103. Risk management

A safe bottle dive is one in which the risks of the operation are understood and managed. An operational technique that I have found to be particularly useful is the McRaven method, which is named after the U.S. Navy S.E.A.L. diver who first popularised it.

In this approach a graph is produced which maps the probability of operational success (the y-axis) against time (the x-axis). The origin of the graph represents the start of the operation. The point at which overall success is more likely than failure is known as the relative superiority line (R.S. line) and by definition it must lie exactly half way up the y-axis. Key events of the operation are then qualified in terms of their risk (probability multiplied by impact), and their contribution to the overall probability of success at the associated point in time. They are marked on the graph accordingly.

The key feature of this model is the way it allows the area above and to the left of the plot-line (the area of vulnerability) to be correlated to the tipping point at which relative superiority occurs. It is clearly desirable to reduce the area of vulnerability as far as possible, regardless of whether or not it lies above or below the R.S. line. However, in practice it is especially important to reduce the area below the R.S. line because that means lessening the time of exposure to serious risk. You want to reach a point of manageable risk as quickly as possible - and that point is represented by intersection of the plot line with the R.S. line.

As an example, let us consider a dive where the site can only be accessed by negotiating a series of submerged and partially submerged obstacles. These obstacles lead out to a silty bottom, which lies beneath a ledge from which the bottles are thought to have been thrown. By carrying gear (such as the line bag and dive float) past these obstructions, we increase the complexity of the journey and the risks associated with it. A failure to negotiate each hurdle before we get to waist depth can be expected to result in the dive not going ahead. However, once we are at that depth it becomes possible to swim out and around any further obstacles and get on with the dive, which therefore puts above the R.S. line.

Once we have a model we can start to optimise it. For example, we could reduce our exposure below the R.S. line by lowering the bag and float into the water from the ledge. This decreases the time it takes to get out to waist depth and also increases the chances of being able to negotiate the various impediments successfully.

Note that “getting above” the R.S. line does not mean that success is guaranteed...it just means that once you reach that point, a careful handling of each risk is likely to result in overall success. Thenceforth, it is improbable that the mission will have to be aborted.

There is an important caveat to all of this. A hazard above the R.S. line might become compounded in ways that were not foreseen, such that it cannot be dealt with, and which might thereby still lead to overall failure. For example if the silty bottom gets churned up before the line is deployed, and does not clear, then it will not be safe to continue. In other words you can't plan for everything. A risk profile can only be an approximate model of the real world.