Plastic design of breasting dolphins

Abstract

Most liquid bulk terminals are equipped with a jetty as berthing facility. The ship mostly berths to dedicated breasting dolphins, which can be single-pile flexible dolphins or multi-pile rigid dolphins. In design methods for flexible dolphins, the yield limit is approached more and more over the years to employ the load-bearing capacity of the dolphin more optimally. In recent design guidelines (EAU 1996 and PIANC 2002) also the plastic yielding capacity is implicitly or explicitly included in the ultimate load-bearing capacity. This movement towards plastic design however is not accompanied by the development of calculation models and design criteria to assess the plastic load-bearing capacity. Damage cases in recent years seem to support the conclusion that the completeness and safety of those standards and guidelines is questionable. The first objective in this thesis project is the development of a calculation model for the prediction of the nonlinear structural behaviour of a dolphin. The most important output of the model is a complete load-deflection curve up to failure, including all relevant failure mechanisms, with which the elastic and plastic load-bearing capacity in terms of energy absorption can be assessed. The model, which is called the Bruijn model, is developed for the system of a single steel pile with uniform cross-section and steel grade sufficiently embedded in non-cohesive soil of uniform properties, under influence of a static horizontal load applied at the pile-head. As the second objective an evaluation of the currently effective design standards and guidelines for dolphins is carried out, in order to assess the safety which can be realised in the plastic range. A representative case is chosen with varying diameter-wall thickness ratios (D/t ratio of 83, 63 and 42), which is calculated according to all relevant standards and guidelines, and compared to the load-deflection curve generated by the Bruijn model. The case study leads to the conclusion that sufficient plastic yielding capacity is confirmed for a D/t ratio of 42, where an increase in the elastic energy absorption capacity of up to 1,33 times the original elastic energy absorption capacity can be obtained after some plastic yielding. At larger D/t ratios (60-80) this plastic yielding capacity is significantly smaller due to a much higher buckling sensitivity. At a D/t ratio above 80 buckling in the elastic range can be expected, reducing the ultimate energy absorption capacity to less than the full-elastic energy absorption capacity. For the standards and guidelines it can be concluded that the employment of the plastic yielding capacity should be accompanied by an assessment of the safety against failure for the failure modes stresses, strains, buckling and ovalisation. This means that the current standards and guidelines should be expanded with limit state criteria for failure modes buckling and ovalisation and should be adjusted for failure mode yielding. By doing this the safety of a design for any value of the D/t ratio will be ensured. It is recommended to evaluate and further develop the Bruijn model, and release such a model to the design environment. Such a model is easier to work with and more suitable for the specific design requirements than complex FEM analysis packages.