UNITED KINGDOM:

Thermal buoyancy materials for deep water

Deepwater oil fields provide the industry’s best hope for replacing depleting reserves around the world. But exploiting such fields requires systems and equipment that can withstand extreme thermal conditions.

Aberdeen-based Balmoral Group has developed solid and liquid-based syntactics (foam polymers) that provide buoyancy during installation and thermal properties during operation. These materials feature optimum insulation performance, temperature tolerance, collapse resistance and hydrothermal stability in depths to 3,000 m (10,000 ft) and temperatures to 90°C (194°F). Balmoral’s Ultratherm can accommodate direct water exposure at the highest temperature without any form of protective coating. Water ingress is low (<2% over 20 years at 90°C /194°F) and triaxial creep unmeasurable under practical conditions.

These syntactic systems have various benefits such as: cast-in-place capabilities for complex steel geometries or field joint locations where precast sections are not suitable; greater flexibility to accommodate pipe movement in expansion loops or the stresses and strains of reelship installation; and filling capabilities of a liquid with the best characteristics of a rigid syntactic material, which is particularly valuable in pipe-in-pipe insulation systems.

Applications. These new materials have been used in several projects, the best known being the giant Girassol field offshore West Africa in depths of 1,350 m (4,428 ft). The tremendous pressures found at such depths greatly challenge materials used in the production system such as risers, flowlines, pipelines, mooring lines and subsea equipment.

Installation and operation of riser and flowline systems is a technically challenging endeavor. Low wellhead temperatures, due to seabed temperatures of 4°C (39°F), tend to increase oil viscosity and promote paraffin and hydrate formation.

Traditional subsea buoyant insulation systems can readily be separated into two service-related groupings:

• Low-density blown foams typically based on polyurethane or PVC, with good / excellent insulation performance but poor depth capability (<300m/984 ft)
• High-density elastomeric or thermoplastic solids, dense foams or syntactics with poor / modest insulation performance but medium / good depth tolerance (>300m/984 ft).

For the ultimate combination of deepwater capability and good insulation performance, the subsea design engineer has traditionally only had steel "pipe-in-pipe" systems at his disposal. With the increasing interest in extreme-depth developments, the wall thickness of the jacket pipe necessary to handle this type of hydrostatic pressure creates problems with lay barge tensioner loads and potential tensile failure of the pipe catenary during installation.

Engineering contractors have been quick to recognize the major advantages offered by the low density and excellent insulation performance of epoxy syntactic foams in addressing these previously near-irresolvable problems.

Girassol. This project employs strap-on syntactic foam modules to meet the combined buoyancy and insulation requirements of both the flowline bundles and hybrid riser towers.

The basic service conditions are themselves extremely challenging – hydrostatic pressure tolerance to 160 bar (2,320 lb/in.²) and temperature tolerance to 90°C (194°F). Additionally, the strap-on "flooded modules" design necessitated an insulation system capable of delivering minimal loss of buoyancy and insulation performance over 20 years in combined "hot/wet" service.

Heat-stabilized derivatives of the standard, minisphere-containing, syntactic foams currently used as subsea buoyancy appear suitable for a Girassol- type project, but they are inherently susceptible to base-catalyzed hydrothermal degradation. Also, minispheres inevitably have unacceptable long-term permeability at certain temperatures.

Recognizing the nature of the material performance requirements for Girassol, Balmoral developed a new syntactic system to achieve maximum guaranteed performance without any preconceptions as to the chemical or mechanical structure. The result was Ultratherm, which has:

• Ultra-high Tg, aromatic amine-cured epoxy matrix, providing long-term hydrothermal degradation resistance to temperatures in excess of 100°C (212°F)
• Pure (solid) syntactic construction, to provide insulation performance with minimum long-term water-ingress resistance and elastic / inelastic compression.
• Computer-controlled cure management system to optimize mechanical properties and allow manufacture of pure syntactic castings of dimensions previously considered impossible in a production environment.

Balmoral provided the insulating buoyancy for three riser towers and seabed flowline bundles. Each tower is 1,300 m (4,265 ft) long and 1.45 m (4.75 ft) in diameter. Six foam sections enclosing 12 lines within the tower structure resulted in 3,876 standard tower modules of volumes up to 1.4 m³ (49.44 ft³).

The flowline bundle modules are contained within a flooded carrier pipe and are 700 mm x 5.7 m (27.56 in. x 18.7 ft) long. With an aggregate bundle length of 19 km (11.78 mi), this provides a total of 7,612 flowline bundle modules, up to 1.35m³ (47.67 ft³) each.

In addition to the "standard" riser tower modules, special units were required to provide insulation and buoyancy over the transition from the insulated line pipe spools at the tower base to the main riser tower systems.