Insights

Extend the Life of Offshore Structures with a Simple Sensor and Smarter Data

Why Fatigue Matters in Offshore Engineering

Fatigue is a critical design factor in offshore environments, influencing the lifespan of structures such as monopiles and wind turbine towers. It arises from repetitive motions that initiate cracks in steel, which grow over time and may lead to sudden failure. To ensure safety, designers often rely on conservative fatigue estimation methods for both the monopile and the supporting structures during transport and installation. While effective, these approaches can result in overdesign, leading to increased steel usage, heavier monopiles, and higher production emissions.

The Transportation Phase: A Missed Opportunity

Despite its significance, fatigue damage during transport remains under-researched. Current design practices typically use simplified Weibull-based predictions from DNV standards in combination with conservative motion assumptions and unfavorable cycle counts.

To improve accuracy, TWD has developed a more advanced fatigue prediction method using stochastically simulated vessel motions and stress calculations. Although these simulations offer improved realism, they still rely on conservative (but more realistic) assumptions to maintain acceptable risk levels for transport fatigue.

Real-Time Motion Monitoring: The Power of Data

To validate and refine these models, TWD is conducting motion monitoring campaigns during transport. Using purpose-built sensors, placed close to the monopile, true motion data is recorded, which is then postprocessed and compared to pre-voyage fatigue simulations.

In many cases, actual fatigue damage is significantly lower, sometimes by an order of magnitude, than predicted. These insights not only help to improve fatigue models and enhance future simulations but can also be used to extend the lifetime of the monopile. Because the actual fatigue damage is now known, the unused fatigue “budget” of the transport phase can be shifted to the operational budget, extending the lifetime of the wind turbine when producing power.

Ultimately, this continuous feedback loop of measuring and improving the fatigue estimation methods will help optimize the design of the monopile itself.

For transporters and equipment designers, there is also a benefit; the fatigue lifetime of the seafastening can be significantly stretched, assuring a longer use or potential re-use in other projects.

A New Method in Fatigue Assessment: How It Works

Our improved fatigue method integrates the following inputs to assess fatigue damage:

•  Sensor data
• Vessel motion simulations
• Nonlinear finite element (FE) models
• Fatigue curves.

Figure 1, Overview of TWDs improved fatigue method

Simulated vessel motions, based on vessel geometry, route, sea conditions, and weather, predict motions and loading cycles, which are applied to a tailored FE model to calculate stress levels. This process can be displayed in the Figure below.

From Raw Signals to Actionable Insights

To ensure accuracy, raw translation and rotation sensor signals undergo extensive post-processing using tailored filtering techniques. From the filtered motion data:

• Loading cycles are identified.
• Phase relationships between surge, sway, heave, roll, pitch, and yaw are examined
• The dynamic relations of the motion and their interactions with the monopiles are better understood

This processed data is then fed into a tailored FE model of the monopile and its sea-fastening to determine experienced stress levels. By combining calculated stresses and cycles with DNV fatigue curves, both predicted and experienced fatigue damage during transport can be evaluated.

The Bottom Line: Design Better, Last Longer

Ultimately, TWD’s simple sensor solution measures actual transport motions while combining them with innovative data analysis and detailed engineering models. This unlocks the potential to optimize monopile transport fatigue budgeting, allowing for longer service lifetime or more cost-efficient monopile and seafastening designs.