Extending subsea tiebacks with OTEC provides a sustainable option for deepwater marginal fields
Key Highlights
- OTEC floating installations (OFIs) can reduce offshore tieback costs by up to 48% beyond 20 km, especially when supporting power, chemicals and water injection.
- The technology eliminates the need for long-distance umbilicals and booster pumps, decreasing infrastructure complexity and operational risks.
- Initial fixed costs for OFIs are higher, but they become more economical than traditional systems at distances over 20 km, with breakeven points around 14.5 to 21 km depending on system size.
By Dan Grech, Global OTEC; and Yann Helle, 2H Offshore
The economic viability of marginal offshore oil and gas reserves often depends on the costs and complexities of long subsea tiebacks. Once a host facility is established, operators are naturally inclined to maximize its use by linking nearby satellite fields. Tiebacks extend the life of existing infrastructure, boost recovery rates and unlock additional value from surrounding resources, all while generally requiring a smaller incremental footprint than greenfield developments.
Historically, beyond 50 km, tiebacks start to face much bigger challenges. The delivery of power, chemicals and water injection (WI) over such distances drives up the costs and operational risks.
Conventional setups route energy for well control, pumping and chemical injection from a central floating production unit (FPU), but this often results in oversized umbilicals, significant power losses and costly infrastructure. The longer tiebacks also place additional demand on FPUs, as many were not designed to support new subsea wells. This leads to higher fuel consumption, increased CO2 emissions and added congestion from risers. As a result, their implementation can become economically impractical.
A potential solution for this lies very close to these operations in the ocean itself. Through ocean thermal energy conversion (OTEC), which harnesses the temperature difference between warm surface seawater and cold deep seawater, localized clean baseload power can be generated 24/7 all year round. Through moored platforms, named OTEC Floating Installations (OFIs), the need for long-distance umbilical and WI lines can be reduced or even eliminated, directly influencing costs and technical challenges.
Envisioned as modular units acting like waste-heat-recovery systems, equipped with organic rankine cycle systems and chemical storage tanks, the OFIs can provide local energy for electric subsea trees, pumps and chemical injection. The benefits of this approach include lower infrastructure costs, reduced diesel consumption at host facilities and a smaller carbon footprint.
Case study
According to a case study conducted by Global OTEC and 2H Offshore, OFIs become more cost-effective than conventional setups beyond roughly 20 km. At distances of about 60 km, capex savings can reach up to 48%, particularly when avoiding WI booster pumps.
The basis of this low-temperature differential organic rankine cycle plant was a grid-connected OTEC facility in Hawaii, which Global OTEC has used to model the thermodynamic states and performance. It validated crucial net-power output and techno-economics of using seawater alone to power a plant.
The simulation considered an OFI designed to generate 2 MW of continuous clean electricity and compared multiple tieback configurations across distances of 1 km to 100 km, including variations with and without subsea WI, pipe-in-pipe versus standard production systems, and scenarios with OFIs providing full or partial support.
Using a bottom-up installed cost model, results showed that while conventional systems become increasingly expensive with distance due to pipeline and umbilical costs, OFI costs remain constant. A crossover distance is identified in each case, representing the point at which OFIs become more cost-effective than FPU-based systems. This analysis highlighted OFIs as a potentially scalable, lower-cost and cleaner solution for marginal field development in tropical deepwater regions.
The analysis ran through installed costs across three subsea tieback configurations:
- A conventional base case with no OTEC;
- An OFI providing power and chemicals; and
- A fully integrated OFI including subsea WI.
While the OFI introduces a significant fixed upfront cost ($120 million to $200 million depending on configuration), these expenses are offset as tieback distances increase, due to the elimination of costly umbilicals and WI lines. This advantage becomes especially clear when the OFI supports all three services (power, chemicals and WI) where infrastructure savings are maximized.
Costs rise sharply at 100 km in the base case, reflecting the need for a subsea booster pump, which further strengthens the case for localized OFI solutions at greater distances.
The cost modeling across both standard (8-ft + 3-ft GSPU) and pipe-in-pipe (PIP) production line systems showed that while conventional setups are highly distance-sensitive, OFI-enabled configurations maintain relatively stable costs, with breakeven thresholds emerging as tieback length increases. By excluding booster pump costs, the analysis isolated the impact of OFI deployment versus the base case, allowing for a direct comparison of infrastructure requirements. The results revealed that breakeven occurs at about 21 km for a single-well tieback, falling to about 14.5 km for larger three-well systems. These consistent breakeven points across both standard and PIP lines highlight how OFIs reduce long-distance cost sensitivity by removing umbilical and WI line requirements.
Ultimately, while OFIs demand a higher initial investment, their delivery results in a compelling economic advantage beyond moderate tieback lengths, particularly in standard configurations where lower baseline infrastructure costs accelerate parity.
The main result of this analysis showcased that an OFI can deliver significant cost benefits up to 48% in long-distance subsea tieback applications, for its localized clean power delivery, chemical injection and optional WI at the well site. This can vary depending on the system configuration, but OTEC-enabled solutions become more cost-effective than traditional tiebacks at distances of about 20 km, with continued cost-effectiveness if you go beyond this distance.
The fixed cost nature of the OFI introduces early investment, but it flattens out infrastructure-related cost growth, especially in high-complexity or booster pump-triggered scenarios. Added to that, there is also a strong potential to reduce CO2 emissions and support more sustainable offshore developments, which aligns with the need to decarbonize offshore operations while allowing them to reach wider distances.
Potential for more global deepwater applications
Viable in all tropical and subtropical ocean waters with surface temperature of about 26°C and deep waters of 5°C, OTEC has an immense potential, considering that there are 293 operating or planned deepwater fields located in the waters suitable for OTEC, notably in regions such as Guyana-Suriname, Brazil, West Africa and Southeast Asia.
OFIs could provide a commercially attractive, lower-carbon alternative to conventional tieback strategies, reshaping how offshore reserves are developed in the years ahead. As the offshore sector seeks solutions that balance economic efficiency with environmental responsibility, OFIs powered by OTEC technology may represent a breakthrough—a path to extend the productive life of existing infrastructure while unlocking reserves once considered uneconomical.
About the Author
Dan Grech
Dan Grech founded Global OTEC to advance offshore renewable power solutions for island infrastructure and oil and gas applications. Since 2017, he has built and managed multidisciplinary teams developing ocean thermal energy conversion (OTEC) and organic rankine cycle (ORC) systems from early concept through detailed engineering. His work includes progressing the OTEC Power Module designs.
He has secured industrial collaborations with turbine and heat-exchanger manufacturers, and operator-driven studies to benchmark OTEC against subsea tiebacks and offshore electrification options.
Grech is co-author of “Ocean Thermal Energy Conversion (OTEC)” in Comprehensive Renewable Energy (Second Edition) (Elsevier, 2021), a reference text on renewable energy technologies. He also has led international collaborations and funding rounds that established Global OTEC as a technical reference point for scalable offshore clean energy.
Yann Helle
Yann Helle is group managing director for 2H Offshore and has more than 23 years of engineering and global leadership experience. He holds an Executive MBA from Imperial College London, an MEng (Hons) in mechanical engineering from Kingston University and is a chartered mechanical engineer (CEng MIMechE).
Yann oversees more than 300 engineers across 11 international offices, specializing in riser and conductor systems, minimum facility platforms, and project delivery from exploration through to production. He has a strong track record in engineering, component supply and large-scale project management.
He is currently driving the company’s expansion into renewable energy and energy transition systems, including offshore wind, carbon capture, water intake solutions and advanced cooling systems for energy generation.