Driving pile s for deepwater platforms is a time consuming, costly and - for some types of soil - a practically impossible operation.
The paper deals with a new type of sitting-on-bottom platform suitable for water depths to 200 meters and over. The platform is fully constructed on land, and transported to site floating vertically. Installation is accomplished by controlled flooding and once on the bottom no other operation is required to secure the platform.
The paper presents platform features, design techniques and comparison between theoretical and model basin tests results.
The type of the design and cost of a fixed offshore platform is mainly influenced by the following three parameters:
water depth,
meteoceanographical conditions and
bottom soil properties.
For limited water depths the jacket-pile configuration is the type of design which has been recognized as the most convenient approach all over the world. In this case the parameter which mostly influences the cost of the platform is the water depth. As for the influence of bottom soil properties, piles driving techniques and related equipment are developed enough to give not particularly difficult problems of installation.
However, with the increase of water depths, the influence of all the three parameters on the structural weight and costs of the platform becomes equally important. Fig. I shows the structural weight (jacket and piles) trend in different offshore areas as a function of water depth. The increase of the bandwidth almost in log-scale shows the more and more determinant influence of the meteoceanographic conditions in relation to the structural weight of the platform. Pile driving operations become critical both for the increase of the necessary number of piles and the required driving energy. In addition, the increase of the distance of the installation site from shore, in connection with the size of the structures emphasizes transportation problems.
As a result, the conventional structural design utilizing a steel jacket and pile foundations appears to be no longer the most suitable application for water depths ranging around 200 meters. There is the need, nowadays, for different design and construction installation techniques, to make fixed platforms still attractive for the development of deep water petroleum fields.
The possibility of using fixed platforms standing, on the bottom by their own weight-thus eliminating the piling - has been studied by our Company for many years. These studies were carried out within a group of engineers engaged in concept studies for the development of submersible and sitting-on- bottom structures for various applications (oil exploitation, port facilities, etc.). However, the water depth limits in which offshore petroleum operations were carried out at that time, did not justify the development of this concept. With the ever increasing water depth for petroleum operations the concept was revived and, in 1969, a study development of such structures for water depth over 100 meters was started, and a solution was developed as illustrated in Fig. 2.
