Anders Rosén

Experience from Parametric Rolling of Ships

This chapter reviews three recent full-scale events with parametric rolling for Ro–Ro Large Car and Truck Carriers (LCTC). The events represent three principally different modes of parametric rolling: principal parametric resonance where the period of encounter is half of the roll natural period in following seas (I) and in head seas (II), and fundamental parametric resonance where the period of encounter coincides with the roll natural period in following seas (III). Roll motion, course, and speed recorded during the events are presented and analyzed together with the present weather situation based on recordings, forecasts, and re-analysis. Different aspects of on-board operational guidance for assisting crews in avoiding parametric rolling are discussed in relation to the presented events. Involved complexities and considerations that are normally not included in well defined model tests or numerical simulations are exposed.

Rough water performance of lightweight high-speed craft

Previous studies have shown how the use of composite materials and application of sophisticated design methods can give significantly lighter high-speed craft structures than what is normally achieved for traditional aluminium designs. A reduction in structural mass and a corresponding reduction in displacement improve the craft calm water performance but can be unfavourable regarding the rough water performance. Here, the rough water performance of two versions of a fast patrol vessel, one in aluminium and the other in carbon fibre sandwich, is studied with simplified semi-empirical methods and more advanced non-linear time domain simulations. In speeds up to 30 knots, the rough water performance of the two craft versions is found to be practically equal. At higher speeds, the lighter composite craft experiences higher vertical accelerations than the heavier aluminium craft, which implies less operational availability. Using trim ballast tanks, the rough water performance of the lighter craft is improved, and it is shown that the acceleration levels can be reduced and even lowered relative to the heavier aluminium craft. This means that the calm water advantages of a lighter composite vessel can be utilized with the same ride comfort and operational availability as for a heavier aluminium vessel.

Experimental evaluation of slamming pressure models used in structural design of high-speed craft

In this paper a methodology that enables detailed studies of the momentary slamming pressure distribution and the related structural loads is presented. The methodology involves pressure measurements, spatial and temporal pressure distribution reconstruction, and finite element analysis. Using a set of model experiments with a high-speed planing craft towed in irregular waves, the methodology is applied to evaluate different slamming pressure models regarding their ability to accurately represent the structure loads. The slamming equivalent uniform pressure model used in the prevailing semi-empirical design methods, and non-uniform time-dependent pressure models used in a method for direct calculation, are considered. It is concluded that for smaller structure members, typically found in densely spaced metal structures, the uniform pressure model performs well with respect to accurately representing the structure loads. However, for larger members found in modern sandwich structures, the underestimation of structure loads is significant, especially with respect to the shear forces. The non-uniform pressure modeling technique is concluded to be promising regarding its ability to accurately represent the structure loads, regardless of structure member size. The presented methodology is concluded to be a useful tool in further research regarding the evaluation and development of methods for high-speed craft structural design.

Comparative Life Cycle Assessment of the hull of a high-speed craft

A comparative Life Cycle Assessment is performed for different structural material concepts on a 24-m-long high-speed patrol craft. The study is comparative and determines the differences in and sensitivities to environmental impact, especially in relation to the total impact of fuel burn for the different material concepts. The material concepts are aluminium and various composite combinations consisting of glass fibre and carbon fibre with vinyl ester resin both as single skins and as sandwich with a Divinycell foam core. Commercially available standard Life Cycle Assessment software is used for the Life Cycle Assessment calculations. The study shows that regardless of hull material concept, the environmental impact is dominated by the operational phase due to relatively large fuel consumption. In the operational phase, the lightest carbon-fibre concept is shown to have least environmental impact. Considering the manufacturing phase exclusively for the different hull concepts, it is concluded that the manufacturing of the aluminium hull has a somewhat larger environment impact for the majority of Life Cycle Assessment impact categories in comparison to the different composite hulls. The significant impact on the marine and the fresh water aquatic ecotoxicity originates from the aluminium raw material excavation and manufacturing processes. It is shown that the lightest hull, the carbon-fibre sandwich concept, with a 50% structural weight reduction compared to the aluminium design, can be utilized to reduce the fuel consumption by 20% (775 ton of diesel) over the lifetime with significant impact on the dominating environmental aspects considered herein, abiotic depletion, global warming and acidification.

Motion-based monitoring of racking stresses in ro-ro ships

This paper presents a method for monitoring of racking-induced stresses in ro-ro ships. The approach is built on the assumption that the racking stresses are mainly induced by the roll and sway motions and therewith related inertia and gravity forces. The approach involves real-time measurement of the ship motions and scaling of the measured motions with pre-calculated structural responses from finite element analysis. The method is applied to full-scale measurement data from the Wallenius Lines PCTC Mignon. Derived stresses show good agreement with stresses derived from strain gauge measurements, indicating that the method has potential as an alternative to conventional strain-gauge-based monitoring. Motion-based stress monitoring has several potential areas of application such as providing data for decision support, for live assistance and short-term route planning, structural condition reports and for supplying feedback to the design process.

IMPACT PRESSURE ANALYSIS ON HIGH-SPEED CRAFT IN WAVES, THROUGH FE-ANALYSIS ON FULL-SCALE MODEL MEASUREMENT DATA

This paper discusses a method to re-construct three-dimensional propagating impact pressure distributions, from recordings with an arbitrary matrix of pressure transducers on a hull. With pressure measurement conducted with several transducers on a small high-speed craft in both full-scale and model scale, the method is used to formulate realistic load cases for finite element analysis (FE). The resulting structural responses are calculated and compared with full-scale strain measurements that indicate that the reconstructed pressure distributions resemble the actual load very well. Further use of the method is discussed, for example, the investigation of real pressure distribution influence on panel boundary conditions and time domain simulation of structural response when the method is used together with methods for seakeeping simulation.