This paper presents how non-linear waves and wave loading induce non-Gaussian dynamic response of fixed offshore structures, and the effects this may have to fatigue life and extreme dynamic response of such platforms. Furthermore, the validity of different theoretical wave and wave load models is discussed. The basis for the paper is full-scale measurements of waves and dynamic response recorded on a jacket platform in the North Sea and time domain computer simulations of the response of this platform using a finite element structural model.
Fixed deep water platforms will because of the large water depths and high deck masses behave highly dynamically. Such platforms must therefore be designed against irregular environmental load processes with the energy distributed over a wide frequency range. This demands the use of stochastic methods for dynamic response analysis both for prediction of fatigue life and extreme load effects. The practical procedures for such analysis which are available today are based on Gaussian environmental excitation processes, linearized load models, linear structural models, and, consequently, on Gaussian dynamic response. Full-scale measurements from existing platforms have, however, indicated that the dynamic response of fixed structures generally is non-Gaussian [1, 2, 3]. In this paper it is, based on the results of systematic analysis of full-scale measurements of waves and dynamic response, discussed which are the sources of non-Gaussian dynamic response and what are the consequences of this to fatigue life and extreme response. Furthermore, the capabilities of existing methods for non-linear dynamic response analysis is outlined through comparison of measured and simulated dynamic response. The effects of the observed type of non-linear load effects to fatigue life and extreme response of deepwater platforms are discussed in [4, 7].
The applied data are recorded on the Val hall quarters platform. This platform is four legged and x-braced double corner piles. The water depth is 70 m. Typical dimensions for wave loaded structural elements are 1.5 m for the legs and 0.7 m for the bracing.
The platform is connected with a bridge to two neighboring platforms.
The platform is instrumented for measuring of wind speed and direction, wave heights, current speed and direction, strain in one of the braces of an x-joint and platform accelerations at cellar deck and mudline levels.
The platform is shown in Figure 1 together with the two connected platforms. In Figure 2 a more schematic picture of the platform is given which also indicates the location of the different sensors.
The data involved in the study are the wave data, current data and strain data in the location indicated in Figure 3. Key parameters for the different data types are given in Table 1. The standard length of each recording period is 20 minutes.
The study included 28 recording periods, all representing sea states where the mean wave direction was within the 30 degrees sector indicated in Figure 3. The distribution of the recording periods with respect to sea state severity is given in Table 2
