Structural Analysis of Ships in Waves using Mesh-Free and Finite Element Methods

Ships are expensive and complex assets. The design and through-life maintenance and sustainment of a ship relies on understanding the complex interaction between the ship structure and surface waves. Although the initial design is likely to be adequate, exposure to long-term fluctuating loads, occasional exposure to extreme waves, and modification and upgrades to the ship through its life can change the response of the ship, potentially compromising crew, asset safety and operational availability.

The research here uses physics-based modelling in commercial software to predict the complex interaction of the ship and the waves. The interest is in the response of the ship structure at rest in calm water, and at speed in waves. The ship used here is a finite element model of a ship that was intended for structural analysis, not hydrodynamic analysis. The present technique is such that as the structural finite element model of the ship interacts with the waves, the time-varying motions, internal forces and loads are revealed.

For the work here the water is represented by the mesh-free finite element technique known as Smoothed Particle Hydrodynamics, SPH. SPH can represent calm water or using a technique of synchronised orbital floor motions in a shallow tank, can develop deep-water-like waves. The shallow tank orbital floor motions result in reduced computational effort compared to a full depth tank and ensure surface wave propagation without decay in wave height. The waves developed with the orbital floor motions are shown to produce waves with similar qualities to deep water waves.

The results presented here are for a 109m long naval frigate at selected operating conditions from stationary in calm water up to 24 knots in waves of 6m wave height across various wavelengths. The correlation of the ship motions and the midship bending moments of the numerical simulation to the experimental results from a segmented scale model improve as the SPH particle size decreases, showing good correlation at 0.6m diameter particles.

The computational effort increases significantly with smaller SPH particle sizes, but the compromise between accuracy and time of computation is demonstrated to be acceptable for high-end users with access to industrial computers of today.

The numerical method here has demonstrated that good correlation to experimental results for ship motions and bending moments can be achieved with manageable computational resources. Further, no special treatment within the technique was required for extreme operating conditions, such that the technique should be applicable to a wider variety of ship types and operating conditions than shown here. The prediction of ship motions and bending moments would be suitable for use at design stage, or at a mid-life stage with modifications to the ship model to reflect a structural or operational change of the ship.

The work presented here is based on prior knowledge of the author as well as that developed by the author and his advisory team within the two three-year projects funded by Pacific-ESI, Defence Science and Technology Group and the Australian Research Council under Linkage Projects LP160100391 and LP190101283.