In order to investigate the local response of a ship structure, it is necessary to transfer the seakeeping loading to a 3DFEM model of the structure. A common approach is to transfer the seakeeping loads calculated by a BEM method to the FEM model. Following the need to take into account the dynamic response of the ship to the wave excitation, some methods based on a modal approach have been recently developed that include the dry structural modes in the hydro-structure coupling procedure and allow to compute the springing and whipping response of the ship structure to the seakeeping loads. In the context of the fatigue life assessment of a structural detail, a very fine FE model is required. A very large number of seakeeping loading cases also need to be considered to account for all the conditions encountered by the ship through its life. It becomes then clear that because of the CPU time issue, the whole FE model can not be very fine. This is why a hierarchical top-down analysis procedure is commonly used, in which the global ship structure is modelled in a coarse manner using one finite element between web frames. The structural details are modelled separately using a fine meshing. Such top-down methods are commonly used for the estimation of the quasi-static response of structural details to the seakeeping loads. This paper presents a methodology in which a top-down method is used to estimate the springing response of a ship structural detail loaded with wave pressure, and its fatigue life. The global dry structural modes are transferred to the detail fine model using the shape functions of the finite elements of the global model. The hydrodynamic pressures are computed directly on the fine mesh model, avoiding any interpolation error. The imposed displacements at the fine mesh boundary are computed using the same method that is used to transfer the structural mode shapes, and the local pressure induced loads and inertia loads are applied on the fine mesh nodes. This method is applied for the calculation of the elongation of a strain gauge which is installed in the passage way of an ultra large container ship.

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In this paper, a general method is presented which combines strain gauges’ data with 3D Finite Elements analysis of a hull structure, allowing the complete reconstruction of the structural response everywhere in the structure, based on the measurements from only a few sensors. By using sensors, one is getting rid of all the usual assumptions on wave loads and structural response which are made in standard desktop analyses. Usually, the main drawback of using sensors is that only a limited number of them can be used so in most current implementations, the structural response is only monitored in a few selected areas. The new method developed here allows to rebuild the response everywhere from just a few sensors measurements. The method is based on earlier works using a conversion matrix approach and a linear decomposition of the structural response on a base of a few modes. The measured time series at the strain gauges are then used to reconstruct the linear combination of modal responses which gives at those locations the same values on the 3D FE model as measured. The modes are defined as the response of the structure on selected load cases and a specific methodology is developed for the selection of those modes since their selection is fully dependent on the number and location of the strain gauges in the hull. The method is also used to automatically derive an optimized strain gauges’ setup by looking at correlations between strain gauges’ measurements in numerical simulations. The method is validated numerically by simulating measurements in an analysis and using them to reconstruct the complete response. Then actual strain gauges’ measurements on a hull are used to validate the method on a real case. Both fatigue damages and extreme stress values are compared, and it is found that on a real case, the fatigue damage and extreme stresses can be predicted with good accuracy in most of the hull structure based on less than 8 strain gauges’ measurements. From that new insight, an optimized inspection and maintenance plan can be developed and updated throughout the life of the structure, leading to safer and more cost-effective operations. Another key benefit to operators is the possibility to keep track of the remaining life of the structure and being able to demonstrate it, which is crucial when it comes to selling or redeploying an asset. The method has been used by DNV on commercial projects for various offshore structures including flare towers, MOPU platforms and sloating structures.

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Ship transport is growing up rapidly, leading to ships size increase, and particularly for container ships. The last generation of Container Ship is now called Ultra Large Container Ship (ULCS). Due to their increasing sizes they are more flexible and more prone to wave induced vibrations of their hull girder: springing and whipping. The subsequent increase of the structure fatigue damage needs to be evaluated at the design stage, thus pushing the development of hydro-elastic simulation models. Spectral fatigue analysis including the first order springing can be done at a reasonable computational cost since the coupling between the sea-keeping and the Finite Element Method (FEM) structural analysis is performed in frequency domain. On the opposite, the simulation of non-linear phenomena (Non linear springing, whipping) has to be done in time domain, which dramatically increases the computation cost. In the context of ULCS, because of hull girder torsion and structural discontinuities, the hot spot stress time series that are required for fatigue analysis cannot be simply obtained from the hull girder loads in way of the detail. On the other hand, the computation cost to perform a FEM analysis at each time step is too high, so alternative solutions are necessary. In this paper a new solution is proposed, that is derived from a method for the efficient conversion of full scale strain measurements into internal loads. In this context, the process is reversed so that the stresses in the structural details are derived from the internal loads computed by the sea-keeping program. First, a base of distortion modes is built using a structural model of the ship. An original method to build this base using the structural response to wave loading is proposed. Then a conversion matrix is used to project the computed internal loads values on the distortion modes base, and the hot spot stresses are obtained by recombination of their modal values. The Moore-Penrose pseudo-inverse is used to minimize the error. In a first step, the conversion procedure is established and validated using the frequency domain hydro-structure model of a ULCS. Then the method is applied to a non-linear time domain simulation for which the structural response has actually been computed at each time step in order to have a reference stress signal, in order to prove its efficiency.