Motion performance of floating systems offshore Western Australia

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Article by Shell Global Solutions BV and The University of Western Australia.

Floating offshore structures have been an important solution for unlocking offshore oil and gas reservoirs. In this post, we are going to address the motion performance of the Prelude FLNG facility, which is the world's largest floating structure ever.

Motion performance in extreme sea states: For a novel technology, it is common to ask what its motion performance will be compared to other better known floating systems in extreme sea states? To answer this question, the motion performance of the Prelude FLNG and an FPSO operating in North Sea are compared. For any wave height and period, all motions of FLNG are lower than those of an FPSO.

Secondly, how do conditions under a typical category 5 cyclone compare with harsh environments in other locations? To answer this question, the most probable maximum motions of the Prelude FLNG and an FPSO are compared for both the North Sea and offshore Western Australia (West Australia Shelf). The Prelude facility would experience much lower motions in extreme cyclones than an FPSO in the North Sea. For instance, the most probable maximum heave with a return period of 10,000 years of the FLNG is estimated as 3m whereas the corresponding value for an FPSO in the Northern North sea is about 3 times higher around 11m.

Offloading operations: Unlike oil offloading whereby tandem configuration is often used, a promising solution for LNG offloading is a side-by-side arrangement whereby the two vessels are placed close to each other at a separation of about 4m. In this configuration, it avoids the modification of available LNG Carriers (LNGC).

In the scenario, the FLNG unit is relatively stable due to its large size, but LNGCs move more. The most significant motion of the LNGCs affecting offloading lies in roll. This is especially so at OWA because the vessels tend to experience beam-sea swells in the winter months of July-September, as shown in Figure 4.

 

Figure 4: A typical vessel heading under the combination of swell, wind and wind seas.

 

 

A huge amount of research has been carried out to increase the applicability envelope of side-by-side offloading.

The main elements have been:

  • Development of a dedicated cryogenic loading arm for the side-by-side arrangement.
  • Extensive testing to help development and validation of analytical tools.
  • Provision of 3 thrusters on the FLNG vessel to enable change of heading.

The combined effect of the above efforts has been to increase the confidence that offloading operation at Prelude will achieve good operability.

Activities at UWA: A number of open technical questions still remain which are currently the research focus at University of Western Australia (UWA). Resolution of these issues will increase the accuracy of predicting offloading operability and extend the applicability envelope. These research focuses include:

  • Roll motions of LNGCs and how the liquid cargo motion affects the roll response;
  • Amplification of wave surface in the gap between the two vessels due to resonance effects.

Furthermore, this team has been awarded the ARC Hub for Offshore Floating Facilities (led by Prof. David White), researching critical engineering challenges for oil & gas projects in remote offshore locations. This will lead to safer and more economic offshore engineering projects.

Article by Wenhua Zhao, David White from the University of Western Australia & Mike Efthymiou from Shell Global Solutions BV (Shell) and University of Western Australia.

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Featured picture: Team from the Shell EMI Chair in Offshore Engineering at UWA.