operations and maintenance for the floating facility for the future shall be based on a normally unmanned concept with a remote operated platform from an onshore operations center (OOC).

It is recommended that the OOC support several different installations. Some personnel will be dedicated to one single installation/field, and some personnel will support several installations, organized in “support pools.” This will ensure necessary specific installation competence while also retaining great flexibility. This organizational structure will also give an opportunity to organize emergence response teams as required.

The specific facilities will be operated with no permanent offshore personnel and all maintenance will be organized in campaigns. All operational aspects required for daily operations and basic design requirements as black start-up, emergency and planned shut downs, environmental aspect etc. will be controlled and ensured by the OOC. The onshore operations center will also ensure all HSE-related aspects according to legislative requirements.

The platform will be supported from a multipurpose service vessel, fully furnished to provide the necessary assistance during revision stops, hence only a shelter and a minimum number of workshops and offices are foreseen onboard the facility.

Further, it is assumed that the PUA will be covered by a new set of legislative regulations written particularly for this type of area. It is envisioned that this will have a positive effect on investment cost related to amount of bulk material, number of decks and overall space requirement.

The facility will apply new compact technology and process concepts, which are simpler to operate, more robust and space saving. To avoid unplanned shutdowns, an advanced simulation and trend-based control philosophy will be applied, as well as condition monitoring of heavy rotating equipment and stationary equipment. This will be integrated into the maintenance philosophy.

Layout philosophy

As the Floating Facility for the Future will be permanently/normally unmanned, they will need to be arranged with a different view of locating the various types of equipment than the approach one would take in the layout of a conventionally manned unit.

As the intention for the more distant future is to provide platforms with larger parts of the process being permanently unmanned, equipment location on the platform will be driven by equipment category rather than system functions.

The permanently unmanned areas need to be located at the far end of the platform away from the safe havens, and, for obvious reasons, the areas to be targeted to first achieve a permanently unmanned status will be the ones containing equipment with a lower volume of maintenance such as vessels, exchangers and pumps.

The normally unmanned areas containing more sophisticated equipment such as compressors and turbines will require more attention and shall therefore be located in a closer vicinity to the utility areas and safe havens. This approach will consequently result in larger contents of hydrocarbon in a more forward location, which is an aspect that needs to be carefully addressed in the design.

In order to account for the development and use of remote operated intervention tools, it is essential that the future units are designed to be as open as possible with “easy-to-get-to” equipment. It is fair to assume that a single level process area will best satisfy such a requirement.

Considering that human attendance only is required during shut downs in the PUA, the equipment can be packed differently and in general tighter thereby reducing area requirement.

The modules shall be accessible from the platform crane to ensure easy removal, which also indicates that a one-deck solution is the optimum layout solution.

Furthermore, the units shall be equipped with a shelter and only a minimum of workshops and stores in order to facilitate light maintenance. Revision maintenance for such units is assumed to take place under the support of multi purpose service vessels, which in addition to having accommodation facilities, will carry main workshops and stores.

Case study

A case study for a semisubmersible platform with crude processing capacity of nearly 16 MMcm/d, gas capacity of 19 MMcm/d and water capacity of 6.4 MMcm/d has been performed with basis of the concept of maximum use of compact processing technology and fully remote operation of the main separation systems (PUA design). The revised design was benchmarked with a conventional semisubmersible platform with three-stage gravity separation and other conventional system design

The revised process includes full crude separation and stabilization, water separation, cleaning and discharge to sea, gas dehydration and compression for export and gas injection.

The oil water separation includes an inlet separator with advanced electrostatic internals for full crude dehydration in the first stage separator. Inline flash separators are provided on the second and third stages with small size vertical separator to efficiently handle flash gas removal at process turndown and prevent gas carry under.

The gas dehydration system is based on an inline TEG gas contactor but with a conventional regeneration system. Gas scrubbing is based on inline de-liquidizing and polishing of the gas for compression.

The extensive use of compact separation technology provides large reductions in gas volumes for blow down to only about 10 % of original volumes. The area reduction is significant and the weight reduction for the revised topside design is 36 %, or nearly 7,800 tons. The cost of the revised topside and hull, including mooring, risers and marine operation, is reduced by close to $400 million compared to conventional design.

The revised facility design is based on fully remote operation and the estimated yearly manhours for maintenance is reduced from originally 72 hrs per year (conventional design) to only 25 hrs per year (revised design).

The yearly operating cost based on revised design and onshore operation is reduced by $30 million per year.

Considering a field development for a 20-year production period, the difference in life cycle cost (∆ NPV) in favor of the revised concept based on new technology and remote operation is estimated to be $650 million.

Conclusions

The desktop study has confirmed that a Floating Facility for the Future is fully feasible, and though further development is required, the main building blocks have been developed and several of them have been tested during pilot studies.•