Riser tower buoyancy

Balmoral riser tower buoyancy

Water depths, seabed conditions, high pressures and high temperatures are all factors to be considered when designing systems for the extraction of hydrocarbons in deep and ultra-deepwater fields.

Ultra-deepwater fields have attracted much attention in recent years and have generated a range of riser solutions to transport the produced hydrocarbon fluids to the surface processing facility. In some cases these solutions reflect the particular expertise of the main contractor while project-specific requirements dominate in other scenarios.

Established solutions include steel catenary risers, flexible flowlines in wave or MWA configuration, single leg top-tension risers and hybrid riser towers. The last-named refers to multiple risers grouped around a single core pipe where the entire assembly is supported in vertical orientation by a massive buoyancy assembly. Due to operational depths going to some 2000msw the majority of solutions require buoyancy to provide hydrodynamic stability and/or riser tension reduction.

With the particular field configurations, depths of reservoirs and relatively benign sea states found offshore West Africa, the hybrid riser tower (HRT) has assumed a dominant position. With the proven success of HRTs in four major West African projects to date the same solution offers significant potential for other deepwater projects both there and elsewhere in the world.

The hybrid riser tower typically comprises 4-12 individual risers grouped around a central core pipe anchored to the seabed and supported by a massive air can at the top. Further to this tower-top air can, buoyancy is routinely provided along the length of the bundle and on occasion at the upper and lower riser tower assemblies – URTA and LRTA respectively.

The primary purpose of the bundle, URTA and LRTA buoyancy systems is to facilitate surface tow-out of the bundle, however the same buoyancy also contributes to the stability of the vertical HRT when in service.

Durafloat HT™ high temperature buoyancy

In project situations involving low wax-content oil or high wellhead temperatures, ie, where the insulation requirement for process fluids is relatively modest, it is usually most cost-effective to provide the insulation requirement for production and gas lift risers within a hybrid riser tower by means of external ‘wet’ insulation coatings on the individual risers.

In this situation, the HRT syntactic foam is provided for buoyancy purposes only. The primary buoyancy requirement here is to facilitate surface tow-out of the assembled HRT, however the HRT syntactic buoyancy modules also augment the tower top steel buoyancy can in providing long-term stability of the vertical HRT after installation and may also serve to reduce tensile loadings within the central tendon pipe.

Here, and in direct contrast to the situation where the syntactic foam is intentionally providing thermal insulation, the module and riser systems are specifically designed to minimise exposure of the syntactic foam to elevated temperatures as a result of proximity to hot risers. This is because syntactic foam, as a polymer material, loses mechanical properties when the temperature rises. As a consequence, denser, stronger and therefore more expensive foam is required for elevated temperature than would be for service at deep ocean temperature.

Due to the modest thermal efficiency of most deepwater wet insulation materials, and despite design efforts in mitigation, there is often a degree of local water heating surrounding insulated lines which, in turn, results in local heating of adjacent syntactic foam modules.

Through extensive development, qualification work and actual HRT project experience, Balmoral has unrivalled knowledge of the syntactic foam design requirements for the entire spectrum of service depths to 3000msw and foam exposure temperatures to 55°C. The loss of mechanical performance associated with increased temperature is inversely related to the difference between the glass transition temperature (Tg) of the epoxy systems in the syntactic foam and the foam exposure temperature.

For foam exposure temperatures up to absolute maximum 30°C, increased density ratings of the standard syntactic foam components used in drilling riser buoyancy foam may be employed. However, for critical service at temperatures from 30-55°C, Balmoral developed a syntactic foam system containing special high-Tg epoxy syntactic and composite macrospheres which delivers both superior mechanical properties at all temperatures of interest and also shows reduced loss of mechanical performance with increased temperature. This generic foam system, called Durafloat HT, was supplied for use on the hybrid riser tower modules and associated buoyancy for the CLOV project in Block 17, offshore Angola.

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