The Weekly Reflektion Week 46 / 2020

When a decision is made there may be some information that is missing, uncertain or unreliable. To progress the decision-making, assumptions may have to be made. Experience shows us that it is not always clear what is actually being assumed.

When you make assumptions what do you actually assume?

During the installation of a steel jacket for a new platform, the piles that hold the jacket in place on the seabed could not be driven to the required depth. The hammer force was increased but the piles started to deform. It was not possible to hammer the piles in and not possible to retrieve the piles. The design specification required that all piles had to installed to the programmed depth to satisfy the predicted loads on the jacket. The remedial action to install a new pile guide, drive the new piles and recover the upper part of the damaged piles delayed the project by one year and was consequently very expensive.

During the planning of the project a decision was made not to carry out tests on the seabed properties to provide the information required for the pile design. There were other platforms installed close by and topographical and geophysical surveys were available. Geotechnical surveys and soil testing had been carried out. The other jackets were secured with piles and there had been no problems driving the piles. It was assumed that the properties of the seabed were well understood and verified by experience. 

The engineering contractor responsible for the jacket design was supplied with the seabed properties and they designed piles that would penetrate the soil to the required depth based on this information. The piles ended up being under-dimensioned. What happened? Were the assumptions wrong? To answer this question, we need to be clear about what the assumptions were. They assumed that the soil properties were correct since there had been no problems with piling in the previous projects. 

On the previous projects the tools used in pile design were less sophisticated and not so accurate. The engineering contractor for the previous projects used a ‘robust’ pile design to account for the uncertainty. The engineering contractor on the more recent project had more accurate tools and designed a pile that was suitable for the assumed seabed properties. This design was less robust and cost less than the previous design.

What should they have assumed? Or rather how should they have formulated their assumption? Perhaps, they should have considered, we assume that the piles used in previous projects were suitable for the seabed properties in this area. Therefore, if we use the same pile design, we should achieve the same result. 

When making assumptions make sure you understand what you are actually assuming. Use some time to formulate the assumption and then verify that it is logical and that it reflects accurately the information that you are basing the assumption on. Make sure you understand what the assumption is related to, the uncertainties and the consequences if the assumption turns out to be wrong. Don’t be an ass.

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