This paper describes an experimental and theoretical study of wave forces and overturning moment acting on two geometrically similar concrete oil production platforms designed for North Sea deployment. The theoretical predictions are based on a modified potential flow (or diffraction) theory and are compared with accurate model test results. The theoretical and experimental results are found to be in excellent agreement. The method applied is proposed as a general method. which is valid for any structure similar to the ones considered herein which are composed of a large base unit with a superstructure- of smaller members.
A primary concept employed in the production of oil in the North Sea centers on the use of large concrete gravity structures which function as storage and loading facilities. One of the critical factors in such designs is the rather large horizontal and vertical wave forces and overturning moments induced by storm waves. The fact that the wave action decreases with depth often leads to structures which have large displacement bases with superstructures composed of smaller members which are subjected to the greatest wave ac- tion. A method is presented herein for calculating the forces and moments on such structures.
The present paper deals in particular with the wave action on two slightly different CONDEEP* production platforms designed for 110 to 120 meters storm still-water depth for deployment in the Beryl field of the North Sea by Mobil Oil Company. The platforms consist principally of a large, nearly hexagonal base composed of 19 cylinders, three of which are tapered above the base and extend from the top of the base through the free surface as indicated in Fig. 1. One of the platforms is constructed from 18 meter diameter cylinders ana the other from 20 meter diameter cylinders, and are nominally referred to as the 18 and 20 meter structures, respectively. All dimensions other than the diameter of the cylinders are common to the two structures. The theoretical and experimental results presented herein correspond to regular wave design conditions in the range of 14.0 to 17.0 seconds period and 21.0 to 29.0 meters wave height.
The theoretical analysis is essentially based on the linear potential flow theory as outlined in Ref. (1-3). This method of distributedsources is used to evaluate the pressure distribution and resulting forces and moment acting on the large base part of the structure in the absence of the superstructure. Then, using this analysis, the influence of the presence of the base unit on the flow field above the base as encountered by the superstructure is evaluated. The wave forces acting on the super structure, consisting of the three large columns are then evaluated separately by use of the Morison equation on the basis of the incident wave flow field as modified by the presence of the base. The results show that the presence of the base tends to increase the forces on the superstructure significantly.