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The future of floating offshore units

10th April 2013

Leading industry experts give their take on what lies ahead for the FPSO sector, cost-effective systems for safer drilling platforms and the advantages of cylindrical offshore units

The future of floating offshore units
35 per cent of offshore developments will be accounted for by the growing FPSO industry, with Brazil reinstating its leading position in that market – Ryastad Energy. Copyright: Sevan Marine

By 2035, the general consensus among analysts is that global demand for oil is going to increase by 32-35 per cent, while the IEA predicts total demand by 2020 to reach 95 million barrels per day (bpd) of oil.

“Full-speed” ahead for offshore

The industry is currently producing around 90 million bpd of oil from a large number of fields. By 2020, it is expected that these fields will have declined to roughly 70 million bpd. As such, in order to meet the projected demand, the global oil and gas industry must develop close to 30 million bpd, according to energy analysis firm Ryastad Energy.

“This is going to be a stretch,” Anders Hannevik, partner at Ryastad Energy says at London’s Advanced FPSO Forum. “We predict that a further 8 million bpd will come from new onshore fields, 4.3 million bpd from tight and shale oil, 4 million bpd from oil sands and heavy oil and then 16.5 million will be offshore. So for us, the only way to meet this demand is to really continue full-speed on offshore developments.”

The countries where the main offshore projects are expected to take place are Brazil, Norway, Angola, US, Nigeria and Saudi Arabia. Ryastad estimates that roughly 35 per cent of offshore developments will be accounted for by the growing FPSO industry, with Brazil reinstating its leading position in the market.

Preparing for harsh environments

Key to maintaining the integrity of an FPSO and a top priority for operators is of course the efficient monitoring of subsea flexible risers, which bridge subsea field developments with their respective production facilities.

When a flexible riser is being pulled through the vessel’s I-tube section it may not align correctly with the I-tube, which results in damage to the latter or the riser and can even cause project delays. Therefore, any floating offshore unit requires riser visual inspection systems to view and record riser installation activities.

To that effect, manufacturer of subsea vision systems for underwater and harsh environments Bowtech Products has released RVIS, a riser visual inspection system for FPSO riser applications. RVIS has been specifically designed to monitor the pull in and drop off (change-out) of subsea flexible risers. The technology is made up of three pan, tilt / zoom, colour, explosion proof video cameras at different angles; seven explosion proof LED lamps, mounted in clusters on the frame of the tool; and a custom designed controller.

The cameras operate at depths of up to 100 metres and the lights use LED technology to maximise subsea and topside vision. The pan and tilt also provide greater viewing area from a single camera with a full zoom function, which allows for closer inspection. The system’s custom umbilical withstands high pressure and contaminants, carrying the commands down to the tool and the images to the surface.

Finally, the surface control unit is a rack mounted system which powers and controls the visual and other functions of the tool. The video images are displayed on a multifunction video console where the process is monitored and recorded, allowing the inspection of riser condition.

“The RVIS technology is tailored specifically to operate in the I-tube sections on FPSOs which house the flexible risers. RVIS also takes into consideration the layout of the riser system on the FPSO and the size of the I-tube utilised for flexible risers,” says Mike Winstanley, sales director at Bowtech Products. “RVIS is designed to be lowered, in a controlled manner, through a neighbouring I-tube section and locked into place once the camera view is in position.”

For operators using FPSOs in hubs such as the US Gulf of Mexico, Norway and offshore Brazil, the effect of environmental conditions on the station-keeping capability and integrity of the vessels’ mooring systems is also becoming a major concern.

“A mooring failure is a class 1 hazard according to the HSE definition and so the technology is critical for the safety of an oil rig,” says Chris Peterson, lead naval architect of floating facilities at Petrofac. The consequences of a mooring system failure for an FPSO can involve collisions with adjacent offshore installations and might result in major oil spills, not to mention severe property loss.

“A mooring system is inadequate if it is likely to compromise the lines, the risers or the overall structure,” Peterson adds. The naval architect led the design team that corrected the mooring system of the floating production unit (FPU) Northern Producer, located in the Don field in the North Sea, after its failure in late 2011.

Ship-shaped FPSOs have well known advantages such as large oil storage and a high stability margin, but high environmental loads – due to wind, current and waves – which surpass design loads can lead to accelerated fatigue and, consequently, a mooring system failure.

“At the Northern Producer, we had three broken lines, not at the same time of course. However, this occurred in a span of a couple of months after four years of service,” Peterson says. Two of the lines were located near the fairleads and the third near the subsea connector.

Petrofac had two choices – to develop a new design or to procure a new mooring system, which can be a considerable investment – and it opted for re-design. The scope of work of amending an existing design is quite complex. Alongside designing a replacement mooring system, reducing the maximum characteristic mooring line tensions is also crucial. All safety factors must be respected as well as the new constraints whilst optimising downtime and costs.

Peterson says Petrofac did not lose a single production day because it kept the same chain and simply added string in between, polyester fibre rope, in addition to some meters of chain down the catenary to use as weight. The company claims this process saves operators and service companies a lot of trouble and money.

The cylindrical FPSO

Sevan Marine seconds Petrofac’s strategy of mooring system redesign. “I agree with this solution. Polyester with mooring lines is a good fit and we have that in all our systems,” says Frederik Major, a naval architect and vice president of business development/R&D at Sevan Marine.

It is not, however, in the mooring system front that the Norwegian company has fostered the most significant level of innovation, but rather in the construction of offshore floating facilities. More specifically, Sevan Marine has been an active developer of a new type of FPSOs which are drawing a considerable amount of attention – the innovative concept of cylinder-shaped platforms.

There are currently 11 projects on cylindrical hulls provided by Sevan Marine, in addition to seven FPSOs and four drilling units. Sevan Marine’s first cylindrical FPSO, the Piranema Spirit – start-up in 2007 – is now operating for Petrobras on the Piranema field in the deep waters of the Brazilian state of Sergipe.

The vessel, which floats on a very shallow draft (as little as five metres) and is therefore an easy unit to bring to shore, is interconnected to six wells and installed at a water depth of 1,100 m. As for specifications, the Sevan 1000 cylindrical FPSO has an oil production capacity of 100,000 barrels per day, a gas production potential of 3.9 million standard cubic meters per day and an oil storage capacity of 1 million barrels.

Construction wise, cylindrical units are similar to large semi-submersibles, with the rule of thumb being that if a shipyard can handle the latter it will most likely be able to build the former. They can also be completed in dock with all of the topsides whilst floating at water depth, which makes it possible for them to be launched off the dock.

In addition, the typical wave-driven fatigue loads commonly experienced in drill ships and semi-submersibles are not an issue when it comes to cylindrical vessels, Major says. Consequently, designing the unit is less of a challenge as they have to be geared not so much toward fatigue loads but rather local loads.

“Our systems have the proven ability of maintaining permanent mooring throughout cyclone or tropical storm conditions, a key box to tick when you consider operations in the Gulf of Mexico,” Major says.

In terms of offloading, cylindrical FPSOs also bear certain advantages over ship-shaped vessels whose mean heading must be aligned with the heading of tankers. In certain sea states such as those encountered in the North Sea, for example, tankers sit on dynamic positioning, so shuttle tankers operating behind a weather vaning FPSO can lead to collisions. With cylindrical FPSOs, collision risk is also reduced largely due to its circular form.

Turrets and swivels are rendered unnecessary as the units are spread-moored – largely considered to be the simplest way to moor an FPSO. This allows cuts in the total cost of the FPSO of between 10-20 per cent, Major claims. Furthermore, with no major orientation problems, the cylindrical FPSO makes it possible for the number of crew to be less than the one required for ship-shaped FPSOs.

Sevan Marine is also working on new concepts for the cylindrical FPSO range. These include an icebreaking unit, a concept for operating with steel catenary risers, an FLNG concept and a floating power plant.