Should we design for the 100-year or 10,000-year storm?
Traditionally we have designed the moorings of floating production systems to ensure an appropriate factor of safety against failure during the worst combination of waves, wind and current, that are expected to occur in a 100-year period.
However, a number of operators have been specifying a 10,000-year condition as part of their basis of design, requiring that the mooring system can safely survive the resulting loads
There’s been some resistance to this approach, possibly because some designers do not clearly understand the basis of the rules using the 100-year condition.
In early considerations of the required safety levels, the intention was always that mooring systems should be able to withstand the 10,000-year storm. The underlying philosophy was that moorings should have an annual probability of failure of 10-4 or lower, and this philosophy was translated into designing for the 10,000-year storm.
The difficulty in implementing this approach was that we didn’t have metocean data for the 10,000-year storm for most projects. So to resolve this problem, studies were carried out to determine the ration of 10,000-year conditions to 100-year conditions, based on data available for the North Sea. My understanding from discussions with industry veterans is that this was performed by the American Petroleum Institute (API), though I’ve not been able to verify this.
The studies indicated an increase in wave height by a factor of around 1.3. Since mooring loads are roughly proportional to the square of the wave height, an increase in wave height of 1.3 leads to an increase in loads of 1.32, ie 1.69. So, what we do in practice, is to apply a factor of safety of 1.69 to the 100-year storm condition, but our target is actually to ensure the design can withstand the 10,000-year condition (we actually use 1.67 rather than 1,69 because its reciprocal is 0.6 – we design for 0.6 of yield, accepting the design will then reach yield in the 10,000 year condition).
However, perhaps predictably, the ratio of 1.3 is not constant over all geographical regions. The ratio can be considerably higher is some parts of the world, for example offshore Australia and in the Gulf of Mexico. So if we apply a factor of safety of 1.67 to the 100-year case in these other regions, the loads for the 10,000-year condition will be underestimated.
It’s to avoid this underestimation of loads that some operators now require a direct check on the 10,000-year conditions.
However, there is not complete consensus on the need for this approach. The uncertainty arises because when we see failures of mooring lines, they are not due to extreme loads, but rather to factors such as fatigue and installation issues. Although the logic of designing directly for the 10,000-year condition is clear, historically we have not taken this approach, and we’ve not seen failures due purely to extreme loads. So maybe there are other factors, perhaps related to the metocean conditions we use, that make the story a bit more complex.
Ongoing studies aimed at correlating safety factors with reliability levels may shed more light on the appropriate metocean conditions to be considered.
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