By Rie Jerichow
Many offshore platforms in the North Sea are reaching the end of their designed lifetime, but the actual integrity of the structures is only known with some uncertainty.
What do we know about the state of offshore structures?
We have been building offshore structures in the North Sea for decades. We inspect them, and we believe that they will not collapse. But their actual condition will always only be known with some uncertainty. This uncertainty may be reduced by more frequent or extensive inspections, but the structures are difficult and costly to inspect, so the cost will soon become prohibitive. So at the moment, we are not able to say how many years of continued safe operation are left beyond the designed life.
The final goal of the project is to build an “information engine”, based on results from our research project “Extend Life of Offshore Structures”. The “engine” contains information about the actual state of a structure, the fatigue and the loads from the waves – and an assessment of the life span. But the project becomes really fascinating when in the future, it will be part of a “structural asset management system” – a useful tool for business managements, budget managers and maintenance managers. As an example, it might – on a daily basis provide the basis for making sound and correct decisions about maintenance programmes.
One of the main areas of our research is fatigue damage. What have the structures been exposed to throughout their lifetime and how can it affect their current state? Nearly all big structures are highly influenced by this phenomenon. It also applies to aeroplanes, high rise houses and bridges. They sway back and forth and the movements do not, individually, have a big impact, but when you bend and stretch the material a lot of times, it weakens by the repeatedly applied loads.
Since we don’t know the real loads, we don’t know the real amount of damage that the loads apply to the structure. We just know that there is a limit to what the material can withstand.
PhD students Marius Tarpø from the Department of Engineering at Aarhus University and Bruna Nabuco from the Department of Civil Engineering at DTU are doing a joint project about this to create more certainty about the real fatigue and the expected extended lifetime.
By using a model and real time data on movements from sensors on the uppermost part of a structure it is possible to calculate the actual fatigue – not only at the top, but all the way down. By combining actual stress measurements with historical weather data it is possible to estimate the loads that the platform has been exposed to throughout its lifetime and subsequently make a correlated lifetime estimate. The research results show that we can extend the life span by a factor of two. Instead of a life span of 30 years, I would think that the offshore structures can operate safely for at least 60 years.
It may have a huge economic impact. Today, the basic knowledge of the actual fatigue of the offshore structures is poor. For safety reason, the structures are inspected regularly and reinforced or ultimately abandoned. With more knowledge about the actual state of a structure, the fatigue and the loads, you may not need very frequent and expensive inspections or even to abandon a platform. It will save a lot of money. However, our research may also impact other branches than the oil industry. Imagine if you could save 20 per cent of the steel needed to build a wind turbine, it would lead to enormous savings.
Using a model and movement measurements from the uppermost part of the structure, it is possible to estimate the strain response of other parts of the structure.
By combining actual stress measurements with historical weather data it is possible to estimate the loads that the platform has been exposed throughout its lifetime and subsequently make a correlated lifetime estimate.
• Actual stress statistics
• Wave statistics and monitoring data
• Structural monitoring data
• Validation for structural model assumptions
• Load model and updated statistics for extreme, breaking waves
• Probabilistic risk modelling