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This paper presents a numerical and analytical study of pipein-pipe installation in deepwater by the J-lay and Steep S-lay methods. In particular it examines the amount and location on plasticity that may be allowed to occur during the installation phase of construction. The influence of the residual strain to operational performance of the line is discussed. The information in the paper provides guidelines for the installation of pipe-in-pipe systems in deepwater as well as indications of the critical areas that require detailed investigation.A general limit of 1% strain during installation has industry acceptance although few projects have put this to the test. The results presented in the paper show this to be acceptable under certain circumstances and perhaps not so conservative in others. It also highlights the importance of understanding the numerical modeling techniques as well as the influence of assumptions in the loading and materials modeling.

Karst geology is characterized by the presence of sinkholes and voids, which may pose significant risk to existing infrastructure. Sinkhole formation is often observed near active quarries, where dewatering operations can alter regional groundwater flow patterns leading to subsidence and increased void formation. In these areas, identifying locations which may be susceptible to sinkhole formation requires an ability to map dissolution features within the rock. Traditional geotechnical explorations alone are not well-suited to this effort as they only provide subsurface information at discrete points and therefore may miss voids within the rock. Geophysical methods offer a means to produce continuous profiles of the rock surface and possible locations for voids but interpreting the results of these tests in karstic geology can be challenging. This study uses 2D electrical resistivity and seismic surveys at a site with previous sinkhole activity along an interstate in central Alabama. The site is adjacent to a limestone quarry. Resistivity data is collected using 2D dipole-dipole and strong-gradient arrays. The seismic data is processed using a full waveform inversion (FWI) technique. Subsurface profiles interpreted from the geophysical surveys are then compared to borehole data from previous site investigations. Results from the geophysical surveys are found to be consistent with borehole data regarding variation of bedrock depth and identification of possible sinkhole features. Potential limitations and sources of error pertaining to each survey type are considered.

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