Lazy-Wave Buoyancy Length Reduction Based on Fatigue Reliability Analysis

Abstract

When the profit scenario of an industry changes, the continuity of some projects can be at risk. The current downturn of the oil and gas industry force managers to take hard decisions about the continuity of projects, resulting in delays, postponements or even the cancellation of forecasted projects. In order to keep with these projects, the rush for cost reduction is a reality and the industry is pushing the involved parties to be aligned with this objective. The Brazilian Pre-Salt region, characterized by ultra-deep waters, is an example of this scenario. Subsea structures represented by flexible risers, which are responsible for the flow assurance of oil, gas and water, are forecasted to have a demand about 4.000 km in the next years. Usually, in these type of applications, lazy-wave configurations are adopted, increasing the costs of the solution with the necessity of the buoyancy modules acquisition. The smaller the buoyancy length is the cheaper the project become, reducing the necessary amount of buoys and the time spent for its installation. These type of solutions can probably carry with it a high level of conservatism, imposed by the use of standardized safety factors, and can potentially be optimized with the adoption of probabilistic approaches within the chain of analysis. The objective of this paper is to assess the possibility of buoyancy length reduction of lazy-wave configurations by using structural reliability methods of analysis. The focus stays on the evaluation of the fatigue of the armour wires located at the bend stiffener region, one of the most critical failure mode for the design of flexible pipes in offshore Brazilian installations. As already discussed in Ref. [1], many variables can influence on such kind of analysis. Based on this previous study, the first six random variables, identified to be the most important ones, are taken to carry out the analysis. The fatigue reliability approach considers four 6” flexible riser configurations: an original lazy-wave, a lazy-wave with less 30% of buoyance length, another one with less 50% of buoyance length and a free-hanging configuration. Failure probabilities and safety factor calibration curves are shown for each presented configuration and compared among themselves. The results indicate the possibility of defining a lazy-wave configuration with smaller buoyancy lengths, reaching 75% of reduction without changing the preconized high safety class at last year of its operational time. Safety factor curves shows to have similar behavior no matter the configuration considered. Structural reliability analysis comes as a potential method to help engineers to have a better understanding on the driving random variables of the problem, giving a support for the actual cost reduction scenario and for better decision-makings based on quantified risk.