Prelude Side by Side Offloading - Quantifying the Qualitative to Assess Economics and Operational Capability
Authors: McPhail Finlay , Auburtin Erwan , van Haaften Ewoud , Yates Darren , McConochie Jason , Leridon Aurelien , Lefebvre Timothee 
Published: April 2020

The Prelude Floating Liquefied Natural Gas (FLNG) facility is designed to offload Liquefied Natural Gas (LNG) and Liquefied Petroleum Gas (LPG) to carrier vessels moored in a Side by Side (SBS) configuration, using Marine Loading Arm (MLA) technology. This operation is novel and therefore could impact the project economics. The design was subject to extensive verification to ensure it could reliably support operations. Robust methods of quantitatively assessing an entirely novel, and complex, offshore operation were developed.

The high offtake cadence, exposed location, and range of different variables associated with the Prelude operation represented multiple technical challenges. There are few international standards which can be drawn from for this type of operation.

Limiting criteria for the operation have been developed through bridge simulations and consultation with marine technical experts. Additional limiting criteria are derived from marine hardware. Wind tunnel modelling, wave basin modelling, and other experiments have been performed. Innovative modelling techniques for multibody hydrodynamic modelling were developed to provide quantification of operational effectiveness and reliability. Novel operational assessment tools and method have been developed.

Hydrodynamic models have been developed for a range of different LNG and LPG carriers based on potential theory. These models include non-linear mooring characteristics, multi-body coupling, and coupling with sloshing effects. Multiple research projects have been executed to effectively model these different characteristics and validate the numerical modelling.

These models have been analyzed against a set of statistically representative MetOcean data derived from a 39 year hindcast of the Prelude location. The reduction of this data to statistically representative bins simplified modelling overhead. Multiple and increasing permutations of carriers, load conditions, and mooring configurations resulted in more than 100,000 time-domain simulations being required to evaluate the operation.

Combinations of different simulations and stages of loading to derive ‘window’ operability were established based on work with marine experts. Significant effort was applied to develop realistic models and assumptions for input to economic assessment and predictive tools for operational planning.

To mitigate the effects of a range of transient conditions, methodologies for employing the FLNG thruster system were developed in coordination with the marine and operations team. Methods for modelling these operational approaches were derived to enhance the operational effectiveness of FLNG and improve the predictive capabilities for operators.

Prelude represents the first of its kind for such a scale and exposed location. Significant novel methods for effectively analyzing such a complex situation were pioneered and refined for this project. There has also been an important feedback loop between operations readiness, operations, and analysis which has lead to safe and successful first operations. Measurements collected aboard Prelude and during future operations should enable further refinement and accuracy of modelling to support enhanced operational effectiveness.