Presentation on theme: "1 3.6 LOADS – Sources and consequences of non-linearities Sinusoidal wave and sylinder structure: H D d z x * l/D > 5 and H/D = 1 Mass term of Morison."— Presentation transcript:
1 3.6 LOADS – Sources and consequences of non-linearities Sinusoidal wave and sylinder structure: H D d z x * l/D > 5 and H/D = 1 Mass term of Morison dominates * linear wave theory: f(z,t;x=0) = 0.25 c m D 2 (H/2) gk cosh k(d+z)/cosh kd cost Kinematics defined to still water level Total load obtained by integrating to still water surface * Total load t=H/2 sint Force frequency = wave frequency, mean force = 0
2 For irregular sea: - Can be written as a sum of sinusoidals. - Linear wave theory, linear load and linear mechanical system. The load – and response properties can be characterized by the transfer function: H F () = F 0 ()/h 0 (), H x () = x 0 ()/h 0 () If assumptions above sufficiently accurate relflect loads and responses, the problem can be solved in the frequency domain, i.e.: Variance: Short term distribution Long term distribution:
3 What are the approximations? * Integration is carried out to still water level ( mean wetted surface for floaters). * Boundary conditions are fulfilled at the still water surface. * Viscous forces are neglected. If all these terms are accounted for (to some order), the total load will consist of the following terms: F(t) = F mean +F LF (t) + F WF (t) + F HF (t) Mean force Slowly varying force, frequency j – i T > 30s Wave frequency force, frequency j 25s >T > 4s High frequency force, frequency j + i T < 4s Typically: FLF and FHF an order of magnitude smaller than FWF These load processes will primarily be of concern if they hit lightly damped eigenmodes
4 Dynamic amplification Load: F(t) = F 0 cost steady state motion: u(t) = u 0 cos(t – ) where: Static response Dynamic amplification, DAF For = 0, DAF = 1/(2) = 0.01 DAF= 50 =0.1 DAF = 5 Fixed platforms: F WF generally most important, F HF may be important if it hits a natural period. Floating platforms: F mean, F LF, F WF generally most important, F HF can be important for TLP (springing) and ship (whipping).
5 Load on a drag dominated pile f(z,t) = 0.5 c D Du(t)|u(t)| Kinematics above mean free surface is a challenge. 1. One can select a regular Stokes 5th order wave. 2. If a sinusoidal wave is used, Wheeler stretching Is most common. (Will underestimate speed for steep waves. Let us adopt a sinusoidal wave and look at f(z,t) well below the still water surface. sint sint|sint|
6 A Fourier expansion of sint|sint| will show: sint|sint| = 0.85sint+0.17sin3t -0.02sin5t+.. (Inertial term: proportinal to cost) As drag is important, integration has to be carried out to the exact surface: Remember that for slender structures, loads will In addition to the wave frequency also appear at multiples of the wave frequency: For the inertial term:
7 Design of drag dominated structures (jackets and jack-ups) Up to 200m in the North Sea T n = 1 – 4s Up to 100m in the North Sea T n = 2-6s Pinned or fixed, important for natural period. If T n < 2s (ca), platform behaves quasistatially, and a design wave approach is convenient for obtaining design loads. If T n > 2s more detailed analysis may have to be done in order to properly assess the dynamic behaviour.
8 Design wave approach: a) A proper – load is obtained by exposing the structure to a regular wave with height equal to the height and a conservative associated wave period (often most unfavourable of 90% band). b) A Stoke 5th order is most often adopted as the regular wave model. c) A conservative way of accounting for current, is to utilize a current profile. d) Effects of dynamics can be approximated by estimating the DAF given by: (Example: T=15s, T n =1.5s, =0.015 DAF=1.003) e) Quastistatic 10-2 load found as worst load as the Stoke 5th wave is stepped through the structure.
9 Time domain analysis If an accurate assessment of dynamics is to be obtained, the equation of motion has to be solved in the time domain. It is not easy to find a very precise way to perform such an analysis. Most experience is gathered for structures essentially behaving quasistatic. Norsok N-003 gives some guidance. For the Kvitebjørn jacket (Tn = 5s in design calculations), the following approach was followed: 1) quasistatic load was estimated using Stoke 5th order wave and wave height. 2) A number of 3-hour simulations speed and acceleration set equal to zero were performed for the worst sea state along the contour line for h s and t p.
10 3) The 3-hour maxima were identified, and a Gumbel distribution was fitted to the data. The percentile correponding to the extreme obtained by the Stokes 5th was estimated. A value of 95% was found. 4) The same 3-hour simulations were repeated, but now the structure was permitted to move, i.e. a full dynamic analysis was carried out. 5) The 3-hour maxima were again selected and a Gumbel model was fitted to the data. From the fitted model the dynamic extreme value at a percentile of 95% was estimated. 6) The dynamic amplification was estimated by: DAF = x_3hrmax_dyn_0.95/x_3hrmax_qstat_0.95 7) Characteristic design loads are calculated using the values obtained by Stoke 5th wave multiplied by dynamic amplications factor.