Assessment of loads on a jack-up platformADVANCED 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 jackup 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 ana-lyzed under two conditions: first, for a relatively large jacking height of 13.5m above calm wa-ter level, resulting in wave loads acting primarily on the legs, and, second, for a smaller jack-ing height of 8.35m, subject-ing the hull to impact loads in higher waves. Advanced compu-tational fluid dynamics (CFD) techniques based on the Rey-nolds-averaged Navier-Stokes (RANS) equations were used [1]. The investigated jack-up platform has three tubular steel legs spaced 39m apart, forming

an equilateral triangle (Fig. 1). These legs can be jacked up or down by electro-hydraulic ma-chinery. The short front wall of the hull beneath the helicopter deck designates the units bow. Table 1 lists principal particu-lars 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 de-sign codes [3, 4] require that design environmental condi-tions 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 platforms location, this units operating manual specified a significant wave height of 6.24m, a cur-rent velocity of 0.51 m/s, and a wind speed of 58 knots. Cur-rent and wind loads were as-sumed acting collinearly with the waves. The most critical de-sign wave for the survival con-dition was modelled as a deter-ministic long-crested wave of 11.6m height and 13 s period, propagating from the direction of 60 degrees to the longitudi-nal 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

overturning moments. Negative values of base shear represent forces acting in the direction of wave propagation; the corre-sponding values of overturning moment are based on moment arms measured positively up-wards from the ocean bottom. Base shear and overturning mo-ment are the dominant safety

criteria against sliding and capsising of the platform, re-spectively. Fig. 2 shows that the computed time histories are characterised by peaks in the negative direction, correspond-ing to wave crests attacking the structure. Peak (absolute) values increase nonlinearly with wave height. However, this nonlinear-

Fig. 1. Platform plan view Fig. 2: RANS computed base shear (top) and overturning moment (bottom) for different wave heights and wave direction of 60 degrees

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 4033 t

Net tonnage 3209 t

Table 1: Principal particulars of jackup platform

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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 mo-ment occur simultaneously be-cause 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 accord-ing to the SNAME design guide-lines for mobile jack-up units [4]. Figs. 3 and 4 show time series of base shear and overturning mo-ment computed with RANS in comparison with the Morison formula for the four considered wave heights. Both methods pre-dict nearly equal base shear peak values for the three lower wave heights; only for the highest (breaking) wave peak values ac-cording 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 val-ues turned out to be larger than those from RANS computations.A transient nonlinear finite ele-ment analysis of the units struc-ture subject to the considered wave conditions (including cur-rent and wind forces) was per-formed 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 exam-ple, local stresses caused by the action of the leg guides were su-perimposed on global bending stresses in the most loaded (aft starboard) leg. At point 1 in the right graph of Fig. 5, the over-loaded structure is most likely to experience plastic deformation. At point 2, plastic deforma-tion occurs for the 19.9 and the 23.7m wave heights [5, 6].

Fig. 3: Comparison of RANS computed and Morison calculated base shear for the 11.6m wave (top) and the 15.8m wave (bottom)

Fig. 4: Comparison of RANS computed and Morison calculated overturning moment for the 19.9m wave (top) and the 23.7m wave (bottom)

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Small jacking height (8.35m)RANS simulations were per-formed 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, respec-tively [7]. For the subject water depth of 33.5m, these waves con-stituted shallow water waves that tended to break after advancing about one wave length. Waves may hit the platform with differ-ent free-surface inclinations rela-tive to the hull. This inclination can be a smooth wave profile (simulated shortly after initiali-sation) 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 plat-form. During impact, the verti-cal force acts upwards. Later, when the wave crest moves un-der the hull, pressures become negative and result in a down-ward vertical force. The asso-ciated pressure distribution indicates that pressures during impact become negative as wa-ter passes the edges. To avoid unrealistically low pressures, a cavitation model was activated to also account for the com-pressibility of air. High pres-sures 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

wave incidence) yield forces that exceed the forces in 60 de-grees incident waves by more than 20%.Sample time histo-ries in Fig. 8 demonstrate ef-fects 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 nonbreak-ing 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.

ConclusionThis analysis accountedfor a considerably higher jacking height than the required mini-mum height. This greater jack-ing 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 predic-tions obtained from the use of RANS techniques. These com-parative results demonstrated the general usefulness of the Morison formula approach to assess strength-related safety as-pects although only for cases of high hull elevation. Peak values

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

Fig. 7: Horizontal (left) and vertical (right) forces on platform in 0, 60, 90 and 180 degrees incident waves of 23.7m height

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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 di-rectly, the RANS technique inves-tigated 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 Peri, M.:,Computational Methods for Fluid Dynamics. 3rd ed., Spring-er, Berlin, 2003.[2] Germansicher Lloyd, Rules for Classification and Construction, IV Industrial Services, 6 Offshore Technology, Hamburg, 2007.[3] International Maritime Or-ganization, Code for the Con-struction and Equipment of Mo-bile Offshore Drilling Units (IMO MODU Code), London, 1989.[4] Society of Naval Archi-tects 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, 2002.[5] Schellin, T.E., Jahnke, T., and Knzel, J., Consideration of Freak Waves for Design of a Jack-Up Structure. Offshore Technology Conf., Houston, OTC-18465-PP, April-May 2007. [6] El Moctar, O., Schellin, T.E., Jahnke, T., and Peri , M., Wave Load and Structural Analy-sis for a Jack-Up Platform in

Freak Waves. ASME J. Offshore Mechanics & Arctic Engg., Vol. 131(2), 2009, Article 021602.[7] Schellin, T.E., Peri, 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 021303.

The authors:Ould El Moctar, Thomas E. Schellin, Tobias Zorn, Germanischer Lloyd, Hamburg, Germany

Fig. 8: Horizontal (left) and vertical (right) forces on platform in 180 degrees incident waves of 19.9m height

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