Floating Production Systems
Large offshore oil and gas deposits have been discovered and produced since 1896. Water depth, location and technology typically dictate what type of floating production facility is used.
For shallow water development, fixed platforms are commonly used. For deep water development where fixed platforms are not feasible technically and commercially, floating facilities such as Spars, TLPs (tension leg platforms) and FPSOs (Floating Production, Storage and Offloading units) are used. For more about the limits for floating production facilities, consult these published technical articles.
An FPSO (Floating Production, Storage and Offloading unit) is typically a ship-shaped floating production facility that has the capability to process, store and offload the hydrocarbon production from offshore oil and gas wells. The first FPSO, Shell Castellon, was built in 1977. Since then, the FPSO design concept is gaining favor in the industry as it can be anchored and operated in any water depth. FPSOs, when compared against other floating production units, offer lower CAPEX (capital expenditure), shorter deliveries, easy relocation and the ability to be reused.
[tweet_box design=”box_09″]Another significant benefit is the use of an FPSO in environments prone to hurricanes where a detachable turret can be used and the FPSO can sail out of harm’s way.[/tweet_box]
An FPSO is an ideal solution for small and medium oilfields that will be depleted in a few years. Another significant benefit is the use of an FPSO in environments prone to hurricanes where a detachable turret can be used and the FPSO can sail out of harm’s way.
The floating foundation and oil storage for an FPSO is either a newly-built ship or a converted oil tanker. Converted oil tankers are more commonly used as they require lower cost and shorter deliveries of 18 to 36 months. Converted tankers reinforce the existing storage tanks in the hull and replace or revise the accommodations unit to meet the specific project requirements.
To keep the FPSO stationed at the field, the floating vessel is moored to the sea bed. If the oilfield is in seas with a mild weather environment, the FPSO is anchored using a spread mooring. Spread mooring designs maintain the vessel location with a fixed heading at the fore and aft of the vessel (dominant climate direction). This mooring system has low ability of weathervane especially when the climate direction is at the sides of the vessels. If the production field is in a location with a harsh weather environment, the FPSO is equipped with a turret mooring system. The turret mooring system allows the FPSO to station with single mooring point. Turret mooring systems can weathervane into the direction of the climate.
In addition, the turret systems are also used as the interface point between the FPSO and the subsea wellheads in the seabed. All connections between the FPSO and seabed are through the turret system, including lines for incoming production fluids, fluid export, water or gas reinjection, chemical injection, electrical cables, hydraulic piping, utility lines and control signals.
FPSO Topsides Design
New production topsides, which consist of multiple modules, are integrated on the deck of a converted tanker to form a complete production facility. This operating model leverages the expertise of a process company, engineering company and fabrication company to coordinate, design and construct the topsides modules. Some experienced EPC companies can provide most, if not all, of the these and can work as a primary partner with an FPSO contractor. New build FPSOs are custom-designed and built for a specific project, hence, they can easily meet the project and field requirements. In this execution model, the EPC company often contracts directly with the shipyard.
FPSO topsides are equipped with process equipment designed to separate the production fluids and remove contaminants to meet project sales specifications. Once separated and desalted, stabilized oil is stored in the tanker’s cargo tank. Separated natural gas is conditioned, compressed and exported via pipeline or is reinjected back into the well to maintain reservoir pressure.
Produced water is treated to acceptable standards and dumped overboard. if allowed by local environmental regulations. Otherwise the treated produced water is reinjected into the reservoir or disposal well. The topsides facilities also consist of utility and auxiliary systems such as power generation, chemical injection, fuel gas, instrument air, utility air, and closed drain systems. FPSO topsides are equipped with a flare system to relieve high pressure gas during an emergency or shut-down. The flare safely burns off the excess hydrocarbon gas before discharge. Normally, only 30% of the oil in a reservoir can be extracted. To increase the extraction and maintain production, reinjection using produced water, treated seawater or produced gas into the reservoir is employed to increase reservoir pressure and stimulate production.
[tweet_box design=”box_09″]It is critical to take into consideration the motion of the FPSO due to wave motion when designing its process systems. The design must ensure that process parameters will be met even during the worst operating conditions due to local climate.[/tweet_box]
The FPSO process topsides are often modularized into several modules with manageable lifting sizes and weights in order to optimize the project schedule. Topsides process modules can be fabricated independently at the same time as the conversion or life-extension of the hull. All process equipment on the topsides process modules is interconnected with piping, electrical and instrumentation. The modules are usually completed mechanically and pre-commissioned before being lifted onto the converted tanker.
It is critical to take into consideration the motion of the FPSO due to wave motion when designing its process systems. The design must ensure that process parameters will be met even during the worst operating conditions due to local climate. For equipment that contains significant amounts of liquid, such as oil, separators and tall towers should be located as near to the centerline axis of the tanker as possible to minimize wave motion of liquids inside the vessel.
FPSO topside design typically incorporates a central pipe rack that interconnects all topside modules, though some smaller FPSOs can be designed without a central pipe rack. In this case, the interconnecting pipe runs underneath the main deck of the process modules. This design is deemed to be more cost effective but may not always be more time-efficient due to the complex interface between modules with very little tolerance.
The flare system is used as an emergency outlet for the natural gas since it cannot be stored. The flare stack is usually the tallest structure on the FPSO and furthest away from the accommodation. A proper flare dispersion study must be conducted so that the heat and pollution from flaring does not affect the operators on the FPSO.