Experimental And Analytical Studies Of The Elastoplastic Behavior Of Offshore Pipelines During Laying

Abstract

With more and more offshore pipelines being laid in deeper water, the need for larger laying equipment has increased larger barges, stronger tensioners and longer stingers. The primary reason is that, in deeper water, the pipeline is suspended at a steeper liftoff angle, and consequently a longer over bend portion is required between the liftoff point and the welding stations In order to help relieve stress in the pipeline. If the over bend portion can be curved until the plastic region of the pipe is reached, it will be possible to use shorter stingers and lay pipe in deeper water without any special equipment investment. We took note of these possibilities and conducted integrated experiments consisting of material tests, pipe bending tests and pipe well denting tests. We also developed a continuous beam elastoplastic analysis program, a pipe section analysis program and a cylindrical shell analysis program. Using both test data and programs, we established a precise elastoplastic design method for laying offshore pipelines. Measured results show good agreement with calculated results. This elastoplastic design method accurately predicts the capacity of pipe to bear environmental loads, thereby making it possible to set optimum limitations for laying operations. However, it should be noted that, just above the yield point of the pipe, slight variation environmental loads cause significant changes in the strain and deflection. It would therefore be desirable in actual laying operations to use a stinger that permits control and restraint of pipe curvature and deflection. INTRODUCTION The elastoplastic design method for laying offshore pipelines makes it possible to use shorter stingers, which results in reduced liquid forces acting on the stinger pipeline system. This in turn results in improved operating rates and increased water depths in which pipe can be laid, as well as reductions in cost. Moreover, optimum limits set on laying operations by this design method ensure operational safety. Three problems arise when large strains are induced in the pipe during laying. It is essential for the elastoplastic design method to solve these problems. The three problems are:NonlinearityThe bending stiffness of pipe bent beyond Its proportional limit becomes nonlinear due to the nonlinearity of the steel's stress-strain characteristics and the ovaling of the pipe section. Therefore, a nonlinear analysis must be made of the portion of the pipeline between the lay barge and the ocean floor.Local bucklingIn our experiments, all pipe specimens with a D/t ratio of 56 or over failed due to local buckling. In the case of specimens with a D/t of 56, local buckling occurred after the yield point was reached, whereas specimens with D/t ratios of 78 and 97 buckled locally before their yield points were reached. Thus, the elastoplastic design must give full consideration to local buckling.

Critical Buckling Strains in Thick Cold-Formed Circular-Hollow Sections under Cyclic Loading

“…11a, based on a similar figure established by Harn et al (2019). The square symbols represent critical strains obtained from various programmes of monotonic bending tests (Reddy (1979), Gresnigt et al (1996), Del Col et al (1998), Dorey et al (2001), Gresnigt and van Foeken (2001), Peters et al (2015) and van Es et al (2016)), as gathered by the authors from publiclyaccessibly literature, as well as the data of van Douwen et al (1974), Bouwkamp (1975), Sherman (1976), Korol (1979) and Kimura et al (1980) as reported by Gresnigt (1985).…”