A riser conveys fluids from a subsea system to a host floater; however, oil and gas phases may alternate, increasing pipe's stress and damaging downstream facilities. This paper studies the nonlinear planar vibrations of a steel lazy wave riser excited by slug flow. The employed formulations comprise the Euler-Bernoulli beam model and the steady plug-flow model with a time-space-varying mass per unit length in the form of a rectangular pulse train. The equations are solved by a Runge-Kutta finite difference scheme and frequency-response curves are constructed for effective tension, curvature, usage factor and fatigue damage. The results offer a useful insight of the slugging frequencies and slug lengths that may receive attention during the design of risers.

While in service, marine risers are subjected to various types of loads, such as axial tension and bending moments arising from waves, winds, and currents in association with the motions of the offshore platform. They are also subjected to internal and external pressure loads caused by internal flows and external water pressure. The characteristics of the loads on a marine riser are essentially probabilistic in nature, as they involve several uncertainties associated with random variables. The aim of this study is to quantify the probabilistic distribution of loads on a marine riser to aid determination of the nominal values of design loads. Two methods are investigated. The first is to select a set of credible scenarios in association with site-specific metocean data on an offshore platform, and then perform dynamic riser analysis to describe the probabilistic distribution of the loads. The second is to calculate a metamodel to predict the loads as a function of multiple input variables, a method that can also characterize the probabilistic load distribution by running a Monte Carlo simulation. Both approaches are compared via a numerical example of a marine drilling riser in ultra-deep water; the results show that metamodel-based method is the most appropriate to describe neatly the loads at low probability of exceedance. The characteristics of the loads on a marine riser are observed to be highly random and significantly affected by environmental and functional conditions. Hence, the design loads must be determined by considering the marginal probabilistic density function of all such parameters.

Some floating production, storage, and offloading units (FPSOs) possess disconnectable systems to avoid harsh environments. According to a literature survey, the practice is based on perceptions and experiences of operators to judge disconnection; however, this paper offers a rational approach. A life-cycle cost model is proposed to optimize (1) the disconnection criteria and (2) the design of mooring lines under reliability format. Relevant ultimate limit states are considered in association with hull, moorings and green water failure. Effects of future failure costs is considered (downtime, environmental damage, reputation, etc.). Disconnection criteria are then formulated in terms of significant wave height and wind speed limits. Because a permanent mooring system may exhibit excessive resistance, it is possible to reduce the lines' thickness until the cost is optimized for non-permanent service. Results for an example in the Gulf of Mexico show that important savings can be achieved by implementing the proposed optimizations.