Operational Guidance for Thruster Assisted Mooring

Abstract In 2019 DNV initiated the TAM JIP motivated by i) that non-optimal operation of thruster assisted mooring systems is believed to be a contributing factor for the mooring line failures thet the industry has experienced, and ii) that last generation semi-submersible drilling units rely more heavily on their thruster systems. How to safely operate thruster assisted mooring (TAM) systems while also learn how one can do it more cost efficient have been central in the JIP work. This have been done by analyses of real operations, a parametric study on the performance of a TAM operation and experience shared by the participants. This paper presents recommendations for how to operate a TAM unit. The results are also used to give analysis guidance for TAM presented in a companion paper at OTC 2023 /1/. Logged data from the participants was used to analyze real operation and interviews of different stakeholders was undertaken. A parametric study was the main part of the analysis work, this was performed in time domain for 3 TAM units of different sizes at 3 water depths and a range of metocean conditions and controller settings. The models were benchmarked both towards real measurements and a state-of-the-art TAM controller to ensure realistic results. Other inputs from the participants were obtained in several workshops. Based on the results from the parametric study recommendations to DP system settings are given for different scenarios, such as when it is recommended to use damping only mode, and thereby save fuel, when it is recommended to include full position control and when mean thrust is sufficient. In extreme survival sea state, the main objective of the positioning system is to keep the mooring tensions at an acceptable level, and this puts other demands on the control system than in an operational sea state with strict position tolerances. Through workshops in the JIP the need for proper training of TAM operators was emphasized and recommendations to preparation of controlling documents such as mooring analyses and ASOG (Activity Specific Operation Guideline) was discussed and agreed, and this is also presented in this paper. Through the insight given by the results from the parametric study and interactions with the participants a better understanding of how TAM operations should be performed has been achieved, ensuring safer and more cost-effective TAM operations.