VersaBuoy performance has been verified through the-state-of-the-art multibody analyses as well as extensive model test program. (Images courtesy of Versabar)

The VersaBuoy system has a unique articulated joint between the top of each hull and the underside of the deck. This connection allows each hull to pitch and roll independently from the topsides. Each hull is sized and ballasted such that it is stable and requires no other constraint or connection. Mooring is by either conventional catenary lines or a taut leg system attached to fairleads located on each hull. VersaBuoy floaters have been adapted to a wide range of applications:
• Minimal wellhead control platform (MWCP);
• Mobile offshore drilling unit (MODU);
• Dynamically positioned multipurpose offshore support platform;
• Very large floating structure;
• Mobile offshore real estate (MORE); and
• Offshore port facilities.

The VersaBuoy system comprises four spar-like, self-stable hulls supporting the platform topside through articulated redundant pin-in-pin connection. The unique application of the articulated joint between the columns and the topside allows the column rotation relative to the deck and consequently results in a dramatically reduced topside rotation.

The articulated joint

Articulating connections have formed a staple of automotive and other machine equipment and as such there is a large body of knowledge and experience in their design and specification. For current Versabuoy applications under consideration, the articulating joint must transmit an axial thrust of between 907 and 2,268 Mt (1,000 and 2,500 S. tons) and accommodate a range of motion of +/-20°. The joint is located at an elevation of +55 to +60 ft MSL (+16.7 to +18.3 m MSL), outside of the splash zone and accessible for regular inspection. Even given the understanding of material wear predictions under cyclic loading (Archard Equation), an additional safeguard for the articulating connection is proposed in the form of a redundant outer articulating joint. Using this arrangement, each primary connection can be removed and replaced on an as-needed basis during standard operating conditions.

Technology evolution

The Versabuoy system evolved from a study of the installation of single-piece, 18,144 Mt (20,000 S. ton) topsides onto spar hulls offshore West Africa using the Versatruss system. Versatruss lift booms are supported on the centerline of standard deck transport barges with pin supports that allow the barges to roll relative to the booms and hence deck (1° of freedom decoupled).

The motions of the topside supported by the Versatruss system during the various phases of the operation were evaluated. An optimization cycle considered the replacement of the standard deck transport barges supporting the Versatruss booms with four small, cylindrical, spar-type buoys now each supporting a single boom set, with the objective of further reducing deck motions during installation. In order for this configuration to work it was necessary to introduce an additional release between the spar hull and buoy to decouple the hull pitch and roll motions from the booms using an articulating connection. Multibody motions analysis for this configuration showed a dramatic improvement in installation system and topsides motions – such an improvement that the motions of the topside on the installation system were equal to or better than the completed structure.

Model testing and verification

A model test program was performed at a scale of 1:53 at the Offshore Model Basin, Escondido, CA. Four basic configurations were tested for Gulf of Mexico environmental conditions including 10-year winter storm, 100-year hurricane and 1,000-year hurricane as well as loop current conditions.

The four configurations tested were based on a standard Versabuoy module with the following characteristics:
• Two level topsides 76.2 by 76.2 m (250 by 250 ft) with a total variable deck payload of 2,903 Mt (3,200 S. tons);
• The deck is supported by four “jacket” hulls on 125 ft (38.1 m) centers;
• Each hull has a total length 139.3 m (457 ft) and a design draft of 398 ft (121.3 m) comprising 1,179 Mt (1,300 S. tons) of hull steel and 2,177 Mt (2,400 S. tons) of solid ballast. Each hull is fabricated from four 148-in. diameter tubular members on 27 ft (8.2 m) centers running full length with cross tie members every 90 ft (27.4 m). Two of the cross ties supports a conventional heave plate; and
• Eight-leg catenary mooring system (2 legs per hull) modeled using OMB’s SMART system.
The modal analysis of this single module showed a system deck heave period of 27 seconds and a deck pitch/rill period of 34 seconds. The model testing program considered a single module, two combined modules to form a single 250 by 500 ft (76.2 by 152.4 m) module, and a four-module combined to form a single 500 by 500 ft (152.4 by 152.4 m) module.

The three-hour deck extreme responses measured during model testing compare extremely favorably with all other classes of offshore floating development systems. An additional benefit is the low drag loads for the selected jacket hull, which results in mooring loads of 30% to 40% less than for other classes of system for the same displacement. Wave run-up seen on columns and hulls for other systems (reducing local air gap) was not seen for the VersaBuoy system.

Minimal wellhead control platform

The VersaBuoy system shows excellent scale-down capabilities, illustrated by the 1,134 Mt (1,250 S. ton) payload system with a total hull structural steel weight (for all four hulls) of 2,540 Mt (2,800 S. tons). Multiple body motions analysis also shows the extreme wave response of this small system to be very close to that reported for the model test results for the larger unit. This allows silicon control rectifiers and umbilicals to be supported directly from the topsides.

An application under consideration for this scale of system is an minimal well head control platform (MWCP) for single or multiple wells tied back to local deepwater infrastructure. This model calls for a MWCP to be located over the wells supporting a control and chemical injection umbilical and pigging riser. The MWCP supports an autonomous control system that can control well functions as well as redundant communication lines (microwave and satellite) back to the host. The default operating mode is with control from the onboard system. A pigging riser allows pigs to be pushed from the well to the host platform, removing the need for a round trip pipe loop. The platform is not normally manned and can operate autonomously for seven to 10 days.

Cost and development schedule advantages for this system accrue from 1) a shorter control umbilical (2 to 3 miles or 3.2 to 4.8 km from MWCP instead of 20 to 30 miles or 32 to 48 km from host), 2) possible reduction of a pipeline leg and riser from host, and 3) reduced additional payload on the host (chemical injection and control equipment located on MWCP, one less riser on host). Recent proposals have shown this solution to be commercially attractive for a number of cases.

Very large floating structures

VersaBuoy floaters are a perfect solution for the very large floating structure (VLFS) applications. The VersaBuoy floater for this application comprises basic modules; each of them consists of four hulls supporting the topside. The topsides of each module are rigidly connected to the topside of the adjacent module. Due to the reduced topside motion, the connection loads are numerically and experimentally proven to be within a normal practical design range.

System motions improve as the size of the combined modules increase. This offers considerable opportunity for offshore ports, mobile offshore real estate (MORE), staging areas, inspection facilities, etc.