Mid Depth Buoys Design for Long Life Using Syntactic Foam Buoyancy, Redundant and Replaceable Tether Systems
- G. E. Aberle (ICON Engineering Pty. Ltd.) | L. Christensen (McDermott International, Inc.)
- Document ID
- Offshore Technology Conference
- Offshore Technology Conference Asia, 22-25 March, Kuala Lumpur, Malaysia
- Publication Date
- Document Type
- Conference Paper
- 2016. Offshore Technology Conference
- 4.5 Offshore Facilities and Subsea Systems, 4.2 Pipelines, Flowlines and Risers, 4.2.4 Risers, 4.5.3 Floating Production Systems, 4.5 Offshore Facilities and Subsea Systems, 4 Facilities Design, Construction and Operation, 4.5.4 Mooring Systems
- Replaceable Tethers, Syntactic Foam Buoyancy, Redundant Tethers, Mid Depth Buoys, 40 Year Design Life
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Riser systems utilize various configurations depending on water depth, size, quantity, environment etc. In the Ichthys development, three systems are utilized, Fixed-S, Lazy-S and Lazy-Wave system. The riser system design  and the riser support structure  in the Fixed-S configuration are the subject of other papers and the current paper is related to the mid depth buoys in the Lazy-S configurations.
Two large MDBs each supporting five risers were designed for the initial phase of the development, one for each mooring sector of a Floating Production Storage and Offloading vessel (FPSO) which has a 3×7 mooring system.
Each MDB is moored to a gravity base via primary chain and redundant, secondary polyester tether systems. Large syntactic foam modules supported in a latticed buoy structure space frame provide the buoyancy to support the riser system for a 40-year design life.
This paper presents the evolution of the design, featuring:
Securement of syntactic foam in the buoy structure, to accommodate dimensional change of foam.
Dimensional control of the buoy structure and syntactic foam modules, during fabrication.
Configuration of the structures to cater for efficient installation of large volumes of syntactic foam during fabrication.
Tensioning system to manage the redundant secondary tether system.
Contingency "Diverless Tether Replacement Procedure" to design features for future implementation if needed.
Management of the system net buoyancy throughout the design, fabrication and installation processes.
Syntactic foam buoyancy in long life, mid depth buoys avoids the need for the certification and maintenance of pressure vessels, as can be required of steel tanks. Testing verified that bulk syntactic foam exhibits minimal degradation from dropped object, impact damage. Coupled with an efficient structural containment, the sensitivity of the design to "damaged" state conditions was essentially eliminated.
Chain tether systems are ideally suited to conventional mid depth buoy installation techniques. Fully redundant secondary tether systems, can be effectively incorporated into buoy system designs, to enhance greatly the system reliability over long operating lives. The manufacturing length tolerances of tether systems required a unique system for managing the ‘in place’ tension of secondary tethers to ensure their longevity.
|File Size||3 MB||Number of Pages||15|