Resumen de Modeling and measurement of dynamic mooring line tension. Impact on operational thresholds assessment for harbour and offshore operations
Miguel Ángel Cabrerizo Morales
Emerging markets as marine wind energy are imposing new challenges to the offshore industry; Marine wind energy business competitiveness is strongly related to offshore substructures and their logistics. Some of the approaches used for deploying wind turbines offshore are gravity base foundations (GBF) however classic transport and commissioning costs cannot be afforded by the renewables business. With the aim of reducing installation cost, new or unusual operations are being planned and executed in an effort to avoid the use of specialized and expensive vessels used in traditional offshore industry as oil&gas.
One of these cases is the DEMOGRAVI3 project, a Horizon 2020 funded effort to prove the feasibility of a singular GBF design through the construction, transportation, installation and exploitation of a full- scale prototype. One of the singularities of this design, also one of its main advantages, is the capability of transport, self-installation and decommissioning with the sole assistance of regular tugs. This avoids the need for specialized heavy lift vessels or tailored built craft, thus reducing both costs and economic uncertainties related to installation and decommissioning phases.
Self-installing foundations as the ones mentioned before also use tugs as external assistance for precise positioning over the seabed, this approach motivates the design of operations that conjugate several vessels or floating structures joined together by mooring and anchoring lines. This type of maneuver is not common and requires extensive analysis, modeling and if possible, training of the crew. In a context of cost reduction, vessel size and linking elements must be optimized according to operational requirements. Ideally, operators will pursue the use of the smaller tug, avoid the use of complex and heavy towing line set ups, like clump weights, and have the best control possible by using the shorter distance between tugs and the structure as possible. On the other hand, installation crews also need operational windows as wide as possible. All the listed elements can lead to taught-slack events on the mooring/towing lines. In this context line tension analysis and prediction must be assessed. .
This thesis approaches the analysis of mooring line tension as a key aspect on the loop around optimization of positioning means design, costs, operational requirements and climatic windows.
It presents a review strategy to analyze mooring line tension and identify impulsive or snap load occurrence; it also proposes a methodology define the maximum load thresholds for a given line set-up and compares it to the methodology found in DNV-GL standards.
To achieve this a global scale simulator is implemented with emphasis on a dynamic mooring line control and monitoring device. The methodology is developed and applied on three case studies.
