Experimental validation of a dynamic mooring lines code with tension and motion measurements of a submerged chain

- A dynamic mooring lines simulation code has been successfully validated against tension and motion experimental data.
- The added mass and drag coefficients, obtained from guidelines, in general represent with accuracy the hydrodynamic loads.
- Drag coefficients from guidelines may not be tuned for highly dynamic conditions present in the test cases.
- Dynamic effects are very important on the prediction of the fatigue and ultimate loads of the mooring lines.
- Computations are sensitive to the cable-seabed interaction that has to be accurately modelled in highly dynamic conditions.

This paper shows a complete study of the dynamics of a mooring line: first, a simulation code based on a lumped mass formulation was developed and tested under different setups and second, an equivalent experimental campaign was performed to compare against the numerical predictions. The tests consisted of a suspended chain submerged into a water basin, where the suspension point of the chain was excited with horizontal harmonic motions of different periods in the plane of the catenary. The code is able to predict the tension at the suspension point and the motions of the line with accuracy. For those cases where the line loses and subsequently recovers tension, the resulting snap load and motions are well captured with a slight overprediction of the maximum tension. The added mass and drag coefficients for chains used in the computations have been taken from guidelines and, in general, predict correctly the hydrodynamic loads. In addition, sensitivity studies and verification against another code show that highly dynamic cases are sensitive to the seabed-cable contact and friction models. The results show the importance of capturing the evolution of the mooring dynamics for the prediction of the line tension, especially for the high frequency motions.