Mooring system design, related to Pioneering Spirit

Improvement of a mooring system to secure a barge alongside Pioneering Spirit in offshore conditions, including an evaluation of the dynamic loads

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Abstract

Continuous development in the wind turbine industry leads to increasing size of wind turbines. Allseas investigates the possibility to enter the offshore wind turbine installation industry with Pioneering Spirit.
For Allseas’ preliminary wind turbine installation design, wind turbines are assembled offshore, requiring wind turbine components to be brought from shore to Pioneering Spirit by means of a cargo barge. This operation requires a proper mooring procedure of which the mooring system is an essential part. The mooring system secures the barge alongside Pioneering Spirit where it has to stay for multiple days.

In this research a pre-defined vessel orientation is analysed where the barge stern is extended 40m in longitudinal direction from Pioneering Spirit stern, to increase the barge area reachable by the unloading crane.
This barge position is challenging due to limited shielding from Pioneering Spirit, leading to excessive environmental loads acting on the barge. Due to the barge extension, also properly connecting the mooring system to Pioneering Spirit is a challenge.
The above leads to the main objective of this thesis: improve the mooring system to secure a barge alongside Pioneering Spirit in offshore conditions, including an evaluation of the dynamic mooring loads.

The main mooring system design requirements are that the system Safe Working Load and motion limits are not exceeded.
Evaluation of the mooring systems starts with a multi-body diffraction analysis with hydrodynamic matrices and other hydrodynamic properties as output. This data is imported in a time domain model which enables capturing non-linearities, e.g. the mooring system and wind and current loads. Validation and verification steps are required to assure realistic outcome and understanding limitations of the numerical models.

Before new mooring systems are introduced, a base case mooring system is defined and modelled in the time domain.
Finding the limiting sea state for which the Safe Working Load limit is reached enables to compute the workability for the reference location defined. Two reference locations are considered: a wind sea area and swell sea area. Wind and current speed is assumed to be constant over time and independent of elevation. Workability is defined as the percentage of time the mooring system is able to operate.

Evaluating the base case mooring system shows performance of 67% workability for wind sea areas and 16% for swell areas. Next to the base case, four mooring improvement concepts are evaluated from which the Cavotec Moormaster® system shows most promising results. This system consists of three main components: ‘fixed structure - hydraulic cylinder - vacuum pad’ (Figure 2) that connects the barge to Pioneering Spirit. This system is modelled as a link with constant stiffness and damping properties within its operational limitations.

The Moormaster system, shows perspective to improve the workability for both wind and swell seas. Besides workability, the Moormaster system improves the entire mooring procedure by quick connection and safe disconnection upon exceeding operational limits.