Control of Large-Scale Impacts by Designed Structural Damping Available to Purchase

Modification of deep-water floaters often involves module installation using a floating crane vessel. The impact forces caused by relative motions between the floating vessels represent a major challenge during set down on the floater deck due to the large inherent variability of these forces. Traditionally the difficulties in predicting impact forces during module installation have been overcome by the use of experienced based rules of thumb rather than accurate simulations and calculations. One has to some degree relied on the indeed present but un-quantifiable effect of human intelligence of the operation supervisor. Traditionally the impact forces are taken either by elastic deformation of the module itself and/or the installation guides or by permanent deformation of intermediate structural elements through e.g. plastic yielding of ductile metal members or crushing of wood members. Designing the module and the guides to be able to take the entire probable range of impact forces is difficult due to the inherent contradiction between wanted flexibility and required strength. The large uncertainties of the impact energy imply that it is difficult to design these intermediate elements to cover all possible impact energy levels. Furthermore, these elements cannot be applied in cases where repeated impacts may occur. An attractive alternative to the traditional solutions is application of industrial shock absorbers. The performance of these is predictable and they can be designed to cover the estimated range of impact energy. This paper will present a more precise and consistent design and analyses methodology that gives a more accurate measure on the reliability of the operation in accordance with code requirements. The paper will show application of industrial shock absorbers as an alternative to traditional solutions for impact handling during offshore module installation to floating vessels, illustrated with experience gained by the installation of two modules on the Visund Semi. Results from multi-body simulations and model tests comparing traditional methods with the proposed solution will be given. The significant benefits obtained with respect to increased operational performance, reduced acceleration loads on the installed equipment, the increased predictability of the operation, and the consistent safety level in accordance with code requirements, will be highlighted. The possibility to apply designed damping for other offshore applications like dropped object protection etc, is also discussed.