Measurements of platform motion, tether tension, air gap, wind and wave conditions have been collected at the Snorre TLP from the time the platform was installed in the North Sea in 1992. Great analytical and experimental efforts were made during the design to document the motion responses, in particular the motions induced by second order wave loading. By comparing measurements of slowly varying horizontal resonant motions. With model tests and numerical results we observe that the full scale motions m some cases (wave spectral peak period below 10s) are significantly smaller than the results from the model tests. This is explained by calculations which include the following modifications compared to the model tests increased horizontal stiffness, wave directional spread, wave frequency distribution different from standard wave spectra, and current. When designing Snorre, the vertical resonant motions were calculated by simplified second order theory, which has been shown to give conservative results compared to measurements [6]. In the present paper, new and more accurate second order calculations are presented. The conclusion then seems to be that the vertical resonant motions are underestimated and other forcing mechanisms, e g third and higher order effects may be significant.


The Snorre tension leg platform (TLP) operated by Saga Petroleum a s., Figure 1, was installed m 1992 at 310 meters water depth m the North Sea The weight of the platform IS 77500 tonnes including riser tensions, and the total tether tension at mean draft 383m, IS about 29000 tonnes. The hull is made up of 4 Circular columns with diameter 25 m, and a square ring pontoon. The column centre to centre distance is 76m. The oil production capacity is about 190000 barrels per day, produced from platform completed wells drilled through the template directly underneath the platform, and sub-sea completed wells on up to three subsea templates, where one is already Installed 8 km from the TLP.

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