Hydrodynamic loading on oversized cylindrical cargo during marine transport

Abstract

With the increase in size of offshore structures and the increase in costs related to the construction of these facilities, a necessity to optimize the construction of offshore facilities arises. An element that contributes greatly to these costs is the transportation and installation of the structures. Transportation costs are proportional to the distance covered and the number of transports necessary, since special vessels or tug boats are required. Installation costs are mainly proportional to time since the day rates of specialized construction vessels are extremely high. One of the most used structural members in offshore construction are steel cylindrical structural members, such as piles and pipelines. With increasing dimensions, also these members increase in size and it is often found wishful from a viewpoint of cost efficiency to transport the members on barges that are smaller in length than their cargo. Not only are standardized cargo barges more widely available then specialized equipment, their use also saves on fuel costs and pollution due to their reduced resistance compared with vessels that are greater in dimensions. Due to forces induced by the sea state the barges move and the oversized cargo undergoes a relative motion with regard to the sea surface. In certain environmental conditions this relative motion can develop in such a way that impact and/or submergence of the cargo occurs. This thesis focuses on the hydrodynamic loads involved with this interaction of oversized steel piles and the sea. The relative motions have been determined from a linearized motion analysis of an industry standard 122m long cargo barge. A radiation and diffraction analysis using potential theory software WAMIT has been performed to 1) determine the hydrodynamic coefficients of the barge, and 2) determine the effect of radiation and diffraction on the fluid pressures in the surrounding fluid field. It was found that the relative motions have a peak amplitude at wave periods that are commonly encountered in marine transport. This holds for approaches with and without taking into account the effects of wave radiation and diffraction. The analysis of the relative motions shows that for large enough wave heights water impact is possible to occur. To be able to quantify the loads an extensive literature review has been conducted on the involved hydrodynamic processes. It was found that knowledge on the governing load mechanisms is available. Water exit effects are less well covered in literature and conservative upper limit assumptions have been made to account for water exit effects. For quantification of the involved hydrodynamic loads, two approaches have been developed. First, a quasi-stationary approach is chosen to analyze the hydrodynamic loads. Four characteristics orientations have been identified which give a good representation of the maximum loading situations. The parameters for this approach are found from a spectral analysis of the relative motions, both with and without taking diffraction and radiation effects into account. By using this approach a lot of additional conservatism is introduced, which is mainly caused by using upper limit values for the relative motion and slamming force coefficient. To gain a more detailed insight in the magnitude of different load contributions therefore a second approach has been developed. This approach consists of a segmented force model that operates on time traces drawn from response spectra of the relative motions. The cylinder is divided into segments iv and the involved loads are determined per segment per time step. The total resulting forces are found by summing the contributions of every segment along the pile length. Using a case study of a recently conducted transport the differences between the two approaches have been investigated. It is found that the maximum loads from the segmented model approach are roughly a factor 2 to 10 smaller than the results from a quasi-stationary approach. Compared with an existing engineering method, the forces from the segmented model are lower for all wave headings except stern waves. The comparison is skewed however because in the current engineering method not all involved loads are accounted for. To gain more confidence in the results of the segmented approach, the model is validated by means of three characteristic situations. The validation shows slight conservatism in the results of the segmented model. Ultimately the segmented model shows that when wave diffraction and radiation are taken into account, a wave heading of 180° is most favorable. With this an additional argument for towed marine transports to weathervane in stormy weather is found. The contribution of this thesis is that it demonstrates that using a detailed approach with regard to hydrodynamics may be beneficial. Although in a segmented model approach the analyses have to be repeated multiple times to gain a statistically reliable result, it is noted that once a model is available the computational costs do not differ a lot from a traditional frequency domain approach. This thesis therefore opens possibilities to reduce costs involved with offshore construction or marine transporting general. Further work should be directed at validating the obtained results by means of full scale or model testing, including effects of more complicated cargo configurations, investigating different cargo geometries, and studying the influence of the dynamics of load transfer to support structures.