Deployment Forces and Dynamics for the Southern Ocean Flux Series (SOFS) Mooring

In March 2018, the Commonwealth Scientific and Industrial Research Organisation's (CSIRO) Moorings team deployed the Southern Ocean Flux Series (SOFS) mooring, an air-sea flux mooring in 4500m of water in the Southern Ocean that has been rotated on a 12-month period for the last 10 years. On this deployment, the anchor strop on the mooring (designated SOFS-7) failed during or shortly after the anchor free fall to the seabed, a phenomenon that has not been previously witnessed with this mooring, or indeed with any other CSIRO mooring. A second deployment was attempted with a backup anchor and a repeat failure occurred. In August 2018, the SOFS mooring (designated SOFS-7.5) was successfully re-deployed, after changes to the mooring configuration. These changes addressed several issues that were identified during the review of the deployment failure, predominantly strop loading and required strength, use of swivels in the anchor section, and anchor configuration. The SOFS-7.5 mooring successfully remained on station for the remaining period of its rotation, before being successfully replaced by the SOFS-8 mooring in March 2019. Mooring analysis is currently a burgeoning field of study for many involved in mooring design. Whilst literature exists on analytical methodology and real-world results for moorings in-situ, relatively little literature exists for the deployment phase, even though deployment analyses exist in some of the commonly used analytical tools for mooring design, including WHOI Cable, Proteus DS and Orcaflex. These events led to a review of our approach to mooring deployment dynamics, and to determine what gaps existed between analysis and real-world behaviour. The SOFS mooring had previously been analysed using both WHOI Cable and Orcaflex. However, whilst we had completed some validation of model inputs against in-situ behaviour of the mooring, nothing had been completed for deployment behaviour. This work looks to address that gap and determine the process for validating deployment forces and dynamics such that future models can be reliable. The SOFS-7.5 mooring anchor section was instrumented with an inertial measurement unit (IMU) and pressure sensor, attached at or close to the releases, to learn about the behaviour of the anchor section during deployment. In addition, on the voyage to recover SOFS-7.5 and deploy SOFS-8, a separate instrumented test anchor was also deployed and recovered to gather similar data from a different configuration, with the addition of tension data just above the releases. This data has been compared to simulations for both mooring configurations using WHOI Cable, with the following data used to verify model inputs: · Descent rate of the anchor section · Subsequently, the drag force acting on the system · Drag force of the float packs through the water (test anchor only) · Overshoot of the releases Additionally, rotational behaviour of the anchor section, an item of much contention following the mooring failure, was also observed and found not to be an issue on either the SOFS-7.5 or test anchors. The drag coefficient and cross-sectional area for all major components of the anchor section were updated and/or verified following the comparison of the model results with the captured data. A method for further refining the model is discussed. This data should now allow us to analytically determine the strop size (strength and length) required on all our moorings, given the anchor and flotation sizing used on the mooring. Further work in this area will involve validation of the WHOI Cable model to accurately predict deployment forces on other mooring systems.