Ds Holloway

The effect of slamming and whipping on the fatigue life of a high speed catamaran

Abstract In order to optimise the structural design of lightweight high-speed vessels, knowledge is required of the effect of sea loads on their structures, with respect to both ultimate strength and fatigue life. This paper reports on an investigation into the influence of slamming and whipping on the fatigue life of a large high-speed catamaran. Full-scale measurements of slam events were conducted on a 98m Incat catamaran to investigate its slamming behaviour in a variety of sea conditions. The full scale results were then used to determine the influence of the presence of slam events on fatigue life. In addition, the effects of significant wave height, slam occurrence rates, slam peak stresses and whipping behaviour on fatigue life were examined. The fatigue life was found to reduce significantly with the presence of slam events. Therefore, a reduction in slamming may prolong the fatigue life of a vessel markedly. The fatigue life was found to reduce significantly as wave height increases, as slam rate increases and slam peak stress increases. However, small slam events were found to have little or no influence on fatigue life. Whipping behaviour, in particular decay coefficient, may also strongly influence fatigue life.

The influence of hull form on the motions of high speed vessels in head seas

Abstract Prediction of ship motions at high Froude number is carried out using a time domain strip theory in which the unsteady hydrodynamic problem is treated in terms of the motion of fixed strips of the water as hull sections pass through it. The Green function solution is described and the integration of the ship motion carried out by an averaging method to ensure stability of the solution. The method is validated by comparison with tank data for conventional slender hulls suitable for catamarans, small water area twin hull (SWATH) forms and hulls suitable for high-speed monohulls. Motion computations are then carried out for 14 designs with an operating speed of 40 kts and a displacement of 1000 tonnes. The vessels are assumed not to be fitted with motion control systems for the purposes of this comparative study. Motion sickness incidence is predicted to rise to between 42 and 72% depending upon the hull design in 3 m head seas of average period 7.5 s. MSI values reduce in smaller seas with a shorter average period to be less than 15% in all cases in 1m seas with an average period of 5.5 s.

Experiments on Dented Steel Tubes under Bending

Steel tubular sections are extensively found in many kinds of civil and mechanical engineering structures such as columns and bracing elements, offshore industry, bridge elements, etc. Due to the extensive usage, a considerable amount of research has been carried out on such structural members. Although many studies focused on the bending stability of steel members, very little research can be found on the effect of surface defects, such as dent imperfection, on the bending behavior of such tubular members. This research aimed to evaluate the effect of dent-shaped defects on the flexural capacity of CHS members. The data obtained in this paper can be applied to evaluate the capacity of large scale CHS members with similar D/t ratio when a dent is formed on the tubes during the service life of such members. An interrelation between the capacity and the details of the dent, i.e. size and position of the dent, was proposed.

Concrete-filled circular steel tubes with a timber infill under axial compression

Over the past two decades, there has been significant interest in research relating to concrete-filled tubes, and a corresponding penetration of this technology into practice. This paper aims to expound upon the effect of timber cores on the structural response of concrete-filled circular tubes under compression. A timber infill with different shapes and geometries surrounded by concrete and encased in a steel tube was employed. The effects of the combination of infill elements on the failure, axial capacity, ductility, and structural efficiency (weight versus capacity) are exhaustively set forth. For the specimens with the highest timber to concrete ratio, the capacity was enhanced by about two times the capacity of the hollow steel specimens. For these specimens a significant reduction in the total weight of the composite element was obtained relative to the fully concrete-filled specimens. These specimens showed the highest ductility among the other specimens. In addition, greater ratios of energy absorption to the mass were obtained for the specimens with different timber cores in comparison to the equivalent values for fully concrete-filled tubes, which is quite desirable in many practical scenarios. It is found that the use of timber as an inner core element in this new composite yields promising results in decreasing the weight and yet enhancing the capacity, ductility, and energy absorption, and can be a good alternative to double-skin concrete-filled steel tubes.

A comparison of the motions of trimarans, catamarans and monohulls

Abstract The seakeeping motions of catamarans and trimarans are computed using a time domain seakeeping method, which has been accurately validated across a range of Froude numbers from 0.2 to 0.8. Both vertical and horizontal accelerations are evaluated. No motion controls are fitted. It is found that the rolling of a trimaran increases as the buoyancy in the outboard hulls is reduced, and exceeds the rolling of an equivalent catamaran by a factor of two when 5% of the total displacement is in the outboard hulls. In head seas, the greater length of the trimaran reduces the maximum heave motions relative to wave height as a consequence of the reduced length-based Froude number.

Experimental analysis of the wet flexural mode response of an NPL 6A hydroelastic segmented model

Abstract This paper investigates the effects of stiffness and mass distribution on the longitudinal flexural natural frequency response of an NPL 6A segmented monohull model in still water at zero speed. Experimental tests were undertaken in order to establish the parameters that affect the whipping frequency at model-scale so as to replicate the vibratory response of a full-scale vessel subject to slamming. The model was cut into two halves at the longitudinal centre of buoyancy and connected by a backbone beam with an elastic hinge joining the two segments. Wet vibration tests conducted on the model showed significant influences on the flexural natural frequency response through variations in stiffness and ballast mass distribution. The whipping frequency was predicted with a two degree of freedom theoretical model using an added mass approximation to provide good correlation with measured experimental data. Damping ratios of the wet transient response are also presented with respect to variations in the condition of the void separating the two model halves.

An experimental investigation of ride control algorithms for high-speed catamarans Part 1: Reduction of ship motions

Ride control systems are essential for comfort and operability of high-speed ships, but it remains an open question what is the optimum ride control method. To investigate the motions of a 112-m high-speed catamaran fitted with a ride control system, a 2.5-m model was tested in a towing tank. The model active control system comprised two transom stern tabs and a central T-Foil beneath the bow. Six ideal motion control feedback algorithms were used to activate the model scale ride control system and surfaces in a closed-loop control system: heave control, local motion control, and pitch control, each in a linear and nonlinear version. The responses were compared with the responses with inactive control surfaces and with no control surfaces fitted. The model was tested in head seas at different wave heights and frequencies and the heave and pitch response amplitude operators (RAOs), response phase operators, and acceleration response were measured. It was found that the passive ride control system reduced the peak heave and pitch motions only slightly. The heave and pitch motions were more strongly reduced by their respective control feedback. This was most evident with nonlinear pitch control, which reduced the maximum pitch RAO by around 50% and the vertical acceleration near the bow by about 40% in 60-mm waves (2.69 m at full scale). These reductions were influenced favorably by phase shifts in the model scale system, which effectively contributed both stiffness and damping in the control action.

Flexural behaviour of locally post-tensioned reinforced concrete beams

Abstract This study investigates the behaviour of locally post-tensioned reinforced concrete beams. Local post-tensioning is an innovative alternative to existing conventional post-tensioning methods that achieves prestressing of concrete beams using conventional reinforcing steel. Four concrete beams were locally post-tensioned and tested. The theoretical predictions were made based on AS3600-2009. A new theoretical approach is proposed to determine the cracking moment of tested beams, since the current design guide was not applicable to this post-tensioning method.

An investigation into the effect of roughness conditions and materials on bond strength of CFRP/steel double strap joints

Abstract Different combinations of carbon fibre-reinforced polymer (CFRP) and adhesives were employed to connect two steel plates in double strap joints with various roughness levels. Three different bond lengths were also examined to decide which one was preferred for further testing. All specimens were subjected to static tensile test until failure. It was found that variations of surface roughness level between 0.73 and 7.75 μm had no significant effect on the ultimate loading capacity of the specimens. However, the CFRP type played a very important role in the bond strength and the failure modes. The best performing CFRP type was the CFRP plate. It was also found that the effective bond length was approximately 100 mm when using CFRP plates.

Comparison of Measured and Computed Non-linear Wave Loads Using a Time Domain Method for an 86m Catamaran

Abstract The time domain, Green function solution of the encountered wave response problem in spatially fixed co-ordinates for slender high speed hulls is used to show the effect of hull form on non-linear motion and load responses. It is found that the non-linear effect on motions is small for round bilge and SWATH type hulls, but is strong for a hard chine hull with significant dead rise angles. In particular, the frequency of maximum heave is significantly reduced in large seas. Non-linear effects on bending and shear are negligible for a round bilge design with wall sides at the water line. For a SWATH design, unsteady loads per unit wave height increase modestly by up to 13% as the wave height increases so as to expose broader immersed hull sections, but reduce again at the largest wave heights. For the hard chine design the bending and shear per unit wave height reduce by about 40% as wave height approaches the draught. Unsteady stress and encountered wave data measured on a 86m catamaran ferry give good agreement with predicted loads over a wide range of speed provided that the appropriate wave height is used in the computations.