BUOYANCY: Composite syntactic foam mid-water arch (MWA)21 Feb 2021
Ichthys Gas Field, West Australia
Composite syntactic mid-water arch
- World’s largest syntactic foam mid-water arch
- 170 msw depth
- 40 year design life
- Modular design
Mid-Water Arches (MWA) are large subsea structures suspended between the seabed and the surface to support and provide catenary shape to flexible pipelines or cables as they transition from the sea floor to the surface. Typical MWAs are constructed from steel with steel pressure vessels providing the necessary buoyancy to suspend the structure in the water column. They operate in relatively shallow water (50 to 100 m) and have a design life of up to 20 years.
The Ichthys field required MWAs to be situated in 170 msw of seawater for a design life of 40 years. The deeper the water, the greater the wall thickness needed for conventional steel pressure buoyancy tanks. Coupled with a large corrosion allowance for a 40 year design life, the cost and lead time of a conventional steel buoyancy tank would have made the MWA project technically and economically unviable.
Matrix Composites and Engineering, experts in providing composite solutions in harsh marine environments, were approached by McDermott’s to determine if composite syntactic foam material would provide the required buoyancy for the structure for 40 years.
Composite syntactic foam is used extensively in deep water drilling applications to provide buoyancy to drilling risers that stretch from the surface to seabed, distances that can often exceed 3 km. They are also used in long term immersion applications for discrete buoyancy modules located directly on flexibles.
Composite syntactic material does not corrode, is extremely lightweight and has miniscule loss of buoyancy even after 40 years continual submergence at deep ocean depths.
Unlike conventional steel buoyancy tanks, there is also no catastrophic failure mode of the structure due to the loss of integrity of a pressure containment wall, such as those occurring from an impact.
Composite syntactic foam is made from macroscopic and microscopic, pressure rated, hollow spheres encased in a binder. Should one sphere fail, the potential volume loss is virtually negligible in relation to the overall volume of the surrounding structure.
Steel tanks are baffled but each baffled section is still a large percentage of the total volume. A large amount of volume for buoyancy redundancy is required to compensate for any flooding of a compartment.
Each arch contained 40 large blocks made up of six smaller blocks, all containing syntactic composite foam.
A total of approximately 490 mT of buoyancy per arch was supplied. The use of smaller sub blocks enabled hydrostatic testing of the buoyancy to confirm that the correct amount of buoyancy was supplied and that the correct amount was still going to be maintained after 40 years field life.
Matrix constructed the syntactic foam blocks for the project on time and on budget, and met or exceeded the clients demanding quality and technical requirements.