The paper presents results from analyses of full scale measurements of the dynamic response of two steel jacket platforms in the North Sea.
The measurements discussed in the paper were, for both platforms, recorded in extreme environmental conditions.
The main emphasis is on describing the structural system behavior, on the composition of the dynamic response and on the evaluation and characterization of the environmental loading mechanisms.
It is found that the stress response is mainly wave induced and quasi-static. The quasi-static response is non-Gaussian and consistent with theoretical predictions based on Morison-type wave loading. Several other non-linear wave load effects are also apparent. The structural system is close to linear in stiffness and mass. The resonant response is, however, non-Gaussian due to nonlinear damping mechanisms.
The paper discusses also the implications which the observed non-linear wave loading and structural damping may have for potential deep water jackets.
In compliance with the requirements of the Norwegian Petroleum Directorate seven fixed platforms in the Norwegian sector of the North Sea have been instrumented for collection of environmental data and platform performance data (E- and P- data). The main purpose of the collection and analyses of this data is "through a check of the design assumptions and platform behavior, to be able to assess the safety of the load carrying structure and foundation in the production phase". An additional purpose is to gain experience with operative platforms that may be useful when considering new platform concepts
The data collection programmes which lasted for periods of two to three years have created a valuable data base for analysis and research on platform behavior in different environmental conditions.
This paper describes the dynamic behavior of two steel jacket platforms in both moderate and extreme environmental conditions, as evaluated from the full scale measurements. Similar results for gravity structures and an articulated tower have been presented in ref. /1/ and /2/.
The main emphasis of this paper is on the description of the behavior of the structural system including the soil-structure interaction, on the composition and nature of the dynamic response and on characterization of the waves and the hydrodynamic loading.
Furthermore, the nature of the observed behavior is compared with theoretical models of the platform behavior arising from the commonly used design assumptions.
The two platforms dealt with are Ekofisk 2/4H and Valhall QP. Sketches of the platforms are given in Figure 1.
Both platforms are four legged quarters platforms situated in approximately 70 metres water depth. Ekofisk 2/4H is K- braced with single corner piles while Valhall QP is X-braced with double corner piles. Both platforms are connected to other platforms with bridges.
