| April 30, 2007
( Houston, TX) Leading offshore classification society ABS told attendees at this year’s Offshore Technology Conference in Houston, that they are seeing a trend toward hybrid designs for offshore floating production installations.
ABS marine engineers and naval architects conducting plan design reviews worldwide report the designs are combining the basic ideas from Spars, tension leg platforms (TLPs) and semisubmersibles and arranging them in new configurations.
“We see a crossbreeding between traditional floating production installations resulting in entirely new designs,” says ABS Vice President of Energy Development Kenneth Richardson. “These hybrids are unlike anything we have ever seen. From a class society perspective the issue is which of our existing Rules or risk methodologies should be applied so review of these new structures can move forward offering the same safety equivalencies to more traditional designs.”
Most of the hybrid designs are destined for the Gulf of Mexico (GOM). However, ABS offshore project development staff says they are beginning to see increased interest in novel concepts in other parts of the world. Here are some examples of hybrid designs currently under review by ABS.
Behaves Like A Spar
Technology currently owned by Durward International, a joint venture of Keppel FELS and TexBASS, has created a design that is a cross between a semisubmersible and a truss spar. The MinDOC3 design, which incorporates three generations of optimization from its initial conception, is intended for the ATP Mirage Field in Mississippi Canyon 941 of the GOM. It is currently under review by ABS.
The first iteration of the design received Approval in Principal from ABS in 2001. It is a concept originally conceived by Alden "Doc" Laborde and William Bennett and subsequently developed over a period of eight years by a consortium of industry-leading firms.
The MinDOC3’s three vertical columns, arranged in a triangular shape with columns connected to pontoons, at first looks like a semisubmersible. However, engineers analyzing the design say the structure behaves like a Spar in terms of stability.
“We had to determine what requirements or Rules we would apply to this unique design,” said Luiz Feijo, ABS Project Manager for MinDOC3. “In our preliminary analysis of the hull we concluded that the in-place stability of the structure indicated it behaves like a Spar.” According to Feijo, the position of the center of buoyancy in relation to the center of gravity is the main characteristic of the Spar’s geometry and configuration: “With a Spar the center of gravity is well below the center of buoyancy so this bottom heavy arrangement prevents the structure from tipping over or capsizing,” he explains.
ABS’ experience with Spars is well-documented as it was the first classification society to offer detailed technical guidance for Spars and tension leg platforms in its supplement to its “Guide for Building and Classing Floating Production Installations” (FPI) in 2003. “We recognized that there were global performance issues and motion characteristics unique to these floating structures,” said Richardson. The supplement outlines global performance requirements, load and environmental issues, stability parameters and structural strength criteria.
In terms of the processing facilities, Feijo says the T-shaped topsides arrangement of the MinDOC3 is also different than traditional floating production systems. The risers will be tensioned by a hydraulic system rather than air cans or buoyancy cans commonly used on Spars.
Multi-Column Floater on the Drawing Board
Calling it a cross between a cell Spar and a semi submersible, ABS is currently reviewing the conceptual drawings for a multi-column floater (MCF) from AGR Deepwater Development Systems, Inc (AGR DDS). The floater is intended for drilling and production in deepwater, high pressure, high temperature fields. President of AGR DDS Edward E. Horton III is the original developer of the TLP, and the three generations of Spars - classic, truss and cell.
“The MCF is in fact a combination of a semi submersible and a spar,” confirmed James V. Maher, Chief Technology Officer, AGR DDS. “The MCF hull is a deep draft semi submersible with longer columns than the conventional semi submersibles and each column is made up of four smaller diameter, closely spaced tubulars like that of the cell spar.”
AGR’s method of installing the deck near shore using a float-over method will allow the MCF to have motion characteristics similar to that of a Spar rather than a conventional semi submersible.
According to AGR the columns are affixed to the base pontoon which is normally fully flooded when the MCF is at its operating draft. The hull is towed out to the deck installation site in about 250 ft. water depth without the deck and ballasted down to a depth where the columns are just above the sea surface. The deck, supported on a deck barge, is floated over the hull between the columns. Then the hull is de-ballasted raising the deck to its tow-out draft.
Another feature incorporated in the MCF design includes a vertically restrained well deck which supports drilling and production risers. The well deck sits on top of a large buoyancy module that is restrained vertically by risers/tendons affixed to the sea floor.
“The buoyancy module is guided between deck and the pontoon base such that the MCF hull can move without imparting high stresses on the riser. The buoyancy module can be thought of as a small tension leg spar vertically restrained by tendons/risers surrounded by a deep draft semi submersible,” commented Maher.
Other features incorporated in the MCF design include a mooring system designed for the 100 year storm with fast hook up to pre-installed anchors and mooring lines and a large deck able to accommodate drilling and process equipment to meet a wide range of functional reservoir demands. “Primarily we wanted to develop a design that is capable of drilling and producing in a Gulf of Mexico deepwater reservoir at lower risk and lower cost for operators,” said Maher.
Design Reduces Risk
ABS categorizes the deep draft semi submersible for Chevron’s Blind Faith field in Mississippi Canyon 695 and 696 in the GOM a hybrid because of its hull characteristics, hull subdivision and also its ballast system. It ballasts not like a semi but like a Spar.
“This design is what’s called a ‘passive hull’ because it does not require frequent ballasting operations. Most semis have their pump rooms and sea chests in the pontoons,” says Feijo. For Blind Faith the ballast water comes from pumps attached to the outside of the hull and the hull tanks are designed independently. The square pontoon shape design is divided into four independent quadrants with no interconnection between quadrants and no ballast pumps inside the pontoons. Feijo says the operators chose this ballast system to reduce risk from pump or valve failure.
The Blind Faith deep draft semi submersible will be the first semi in the GOM with this ballast system.
A Round FPSO
Deepwater exploration and production leader Petrobras has developed two new hull production unit concepts, the MonoBR and FPSOBR.
The MonoBR concept is a non ship-shaped floating production storage and offloading facility (FPSO) that breaks with the tradition of converting existing tankers into FPSOs. ABS reviewed both designs and provided its “Approval In Principal” or AIP in 2005. The MonoBR is a short cylindrical mono-column floater known also as MPSO and it is being considered for sites offshore Brazil and, possibly, the GOM.
“A round FPSO design was unheard of ten years ago,” commented Feijo who also serves as ABS Project Manager for the MonoBR. “Today we are not surprised by the designs being put forward for classification review. If we are not able to review a design to prescriptive rules then we take a risk-based approach for determining criteria.”
For example, the MonoBR has some characteristics of a Spar but a much shallower draft, he explains. The design is such that it minimizes heave and pitch making it more suitable for the application of steel catenary risers (SCRs). With water depths pushing the 10,000 feet mark in some field developments, industry has concern with the riser fatigue caused by the facility motions. Feijo says the new Petrobras hull design limits or reduces heave thus, in turn, lessening the fatigue on the SCRs.
The FPSOBR was designed for a site specific location offshore Brazil. ABS assessed the strength of the hull by evaluating load components, hull strength, fatigue assessment and the still water bending moment (SWBM) and loading patterns.
Hybrid Designs Challenge Review Process
For ABS offshore design innovation drives the society to supplement its current Rules, develop new Rules or look for new ways to provide design review when there is no in-service experience or historical data to draw upon. “While we still have our prescriptive requirements,” said Richardson “today we often review new and novel concepts by taking a risk-based approach.”
Some of the hybrid designs reviewed recently underwent design review using the ABS Guidance Notes on Review and Approval of Novel Concepts. Richardson points to reviews on Compressed Natural Gas (CNG) carriers, liquefied natural gas (LNG) tank containment designs and FPSOs as further examples. He adds that often the technology is not new but the way in which the operator has packaged the technology is different, calling for close review to determine safety equivalencies.
Feijo says innovation for extracting oil and gas in deepwater in both a technically feasible and economical way will continue to spur the advent of new designs. The issue for class societies will be the manner in which they anticipate and develop Rules for the next generation of offshore installations in order to continue to facilitate offshore frontier development.
Founded in 1862, ABS is a leading international classification society devoted to promoting the security of life, property and the marine environment through the development and verification of standards for the design, construction and operational maintenance of marine-related facilities.
Editor’s Note: Photos are available for download from the ABS website at http://www.eagle.org/news/press/otc2007/
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