Component Description
Our Distributed Buoyancy Modules comprise of an internal clamp and a buoyancy element together with a tensioning assembly and fastening system. They are installed using an efficient, fast and safe offshore installation procedure.
This is bolted directly onto the riser to securely lock the buoyancy module axially onto the riser. Each clamp assembly consists of the following components and incorporates several key patented features. It is without doubt the most effective means developed to date of securing buoyancy modules to flexible pipes.
Cast from a tough and rigid epoxy resin composite, the buoyant clamp body has a cylindrical geometry and is split into individual segments to allow installation around the riser. This segmented configuration allows the clamp to fit a range of pipe diameters within a specified manufacturing tolerance. The bore of the clamp can be engineered with a smooth or profiled surface to suit the particular pipe structure it is being clamped to. The clamp body is also used to locate the circumferential strap.
In order to generate the clamping pressure required to ensure the clamp/buoyancy module does not slip when subjected to external loadings (e.g. wave slam), a tensioned titanium circumferential flat strap is applied around the circumference of the segmented clamp body. The design of the strap is:
- Capable of withstanding high tensile loads
- Capable of storing elastic energy to cope with pipe contraction
- Strong enough to withstand increases in tension due to pipe expansion
- Corrosion resistant in seawater
- Lightweight
The titanium circumferential strap is tensioned using a patented titanium cross bar and screw arrangement. Rotation of the screw with a defined torque causes it to rotate, pulling the cross bars together and hence generating the required tension. Locknuts are then tightened against the cross bars to prevent loosening during the design life. Alternatively the strap can be pretensioned using a hydraulic tensioning system and held in place using tension bars and lock nuts.
The buoyancy element, which generates an upthrust to the flexible riser, is normally supplied in two identical halves comprising of a syntactic foam core with a polymer external skin which fits around the internal clamp securely fastened together by a system of Kevlar straps and super duplex tensioners.
Syntactic foam core - each half of the buoyancy element has a composite syntactic foam core, the properties of which can be adjusted to suit the full range of seawater depths from 100msw to 6,000msw and beyond
Polymer external skin - the syntactic foam core is fully encapsulated within a high performance polymer external skin to:
- Provide a continuous and aesthetically pleasing, highly visible surface
- Protect the foam core from abrasion, external impact and marine growth
- Provide the required external profile
The buoyancy element is of a cylindrical configuration split axially. The internal bore is provided with a recess which accommodates the internal clamp. The bore also flares towards the module edges which ensures that contact between riser and element does not occur even when the riser is at the minimum operating bend radius.
A set of holes through the external skin and foam core in the axial direction provide useful handling points, and the external diameter of the module is fitted with a set of circumferential grooves and recesses which accept the fastening system.
When fitted around the riser and internal clamp the buoyancy element halves require to be secured together. This is achieved by fitting two sets of securing straps and tensioning assemblies.
Each securing strap consists of a flat band (usually Kevlar), tensioned by a conventional titanium cross bar and screw arrangement with the fibres (not twisted or braided) parallel and longitudinal to the applied tension. The straps fit into grooves in the element's external diameter so that the assembly profile does not project beyond the external skin of the buoyancy element at any point.
After tensioning the toggle assembly is secured by means of a locknut to prevent loosening during the design life.