Large Scale Model Testing Of Deep Sea Risers

Abstract

Abstract The paper describes a series of tests with 90 m long models of deep sea risers, performed at an open air test site in a Norwegian fjord. The main purpose of the tests was to provide experimental data for the vortex induced vibrations (VIV) of such risers in a shear current. The test results can also be used to study collision criteria, drag coefficients, etc. The paper concentrates on a discussion of one of the phenomena observed in the tests, the axial vibrations of the risers. A theory for estimating the fatigue stresses due to such axial vibrations, induced by the VIV, is developed and presented in the paper. If the VIV frequency coincides with half the frequency of the first axial vibratory mode of the riser, the theory predicts extreme stresses to occur. The phenomenon of axial vibrations may become an important aspect of future deep sea riser designs. Introduction The riser system represents one of the main challenges in the design of safe and cost-effective concepts for production of oil and gas at sites with ocean depth of one to two thousand meters or more. The loads and responses of deep sea risers due to current can to a certain extent be predicted by numerical methods. However, for verification of the theory, and for detecting possible unexpected phenomena or problems encountered when designing for conditions beyond previous experience, model testing is the best, if not the only possible tool. One of the problems in the design of such systems is that there is no model tank: in the world today deep enough to model the complete riser and mooring systems at a scale which has traditionally been considered necessary to achieve reliable predictions. One possibility is to do model tests in fjords or the open sea. In Norway an extensive test program was done in August 1997 at an open air site with 90 m long models of steel risers, at a geometric model scale of about 1:15. The present paper describes the tests and shows examples of results. An interesting aspect of the results was the considerable axial tension fluctuations induced by the vortex induced transverse vibrations (VIV). This triggered the authors to study VIV-induced axial vibrations in deep sea risers in more detail. The results indicate that for steel production risers of the order of 1500 m length the VIV may induce resonant axial vibrations causing such dramatic stresses that it becomes an important aspect of future deep sea riser designs. Objective The objective of this paper is to present a recently conducted research project, including a series of model test on deep sea risers, and to show some examples of the results. A main part of the objective is to present the development of a theory for VIV-induced axial tension fluctuations. Large scale tests Test philosophy, test site. Since there are no model tanks deep enough for complete modeling of deep sea systems at what has traditionally been considered a suitable model scale, one has to look for other possibilities.