ABS is classing ATP Oil & Gas Corp.’s Cheviot FPU, which will work in the UK Sector of the North Sea. (Image provided by ABS)

Class design review at ABS was prompted by some recent innovative floating production units ABS is classing, one of which is ATP Oil & Gas Corp.’s Cheviot floating production unit (FPU), a Moss Maritime Octabuoy design, which is headed for the UK North Sea.

“This semisubmersible hull with extremely deep draft has very good motion characteristics,” said Luiz Feijo, ABS project manager. The Octabuoy, named for its octagonal ring pontoon, minimizes the parametric motions (pitch, roll, heave) due to the unique geometry of its columns. “The unit can accommodate the use of top-tensioned risers for dry-tree completion and provides a very favorable platform for the use of steel catenary risers (SCRs),” Feijo said.

The MinDOC3, a cross between a semisubmersible and a truss spar, will be classed by ABS when it is installed for the ATP-operated Mirage field in Mississippi Canyon in the Gulf of Mexico (GoM). ATP has ordered a second MinDOC3 for the Telemark field, also in the GoM.

“In our preliminary analysis of the hull, we concluded that the in-place stability of the structure indicated it behaves like a spar,” said Feijo, who also acts as project manager for the MinDoc3. In terms of the processing facilities, he says, the T-shaped topsides arrangement of the design is also different from traditional FPUs.

The growth trend for production floaters continues to rise as operators look for ways to meet the world’s increasing demand for energy. The role of class societies will evolve to anticipate rules and standards for the next generation of offshore installations.

Floating production advancements have challenged traditional thinking and rules. What’s next is limited only by imagination.

Changing times

These innovative units emphasize that nothing is “standard” anymore when it comes to offshore exploration and production design. Advances in the industry mean units no longer fit neatly into categories such as spars, tension-leg platforms, and floating production, storage and offloading vessels (FPSO). Today, ABS is reviewing more designs that are either a combination of basic ideas packaged into new and novel configurations or completely new approaches like the Octabuoy.

With a near 50% share of the floating production market, ABS is a leader in offshore classification and has a unique perspective on global offshore technology.

“Offshore production is going into deeper waters and harsher environments,” said Kenneth Richardson, ABS vice president of energy development. Designers are facing cold weather environments and water depths of 12,000 ft (3,658 m) and beyond that demand different technologies and materials.

“Another significant mindset change is to separate the well fluids on the seabed floor as opposed to separation taking place in process facilities on the topside of the units,” Richardson said. Advocates of this approach say there is a significant potential for cost savings by moving some of the traditional topsides fluid processing to the seabed.

The changing shape of FPSOs

Although ship-shaped FPSOs, many of them conversions, dominate the FPU category, newbuilds and novel configurations are becoming more common.

A round FPSO was unheard of 10 years ago, but offshore pioneer Petrobras has proposed two hull production unit concepts, the MonoBR and the FPSOBR, that are non-ship shaped, breaking the tradition of converting existing tankers. ABS reviewed both designs and provided Approval in Principle (AIP) in 2005.

The MonoBR is a short cylindrical mono-column floater that is being considered for use offshore Brazil and in the GoM. Round hull concepts by Sevan Marine and OPE are also on the market.

The ABS classed Agbami FPSO looks more like a barge than a traditional FPSO. The largest FPSO constructed to date has 13 topsides modules weighing 35,000 tons, which required close review of the loads as well as calculations for the hull’s strength and a detailed fatigue assessment.

“The FPSOs of today and the future require us to think differently,” said Xiaozhi Wang, managing principal engineer, ABS corporate research
and product development. Previous class requirements and rules for these vessels were based on those for trading tankers. “This has certainly served the industry well for many years,” Wang said, “but our knowledge of these structures and the detailed analyses we have conducted have provided us with the information needed to develop rules that are more specifically focused on the operational demands that are placed on these vessels.”

Wang and her team say the new requirements apply FPSO-specific loading conditions and prescribe strength assessment procedures to be followed. The new structural analysis of FPSOs will be contained in a major revision and update of the ABS Guide for Floating Production Installations.

Wang and her team of hydrodynamic and structural engineers have been working on criteria development and validation, addressing inspection and repair load cases, refining operational loading conditions, revising fatigue assessment to include low-cycle fatigue, and developing new procedures for topside and hull interaction analysis and for position mooring and hull interface.

For conversions, the strength evaluation consists of evaluating the hull structure as a trading tanker prior to conversion and then as an FPSO. This involves considering the sea environment encountered as a trading tanker as well as the conditions encountered en route to the FPSO site and the sea environment at the site itself.

Loads encountered in FPSO operations include variations in tank loadings as well as the sea waves and swell. Wave and swell loadings subject the hull structure to high cycle fatigue loads, whereas the loading and unloading of the cargo tanks subject the structure to low-cycle fatigue loads.

ABS anticipates the new structural requirements will be beneficial to industry, particularly given that industry predictions indicate the FPSO fleet will grow by 25 units this year alone.

New materials

Polyester moorings are seeing wider application. At Independence Hub, for example, there is a 12-line taut polyester/chain mooring system connected to 12 suction piles. The challenge for designers is to identify the most effective and, in this case, lightest mooring system to match the range of environmental loads the installation faces.

“Synthetic mooring lines are lighter weight than steel moorings in sea-water, offering the deepwater market neutral buoyancy and a much lighter load on the overall platform system,” Richardson explained. “As a result, operators can dedicate more space to payload such as production equipment and less to supporting the weight of the heavy steel strand.”

Key considerations in choosing a mooring system are the environmental loads resulting from wind, waves, and current. These factors pose increasing risks and technical challenges as activity moves farther offshore and into more hostile environments. Independence Hub, for example, is moored in 8,000 ft (2,438 m) water depth, making it both the deepest offshore platform installed to date and the deepest project using polyester moorings.

Synthetic moorings have become more commonplace since the first permanent use of polyester moorings in the GoM for BP’s Mad Dog truss spar. Prior to use in the GoM, synthetic rope was used offshore Brazil by Petrobras when it installed the P-27 semisubmersible in 1998.

Ultra-deepwater installations proposed for GoM developments, coupled with concern for mooring lines in light of hurricane threats, require a great deal of scrutiny of the mooring systems, Richardson said.

Polyester has paved the way for other materials to gain acceptance, including aluminum, titanium, fiber reinforced plastics, and composites. ABS certified the use of aluminum alloy for risers (AAR) for Noble Drilling in 2003. Since that time, risers made of this material have been deployed on vessels in the GoM and offshore Brazil. Studies by Noble Drilling show more than 50% weight savings when AARs are submerged.

Industry is now testing deployment of ultra-deepwater AARs that can be installed in 10,000 ft (3,048 m) water depth.