2007 News releases
10 October 2007
Balmoral Offshore Engineering launches new buoyancy system
With the continuing race by operators to find and develop reservoirs in ever deeper water the challenges to bring these fields on stream become greater. New deepwater discoveries are typically larger, with higher flow rates, where new riser technologies are playing a key role in concept selection and in meeting flow assurance demands.
Significant industry experience in design, qualification and manufacturing techniques has been gained in developing riser solutions on projects such as the use of syntactic foam to provide both the insulation and buoyancy for the hybrid riser tower on the Total Girassol project.
On subsequent projects the insulation and buoyancy were separated allowing the use of derivates of standard drilling riser buoyancy to be used. However, these systems can normally only tolerate temperatures up to 50ºC and can become susceptible to chemical degradation resulting in unsatisfactory through-life riser performance. Additionally, their mechanical performance relative to the massive uniaxial loadings is generally marginal.
The new challenges are to deal with greater water depths and higher temperatures as well as critical system design requirements including tolerances and high loads.
Bundle hybrid offset riser buoyancy
The use of bundle hybrid offset risers (BHOR), where a number of process pipelines are grouped together around a central tendon, is increasing as the industry strives to extract hydrocarbons from larger, more complex fields.
To meet BHOR performance requirements, Balmoral Offshore Engineering developed its Supatherm™ high temperature buoyancy system. BHOR buoyancy operates in a production – rather than drilling – environment, meaning the syntactic foam is exposed to much higher temperatures.
As Supatherm is a ‘pure’ syntactic material, ie, macrosphere-free, it tolerates temperatures exceeding 90ºC without significant buoyancy loss. The new material also demonstrates substantial mechanical advantages, such as compressive strength. When combined with other key advantages of Supatherm the highest possible safety factors required in critical service life patterns are delivered. See Figure 1: Supatherm™ combined strength illustration*.
Figure 1: Supatherm™ combined strength illustration*
In standard BHOR designs, buoyancy forces react against a tower top bulkhead rather than being transferred to the central core pipe every 15-25m as seen in drilling riser systems. This has introduced a new and critical system design requirement, specifically, the tolerance of massive uniaxial compressive forces simultaneous with the routine triaxial stresses from hydrostatic pressure.
BHOR buoyancy also differs from drilling riser buoyancy in terms of performance demands. It is designed to provide a far longer service life as it cannot be maintained, replaced or have any performance loss compensated for by tensioners, etc.
Balmoral’s technical director, Dr Robert Oram, was responsible for the development of Supatherm and said: “Critical manufacturing parameters such as time, tolerance, temperature, cure and post cure control are vital to the successful production of these specialised modules. The knowledge bank retained at Balmoral through extensive involvement in previous projects of this nature has benefited the development of Supatherm.”
The new material is available in three standard grades at operating temperatures up to 90°C: Supatherm 560, rated to 1000msw; Supatherm 580, rated to 1600msw; Supatherm 610, rated to 2000msw.
A highly flexible system, Supatherm can be cast into large complex modules, is readily machinable and can be site-trimmed for late design changes.
Global market potential
Offshore west-Africa is where this type of system is most likely to be deployed currently, but with opportunities arising in other deepwater provinces such as Brazil and south-east Asia, the worldwide potential for Supatherm is considerable.
Jim Hamilton, Balmoral Group director, spoke about the market opportunities for Supatherm: “We are bidding for a number of contracts where this material would be ideally suited. However, at this stage this information is commercially sensitive and we can only say that the projects involved lie in the deep waters offshore west-Africa.
“Working in alignment with the exploration and production industries we are confident that Supatherm will make a major contribution to the development of future deepwater high temperature projects.”
Pic: Typical bundle hybrid offset riser field layout
* Figure 1: Supatherm™ combined stress illustration
• Uniaxial Compressive Strength (UCS) That which resists the compressive forces developed by the buoyancy acting up the tower, reacting against the top reaction plate
• Hydrostatic Compressive Strength (HCS) The compressive forces acting in all directions from hydrostatic pressure
The graph shows the compressive forces acting on the syntactic foam at various positions (heights) up the tower. Values are quoted as ‘% Utilisation’ of the ultimate compressive strength at failure – the lower the % utilisation, the higher the safety factor.
Near the tower bottom, hydrostatic pressures are highest, but uniaxial compressive forces are lowest (as there is no buoyancy beneath). At the top of the tower, hydrostatic compressive forces are low (as water depth is low) but uniaxial compressive forces are high (as there is a lot of buoyancy below, acting on the material at the tower top).
The foam sees a combination of both uniaxial and hydrostatic forces so these have to be combined (providing the ‘Combined’ value shown) in order to identify how stressed the foam is overall.
The fundamental difference between Supatherm and traditional high temperature composite foams is that the composite foams are operating much closer to their failure values. Indeed, denser, stronger foams than the depth rating requires have to be specifically designed to avoid failure. Supatherm is far stronger and therefore operates a long way from any failure point.
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