Fitness for service assessment

The challenge

During routine maintenance, damage was found to have occurred to the inner bore surface of a branch connection in a Water Injection Header.

Our client identified between 20 and 30 similar connections on the same header which could have suffered similar damage. This was an unexpected problem and only a very short period of time was available to inspect the branches and substantiate their integrity before production would be affected. The branch connections were quickly identified to be a bespoke design and no detailed drawings were available.

Our solution

We developed a simple, flexible and effective approach to quickly identify those branch connections that had suffered damage attributed to a combination of corrosion and erosion damage.

Our structural analysis team identified the key dimensions and the necessary inspection grid for Non-Destructive Testing (NDT) thickness measurement. The offshore inspection team then used this to gather the data from the branches most at risk.

We developed an initial screening assessment against which the damaged branches could be assessed, which involved;

• API579 level 2 assessments, using a simple area replacement approach to better understand the extent of the damage on each branch
• When the results of the analysis produced low reserve factors a more detailed finite element model was built in accordance with a level 3 assessment to demonstrate protection against plastic collapse
• The wall thickness of these detailed models was then reduced until the limits of the solution for the applied loads were reached
• The mesh of the finite element model was built to match the inspection grid so that individual elements could be removed easily and the associated damage on each branch assessed
• When branch connections were in close proximity to major structural discontinuities, we adapted the model to consider these scenarios.

The benefits

The initial assessments and finite element model were developed in a few days allowing the analysis to drive the inspections. These models were developed so that individual branch connections and their associated damage could be assessed without the time-consuming process of re-building separate models.

By quickly mobilising the right team with many years of experience, we demonstrated the fitness for service of all the damaged connections before the completion of the outage resulting in no further loss to production.