La Houille Blanche
Number 2, Mars 1970
|Page(s)||111 - 115|
|Published online||23 March 2010|
Consignes et aspects nouveaux dégagés par le congrès de navigation de juin 1969 sur les problèmes d'observation et de dépouillement de houle application aux plates-formes de forage
Chef du Service maritime de la société Grenobolise d'Etudes et d'Application Hydrauliques, Grenoble.
Subject 5 in Section II at the Twelfth Navigation Congress held in Paris from the l5th to 21st June 1969 covered "wave characteristics to allow for in the design of maritime structures". This note starts with a brief review of the main points covered in the nine reports which were read on the subject, from which the following rules and remarks emerged : Wave recordings are required in large quantities and should be obtained with the aid of digital output equipment so that they may be suitable for statistical analysis by computer. In processing this information the problem arises of characterising each storm by a limited number of parameters. One may hope to find a "standard spectrum" defined by a single wave period an height for each site. In seeking to determine wave value limits to allow for in the design of a structure for a given site, one has to attempt to apply theoretical relationships (e.g. exponential or Galton's), but because of the physical limits attached to natural phenomena, considerable care is required in the use of such relationships to determine ten-yearly or hundred-yearly storms from data observed over a limited period. The Congress expressed the wish that an international body be set up to centralise the data obtained and to standardise their processing. Current practice in studying wave action on man-made structures is to consider a characteristic wave height for storms, but the effect of wave period, length of action and irregularity require closer attention. Attempts should be made to estimate damage risks from storm damage suffered by actual structures and to avoid designing overdimensioned maritime structures. In the final part of this note sorne of the considerations in the papers read before the Congress are applied to drilling rigs as regards both the risk of destruction and the limitation of working conditions, the latter for floating rigs. It is recommended that, in designing structures of this type, one should not apply a single safety factor but, instead, a set of coefficients related to the uncertainty associated with each factor. The problem of structural fatigue due to repeated loads also requires attention, even for loads smaller than the static breaking load. Optimising the dimensions of floating drilling rigs in order to ensure the maximum number of operational days in the year without excessively increasing construction costs is a very tricky problem as the rolling, pitching and heaving periods of such structures depend on the considered wave spectra and, therefore, the optimisation calculations involve successive approximations and require a sound knowledge of the storm characteristics.
© Société Hydrotechnique de France, 1970