Wave loads on a typical

Transcription

1 OFFSHORE & marine Technology ocean & offshore engineering Assessment of loads on a jack-up platform ADVANCED OFFSHORE ANALYSIS Growing importance of the offshore oil, gas, and renewables sectors imposes increasing safety challenges, especially for new kinds of wind turbine platforms and jack-up units. A promising way forward is the application of advanced analysis methods based on CFD techniques. Ould El Moctar, Thomas E. Schellin, Tobias Zorn Wave loads on a typical self-elevating jack-up platform were analyzed under two conditions: first, for a relatively large jacking height of 13.5m above calm water level, resulting in wave loads acting primarily on the legs, and, second, for a smaller jacking height of 8.35m, subjecting the hull to impact loads in higher waves. Advanced computational fluid dynamics (CFD) techniques based on the Reynolds-averaged Navier-Stokes (RANS) equations were used [1]. The investigated jack-up platform has three tubular steel legs spaced 39m apart, forming Fig. 1. Platform plan view Moulded hull length 46.0m Moulded hull breadth 47.6m Moulded hull depth 5.5m Leg diameter 3.66m Overall leg length 64.0m Gross tonnage Net tonnage 4033 t 3209 t Table 1: Principal particulars of jack-up platform an equilateral triangle (Fig. 1). These legs can be jacked up or down by electro-hydraulic machinery. The short front wall of the hull beneath the helicopter deck designates the unit s bow. Table 1 lists principal particulars of the platform. Large jacking height (13.5m) The platform was analysed for an operation in the North Sea at a water depth of 33.5m. Class rules [2] and offshore design codes [3, 4] require that design environmental conditions be based on a significant wave height with a period of recurrence of at least 50 years for the most severe anticipated environment. For the survival condition at the platform s location, this unit s operating manual specified a significant wave height of 6.24m, a current velocity of 0.51 m/s, and a wind speed of 58 knots. Current and wind loads were assumed acting collinearly with the waves. The most critical design wave for the survival condition was modelled as a deterministic long-crested wave of 11.6m height and 13 s period, propagating from the direction of 60 degrees to the longitudinal axis of the hull, (Fig. 1). An analysis based on the SNAME Guidelines [4] showed that these wave parameters resulted in the highest base shear acting on the structure. In addition, three steeper episodic waves of the same period and direction and heights of 15.8m, 19.9m and 23.7m were investigated. Fig. 2 shows time histories of computed base shear forces and Fig. 2: RANS computed base shear (top) and overturning moment (bottom) for different wave heights and wave direction of 60 degrees overturning moments. Negative values of base shear represent forces acting in the direction of wave propagation; the corresponding values of overturning moment are based on moment arms measured positively upwards from the ocean bottom. Base shear and overturning moment are the dominant safety criteria against sliding and capsising of the platform, respectively. Fig. 2 shows that the computed time histories are characterised by peaks in the negative direction, corresponding to wave crests attacking the structure. Peak (absolute) values increase nonlinearly with wave height. However, this nonlinear- 52 Ship & Offshore 2011 N o 2

2 Fig. 3: Comparison of RANS computed and Morison calculated base shear for the 11.6m wave (top) and the 15.8m wave (bottom) ity is less pronounced between the two highest waves because the increase of the wetted areas of the legs decreased when the highest wave started to break [6]. Positive peak values of these time histories are nearly equal for all four wave heights. Peak values of base shear and overturning moment occur simultaneously because the overturning moment is a direct result of multiplying the horizontal loads with their respective moment arms. For comparison, wave loads were also calculated with the Morison formula with coefficients according to the SNAME design guidelines for mobile jack-up units [4]. Figs. 3 and 4 show time series of base shear and overturning moment computed with RANS in comparison with the Morison formula for the four considered wave heights. Both methods predict nearly equal base shear peak values for the three lower wave heights; only for the highest (breaking) wave peak values according to the Morison formula exceed peak values from RANS computations by about 15%. Regarding overturning moment, both methods yield nearly equal peak values only for the 19.9m wave; for the 11.6m wave, the Morison formula overpredicts peak values by about 15%, and for the 15.8 and 23.7m waves the overprediction is about 25%. Thus, Morison peak values turned out to be larger than those from RANS computations. A transient nonlinear finite element analysis of the unit s structure subject to the considered wave conditions (including current and wind forces) was performed for the considered wave heights. The left graph in Fig. 5 (page 54) shows the global finite element model. For the 15.8m wave height, for example, local stresses caused by the action of the leg guides were superimposed on global bending stresses in the most loaded (aft starboard) leg. At point 1 in the right graph of Fig. 5, the overloaded structure is most likely to experience plastic deformation. At point 2, plastic deformation occurs for the 19.9 and the 23.7m wave heights [5, 6]. Reliability in any condition Fig. 4: Comparison of RANS computed and Morison calculated overturning moment for the 19.9m wave (top) and the 23.7m wave (bottom) info@hatlapa.de Ship & Offshore 2011 N o 2 53

3 OFFSHORE & marine Technology ocean & offshore engineering Small jacking height (8.35m) RANS simulations were performed for wave directions 0, 60, 90 and 180 degrees in wave heights of 15.8m, 19.9m and 23.7m with wave lengths of 221m, 229m and 237m, respectively [7]. For the subject water depth of 33.5m, these waves constituted shallow water waves that tended to break after advancing about one wave length. Waves may hit the platform with different free-surface inclinations relative to the hull. This inclination can be a smooth wave profile (simulated shortly after initialisation) or a breaking wave. Fig. 6 shows water running high up the platform hull after wave impact for the 19.9m wave height. Once the wave crest is under the hull, the entrance of water on deck stops and green water starts flowing off the platform. During impact, the vertical force acts upwards. Later, when the wave crest moves under the hull, pressures become negative and result in a downward vertical force. The associated pressure distribution indicates that pressures during impact become negative as water passes the edges. To avoid unrealistically low pressures, a cavitation model was activated to also account for the compressibility of air. High pressures also act on platform legs. Figure 7 shows sample time histories of RANS simulated horizontal (X-) and vertical (Z-) forces acting on the platform for the 23.7m wave. For all wave heights considered, cases in following waves (180 degrees Fig. 5: Global structural finite element model (left) and stress distribution (right) in the most loaded leg for the platform in 15.8m wave Fig. 6: Free surface shape (left) and pressure distribution (right) in 60 degree incident 19.9m wave wave incidence) yield forces that exceed the forces in 60 degrees incident waves by more than 20%.Sample time histories in Fig. 8 demonstrate effects of the 19.9m wave on total forces acting on the platform. The horizontal force at time 17 s, when the wave breaks, is about twice as high compared with the force at time 4 s, when the nonbreaking wave crest passes the platform. Regarding vertical force, the nonbreaking wave (time 4 s) first causes a large upward force equal to about the platform weight and, a short time later (6 to 7 s), a large downward force equal to about 75% of the platform weight. The situation is similar at 17 s when the breaking wave passes the platform. Conclusion This analysis accountedfor a considerably higher jacking height than the required minimum height. This greater jacking height was selected because operating assignments often call for a high hull elevation. Base shear and overturning moment of the platform in the highest freak waves based on the use of the Morison formula differed by less than 25% from predictions obtained from the use of RANS techniques. These comparative results demonstrated the general usefulness of the Morison formula approach to assess strength-related safety aspects although only for cases of high hull elevation. Peak values Fig. 7: Horizontal (left) and vertical (right) forces on platform in 0, 60, 90 and 180 degrees incident waves of 23.7m height 54 Ship & Offshore 2011 N o 2

4 Fig. 8: Horizontal (left) and vertical (right) forces on platform in 180 degrees incident waves of 19.9m height of overturning moment differed more than peak values of base shear. This was brought about by the more accurate distribution of RANS-based wave forces acting on platform legs, especially for the higher (breaking) waves. For the reduced jacking height with waves attacking the hull directly, the RANS technique investigated wave-in-deck loads. High forces and moments were caused by impact-related wave-structure interaction. The changing wave surface profile was shown to be significant. References [1] Ferziger, J.H. and Perić, M.:,Computational Methods for Fluid Dynamics. 3rd ed., Springer, Berlin, [2] Germansicher Lloyd, Rules for Classification and Construction, IV Industrial Services, 6 Offshore Technology, Hamburg, [3] International Maritime Organization, Code for the Construction and Equipment of Mobile Offshore Drilling Units (IMO MODU Code), London, [4] Society of Naval Architects and Marine Engineers (SNAME), Guidelines for Site Specific Assessment of Mobile Jack-Up Units, Technical & Research Bulletin 5-5A, Jersey City, 1st Ed., Rev. 2, [5] Schellin, T.E., Jahnke, T., and Künzel, J., Consideration of Freak Waves for Design of a Jack-Up Structure. Offshore Technology Conf., Houston, OTC PP, April-May [6] El Moctar, O., Schellin, T.E., Jahnke, T., and Perić, M., Wave Load and Structural Analysis for a Jack-Up Platform in Freak Waves. ASME J. Offshore Mechanics & Arctic Engg., Vol. 131(2), 2009, Article [7] Schellin, T.E., Perić, M., and El Moctar, O., Wave-In-Deck- Load Analysis for a Jack-Up Platform, ASME J. Offshore Mechanics & Arctic Engg., Vol. 133(2), 2011, Article The authors: Ould El Moctar, Thomas E. Schellin, Tobias Zorn, Germanischer Lloyd, Hamburg, Germany built with SHIPCONSTRUCTOR cad/cam software Offshore Freedom LeTourneau Super 116E Jack Up Rig Lamprell plc Ship & Offshore 2011 N o 2 55

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7 Content March/April Offshore & Marine Technology Oil & gas 46 Produced water by oil and gas operations Ocean & offshore engineering 52 Assessment of loads on a jack-up platform 56 Safe access to platforms at sea 58 HiLoad DP completes sea trials Offshore & Marine Technology Offshore & arctic technology 60 Robotised rig in the Arctic Industry news 62 Cooperation on lifeboat Versatile conference and short courses Hyundai chosen to build super vessel Ship Operation Navigation & communication 74 SharpEye radar takes aim at pirates 75 Marine terminal for small vessels Industry news 76 European Flagship transport project sees progress 78 Bulk containers for lease XX german offshore equipment The second edition of the German Offshore Equipment directory, an initiative of German Engineering Federation VDMA in cooperation with the German Association for Marine Technology (GMT), is enclosed to the copies of this Ship&Offshore issue 57 Regulars Comment... 3 news & facts... 6 Buyer s Guide Imprint Ship & Offshore 2011 N o 2 5