Side by side: offloading LNG on the open seas

Prelude, Floating Liquefied Natural Gas

Shell’s Prelude

A new and innovative technology; Shell’s Floating Liquefied Natural Gas (FLNG) facility, Prelude, is being deployed to meet the increasing global energy demands.

With a hull measuring 488metres, Shell’s Prelude is the largest floating gas facility in the world.

Anchored to the sea bed in 250m water at its location 475km north-north east of Broome, Western Australia; it has the capacity to produce at least 5.3 million tonnes of hydrocarbon liquids per annum.

“FLNG is a game changer in offshore hydrocarbon development”

Dr Wenhua Zhao

Prelude is anchored 475km off the coast of WA

Remaining at sea, Prelude will extract, liquefy and store liquefied natural gas (LNG) until it is offloaded to a shuttle tanker and exported to customers globally and directly. It is predicted that by 2019, Australia will be the largest single exporter of LNG in the world (APPEA, 2016).

The novel technology promises safer and more secure energy, reduces costs and has less environmental impact than traditional energy sources. However, remaining in deep water and with higher well temperatures, offloading LNG offshore presents new technical challenges.

Researchers at UWA’s Centre for Offshore Foundation Systems (COFS) work closely with Shell to meet challenges associated with reliable offloading of LNG; a critical component for the successful implementation of the technology.

Side-by-side offloading process

FLNG & LNG carrier in side-by-side offloading operation

Floating production storage and offloading (FPSO) units typically offload crude oil via an offloading line which sits on the sea surface and connects to a shuttle tanker approximately 100m downwind of the FPSO.

Prelude’s LNG will be offloaded using cryogenic loading arms to an LNG carrier parked in close proximity (roughly 4 metres) in a side by side vessel configuration. The LNG remains in a liquefied state at minus 162 degrees making it easier and safer to transport.

Crucial to its success, is discovering a reliable offloading process to transfer liquid cargo onto a carrier.

“This will be the world’s first side-by-side offloading operation in the open seas”

Dr Wenhua Zhao.

Dr Zhao with a side-by-side model in wave basin

Offloading is sensitive to variable weather and ocean conditions (e.g. beam and quartering seas), side-to-side rotation of the LNG carrier, wave motions and different filling conditions of a liquid cargo.

In 2014, Dr Zhao together with Profs M. Efthymiou, P. H. Taylor and Drs. H Wolgamot, S Draper focussed their research on hydrodynamics of multi-body systems in side-by-side offloading configurations to predict the performance of LNG offtake from FLNG to LNG carriers.

Roll motion

Vessel model moves in the wave basin in Shanghai

A specially scaled vessel model was designed to enable researchers to simulate unique issues including:

• vessel motions (roll in particular) in a side-by-side configuration (coupling)
• transferring liquid inside pairing tanks sloshing in a non-linear motion
• the effect of partially filled tanks containing liquid cargo on the overall roll response of a vessel
• Variable loading and oceanic conditions

Gap resonance

narrow gaps can hinder offloading

A series of scaled model tests were conducted to determine the likely behaviour of fluid oscillating within a narrow gap between ships operating at sea (Response Amplitude Operator) with a view to application in numerical modelling. This UWA funded fundamental research was conducted in a world-class large wave basin, and focused on:

• two side-by-side fixed bodies in deep water
• narrow gaps during side-by-side cargo offloading

For the first time, it is observed that the viscous damping in the gap has a linear form (at least) at the large model scale testing. This valuable observation has facilitated the numerical predictions as shown in the academic paper through an international collaboration with the ITRH Hub (http://offshorehub.edu.au/) Partners from LR Canada, BV Singapore, Shell and Academic from Oxford University UK. Varying ocean weather conditions were also simulated during the basin model tests.

This research was funded in part by Shell and UWA and was conducted in the Deepwater Offshore Basin at Shanghai Jiao Tong University as a joint program with the University of Western Australia. The work has been continuing with the support through the ITRH Hub jointly funded by ARC, Shell, Woodside, LR and BV.

In conducting this UWA research, we are helping to provide more confidence in offshore operations.

Dr Wenhua Zhao

Safer and more secure energy

Shell EMI Chair visiting the FLNG model in Shell office (Jan Flynn, Wenhua Zhao, David White and Mike Efthymiou)

For the offshore hydrocarbon industry, the deployment of Prelude marks a turning point, with UWA’s research contribution boosting industry confidence in adopting new practices for future projects within the LNG industry. Close collaboration between Shell and UWA, reinforced through the Shell Chair Team, will continue to be important during the operational phase of Prelude as full-scale data from monitoring systems installed on the facility will provide valuable information for validating models and improving design methodologies.

FLNG technology can unlock gas resources from underwater gas fields previously uneconomic or too challenging to reach and provides end users with outcomes of increased efficiency, safer operations and a reduction in the cost of building and infrastructure maintenance. For onshore communities, the technology brings long term employment opportunities and business prospects for local businesses supplying the facility with consumables and equipment; having a positive impact on the local economy.

Research adopted for use on Prelude, such as undertaken by Dr Wenhua Zhao and his colleagues, has allowed for reliable offloading operations to remain at sea. Without the need for onshore works such as dredging, the offloading process is safer for the community and significantly limits the disturbance to the surrounding environment.

As well, the fundamental research provides a foundation for further adoption in the hydrocarbon industry and is paving the way for new open innovations to be developed to help meet ever growing consumer needs.