Nonlinear Simulation of Jackup Platform Models
- Ping Liu (John Brown-Zeetech Engineering) | W.W. Massie (Delft U. of Technology) | J.G. Wolters (Stork Protech Engineering) | Johan Blaauwendraad (Delft U. of Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 1993
- Document Type
- Journal Paper
- 118 - 124
- 1993. Society of Petroleum Engineers
- 4.5 Offshore Facilities and Subsea Systems, 4.1.2 Separation and Treating, 5.1.8 Seismic Modelling, 1.6 Drilling Operations, 4.3.4 Scale, 4.5.2 Platform Design, 5.6.3 Deterministic Methods, 4.1.5 Processing Equipment
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This paper summarizes laboratory studies and associated computer simulationsfor three jackup models. Comparison of computed results with measured datashows that such simulations are reliable. Further results demonstrate thathydrodynamic interaction must include relative velocities, the P-6 effect isessential, and damping and stiffness at the deck/leg connection can be"traded off." Parameter studies, such as the influences oflinearization, free-surface treatment, different wave theories, andhydrodynamic force cancellation, also are presented.
The primary objective of this work is to investigate the influence ofstructural and hydrodynamic nonlinearities on elevated jackup rigs. This helps"unravel" the problem of the various (conflicting) existing bodies ofknowledge on performance of elevated jackup platforms. Three principle modelswere tested in a wave tank. platforms. Three principle models were tested in awave tank. Associated simulations were carried out in the time domain with anonlinear, dynamic, multiple-degree-of-freedom software with varioushydrodynamic interaction options. The present work forms a part of a Delft U.of Technology project to increase the detailed knowledge of the behavior ofsuch project to increase the detailed knowledge of the behavior of suchplatform components and to predict the overall structure's elevated platformcomponents and to predict the overall structure's elevated behavior and(remaining) lifetime. An earlier paper gave an overview of the jackup problem,and Ref. 2 summarizes the progress of the project. This paper concentrates onthe experimental study, which uses regular waves, and the associated computersimulations. Results concerning the irregular-wave tests have been presented ina separate paper. 3
Testing of three principle jackup models was carried out in the wave tank atShip Hydromechanics Laboratory, Delft U. of Technology, during Summer 1988. Allmodel designs were simplified to concentrate on the physical processes to bestudied. While some discussion of model scales is relevant, we did not attemptto reproduce actual field conditions in the models. Instead, the physicalmodels should be seen as full-scale structures. physical models should be seenas full-scale structures. A convenient tank water depth was 2.0 m. Possiblewave frequencies in the tank range from about 0.6 to 1.3 Hz with wave heightsup to 0.080 m. (Higher frequencies were reached for lower wave heights.) Wechose a natural frequency of around 1 Hz for the structures tested. The modelleg spacing was chosen to include a reasonable hydrodynamic-force-cancellationeffect. The design approach was to choose a leg stiffness so that the modelplatform had a quasistatic deflection of 2% of the water depth at deck level ifthe peak force resulting from a design wave was applied to all three legssimultaneously. By choosing different leg materials and adjusting deck masses,essentially retaining the natural frequency and quasistatic deflection (asoutlined above) while using two different types of legs proved possible. Fig. 1illustrates the model geometry, and Table 1 lists the most important physicalparameters for each of three models. parameters for each of three models. Thedifferent test models were chosen to segregate the several types ofhydrodynamic and structural nonlinearities. Model 1 was designed withrelatively large-diameter legs yielding inertia-dominated forces (thedrag-force amplitude was about 3.5% to 10% of the inertial-force amplitude inthe waves tested); Model 2 had more-slender legs and more-drag-dominated forces(the drag-force peak was about 80% to 250% of the inertial-force peak). peakwas about 80% to 250% of the inertial-force peak). Additional testing of Model2 with extra deck masses (then called Model 2-M) was carried out to expose theeffects of deck load eccentricity: the P- effect and the effects of a variationin the natural frequencies of the models.
The model test results reported are based on static tests, free vibrationsin air and in water, and exposure to regular waves. The testing program alsoincluded irregular, unidirectional, long-crested waves and, as a special case,a superposition of two regular waves. All the experimental data (including waveelevation, base forces along three axes, deck displacement, and acceleration)were recorded in an analog form on magnetic tapes. Some of the water elevationand deck displacement data also were recorded on paper with an ultraviolet (UV)recorder. The present work, is limited to hand-processed data from the UVrecorder. Inconsistencies between static and dynamic experimental results wereobvious. The overall structural stiffnesses derived from the free vibrationtests and computed with simplified theoretical models were up to four timeshigher than those from the static tests. The theoretical and dynamicstiffnesses, however, were of about the same order. Additionally, Models 2 and2-M exhibited damping ratios up to 23 %. Both observations indicate that thedeck/leg connections were different from their originally intended design(perfectly clamped). These inconsistencies can be explained by the connectionstiffness and damping tradeoff phenomenon. With the structures tested(especially Models 2 and phenomenon. With the structures tested (especiallyModels 2 and 2-M), a large amount of damping was concentrated locally in thedeck/leg connections; relative dynamic movement between the deck and legs wouldgenerate remarkable resistance, and this resistance would increase as relativevelocities increased. Hence, the effect of high damping in the connection wasanalogous to a fixation against dynamic loading and thus equivalent to a large"dynamic stiffness. " As such, the localized high damping at theconnections not only influenced the overall structural damping behavior butalso increased the apparent dynamic stiffness noticeably and thus increased thenatural frequency, while the structure showed an appreciably lower staticstiffness.
Numerical Simulation Tool
The nonlinear offshore structure dynamic analyzer (NOSDA) is aspecial-purpose time-domain software for stochastic (or deterministic)nonlinear dynamic analysis of offshore structures. NOSDA possibilitiesimportant for later simulations follow. The hydrodynamic interaction optionsare relative or absolute velocity field, linearized (Borgman ) or quadraticdrag, free-surface choice, and wave theory choice. Structural dynamics optionsare P- elements and local damping.
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