Floating Giants: Taming the High Seas with Cutting-Edge FLNG Technology

The world of offshore energy is undergoing a revolution, spearheaded by the advent of Floating Liquefied Natural Gas (FLNG) facilities. These extraordinary structures represent a groundbreaking leap in offshore engineering, combining the entire process of natural gas liquefaction, storage, and offloading directly at sea, above offshore reservoirs. Imagine a floating city, hundreds of meters long—vessels like the Prelude, approximately 500 meters in length, are among the largest floating facilities ever built, pushing the boundaries of what's possible in marine technology. This new frontier in energy production is not just about scale; it's about unparalleled complexity and the brilliant application of advanced technology to conquer the challenges of the open ocean.

The Challenge of Floating Giants

For the industry, side-by-side offloading has emerged as the preferred method for transferring liquefied natural gas from FLNG facilities to LNG carriers. This choice is driven by the extremely low temperature of LNG, stored at around -167 degrees Celsius, as well as the need for safety and quickness in operations. However, positioning two massive vessels in such close proximity—sometimes with a gap as narrow as 4.5 meters—introduces significant hydrodynamic complexities that demand meticulous analysis.

The Core Hydrodynamic Hurdle: Gap Resonance

One of the most critical challenges is a phenomenon known as "gap resonance". While FLNG operations are typically conducted in relatively calm, "benign seas," the ocean is never truly still. Long-period swells, originating from distant storms, can travel thousands of miles to the operational site. Intriguingly, these low-frequency swells, which might not linearly affect vessel motion, have been shown in both literature and model tests to quadratically couple with gap resonance.

When this coupling occurs, the wave amplitude or height within the narrow gap region can become significantly elevated—far higher than the waves in the surrounding ocean. This creates what appears as standing waves in simulations and can lead to severe adverse effects on the motion of both the FLNG and the LNG carrier. Given that these multi-billion-dollar FLNG facilities are designed for years of continuous operation with frequent offloading, this "hidden danger" of gap resonance poses a critical safety concern, potentially disrupting crucial offloading activities. The focus, therefore, is on prevention rather than waiting for a real-life incident.

Industry 4.0 to the Rescue: Simulation, Validation, and Intelligent Design

To navigate these complex hydrodynamic interactions and ensure safe, efficient operations, engineers are employing a powerful blend of cutting-edge numerical simulations and rigorous physical model testing, embodying the very essence of Industry 4.0 principles:

• Sophisticated Numerical Simulations: Extensive numerical investigations are performed using advanced software platforms like DNV GL's SESAM (including HydroD and Sima). These simulations operate on potential theory, which models the fluid as incompressible, irrotational, and without viscosity, allowing for detailed analysis of wave interactions.

  • Frequency Domain Analysis is used to understand the fundamental hydrodynamic behavior of both single vessels and the coupled multi-body system.

  • Time Domain Analysis then simulates the global performance of the FLNG and LNG carrier hulls, along with their vital connecting systems. These critical systems include eight hawsers (thick cables providing linear force elongation coupling), four fenders (non-linear compressive elements preventing collisions), and a complex arrangement of fifteen mooring lines. A turret mooring system is employed, allowing the entire FLNG facility to "weather vane" and rotate in response to varying wind, wave, and current directions, maintaining optimal alignment.

  • A key challenge in numerical modeling is that potential theory tends to overpredict resonance responses in confined areas due to the absence of viscous damping. To achieve accurate results, a "damping lid" is imputed into the simulations, and second-order effects (Quadratic Transfer Functions, or QTFs), representing non-linear wave interactions, are incorporated using updated software versions.

• Rigorous Model Testing: To acquire the precise damping factors needed for accurate simulations and to validate their numerical predictions, engineers conduct physical model tests in specialized hydrodynamic laboratories, such as the facility at Newcastle University.

  • These experiments involve scaled models of the FLNG and LNG carrier, meticulously fixed at various drafts and gap distances. They are then subjected to carefully controlled regular and irregular waves from different directions, including head sea, oblique sea, and beam sea conditions.

  • Advanced wave probes and internal cameras precisely measure wave elevations within the gap, especially during resonance conditions where wave heights become "extremely elevated". The invaluable data collected from these tests is then used to select the optimal damping factors for refining the numerical models, bridging the gap between theoretical prediction and real-world behavior.

This integrated, data-driven approach allows engineers to thoroughly analyze how different wave directions impact vessel motion and the loads on critical connecting systems. For instance, studies have shown that aft hawsers and forward fenders can experience the highest loads under specific environmental conditions. Furthermore, innovative solutions like TechnipFMC's HiLoad LNG Parallel Loading System are being developed from proven technologies, designed to enable offloading even in harsh environmental conditions.

The Impact: A Safer, More Efficient Future

The insights gained from this advanced research are not merely academic; they are directly adopted by the industry, leading to significantly enhanced safety in side-by-side offloading operations. By developing algorithms that can predict when gap resonance will occur, and by integrating this intelligence into dynamic positioning systems that can help vessels adjust their position to reduce excessive wave elevation, these studies are actively preventing potential damage and collisions involving incredibly expensive FLNG vessels.

Beyond safety, the pioneering concept of processing natural gas offshore offers substantial cost reductions and environmental benefits by eliminating the need for extensive onshore infrastructure. This groundbreaking work also plays a vital role in job creation and has a positive impact on both local and global economies.