While the industry is rapidly developing experience with subsea developments this chart shows the vast majority of such fields are tied back less than 15 miles.
A scaled-down mini-tension leg platform (TLP) solution for subsea developments that requires extended tiebacks is being offered by Atlantia Offshore Ltd. The system, known as the MicroStarRegistered utility platform is a successor to the SeaStarRegistered prod-uction platform.
The new structure offers the motion characteristics of a tension leg platform with a smaller footprint and lower cost. At present, there are two SeaStar production platforms installed in the Gulf of Mexico, and a third is scheduled to be completed and installed in summer of 2001. Applying the same design characteristics to an even smaller platform, Atlantia hopes to break into the control buoy market.
While the MicroStar is a mini-TLP, and not a buoy, it fits into an emerging category of floating solutions called control buoys. Combined with a subsea tieback, these platforms can enhance reliability and reduce costs by solving a number of outstanding problems.
The majority of subsea tiebacks are less than 10 miles from a host platform. Only a few extend beyond the 15-mile mark because of inherent technical and flow assurance concerns. It is in this category of long-range tiebacks that control buoys can really enhance existing capabilities.
Tieback distances
With such an extended tieback, communication, injection of chemicals, and control of the subsea equipment become an issue. An umbilical carrying electric power, hydraulic lines, and chemical injection lines must be run between the subsea installation and the host platform. It is expensive to build and install such a line. David Snell, Vice President of Marketing and Sales for Atlantia, estimates the cost of such umbilicals to be $60-100/ft installed.
Hydraulic response slows over such lengths and greater pressure is required to transmit the hydraulic signal across 15-plus miles of line. Similarly, higher pressures are required to transport the injection chemicals needed to ensure flow assurance.
In order to pig a traditional subsea tieback, two lines must be laid over the length of the tieback.
One line carries the pig to the subsea manifold and another provides a return route. While one line is unavoidable because it provides for the production flow from the wells, the second line exists to accommodate the pigging loop. When the subsea well is plugged and abandoned, the umbilical and two flowlines are also abandoned.
Control buoy concept
A MicroStar control buoy is a minimal, normally-unmanned platform that is moored near a subsea well(s) or template. The facility offers an alternative to laying a control umbilical back to the host. Models have been tested to a water depth of 6,000 ft. The unit can provide electric and hydraulic control of the wells through an umbilical that is little longer than the depth of water. The loss of power over this distance is negligible and the pressure required to transmit a hydraulic signal is greatly reduced.
Because of the short distance from the buoy to the seabed, a direct hydraulic signal can be used, compared with a more costly and complex electro-hydraulic (MUX) signal currently used on long-range tiebacks. This saves on the installation cost and adds to reliability, because it is a much simpler system. Using a radio signal, the host platform can communicate directly with the control buoy to control and communicate with the well.
Low pressure pigging
The pig is introduced into a low-pressure riser and driven down using an available fluid such as diesel fuel. Normally, the pigging line would have to buck the well-stream pressure before being introduced into the flow. Atlantia designed a system that uses a series of valves to first isolate a section of the pigging loop, so the pig can be lowered down the riser line to the manifold. Returns are taken up a separate return line.
Once the pig is in place, valves are closed at the base of the lines isolating the section of pipe containing the pig. A valve on each end of the pig is then opened. Another valve in the well stream is closed, diverting flow through the parallel section containing the pig. In this way, the pig is introduced into the well stream. The pig then rides the well stream back to the host platform where it is recovered. This system not only eliminates the need for a pig return line, but also high-pressure equipment on the MicroStar.
Motion characteristics
Because the MicroStar is a TLP, it has the stable characteristics of such a platform. Snell said the MicroStar would be anchored to the bottom by steel tendons of varying OD and wall thickness, depending on the water depth. Once installed, these tendons are designed to be nearly-neutrally buoyant to minimize their weight on the platform.
At the same time, Snell said, the tubular steel tendons do not strain (stretch) as much as the wire rope tendons used on other control-buoy concepts. Because of its stability, the MicroStar can support topside power generators and other rotating equipment that do not perform well under extreme motions. MicroStar's design approach permits placing all topsides equipment on a traditional open platform deck, rather than in a sealed compartment located below the waterline. This eliminates safety and access concerns associated with such confined spaces and reduces costs.
In situations where subsea multiphase pumps are not the option of choice, Snell said pumps and compression on the unit could boost production going into the flowlines. Typically, a subsea tieback is reliant on the natural energy of a reservoir to drive the production stream back to the host platform. If production is run through the MicroStar, it could be boosted by a pumping and compression system before it travels to the host.
Applications
The West Africa market has a huge potential for the use of subsea tiebacks due to its lack of on-shelf infrastructure. As more very large fields are brought online, the region is developing an infrastructure that will lend itself well to tiebacks from marginal fields. Snell said this market is still a few years off because there are still a large number of massive discoveries to be brought on line.
Once this market begins to level off, subsea tieback will be a natural solution since there is virtually no infrastructure to tie-in marginal fields. However, Snell said the deepwater Gulf of Mexico is the most attractive market. With a number of large finds in deepwater, a hub and spoke pattern has emerged. Unfortunately, many of the platforms being installed in deepwater were not built with excess capacity in mind.
These fields were often looked at as freestanding entities. The benefit of tying back production from future marginal fields was not considered in the economics of such platforms. That means production has to level off and decline before there is the capacity and desire to bring on production from another field. Snell said such a cycle could take as long as 3-5 years to run its course. In the meantime, tiebacks in deepwater will be hosted by platforms in conventional depths on the continental shelf. This means a lot more long-distance tiebacks.
The more economical the equipment used in these subsea solutions, the greater the number of viable fields in deepwater. Lessons learned from shorter tiebacks can contribute to cost savings on future projects, but eventually, the distance will overcome the practicality of running control umbilicals and twin flowlines between a host and a remote field. Step change technologies such as the control buoy will be needed to further extend the reach of subsea tiebacks and broaden the range of fields developed in deepwater.
