Getting production from deep fields to surface relies on riser and flowlines. This is where some of the industry's latest technology effort is being expended.

ChevronTexaco's Agbami project off Nigeria could become the fourth field in deep West African waters to use Steel Catenary Risers (SCRs) to take production from subsea wells to the surface. It will follow Shell's Bonga development, due to produce first oil this year, ExxonMobil's Erha project and the Akpo development, once it gets moving.

Current planning for Agbami is based on floating production, storage and offloading (FPSO) vessel in water depths ranging from 4,200 ft to 5,400 ft (1,280 m to1,650 m) off Nigeria in Offshore Production Licenses blocks 216 and 217.

Intec Engineering, owned by the Heerema Group, performed Front End Engineering and Design for the subsea layout on Agbami involving development of EPIC bid packages for both an FPSO and subsea systems, including flowlines, risers and an export loading system.

Ten-in. SCRs will be used on the project. Four-well clusters are to be developed on the sea floor, with a central manifold commingling production before it is carried to surface.

Dual 10-in. flowlines, insulated with cast syntactic foam, will carry production from the manifold to the SCRs. Flowlines will be hooked up to the manifold using conventional U-type jumpers.
In July last year Chevron Texaco finally cleared the way to proceed with Agbami by agreeing with the Nigerian National Petroleum Corporation and partners, Star Deep Water Petroleum and Petrobras plus local Nigerian oil company Famfa Oil Ltd.

Announcing that agreement, George Kirkland, then president of ChevronTexaco Overseas Petroleum indicated first oil from Agbami would be targeted for 2007 with peak oil output of 250,000 b/d. Project cost is around US $2.5 billion.

Recoverable reserves were put more than one billion boe and Agbami is ranked as the biggest oil discovery off Africa for years. Pre-qualifications from project supplies are with partners for evaluation and the next project stage is to put technical bids out to the market, a ChevronTexaco spokesman said. These involve an FPSO and subsea hardware. Local content was emphasized as a "critical" element for contracts after 13 groups submitted tenders, but only seven pre-qualified.

SCRs have been the benchmark production transport tool from subsea wells to FPSOs. They first debuted on Shell's Auger TLP development in the US Gulf in 1998. They have been a favored technical solution since and have been in common use for up to 15 years.

Total turned to bundled riser buoyancy towers - a bundled riser suspended with buoyancy cans near the surface - for its Girassol project. Now, riser technology is being taken a stage further with evolution of what 2H Offshore call's a third-generation riser, a single line offset riser (SLOR) which will used on the P51 semisubmersible platform for the Marlim Sul development in Brazil's Campos Basin. Engineering is underway after 2H won a contract to carry out detailed design for the P51 using an 18-in. free-standing oil export SLOR which will operate in a water depth of 4,116 ft (1,255 m).

Depth and diameter requirements for the P51 are outside the range for qualified flexible risers and the SLOR provides fatigue and weight-saving benefits compared to an SCR. "Both flexible risers or steel catenary risers will mean putting a large payload on the platform," Frank Lim of 2H said. "If would be nicer if Petrobras did not have to accept that payload, so it is one of the reasons they decided to go for a free-standing hybrid riser."

There are other reasons why an SCR was ruled out for P51. One is that the vessel motions inflict unacceptable fatigue damage to the large diameter riser at the touch down point, and SCR installation requires the host platform to be in place first.

For the P51 this is not possible since there have been delays to tendering for the P51. Without the built vessel, it's motion characteristics and payload cannot be modeled and without those, an SCR cannot be designed.

Petrobras has an option with 2H for designing an oil export riser for a sister semi-submersible, the P52.
In May 2003 Saipem announced it had won a second major riser and flowlines package for deep West African fields including engineering, procurement construction and installation of risers and flowlines for Exxon's Kizomba B project in Block 15 offshore Angola in a water depth of 3,600 ft (1,100 m). Along with a deal to provide topsides for Total's Dalia FPSO project in Angola's Block 17 in alliance with Stolt Comex, Technip plus Samsung and DSME, Saipem's deal was worth $490 million. Earlier, Saipem won a contract to supply risers for Kizomba A after engineering by 2H which involves five single line offset risers (SLOR) for water and gas injection.

Installation of these SLOR for Kizomba A was taking place and another five for Kizomba B are being engineered and due for installation in the third quarters 2004 and 2005.

Meanwhile specifications for ancillary flexible pipe equipment are being refined under a joint industry initiative backed by MCS International and five oil majors - ExxonMobil, BP, ChevronTexaco Petrobras and Statoil. The JIP is looking at standard specifications for bend limiters/restricters, connectors, subsea buoys and buoyancy modules, clamps, riser and tether bases, installation aids, vent systems plus tube seals.

Participants include TechnipCoflexip, Wellstream, NKT Flexibles, plus equipment suppliers, CRP Marine and Dunlaw Engineering, both members of the Scottish Subsea Technology group, plus Single Buoy Moorings and regulatory agencies API and the Norwegian Petroleum Directorate. MCS was visiting project participants to gain a consensus on specifications.

An expert at London and Houston, Texas-based engineering studies and risk assessment consultancy, e-p-consult, has described SCRs as a maturing technology that has been in use for 10-15 years.
For larger diameter risers in deeper water, SCR technology has proven to be a practical and cost-effective alternative to flexible risers. "It is a technology that is still developing and there are many uncertainties to manage. As a result, large design safety factors are traditionally applied to reduce project exposure." Martin Larsen, Managing director, e-p-consult in London, told E&P magazine.

"The structural analysis of SCR response is extremely interesting. There are a multitude of issues and parameters that play a big role in the SCR response behavior via the vessel movements and the riser hydrodynamic loading. Fatigue is always an issue, especially in the ever-moving touchdown zone and, to a lesser extent, near the hang-off point. Vortex-induced vibration (VIV), leading to rapid SCR fatigue, is of utmost importance and may be suppressed by fitting strakes to the riser when analysis suggests it could become a problem."

Produced fluid properties - pressure, temperature and fluid composition - are important to the required features of SCR design. Issues of corrosion and thermal management may lead to the use of exotic or CRA piping and or pipe-in-pipe or lined solutions. The material grade, wall thickness and the gas oil ratio (GOR) also have big cost and weight impacts on SCRs.

He said SCR design requires an iterative approach to establish the right balance between the numerous input and output parameters. The overall goal of long-term integrity is acquired in today's design by larger than necessary safety factors, although new and more sophisticated approaches are being developed. "Detailed structural risk and reliability analysis are increasingly applied in optimizing solutions for SCR design," added Larsen.