First Buoy Supported Riser System Installed on Guara Lula Project (Brazil)

Balltec has developed and commissioned a pioneering subsea chain tensioning system and tether top connector for the first Buoy Supported Riser (BSR) System installed on the Guará-Lula Project. Balltec took on their largest project to date for the design, manufacture, assembly and testing of 36 tether top connectors and six subsea chain tensioners for the BSR system.

The Guará-Lula project is the largest engineering, procurement, installation and commissioning (EPIC) Subsea, Umbilical, Riser and Flowline (SURF) contract ever to be awarded in Brazil. The project, operated by Petrobras, is located in the Guará and Lula pre-salt fields in Santos Basin. The BSR system features four large subsea buoys which, when moored and tensioned, will support 27 steel catenary risers. Each buoy weighs approximately 2,000 tonnes and is installed 250 metres below sea level.

Balltec was invited by Subsea 7 to participate in a design competition against more established oil and gas engineering companies. Balltec utilised its mooring expertise and extensive engineering abilities to develop a workable conceptual design for the connectors and tensioning system to moor and tension the subsea buoys into their final position. The top tether connectors were designed to accommodate 162 mm chain. Balltec’s proposal was selected as the ideal solution and secured the contract for the company. The chain tensioning system and tether top connector were successfully taken from concept design through to delivery within 18 months of the contract being awarded, which is considered an aggressive lead time for the size and demands of the project.

During the design and engineering process a significant amount of time was invested into research and development of the connectors and tensioning system. As the concept was a world’s first, detailed FEA and fatigue analysis were carried out to ensure the connectors and tensioners were technically viable and could adhere to project specific requirements. Additional research was carried out in chain tribology, out of plain bending and corrosion analysis, which included full scale prototype testing of both the connector and the tensioner. Det Norske Veritas (DNV) reviewed and certified the connector and tensioner throughout various stages of the project from design approval to construction and testing.

The tether top connectors consist of two parts; the chain management structure and the central structural core. The central structural core is the main load bearing assembly in the tether top connector. It is designed to articulate to a maximum angle of +/- 15 degrees in a 360 degree orientation to allow buoy movement during service and to accommodate a survival load case. The chain is locked into position by a series of chain dogs, at the bottom of the central structural core, offering a one link tensioning resolution. The central structural core is designed to accommodate a maximum chain tension of 9,380 kN (956 mT). The chain management structure is a stationary assembly that interfaces with the submerged buoy, provides lifting points to deploy the tether top connectors subsea and manages any excess chain, ensuring that no collision occurs during the maximum articulation of the central structural core. The tether top connector was designed for a 27 year service life and a 270 year fatigue life. The connector was also provided with its own cathodic protection system that was calculated and designed by Balltec.

The tensioning system developed by Balltec is a temporary subsea installation device that can be docked onto the tether top connectors and tensions or slackens the tethers at an operating depth of up to 300 metres. The tensioner was also designed to a maximum tension capacity of 9,380 kN (956 mT) with a 1,750 mm stroke. The tensioner is controlled via dedicated PLC software and a control system developed by Balltec enabling the tensioner to be operated either from the surface or subsea using an ROV skid to provide the power and communications line.

Since starting the Guará Lula project, Balltec has grown as a business to accommodate the larger contract whilst maintaining the delivery of core products. In 2011 Balltec moved to larger premises totalling 75,000ft2 (6,968m2) and expanded their workforce across all departments. A 24/7 working pattern for the production and testing of the connectors and tensioners was introduced during the project execution. The additional space enabled Balltec to design and build the required rigs (for assembly, R&D, proof load and function tests) that enabled the successful delivery of the contract within 18 months.

The first buoy and set of connectors has been successfully installed, moored and tensioned, all of which was overseen by Balltec’s experienced technicians and engineers. The second buoy is currently being installed and tensioned.

Press Release, February 10, 2014; Image: Balltec