Bundles and towed installation

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Bundles and towed installation

3. Carrier pipe design : The diameter must comply with the geometric restraints of the flow lines and umbilical components. The wall thickness must suit the internal N 2 pressure onshore (but necessary to provide balanced internal pressure so avoiding hydrostatic collapse during tow-out). The wall thickness must be sufficient to accommodate pull forces during the beach launch and the tow offshore to the site location. It may be thickened slightly towards the ends to withstand the bending stresses resulting from the tow-out. Where the carrier is to be used for heating either by hot water flowing through the annulus or heating pipes, a thermal coating may be needed on the carrier. 4. Ballast chains : The centres and number of links are determined to provide near-neutral buoyancy of the bundle and towhead assemblies in seawater prior to and during the tow (minor adjustments using trimming links are made by divers prior to towing). This iteration can only be undertaken when allowance for all other weights have been made. It may involve changing the carrier diameter. Or some of the flowlines may be towed out flooded. The total submerged weight of all pipes, nitrogen, chains etc is just slightly heavy – with the bottom links of the chains only partly supported on the seabed. 5. Bulkheads : The thickness is sized to suit the forces due to internal N 2 pressure combined with the thermal expansion loads (for a number of case combinations), in order to avoid product leakage. Radiuses are used to reduce any stress concentrations. 6. Tow heads : Separately from the above, the tow and trail heads are designed to be near-neutrally buoyant during tow out. They must be stable during towout and transmit all forces into the bundle. They must accommodate and support all the valve and pipework within the structure. In areas of high fishing effort, they must provide protection against impact. Once in place, they should remain stable against environmental forces. This may require the incorporation into the structure of guides for piles. 7. CP: Both the carrier and towheads require anodes to provide external corrosion protection.

BUNDLECARRIERPIPE

BUNDLE CARRIER PIPE

ƒ Sized to contain flowline pipes and umbilicals ƒ Allowance for assembly ƒ Provides buoyancy ƒ C of G below C of B ƒ Wall thickness sizing ƒ To withstand assembly, tug pullout forces and expansion loads ƒ Safely contain internal pressure of N 2 on

Carrier

Roller on main spacer

construction site, suitable for maximum water depth