Directions de recherche du programme actions des éléments du comité d'études marines

Division Forages, Institut Français du Pétrole, Rueil-Malmaison

Résumé

The Institut Français du Pétrole has been running an overall study on the effects of wind, waves and currents on offshore drilling rigs since 1967, with the object of providing oil company engineers with better working methods. This is the first investigation of its kind ever to have been undertaken in France. The work put into this study was amply justified in view of the wide scope offered by oil prospection at sea, the need for more precise knowledge of environmental action and the importance of narrowing down the risk factor in the construction of new rigs. The programme is being carried out with direct participation of the Neptune undertaking and the assistance of " Doris ", IMF Lille, LNH Chatou, the Delft hydraulics laboratory and Franlab. It covers the following subjects : (i) Periodic Waves. Several models have been compared in the light of mathematical criteria and by considering the calculated forces acting on a pile in order to illustrate the resu!ts. The only really satisfactory form of wave representation is based on the stream function. (ii) Random waves, which are characterised by their energy spectrum. In this connection, models by Neumann, Breischneider-Scott-Pierson and Moscowitz will be compared with a view to establishing possible fields of application. (iii) The proccssing of recorded wave data and problems connected with it with a view to determining a wave spectrum (filtering) will be examined. (iv) Wave forces on structures can be calculated by the Morrison O'Brien formula. This is the only convenient method available for small bodies. No precise values are known for coefficients Cd and Cm; these should be expressed in terms of the wave velocity and acceleration field at a given point, which will also require experimental confirmation. A different approach will be required for larger bodies, for which wave diffraction should be taken into account. (i) Random wave forces can be estimated by Borgman's method, which will be applied to a fixed rig. (ii)By studying wind action, closer information can be obtained on the action and structure of gusts, static effects, resonance effects between rolling motion and gusts, and other similar phenomena. (iii) Available strength of materials programmes more specifically designed for petroleum engineering applications should be established ; for example, a study of vibrating structures should provide useful information. As an overall programme of this type should produce concrete results, experimental in situ measurements or on actual operational structures require particular care and attention.