Session: 01-01-01 Offshore Platforms-1

Submission Number: 179901

Low-Draft Semi-Submersible for Floating HVAC Substations in Long-Period Waves Offshore California 

Offshore California hosts the largest number of approved floating wind leases in the United States, reflecting significant interest in renewable energy development along the U.S. West Coast. Many of these commercial-scale floating wind projects are planned for water depths exceeding 1,000 m, where long-period wave conditions impose unique engineering challenges on offshore structures. In these environments, floating offshore substations play a critical role in collecting, transforming, and exporting electrical power from arrays of wind turbines to onshore grids. The design of these substations is particularly complex due to the presence of high-voltage alternating current (HVAC) transformers and associated dynamic export cables. These components are highly sensitive to platform motions, which can induce excessive accelerations, repeated bending, and cyclic loading, potentially compromising operational reliability and dynamic cable longevity. Additionally, offshore environmental factors such as waves, wind, currents, and corrosion further challenge the structural and functional integrity of floating HVAC substations.

To address these challenges, a simple, low-draft, and cost-effective semi-submersible hull concept—originally developed for regular steel catenary risers (SCRs) in the central Gulf of Mexico (GoM) and reported in OMAE 2025—has been adapted and optimized for floating HVAC substations. The adaptation involves tuning the hull geometry, weight distribution, and hydrostatic properties to minimize platform motions while maintaining manufacturability, transportability, and cost efficiency. Offshore California sites, such as Morro Bay, are characterized by extreme wave events, with 100- and 1,000-year storm waves exhibiting peak periods of 20.1 s and 21.8 s, respectively. These conditions present significant design challenges, as conventional hull configurations may exhibit resonant responses, excessive heave, and unacceptably high motions and accelerations that can damage both transformers and dynamic cables.

The adapted semi-submersible hull effectively minimizes heave and controls platform motions. It maintains response amplitude operators (RAOs) below 0.50 m/m for wave periods of 10–22 s and a heave natural period exceeding 24 s, ensuring stable dynamic responses under 100- and 1,000-year long-period storm waves. The hull’s draft is intentionally limited to ≤24 m, balancing motion performance with cost efficiency. These features collectively reduce dynamic cable motions and transformer accelerations, enhancing operational reliability and service life. Furthermore, the hull’s simple geometry and shallow draft simplify fabrication, reduce construction costs, and facilitate offshore transport, integration, and installation.

In addition to its performance advantages, the hull is designed for scalability and compatibility with a broad range of HVAC transformer configurations and dynamic cable layouts. Its modular design enables integration of multiple inter-array and export cable connections, supporting substations of varying capacities. To validate the concept, a case study was conducted for an 810 MW floating HVAC substation at Morro Bay, located in 1,100 m water depth. Preliminary analyses demonstrated that the hull maintains low motions under extreme wave events, meets operational acceleration limits for transformers, and ensures adequate durability for dynamic cables. These results confirm the feasibility, cost-effectiveness, and practicality of the proposed hull solution for offshore California conditions.

Overall, the study presents a low-draft, low-motion, and cost-effective semi-submersible hull tailored for floating HVAC substations in deepwater, long-period wave environments. Its simple and robust design, combined with adaptability to high-voltage transformers and dynamic cables, offers a scalable solution that addresses the technical, economic, and logistical challenges of offshore wind power collection and transmission. By validating the hull through a representative case study, the research provides a foundation for future deployment of floating substations in high-energy offshore regions, supporting the growth of commercial-scale floating wind energy in the United States.

Presenting Author: Jun Zou Floating and Green Energy Consulting, LLC

Presenting Author Biography: Dr. Jun Zou is the founder and president of Floating and Green Energy Consulting (est. 2023) and a former co-founder of Houston Offshore Engineering (HOE, est. 2005). He earned his Ph.D. in 1997 and served as Director of Naval Architecture at HOE and its successor entities for over 17 years. With more than 30 years of experience in the oil and gas industry and over 15 years in floating green energy systems, Dr. Zou has led or contributed to numerous offshore projects worldwide. His expertise spans conventional offshore platforms and innovative floating systems for renewable energy, including wind, hydrogen, and ammonia production. Currently, he focuses on advancing offshore solutions for the energy transition. Dr. Zou serves on the editorial board of Ocean Systems Engineering and has authored or co-authored over 70 technical papers on offshore structures and marine systems.

Authors:

Jun Zou Floating and Green Energy Consulting, LLC