INTECSEA discusses the design and benefits
of its latest “low-motion” FPSOs
April 26, 2017
Once the flexible workhorse of the frontier offshore oil and gas industry, floating production, storage and offload (FPSO) vessels have had a quiet 18 months. Forecasts for use and expenditure sank, and order books lay pretty much empty from mid-2015 until the end of 2016, while low prices marred development in the traditional markets of West Africa, Brazil and Australia.
With a relatively stable oil price now perceived to be at hand, green shoots are emerging. But an industry focused on cost reduction is still in need of some bright ideas and innovation to boost efficiency. Enter engineering consultancy INTECSEA, with a new range of so-called “low-motion” (LM) floaters, aimed at reducing both the up-front capex of floating developments, and the opex over the desired charter period.
First announced in November 2016, the range now includes designs for the LM-FPSO, LM-FLNG and LM-Semi variants. Suitable for use in shallow water and all the way to 3,000m and above, the company claims that the LM-FPSO can dramatically alter the economics of remote offshore fields, with harsh operating conditions, delivering savings of between US$500 million and US$1.2 billion per project. For an industry just making it through the doldrums of FIDs, this may be music to operator ears.
Solid concept “The FPSO has the advantage of providing the required storage in the hull and direct offloading to tankers,” explains INTECSEA marine engineering manager and project lead Alaa Mansour. “However, the inherently high dynamic motions of a conventional FPSO make it an unsuitable host for the simpler, robust, lowest capex and opex riser solutions.”
The key innovation in the company’s new concept is a solid ballast tank (SBT) attached to the floater hull through groups of short tendons. This offers increased stability and reduced motion in wave surge, sway and yaw – less than a third of the motion experienced by a SPAR – and so much so that the company has likened its performance to that of a tension-leg platform (TLP).
Better stability means that the SBT can then be anchored to the seafloor using simpler and less expensive steel catenary risers (SCRs). Traditionally, the high dynamic motions experienced by FPSOs would make SCRs an unfeasible choice in most environments. Moreover, even if a lack of excessive movement could be guaranteed, the vessel would require many, large-diameter SCRs, again pushing up costs.
Likewise, top-tensioned risers (TTRs) provide benefits in allowing direct vertical access (DVA) to production wells and can help improve recovery. But owing to the stroke limitation of qualified tensioner technologies, the use of TTRs requires a floating system with minimal heave response, making them unsuitable for use with existing FPSOs. In dry tree unit (DTU) applications when storage is required, a wellhead platform is typically used alongside an FPSO, adding significant expense to the overall field development cost. Instead, with the LM range, well drilling and completions can be undertaken from the FPSO.
The SBT itself contains dedicated compartments for high density material (concrete, slurry or iron ore) to provide in-water weight and maintain tendons in tension for all design conditions. The remaining SBT volume is completely flooded with seawater at its in-place condition, meaning the LM range can still be integrated quayside and then towed out for service installation.
When not in service, the SBT is retracted to the hull and kept in this position through its buoyancy force. Once the platform reaches the desired field position, the SBT is flooded and lowered using two mooring chains at each corner, used temporarily for this purpose.
The tendons are then upended and installed. No coupling or installation derrick barge is required for the tendon installation, again reducing costs.
Steady as she goes Back above water, the hull itself has a rectangular cross sectional shape, which enables a straightforward rectangular topside footprint with proper separation between the process and living quarters, while allowing for flexible equipment arrangements.
In addition to savings in design and equipment use, being less sensitive to gravity and weight changes also reduces the risk to the project schedule and operating costs. Rough weather and waves have a far less pronounced effect on process safety, helicopter operability and the working time of the crew on board, all of which helps the operator maintain production and reduce unnecessary downtime.
A steadier topside also helps with product storage. “Due to the high stability offered by the shape of the hull and the attachment of the solid ballast tank at the keel level of the hull, the topside can now be arranged vertically and heavy equipment can be located at the top level accessible by the platform cranes for easy maintenance,” Mansour told InnovOil. “Temperature sensitive equipment can now be elevated high enough above any green water effect on top of the hull. Large-diameter risers can be adopted and less sloshing is predicted in storage tanks.”
Another hull innovation helps to shed further costs. Most conventional FPSOs also require a turret mooring system – a central bearing assembly that allows the vessel to rotate around the static portion, which is moored to the seabed. Often, a fast disconnection system is used in the event that the vessel needs to detach quickly from the turret, for example in the event of extreme waves or weather. These are sound safety options, but they add considerable cost to the development. INTECSEA’s design enables the LM to use a conventional mooring system, eliminating the need for this costlier option even in harsh operating conditions. Although the mooring system must be more robust and more expensive than traditional options, by the company’s reckoning the removal of the turret equates to a US$250 million saving straight off the bat.
Float on Unsurprisingly, INTECSEA is excited over the prospects for the LM range. Its scalability – the company says it could be used at fields producing 2 million bpd and above – could see it become an industry-standard blueprint for future oil and gas development, if the industry can be persuaded of its benefits.
In a statement, INTECSEA president Geeta Thakorlal noted: “With the easy oil long gone, operators are increasingly turning to more remote, hostile and often deepwater environments. However, the challenge is the financial viability of these fields. The LM technology is incredibly exciting because it represents a major step forward in improving the economics of these offshore developments.”
Mansour, for one, is equally hopeful. “Since the early phases of the LM-technology development, INTECSEA has been very keen to engage with subject matter experts from the operators, vendors, fabricators and installers to understand their feedback and address any risks,” he said.
Over the past two years, the group has held a number of open industry meetings to help determine the programme direction. Its most recent was a technology risk assessment workshop in February 2017, which followed the successful completion of a five-week model testing programme performed in collaboration with the Korea Research Institute for Ships and Ocean Engineering (KRISO). So far, Mansour adds, feedback has been “very positive” and no major risks have been identified.
As a result, momentum is building steadily. “DNV GL DNV has completed its review of the design and an Approval in Principle (AiP) certificate is imminent. [This] is the last key milestone in the roadmap towards a project-ready LM-FPSO, and the technology is now ready for deployment.” And if INTECSEA can deliver the project savings it expects to, the floater market is about to be hit by some big waves.