Digital twin of mooring line tensions

With a detailed numerical model of the FPS mooring system, the mooring line tensions are calculated with the time domain simulation program aNySIM, using the FPS measured wave frequency motions and the measured horizontal GPS positions in the field as input. These measurements are provided by a Motion Reference Unit (MRU) and an accurate RTK DGPS system. Using the measured response of the FPS avoids the uncertainty resulting from a prediction of the hydrodynamic response. The time domain software, running in real time, generates the static and wave frequency mooring line tensions. This provides the operator with valuable data for the validation of the design. The associated fatigue analysis contributes to knowledge on the integrity of the mooring system.

One of the main benefits of the indirect mooring line tension monitoring system is its robustness, since all monitoring equipment is located on the FPS (and not subsea). This equipment is fully accessible and therefore maintainable and replaceable if needed.
The analysis methodology used in the indirect mooring line tension monitoring system provides an important contribution to the development and maintenance of a digital twin of a mooring system. By integrating the design model and in-field measurements, the long-term distribution of forces in the mooring lines are monitored. This generates a detailed knowledge of the status of the mooring system and provides design feedback for the future.


Contact person photo

Pieter Aalberts

Senior Project Manager


The hardware of an in-direct mooring line tension monitoring system comprises a Motion Reference Unit (MRU), a RTK GPS and a Windows operated PC with data acquisition software. Alternatively the measurements are streamed directly to the cloud. In this case the software runs in the cloud. The MRU sensor measures the 6 DOF wave frequency FPS rigid body motions of the FOWT.

The equipment to measure the FPS positions comprises two GPS antenna’s and one GPS receiver. The receiver receives signals from multiple devices including GPS, GLONASS, BeiDou, Galileo, SBAS / WAAS and ATLAS H10 offshore corrections (commercial service). The GNSS receiver is connected to two antenna’s to obtain, in addition to the FPS position, the FPS heading. With measurements of the position (at the GPS antenna location) and the heading the horizontal position at the fairleads can be determined. With ATLAS H10 offshore corrections a position accuracy at the fairleads of approximately 5 cm is obtained.
MARIN Digital twin of mooring line tensions

Example of in-service application experience

Bluewater decided in 2018 to equip the turret moored Aoka Mizu FPSO with an extensive environmental and structural monitoring system. The new system was installed during the upgrade at Drydocks World, Dubai. To get more insight into the actual mooring loads, Bluewater decided also to monitor the mooring line loads and associated fatigue using the indirect mooring line tension monitoring system while operating in the challenging environment such as the West of Shetlands. Since the implementation of the software in 2020, the calculated mooring line tensions are forwarded and visualized on the bridge.

MARIN experience

For more than 25 years MARIN provides for the oil & gas industry structural monitoring systems with associated integrity management tools for the hull and moorings of floating production systems.