Software tool models ice load and impact on Arctic floaters
Marie-Francoise Renard - Bureau Veritas
To extract oil and gas out of the Arctic basin, new tools are needed to model ice loads and their effect on offshore structures.
Until now, a number of rules and standards for Polar class ships have been adequate to calculate the reinforcement required for different areas of a ship’s structure for operating in specific areas and ice conditions.
However, industry is considering new types of floating platforms and ships to serve high Arctic exploitation of oil and gas, and the rules are not sophisticated enough for these new-generation structures. There is a need for direct calculation of the effects of the ice loads on the structures and on the mooring systems of the floaters. These units and the tankers that serve them will have to be able to operate safely and to keep operating a high percentage of the time, so ice effects modeling has to be accurate.
To understand and define the ice loads that will be applied, Bureau Veritas joined with the State Maritime University of St. Petersburg. We used its ice data and experience, married with our experience of developing modeling tools, to define ice loads with the aim of developing a tool to assess specific designs.
(Above) Bureau Veritas software can calculate mooring loads from ice impacts on ships engaged in transfer operations.(Below) The software uses FEM modeling to calculate structural ice loads for any point on a moored FPSO and also on the moorings.
What the industry needs is a tool to map ice loads, to apply them in a finite element model (FEM) of the hull structure, and then adjust those loads according to the ship’s speed, or to the ice drift speed.
At present, Bureau Veritas engineers have an internal tool for working on FEED studies on Arctic units and ships. However, there are plans to develop it further and roll it out for industry use, probably next year.
All the current class and national rules for ice are based on calculating the design load for a glancing bow impact on ice of a specific thickness. Then, the impact loads on the fore body are converted to loads on other hull areas using an area factor, which in turn is used to calculate the scantlings for different parts of the structure.
The software calculates the ice load for each specific hull area. Using an FEM modeler, it can then calculate the structural response for each area of structure and range of speeds, and also types of impact. The modes for ships include breaking ice ahead or astern, working in a channel, or glancing impacts. For floating units, a range of potential ice collisions can be modeled. Those models decide on the structure required for each element and underpin the disconnection strategy that will be the last line of defense for floating units.
Arctic assets are expensive to build and keep on station, so any downtime or underperformance is costly. The software can show a maximum allowable ship speed for a fixed ice thickness or a maximum allowable ice thickness for fixed ship speed.
FPSOs and other floaters have the same problem but from a different perspective. The ice is moving, not the unit, and even if the unit can rotate under power, it cannot always be determined exactly how the ice will hit. So we have to be able to directly calculate the effect on the FPSO structure of different types of ice impacts in different places at different speeds. Validation of a new tool is also critical. In this case, Bureau Veritas turned to records of damage to aging tankers with a known track record of navigation in Arctic areas. There is a 20-year service history to draw on to correlate the ice conditions and damage experienced with the known structure.
When drifting ice threatens an Arctic floater, disconnection is the ultimate defense. For each unit there has to be a clear disconnection decision strategy, which will depend on the expected structural impact and also the mooring load impact of moving ice of different sizes and speeds. The software tool will help develop those strategies, which, in practice, will depend on guard zones around each floater that trigger the decision to disconnect when breached.
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