Åsgard environment impacts riser buoyancy, workover umbilical

Pressure, temperature extremes elicit
new approach for production design

Balmoral's dynamic flexible riser buoyancy modules developed for the Åsgard project, illustrating the company's internal composite clamp tensioning mechanism.

With exploration and field development in the offshore industry moving into more hostile and deeper waters, the demands on the buoyancy required for these projects becomes ever more rigorous. An order placed by Coflexip Stena Norge with Balmoral Composites in Aberdeen for the design, manufacture, and supply of dynamic riser modules for the flexible production riser system on the Åsgard project off Mid-Norway, proved to be no exception.

Riser buoyancy modules are used to generate upthrust to the riser in order to maintain the pliant wave configuration of the system. The density and composition of the syntactic foam required for the task was selected by Balmoral on the basis of design criteria imposed by elements such as hydrostatic pressure, water ingress, and elastic/inelastic compression.

Current and wave loadings that the riser buoyancy modules must withstand are exceptionally high, requiring overall size restrictions on the modules to minimize the loads inflicted upon them. In addition, multi-axis water channels were incorporated into the rotationally molded module shell to allow convection currents in the seawater to circulate, whatever the orientation of the modules.

Internal clamps are used to prevent the buoyancy modules sliding along the riser. They must achieve this by friction alone, without applying excessive pressure or damaging the outer protective layers of the risers. They must also cater for expansion/contraction (both thermal and mechanical) and creep in the risers. This means that the clamp tensioning mechanism has to function as an elastic spring throughout its operational life, maintaining sufficient pressure in the risers to prevent slippage.

The high reservoir temperatures on the Åsgard project introduced a new dimension to the equation with the need to use high specification materials to prevent localized heat build-up in the riser beneath the modules. The thermal conductivity of the internal composite clamps was increased by using a specially developed syntactic matrix, designed to minimize any increase in density which would have had an effect on the module size and loads.

The size and demands of the order also required radical changes to both production and processing methods, and new processing equipment was developed by Balmoral to meet these needs.

Internal umbilical

Åsgard also provided one of the most testing assignments to date for the UK/Norwegian joint venture of JDR Cable Systems and ODIM. The pair recently delivered their package to main contractor Kongsberg Offshore for the Christmas tree workover control systems for both Åsgard and the Gullfaks Satellites. These comprised 500-meter-long umbilicals, handling winches, electro-hydraulic stabplates, tie bars, and bend restrictors.

For all parties, the main challenge was accommodating the landing string umbilicals within the annulus of the workover riser and the marine riser. This arrangement, which was specified partly for cost-cutting purposes, would impose severe strains on the control systems because of the attendant temperatures and pressures.

Conventionally, an umbilical located outside the workover riser has to cope with temperatures up to 60°C. The internal Åsgard/Gullfaks umbilicals, however, will encounter readings of 135°C at the workover riser wall. Control fluid in the hoses will at times heat to 100°C, while the marine riser annulus fluid will fluctuate between 10-135°C. The Åsgard system must survive intact for 20 years, which is the field complex's predicted lifespan.

Key considerations for Littleport-based JDR were:

Compatibility problems between the service fluids and hose liner materials
Fitting retention with the hoses, in light of the high pressures, as well as temperatures
The limited space afforded by the annulus, restricting the outside diameter of the umbilical to just 85 mm.

JDR's solution was to employ spirally wound, steel-reinforced 10,000 psi polyflex hoses with PDVF liners in the center. Polyflex hoses have a much smaller diameter than conventional thermoplastic hoses, and they also offer low volumetric expansion. However, the sheath itself is made from thermoplastic rubber, which can work at 135°C.

A high temperature test was performed on the sheath material to prove that it could survive the clamping forces, as well as to highlight any potential sheath deformation. Accelerated tests were also executed on the Castrol control fluids.

A new slimline subsea termination also had to be designed, no bigger than the umbilical itself, to fit within the riser annulus. JDR adopted principles used in its seismic exploration gun umbilicals, where terminations must be slim to limit drag as the cables are towed through the water. The resultant configuration on Åsgard/Gullfaks is a fan of 17 hose tails, all with quick release couplers, each capable of withstanding the high temperatures predicted.