Since the pioneering work of Huse (1986, “Influence of Mooring Line Damping Upon Rig Motions,” Proc., 18th OTC Conference), it is well known that mooring lines may account for a large fraction of the overall damping present in a moored floating structure. This paper is concerned with the mooring line damping induced by the low-frequency, quasi-static, horizontal motion of the mooring line fairlead. The main advantage of the quasi-static approach is that it is much faster than the more accurate finite element methods, and, secondly, that it does not require any finite element modeling skills. A new formulation is proposed and is compared to the results of Liu et al. (1998, “Improvement on Huse’s Model for Estimating Mooring Cable-Induced Damping,” Proc., 17th OMAE Conference), as well as to time domain results obtained with FLEXRISER. The improvement with respect to the previous quasi-static methods is quite notable and our results are closer to FLEXRISER predictions. Finally, quasi-static results are compared to mooring line damping values measured during model tests for full mooring systems. The agreement between the two is very encouraging and suggests that the simpler quasi-static approach may, in some circumstances, be a valuable substitute for the more complex and time-consuming numerical tools. [S0892-7219(00)00102-3]
The development of floating offshore wind farms requires the parallel development of suitable floaters to support the wind turbines. These floaters must be economic and must also exhibit good motion characteristics to limit the accelerations and inclinations imposed on the turbines during operation. This paper reviews the required characteristics of these floaters, addressing the requirements of fabrication, turbine integration, tow to site, and offshore installation, as well as the required behavior of the floater once installed. The benefits of an integrated design approach considering all of the above is demonstrated, and consideration is given to the industrialization of the production process for large numbers of floaters for full-scale wind farms.Based upon this review of requirements, an innovative light weight structural solution incorporating tensioned mooring legs has been developed as an economic solution for the floater. The modularity of the design facilitates construction, and offshore installation can be accomplished using standard anchor handling vessel (AHV) means. The floater design exhibits low turbine inclinations and low accelerations due to a combination of its mooring arrangement and its high degree of transparency to waves, which reduces fatigue loads and maintenance issues on the turbine.The floater behavior during towing to site and in the installed condition is described, and key performance characteristics are reported based on analytical simulations and model tests results conducted at 1:40 scale.The paper seeks to clarify the key factors to be considered in developing a floater to support a wind turbine, and to propose a solution that achieves good motion characteristics whilst satisfying the economic constraints of wind farm development.
This paper reviews the issues associated with mooring a ship shaped Floating Production Unit (FPU) in arctic conditions, and presents the development of a novel disconnection and reconnection system for such conditions. The mooring systems of FPUs operating in arctic conditions must be disconnectable to allow the FPU to leave the station to avoid collision with icebergs, or to avoid overloading the mooring legs due to sea ice acting on the FPU hull. In the case of sea ice, the FPU may be required to disconnect under much higher loads than the non-arctic disconnectable systems in operation today. A recent study for the design of an arctic mooring system identified a number of key developments that are required before such systems could be deployed. The disconnection system is a safety critical element, and requires high reliability and redundancy to ensure the FPU can always rapidly disconnect from its mooring when required. In addition, the large number of risers that may be installed for these large field developments, combined with the significant suspended weight resulting from the high capacity mooring system, leads to large buoyancy requirement for the buoy which must support the risers and mooring system when disconnected from the vessel. As a consequence the analysis of the reconnection process must account for the coupled behaviour of the large buoy body, the mooring system, and the risers and umbilicals. Such analysis has shown that using conventional disconnectable turret technology, the large buoy size coupled with the requirement to reconnect in heavy sea states, can readily generate snatch loads that would break the pull-in winch wire. In response to the above, this paper presents two significant advances in disconnectable mooring technology. The first is the development of a new locking system to connect the buoy to the turret, which has been qualified at full scale. The second concerns the design of a new pull-in arrangement that eliminates the risk of snatch loads even in sea states in excess of 3m Hs. The system robustness has been demonstrated through model testing.
This paper presents a new design of chain connector that allows disconnection of the mooring line, and inspection and possible replacement of the chain connector's articulation bushes, without the assistance of divers. This design brings dual benefits: Reduced risk of mooring line failure caused by unexpected wear or damage to chain connector articulations, which could not have been inspected with traditional chain connector designsIncreased safety by removing the need for diver intervention. The paper presents the development of the chain connector, including the latching system that enables connection and disconnection without diver assistance. The development has been performed following a structured process with milestones. The main development steps have been the concept engineering phase, an extensive phase of sub-component testing, a phase of assembly prototype testing at a reduced scale, realization of detailed engineering phase of the component, and the development of full scale prototype test rig. Chain connectors are a key component of any mooring system, transferring the load from the mooring leg to the FPU. The chain connector also forms an articulation between the mooring leg and structure, and its reliable behaviour is fundamental to avoiding Out-of-Plane Bending (OPB) fatigue failure of the chain links. The described development process and especially the system testing demonstrate the functionality of the diverless chain connector having as aim the high integrity of the mooring legs. Furthermore, it allows assessing the envelope of its connection and disconnection functionality, given different mooring line configurations. Testing provides complete and reliable results of connection and disconnection functionality for different values of in plane and out of plane angles, and pretension. In addition to that, the testing results allow sizing the installation winches adapted to different mooring line configurations. The trend towards offshore developments in increasing harsh environments is leading to increased technical challenges in ensuring the integrity of Floating Production Units. In parallel, the design life of these systems is increasing above 20 years, and up to 50 years. The ability to monitor the condition of the chain connectors, and to safely maintain or replace any element in case of unpredicted aging effects is the benefit given by the component development described here.
The objective of this paper is to present the design and performance of an offshore floating wind turbine support structure and associated station keeping system, for a commercial 6 MW turbine. The results reported in this paper are based on a joint desk study performed by SBM and IFPEN for the development of this new floating support structure concept. The proposed system has been extensively analyzed thanks to time domain simulation software. Time domain models incorporate the wind turbine, the station keeping system, as well as structural components of the floating foundation. The system’s behavior has been assessed for a variety of environment conditions and turbine conditions (operating, idling, fault), resulting in an extensive design load case table. In addition to the nominal system, a number of sensitivities have been investigated to test the system response to various effects: marine growth accumulation on the floating support structure, anchor position tolerance, variations of water level. Results produced during this study show the good performance of the proposed floating wind turbine support structure and components. The proposed arrangement is capable of sustaining 20 years of operation with environment conditions up to the 50-year return period. The motions of the floating support structure are beneficial for the turbine performance, with low inclinations and low nacelle accelerations. As a consequence of these floating support structure’s low motions, the floating offshore wind turbine production is only marginally lower than the production of the same turbine on a fixed offshore foundation in the same environment. Production can occur up to the 50-year joint environment conditions. The work presented in this paper formed part of a design dossier independently reviewed by a certification body to obtain an ‘Approval in Principle‘ for the development of the floating support structure. The study has shown that the floater motion characteristics allow similar turbine production levels to be achieved by a turbine on a fixed offshore foundation, providing support to move of floating offshore energy production.
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