DNV GL's Perth-based regional manager Richard Palmer told Energy News that the genesis of the project stretched back five years, with Origin's decision to instigate the BassGas Mid-Life Extension project, which would require lifting new modules onto the existing platform and what was expected to be the drilling of the Yolla-5 and Yolla-6 wells from Yolla-A with a tender-assisted drilling rig.
"That was to get around geotechnical issues, because there had been previous concerns about the soils and their ability to support a jack-up rig," Palmer explained.
Eventually the tender-assisted drilling unit was no longer available, forcing Origin and DNV GL onto a long journey to assess how a jack-up could be used and what qualities it would need.
DNV GL worked with stakeholders to review the soils and collect new data and then helped Origin make the case to the JV partners - AWE, Toyota Tsuho and Prize Petroleum - that the risks could be mitigated.
Detailed work was needed to define types of suitable rigs, and the process gradually narrowed the focus on the West Telesto, which was considered the best fit in an area where the foundations are very sensitive to the weight of the jack-up.
In tandem with that work was a project to determine allowable loading conditions across different seasons and potential storm events, and the best way to safely locate the kit needed for the Yolla project on the rig.
"We needed to look at how much weight we could put on board and where we put that weight, and we came up with an envelope design that allowed the rig crew to effectively manage the operations and the uptime of the project," Palmer said.
The seafloor above Yolla is composed of calcareous soils, essentially old shells and other planktonic life that have been crushed up, and pose issues for long-term loading issues.
In the short term they can support a jack-up's legs, but the longer the rig is on location the greater the risk the structure will slip.
It's not an uncommon issue in Australia, but in the Gippsland Basin there are additional severe weather issues to contend with.
DNV GL also provided reassurance and consultancy services during the recently concluded drilling operations, but its legacy will be the development of a tool that can now be used by jack-up rig operators anywhere around the world.
The tool helps define how much weight can be loaded onto a jack-up and how the weight should be distributed.
"We can determine what the loads and environment would be like, but that isn't a lot of use to non-technical people. How does the driller and operator know if they are right or wrong?" Palmer said.
"Part of that is linking with our mariners during the pre-loading when the jack-up is still floating in the water, so if you have a problem you identify it before you have a real problem.
"We added the theoretical and practical together to come up with marine procedures that can be executed in such a way that is befitting to the situation, that is cognisant of the limitations of the soils and the properties of the jack-up and the platform, and the environment in the Bass Strait.
"It has taken years of theory and practice to transform it into something visual that can be used in other applications. It also tells you what the impacts of things such as the weather."
The tool also helps minimise potential for downtime due to weather to help keep the project running on schedule and within budget.
The method is valid globally and in any location.
A joint paper between DNV GL and Origin will be presented at the dedicated jack-up conference in London later this year.
The complexity of the project has also deepened DNV GL's breadth of knowledge, and led to a number of new projects offshore Australia, he said.
