Rising ambitions

Technological barriers and operator requirements can limit the possibilities available to offshore design teams when it comes to engineering a production riser plan for a deepwater facility. Hybrid solutions and composites will offer some relief as Jennifer Pallanich discovers in OE's latest review of riser technology challenges and talking points.

As Kieran Kavanagh, group technology director for Wood Group Kenny, describes it, ‘our job at MCS Kenny, the riser business for Wood Group Kenny, is to work the engineering'. Because the floating concept and its associated riser design are ‘strongly interlinked', he says, ‘we operate under certain technology limits'. Certain risers work best with certain floaters. The question, he adds, comes down to: ‘Can this riser type work with this type of floating facility?'

And water depth adds its own challenges. For instance in the Gulf of Mexico, Independence Hub in 8000ft of water, Perdido in 7800ft of water and Cascade/Chinook in 8200ft of water, all required steel at the seabed. The Anadarko-operated Independence Hub used a semi with steel catenary risers (SCRs), while the Shell-operated Perdido relies on top-tensioned risers (TTRs)serving a spar and the Petrobras-operated Cascade/Chinook FPSO project plans to use hybrid tower riser systems.

Those projects, Kavanagh says, couldn't work with flexible risers as that design exists today, especially for large diameter export service. The current water depth limit for flexible risers, he says, tends to be around the 6900ft mark. Flexible pipe manufacturers are, however, working to qualify their concepts for well beyond 6000ft water depths. For now, he says, ultra-deepwater risers, especially for export service, need metal, as found with SCRs, TTRs, and hybrid risers. ‘Whether these work or not depends on the degree of challenge that the floating production system puts on it,' Kavanagh adds.

SCRs are the least expensive and simplest riser system, he says, but the trade-off can be in terms of fatigue life. If a tanker is used, Kavanagh adds, vessel motions become a challenge, especially in places like the Gulf of Mexico with its unique metocean conditions. ‘The vessel is just moving too much for the loads you'll impose on the SCR without something creative.' However, he adds, the SCR and tanker solution could work offshore Brazil or West Africa.

The Gulf of Mexico will witness a significant step in riser technology when the Cascade/Chinook wells begin producing via a hybrid riser system to the BW Pioneer, Kavanagh says. They'll not only be the deepest risers in the Gulf of Mexico, but also the deepest hybrid risers in the world, he adds.

In short, he notes, TTRs are selected when a dry tree solution is required, while SCRs or hybrid risers tend to work well for subsea tree scenarios. TTRs to date have been used with spars or TLPs, but not with semisubmersibles, as most semis tend to move too much for the use of dry tree systems, he adds. ‘That's one of the challenges out there, to make a dry tree semi work.'

Other variables come into play beyond water depth: export options, reservoir conditions, metocean conditions, and type of hydrocarbons being produced.

Traditionally, Kavanagh says, the industry turns to variations on an existing concept when it needs to push technology. For instance, Kavanagh says, the need for a heavy riser wall to deal with high pressures and temperatures can pose a challenge for some of the materials in common use. Or, he says, the need to improve fatigue performance or when a riser is being used in sour service may drive the use of inconel welding or pipe cladding.

He notes there are temperature limits for corrosion or insulation coatings, and for HPHT service ‘we may be looking to extend the temperatures at which these coatings work'.

Weight is another issue where new materials may come into play. Kavanagh says an industry wanting to lessen hangoff weight of risers could look to high-strength steels, non-welded steels, titanium and other new materials. For example, Heidrun offshore Norway used the full titanium drilling riser, and titanium, which is stronger and more flexible than steels commonly used for risers, has been used for stress joints in TTR systems for Murphy's Medusa and BP's King projects in the Gulf of Mexico, among others.

Composites can help address not just the weight issue but other problems like material compatibility. Airborne, MCS Kenny and OTM Consulting are involved in a Thermoplastic Composite Riser (TCR) JIP started in 2009 and sponsored by BP, Chevron, Petrobras, SBM Offshore, Shell, Statoil and Total, aimed at proving the feasibility of a TCR in a dynamic riser application. The first phase, to look at the initial suitability and qualification, is complete, and the group is in the second phase, carrying out combined load case testing, with qualification expected to start this year.

Martin van Onna, managing director at Airborne, says qualification – executed in conjunction with DNV – of the TCR is expected to be complete by the end of 2012, yielding full type approval for the novel production riser system.

The technology features a thermoplastic liner reinforced with melt-fused fiber tapes impregnated with the same thermoplastic compound as the liner and coating. Further, it's fully bonded and has a solid wall to support pipe in all load cases, according to Airborne, and the thermoplastic matrix makes the design spoolable without fatigue or cracks. It has a higher ductility resulting in unsurpassed toughness, impact resistance and residual strength, the company says, and it has a smooth bore in a collapse-resistant pipe.

Van Onna says there are a number of clear benefits to using a TCR beyond the goal of reducing weight for deepwater production risers. First, he says, the ‘production technology and composite wall concept allows us to vary the hoop stiffness in the wall of the pipe', which results in a TCR that can support the highest pressures of any system available today.

Second, he says, using thermoplastics such as PA or PVDF as the matrix material in the composite wall allows for spoolability and impact tolerance, a benefit van Onna calls ‘a vital improvement over using conventional thermosetting composites'. Additionally, the chemical resistance of PA and PVDF give the TCR a range of applications for production riser, while the materials themselves have been accepted in the market place, he says. As the liner is completely supported by the melt-fused fibre tapes, it shows low stress levels even at high pressures and temperatures, he adds.

Because the TCR is a fully bonded, solid wall pipe, it has no annulus. This, van Onna says, means the TCR is a simple pipe concept without all the failure modes associated with an annulus. The annulus-free design is expected to offer a 60% reduction in top tension for steep wave, extending the operational envelope to deeper waters, Airborne says, and it has more stretch, thereby avoiding heave compensation. It is designed for waters of 1300-11,450ft and can be configured as free hanging catenary, lazy wave, steep wave, multibore hybrid riser and TTR.

Airborne's history in thermoplastic composite pipe technology started about a dozen years ago when the company began developing it. The company has been producing and delivering pipes now for two years.

‘Market acceptance was slow on this technology many years ago,' van Onna says. ‘Lately however we have seen a tremendous increase in interest, enquiries and sales over the past two years, showing that the industry is ready for this technology and recognizes its value. The future is in composites.'

Composite flexible

DeepFlex offers the FFRP, or Flexible Fiber-Reinforced Pipe. The FFRP is constructed from multiple extruded polymer pressure barrier layers and fiberreinforced/ thermoset matrix composite reinforcement layers. It is not yet in use in deepwater, but it's been used for surface jumpers since 2006 at Chevron's Petronius field in the Gulf of Mexico. The FFRP is also in use as a subsea dynamic riser and flowline at Afren's Okoro field serving an FPSO in 45ft of water offshore Nigeria. It's also been used for export pipeline commissioning in 4265ft of water at Petrobras' Urugua field offshore Brazil. Mark Kalman, VP engineering at DeepFlex, says FFRP serving the shallow water FPSO offshore Nigeria and the Urugua commissioning has seen ‘significant dynamic motions.'

In the late 1990s, Michael Bryant began developing DeepFlex's composite pipe technology, working out of his garage in Katy, Texas. His approach resulted in several patents which now form the core of DeepFlex's intellectual property portfolio. Bryant, who was a co-founder of DeepFlex predecessor DeepSea Flexibles, died in 2010.

Kalman notes DeepFlex is not the first company to consider using composite materials for flexible pipe for armor layers. Other companies have prototyped a design or put a product in the field ‘with mixed results,' he says.

‘It hasn't been commercialized successfully on a large-scale basis yet, but the value proposition is so high we're all working on it.'

The FFRP is about half the weight of unbonded metal flexible, DeepFlex says, and it resists corrosion and chemicals.

‘We're a newer entrant, and we're deploying revolutionary technology,' Duane Moosberg, VP sales and marketing, says, with composites replacing steel.

Kalman says DeepFlex expects the FFRP technology ‘will leapfrog what the other flexible pipe manufacturers are offering'. DeepFlex's design features helical winding with gaps between adjacent wraps so the pipe can be spooled and subjected to repetitive bending in dynamic risers. A tradeoff is that the design is not as strong in axial compression loading as in axial tension loading.

‘As you get into deeper water, compression resistance becomes more and more important,' Kalman notes. ‘What has been tried in the past is just wrapping composite tapes on the pipe, but the composite tapes are very thin and buckle under the compression loading.' The answer, he says, turned out to be a reinforcement stack, which DeepFlex has patented.

The design uses a composite rather than steel for reinforcement ‘so the weight can be half as much as a steel reinforced pipe,' he says. He calls the FFRP the first ‘significant' effort to use composite materials for all the major reinforcement layers.

‘To make composite structures that are flexible is a big challenge, and that is the challenge that we're taking,' Kalman adds. That challenge comes down to the basic nature of steel, which is ‘more forgiving' because it yields while composites are more brittle, he says. A further challenge is steel's track record of hundreds of years of use compared to the half-century or so of history backing composites.

‘To make composite structures that are flexible is a big challenge, and that is the challenge that we're taking,' Kalman adds. That challenge comes down to the basic nature of steel, which is ‘more forgiving' because it yields while composites are more brittle, he says. A further challenge is steel's track record of hundreds of years of use compared to the half-century or so of history backing composites.

Other risers

Petrobras has been involved in developing a hybrid riser for use offshore Brazil, including the self-standing hybrid riser installed at Roncador (OE April 2008), and plans to use a hybrid SCR at the Lula field.

Patrick O'Brien, WGK's group director for strategic business and marketing, says another challenge is emerging where, in certain regions of the world, operators are planning to use large diameter gas steel catenary risers in moderate water depths of between 1970-2620ft. The riser's lack of flexibility is more pronounced in shallower water due to vessel motions, he says. ‘This, I think, is a significant challenge for the future,' he adds. ‘Indeed, the importance of vessel motions and their impact on riser design is a constant challenge for our guys at MCS Kenny.'

Kavanagh notes most frontier projects boast their own list of industry firsts, though ‘some weren't without their problems'. In general, he says: ‘Technology development often tends to be by increment. It's usually not something so dramatic, favoring evolution rather than revolution.' Some of that comes from moving into ever deeper waters. A key question is ‘do our existing methods still work when we go out further?' Kavanagh asks. ‘At some stage, revolutionary design may be required.'

Identifying and solving problems with deepwater design is often done as an industry, and some of that through joint industry projects (JIPs). One design challenge with any water depth, he notes, is slugs or ‘plugs' of liquid pushing through a gas pipe. A new JIP, called SLARP, or Slug Loading and Response in Pipelines, is headed up by Wood Group Kenny's MCS Kenny and MSi Kenny businesses.

Formed in January 2011, the JIP focuses on developing design and analysis techniques to avoid pipeline or riser vibrations and consequent fatigue caused by fluctuating internal fluids transported by the pipeline. Such vibrations can lead to shutdowns or intervention to repair or replace sections of the pipeline. Participants in this group include BP, ExxonMobil, Petrobras, Saipem,Technip, Total and Wellstream, and the group hopes to develop industry design guidelines by early 2012, then move to phase two testing to validate the methods.

Code updates As the technology changes, so too must the codes that guide the use of these evolving technologies. ‘We've had to extend the code of practice to allow us to update our design methods,' Kavanagh says.

In addition to the updates in industry standards, Wood Group Kenny and other companies have been involved in riserrelated JIPs. For example, MCS Kenny and composite specialists Airborne are jointly working on thermoplastic composite risers.

‘They are so light they're almost neutrally buoyant in water, some of them,' Kavanagh says of composite risers. ‘The good thing is that you can lighten the riser system substantially if you use this material.'

To date, the industry has not yet really adopted composite risers for deepwater, and Kavanagh believes it will still be a while before we see a deepwater hydrocarbon riser composed entirely of composite materials – it must first be qualified. ‘It's not easy to get a new concept in the water, and it shouldn't be, but the industry is moving toward such concepts as innovative solutions.'

As well, maintaining the integrity of established equipment must remain a priority. WGK's O'Brien says the company has been involved in a number of JIPs revolving around risers and flexible pipes. One is SureFlex – managed and delivered by Wood Group Kenny under the auspices of industry body Oil & Gas UK. Tasked with examining integrity of flexible pipe globally, the group compiled a database of 1900 risers, 1400 static lines and 315 damage or failure incidents covering 130 field developments across the world. On studying the database, it emerged that 58% of flexible pipe is installed as risers and 76% of flexible pipe has a design pressure of 5000psi or less. Further, 90% of flexible pipe has 10inID or less. Also, 70% of flexible pipe has been designed for temperatures of 80°C or less, and 70% of flexible risers are in 3280ft of water or less.

By far, the main failure mode for flexible comes down to external sheath damage, with 35% of the failures attributed to this cause globally. The group says the increase from 25% of reported failures in 2002 across the UKCS and Norway could be attributed to higher levels of flexible pipe annulus monitoring. The second leading cause, with about 11% of failures reported, is vent system anomaly. Again, the group believes the added focus on monitoring could be behind the increase. Often, the vent system is found to be blocked, malfunctioning or poorly designed.

Corrosion fatigue has also prompted operators to retire flexibles, according to the group, but there is some uncertainty in how to predict the safe remaining life because of corrosion fatigue. Dissections of retired risers suggest remaining life predictions have been too conservative, according to the group, so there is a need to improve the accuracy of corrosion fatigue assessments.

SureFlex, which O'Brien participated in, concludes no single inspection or monitoring technique will provide a full picture of any flexible system's integrity. OE