Summary

In the process of new designs of mechanical structures or systems and control strategy development, a great role for numerical modeling and simulation can evidently be identified. The most comprehensive verification provided by an experiment is naturally the best solution. It is however tedious and costly and in many cases difficult to perform. Offshore structures are often produced as single specimen for a specific order. Carrying out detailed empirical research would raise the final price of a device considerably. Therefore many design companies, including ones in the business of offshore engineering, are interested in access to appropriate calculation software. Such programmes have different purposes. Some of them are suited for strength analysis, others to simulate operation of a device or its control system. In addition to accurate calculations of precise values which are necessary when designing a given machine, companies also need quick and rough simulations, e.g. when preparing an offer (during initial negotiations with a counterparty). Calculations performed at the design stage are not significantly constrained by allowed duration of the simulation. On the other hand, control systems of devices must perform real-time calculations which requires using sufficiently numerically efficient models and methods. In many cases, to obtain satisfactory correspondence to reality flexibility of links must be taken into consideration by their discretisation. In some problems, nonlinear properties of the material or other specific conditions may be important. At present, different discretisation methods are used in calculations of machines’ dynamics. The most widely known is the finite element method. The authors of this book have been involved for many years in the development of the rigid finite element method. Based on their experience, it is their position that this method allows developing models of structures adequately reflecting the actual features of the dynamics involved while keeping the number of generalized coordinates small. It is also fairly simple to implement on a computer. It furthermore enables quick and convenient changes of the number of rigid finite elements in the discretized links. This allows both the calculations in real time (for small numbers of RFEs) necessary for control and more time consuming ones which better reflect the flexibility of the system (assuming more RFEs) to be carried out.