One of the biggest technological challenges facing the offshore industry today as it seeks to exploit oil and gas discoveries in deep and ultra-deep water is the reliable development of more advanced riser solutions.

Risers are a key part of any offshore development, whether in the drilling or the production process, and the challenges posed by the inexorable advance into water depths of up to 3,000 m (9,843 ft) and beyond are many. Not only is there a need for risers to be longer, larger, and stronger, but they also must be lighter, high-integrity, and able to deal with extreme pressure and temperature conditions both internally and externally. On top of all this, of course, operators would like them to be lower cost and more reliable than today’s available products.

This is no easy task, and it is why a concentrated series of studies under the Research Partnership to Secure Energy for America (RPSEA) umbrella have been and are still being carried out to help find new solutions. RPSEA, a not-for-profit organization, has been looking at a number of potential answers, including the following:

  • Lower weight, composite, high-pressure drilling risers;
  • The development of shrink-fit connections for high-pressure production risers;
  • A concept study on risers for high-motion vessels in
  • 3,048 m (10,000 ft) of water; and
  • A qualification program for two different flexible fiber-reinforced high-pressure production risers for use in 3,048 m of water.

A weighty problem

The industry and RPSEA outlined some of this work at the recent Offshore Technology Conference (OTC) 2013 Houston, where the US government-supported organization pointed out an obvious but very real limitation for ultra-deepwater risers – that additional weight is not a good thing.

However, at present steel solutions can only be made so light in weight before metal strength and integrity fail, RPSEA said in one technical paper, while riser margin – the pressure differential between the inside of the riser and the surrounding seawater – also is a big concern. Composites, common today, have never been stressed in the high-temperature, cold-water barrier found at deep-water seabed pressures, it continued.

And as for the longevity of risers, there are still many design and material limitations not completely understood regarding the reaction of the riser, whether steel, composite, or a combination, to commonly found hydrocarbon and associated chemicals. Add high pressures in excess of 15,000 psig and temperatures of more than 163°C (325°F) to the mix, and it is no surprise that no one wants to find out these limits with real-life failures.

“No company can realistically pay all the damages from a large spill. Therefore, mitigation strategies must begin with design,” said RPSEA, which has the general aim of pushing the technology to surpass 15,000 psig and temperatures above 177°C (350°F).

Risers for high-motion vessels

One of the most interesting areas tackled was riser concepts for high-motion (extreme heave, roll, and pitch) vessels, such as FPSO vessels and conventional semisubmersible facilities in ultra-deep water.

A separate OTC paper featuring conceptual research work for RPSEA by Stress Engineering Services pointed out that the steel catenary riser is the simplest riser configuration but that it has limitations when attached to such floating production vessels in water depths approaching 3,048 m. It also examined other types, including steel lazy-wave risers, steep-wave risers, compliant vertical-access risers, tension-leg risers, and hybrid riser towers. It used a design basis with a shut-in tubing pressure of 20,000 psi at the subsea wellhead for the study, which effectively restricted it to rigid pipe construction (steel, titanium, or fiber-reinforced composite) and eliminated currently available flexible pipe projects.

Technology gaps

The study, which had a supporting project working group at RPSEA representing companies including Shell, Petrobras, ExxonMobil, ConocoPhillips, and BP, flagged technology gaps for its design scenario, in particular those that represent concepts or aspects that appear analytically promising but have yet to be field-proven while also identifying preferred riser options. Those gaps were:

  • A swivel capable of withstanding internal pressure approaching 20,000 psi;
  • Thermal insulation capable of service at 177°C;
  • Riser-based flow-path components capable of containing 20,000-psi internal pressure;
  • Use of composite-reinforced pipe as a flowline/ production riser;
  • Use of a captive-turret FPSO vessel in the US Gulf of Mexico (GoM); and
  • Proven inhibitor methodology for 20,000 psi and 177°C.

Stress Engineering and RPSEA said in the technical paper that the next phase of evaluation will cover several different aspects, including analyzing a carbon steel riser sized for 20,000 psi shut-in tubing pressure as a base case for a lazy-wave riser for both a conventional semisubmersible platform and a captive-turret FPSO vessel. Both vessels have riser payload capacities that allow them to support heavy riser systems.

They added that for a disconnectable turret FPSO vessel, the discrete buoy concepts – tension-leg risers and hybrid riser towers – are the most promising from the standpoint of riser payload because the host vessel supports only the jumpers spanning from the turret to the submerged buoy.

The companies also will study riser concepts that meet an assumed 3,000-kip riser payload limit of a disconnectable turret FPSO vessel, including a titanium catenary riser, a composite reinforced pipe shaped as both a lazy-wave riser and a steep-wave riser, and a compliant vertical-access riser.

The end objective, according to RPSEA, is to find innovative ultra-deepwater riser concepts that can tackle the problem of providing acceptable extreme storm and longterm fatigue response on these vessel types.

More solutions in the mix

Other work being undertaken as part of the riser study program includes a novel hybrid (composite/metal) flexible deepwater pipe technology being analyzed for RPSEA by GE Oil & Gas.

Another by DeepFlex has the objective of field-qualifying a single flexible fiber-reinforced pipe riser solution for use in the US GoM. Flexible fiber-reinforced pipe is unbonded flexible pipe with composite reinforcement layers, which has the advantage of lighter weight, high flexibility, and corrosion resistance. This enables a simple, low-top tension riser configuration.

The 7-in. internal diameter, 10,000-psig solution being studied should – in theory – help to enable installation vessels and floating production systems previously limited to water depths of 914 m to 1,524 m (3,000 ft to 5,000 ft) to be used in depths of up to 3,048 m. This would not only significantly expand the number of existing vessels that could be used for such work but also substantially reduce overall project cost and risk.

The results of these various phased riser studies will continue to emerge from RPSEA over the course of the next year or two, driven by the industry’s need to find and extract new oil and gas reserves from extreme water depths and under severe pressure and temperature conditions.

Editor’s Note: Excerpts for this article have been taken from sources including OTC papers 24229 and 24195.