Session: 10-03-01 Anchors and Pipelines I

Paper Number: 81101

81101 - A Whole-Life Anchor Macro Model for Floating Offshore Renewable Energy Systems 

The offshore renewable energy (ORE) industry is developing new solutions to enable floating facilities that can operate further from shore where more ocean space is available and stronger wind resources can be harnessed. However, it is not yet economically viable to develop the commercial scale floating ORE farms needed to achieve net zero decarbonization targets. A step change reduction in the unit price for floating offshore renewable energy (FORE) devices is required to unlock opportunities for further investment and commercial scale development of FORE infrastructure.

FORE infrastructure targets a spatially-spread resource, requiring multiple similar structures with lengthy multi-line interconnections that will span over many different seafloor types. The resulting geotechnical costs for FORE infrastructure are already significant, at ~30% of the ORE project value, and are projected to increase as more challenging ocean and seabed conditions are experienced further from shore. Typical fluid-structure interaction software models the connection of the floating structure to the seabed as a pin connection and the soil-anchor interactions are not included. This current uncoupled approach between characterizing the geotechnical conditions of the seabed and integrating these into the station-keeping (anchor and moorings) design process, results in more conservative and expensive designs because beneficial seabed and station-keeping interaction effects are overlooked.

The conventional capacity methods overlook three sources of potential capacity enhancement. They are

(i)               changes in available seabed strength from consolidation which can result in rises in long term anchor capacity

(ii)              viscous effects that lead to short term changes in available seabed strength

(iii)            added mass effects during dynamic loads (e.g. snatch loads) where extra anchor capacity is created from mobilising the mass of the soil surrounding the plate

Characterising and integrating these beneficial whole-life soil interactions into the analysis of the connected floating structure results in more reliable predictions of the full floating response of the infrastructure, and can lead to cost savings in optimally designing and managing the ORE facility throughout its operational lifetime.

An anchor macro model has been created as part of a Supergen ORE Hub industry collaboration project with the Norwegian Geotechnical Institute (NGI) that is designed to be coupled with existing mooring software numerical analysis packages (NAPs) to enable easy integration of soil-anchor interactions within full system modelling. The anchor macro model uses mechanical analogue parameters (MAPs), such as spring, slider, dashpot and added mass components to represent the averaged soil behaviour around an embedded anchor as a single element or node where the current anchor strength is a function of the history of applied loads as well as the current loading rate. The anchor model is also explicit, so a load history of many cycles is converted to increments of the model state parameters that update over time to predict the through-life changes in anchor capacity of the system.

This paper will present the anchor macro model, demonstrate how it can be connected and benchmarked with existing NAPs and will discuss the results from applying the coupled MAP-NAP model to example cases. Results from the example cases will demonstrate how the coupled model can predict changes in anchor capacity over a multiscale hierarchy of time processes, over wave period loads (10⁰ to 10¹ s), through to geotechnical consolidation durations (10⁶ s), through to full facility life (10¹² s).

Presenting Author: Katherine Kwa The University of Southampton

Authors:

Katherine Kwa The University of Southampton
Nallathamby Sivasithamparam Norwegian Geotechnical Institute
Andrew Deeks Norwegian Geotechnical Institute
David White The University of Southampton