1. Body components The data group BODY COMPONENTS can optionally be specified together within the data defining each body, and includes all information about equipment at the attachment points of coupling elements that can be changed during an analysis. One feature is winch running. 1.1. Application The BODY COMPONENT group contains a description of attachment points on a body: Name and location (in local body coordinates). Point characteristics (whether the end of a coupling wire is fixed at the attachment point or if the body slides along the wire span, cf. Guidewires). Winch data (maximum acceleration and speed, wire lengths paid out and remaining on the drum, sequences of winch running). Winch running can be applied to the following types of coupling elements: Table 1. Coupling types for winch running SIMPLE WIRE COUPLING Wire cross section stiffness (EA) is given (Winch running also in previous versions) MULTIPLE WIRE COUPLING Several wire segments connected to a branch point (Winch running also in previous versions) FIXED ELONGATION COUPLING Spring with specified length - force relation (Winch running modifies reference length) USER DEFINED COUPLING 1.2. Guidewires Brief description An arbitrary number of guidewires can be defined and used for leading a module towards a foundation, or towards guideposts. The guidewire model is applicable both on 3D bodies and 6D bodies. On 3D bodies each of the lines passes through one point/ring (modelling the guide funnel), and for a 6D body two points/rings can be defined for each guidewire, representing the end openings on the respective guide funnel. The ring(s) are specified by local position, diameter and orientation. A principal sketch is found in Guidewire alternatives. The ends of the guide funnel are given in local body coordinates, and these two points define the centre of the guide funnel. The force from the guidewire on the body at each end of the guide funnel is shown in the same figure. Figure 1. Guidewire alternatives. The model is also suitable for describing e.g. the pull-down of a buoyant module, using a wire through a sheave on the sea bed, see Pull-down. Figure 2. Pull-down. 1.3. Example An example with use of coupling points and a guidewire is shown in System with 4 coupling points. Figure 3. System with 4 coupling points. Both VESSEL_A and VESSEL_B are of body type 4 which means that they will not move during a simulation. The coupling point Point_on_VESSEL_A has a winch named WINCH1. This starts to run at \(t=10~\mathrm {s}\) and stops at \(t=160~\mathrm {s}\) with a velocity of \(0.8~\mathrm {m/s}\). The initial position of body LOAD is set to \(-58~\mathrm {m}\) and the static equilibrium position at \(\mathrm {t=0}\) is found to be \(-63.5~\mathrm {m}\). For a simulation with length \(200~\mathrm {s}\) the vertical position is plotted in Vertical motion of body LOAD. Figure 4. Vertical motion of body LOAD. Because body LOAD slides along the coupling line without friction, the body does not get any motion in other degrees of freedom. The SIMO "sys-file" for the system in System with 4 coupling points is: '************************************************************ SYSTEM DESCRIPTION SIMO '************************************************************ ' txsys, 3 lines Test of guide points 2006-03-01 9:03 May 2002 'LENUNI TIMUNI MASUNI GRAV RHOW RHOA m s Mg 9.81 1.025 0.00125 'DEPTH DIRSLO SLOPE 1000. '************************************************************ BODY DATA SPECIFICATION '*************************************** V E S S E L _ A **** 'CHBDY VESSEL_A Text line # 1 Text line # 2 Text line # 3 'IBDTYP 4 '============================= BODY LOCATION DATA '============================= ' Xglob Yglob Zglob Phi Theta Psi 0.0 0.0 0. 0.0 0.0 0.0 '====================================== BODY COMPONENTS ==== BODY COMPONENTS '============== COUPLING POINT '-------------- Attachment point for coupling line at body VESSEL_A 'Coupling point of type FIXED ' chcopo cpltyp xcpl ycpl zcpl Point_on_VESSEL_A FIXED 0. 0. 0. COUPLING WINCH ' chcowi cwico WINCH1 NRUN ' waccm wvelm druma druml nrun .1 1. 100. 200. 1 ' tstart tstop runvel 10. 160. .8 '*********************************************************** BODY DATA SPECIFICATION '************************************** V E S S E L _ B **** 'CHBDY VESSEL_B Text line # 1 Text line # 2 Text line # 3 'IBDTYP 4 '============================= BODY LOCATION DATA '============================= ' Xglob Yglob Zglob Phi Theta Psi 100. 0.0 0. 0.0 0.0 0.0 '************************************************************ BODY DATA SPECIFICATION '************************************************************ 'CHBDY LOAD Text line # 1 Text line # 2 Text line # 3 'IBDTYP 1 '============================= BODY LOCATION DATA '============================= ' Xglob Yglob Zglob Phi Theta Psi 50. 0.0 -58. 0.0 0.0 0.0 '============================= BODY MASS DATA '============================= 'txmass, 2 lines Example 2006-03-01 9:03 'xcog ycog zcog 0. 0.0 0. '----------------------------- MASS COEFFICIENTS '----------------------------- ' rm rixx riyx riyy rizx rizy rizz 1000. 10000. 0.000 40000. 0.000 0.000 40000. '----------------------------- GRAVITY FORCE INCLUDED '----------------------------- LINEAR DAMPING txt1 txt2 ' dl1 dl2 dl3 dl4 dl5 dl6 50. 0. 0. 0. 0. 0. 0. 50. 0. 0. 0. 0. 0. 0. 50. 0. 0. 0. 0. 0. 0. 200. 0. 0. 0. 0. 0. 0. 200. 0. 0. 0. 0. 0. 0. 200. ' ====================================== BODY COMPONENTS ==== BODY COMPONENTS ' ============== '2 coupling points of type GUIDE COUPLING POINT ' -------------- First guide point at body LOAD ' chcopo cpltyp xcpl ycpl zcpl Guide1_at_LOAD guide -14. 0. 10. ' dia dv1 dv2 dv3 .11 2. 0. 0. ' ============== COUPLING POINT ' -------------- Second guide point at body LOAD ' chcopo cpltyp xcpl ycpl zcpl Guide2_at_LOAD guide 14. 0. 10. ' dia dv1 dv2 dv3 .11 2. 0. 0. '********************************************************** COUPLING DATA '********************************************************** CPL1 ' =================== Coupling between VESSEL_A and VESSEL_B ' =================== SIMPLE WIRE COUPLING ' =================== ' chbdy1 xbdy1 ybdy1 zbdy1 ' VESSEL_A 0. 0. 0. Point_on_VESSEL_A ' ------------------------------------ Load ' chbdy2 xbdy2 ybdy2 zbdy2 VESSEL_B 0. 0. 0. ' ------------------------------------ ' nguide 2 Guide1_at_LOAD Guide2_at_LOAD ' EA RLEN FLEXC DAMPSW 8.8E4 147. 0. 1000. ' ifmoco ftime btens 0 0. 100. '********************************************************** END 1.4. Heave compensator A heave compensator (HC) is used to minimize the motion of a suspended load, by paying out or taking in crane wire and thus compensating for the vertical crane motion. The HC has been implemented as an attribute to the BODY COMPONENT / COUPLING POINT of the type FIXED. To a coupling point equipped with HC, it is possible to connect coupling elements of the following types: "Fixed elongation coupling" "Simple wire coupling" "Multiple wire coupling" "Liftline coupling" The numerical model of the HC is to be based on a method previously developed by Hans Berntsen, SINTEF. The intention is to facilitate modelling of an active HC, where the vertical motion of the suspended load is minimized, preferably so that the HC piston moves around its mean-stroke position. It is not possible to combine the active HC with a tensioner, and is not possible to "de-couple" a coupling point to which a HC is attached. The modelled active heave compensator is a cylinder type device, where oil is supplied in the pressure side of the piston, controlled based on measured vertical position of the crane top. A principal sketch is shown in Sketch of heave compensator system. In the present implementation a detailed model for internal function of the compensator is not included. The following features are disregarded: Mass of the moving internal components (piston, rod, sheave, etc.) Internal oil flow resistance Friction between piston and cylinder and in sheaves Total volume and elasticity of the oil Large motion of the crane top due to crane manoeuvres or ballasting is not compensated. Figure 5. Sketch of heave compensator system Table 2. Key parameters \(\mathrm {Z_{cr}}\) Instantaneous crane top displacement from initial static position \(\mathrm {[m]}\) . \(\mathrm {Z_{cr0}}\) Initial / static crane top position \(\mathrm {[m]}\) \(\mathrm {Z_{p}}\) Piston position \(\mathrm {[m]}\) . It is assumed that the piston moves symmetric ally about its half-stroke position, where \(\mathrm {Z_p=0}\) \(\mathrm {N_{wc}}\) Number of wire parts at compensator \(\mathrm {N_{w}}\) Number of wire parts between crane top and hook as modelled in SIMO \(\mathrm {A_{c}}\) Compensator piston area \(\mathrm {[m^2]}\) \(\mathrm {S}\) Cylinder stroke, allowable motion range of the piston \(\mathrm {[m]}\) \(\mathrm {q}\) Oil flow into / from cylinder \(\mathrm {[m^3/s]}\) \(\mathrm {FACTOR}\) Fraction of the crane motion that shall be compensated \(A_{\mathrm {MAX}}\) Clipping amplitude of piston motion 1.4.1. Controller The controller uses two methods for heave compensation: Feedback loop PD control Forward-feed control with a Kalman-type filter Feedback loop control of compensator piston motion Difference between desired piston position and present position: \[\Delta Z_{p_k}=Z_{cr}\frac{N_w}{N_{wc}}-Z_p\] Differentiated and LP filtered: \[\Delta \dot Z_{p_k}=\Delta \dot Z_{p_{k-1}}+\eta _f[\frac{(\Delta Z_{p_k}-\Delta Z_{p_{k-1}})}{dt}-\Delta \dot Z_{p_{k-1}}]\] where: \(\eta_f\) \(=1-\exp{(-\displaystyle \frac{\mathrm{d} t}{T_f} )}\) \(T_f\) time constant in LP filter \(dt\) time step \(\mathrm{[s]}\) Reference from feed-back control: \[U_k=K_p(\Delta Z_{p_k}-T_d\Delta \dot Z_{p_k})\] where: \(K_p\) gain feedback loop \(T_d\) feedback derivative time Feed-forward loop control, using estimated crane velocity and acceleration Vertical crane velocity (if \(\mathrm {Z_{cr}}\) is not reduced by \(\mathrm {FACTOR}\) or clipped by \(\mathrm {A_{MAX}}\) ): \[\dot Z_{cr_{k}}=\frac{Z_{cr_{k}}-Z_{cr_{k-1}}}{dt}\] Vertical crane acceleration: \[\frac{\ddot Z_{cr_{k}}=(Z_{cr_{k}}+Z_{cr_{k-2}}-2Z_{cr_{k-1}})}{dt^2}\] Reference from feed-forward control: \[U_{f_{k}}=U_{f_{k-1}}+\eta _{f}[K_f(\dot Z_{cr_{k}}+T_{fd}\ddot Z_{cr_{k}})-U_{f_{k-1}}]\] where: \(K_f\) gain in forward loop \(T_{fd}\) feed-forward derivative time Control of oil flow Total control reference velocity: \[U_{t_{k}}=U_k+U_{f_{k}}\] Capacity utilization found from low-pass filtered velocity: \[\phi _{k}=\phi _{k-1}+\eta _{q}(U_{t_{k}}-\phi _{k-1})\] Limitation: \(\mathrm {-1\le \phi _{k}\le 1}\) \(\eta_q\) \(=1-\exp (-dt/tq)\) \(T_q\) time constant for hydraulic valve Oil flow into the cylinder: \[q_{k}=K_q(\phi _{k}+\phi _{k-1})\] \(K_q\) hydraulic valve characteristics Piston position: \[Z_{p_{k}}=Z_{p_{k-1}}+dt\frac{q_{k}}{A_c}\] Limitation: \(\mathrm {|Z_{p_{k}}|\le S/2}\) The length adjustment of the crane wire is the output from the compensator model, to SIMO: \[\Delta L_{k}=Z_{p_{k}}\frac{N_w}{N_{wc}}\] 1.4.2. Selection of compensator parameters Cylinder cross section area If we neglect friction the mean compensator force must counteract the weight of the suspended load, \(\mathrm {W_{l}}\) , the hook, \(\mathrm {W_{h}}\) , and the wire below the crane top, \(\mathrm {W_{w}}\) : \[F_c=\frac{N_{wc}}{N_w}(W_l+W_h+W_w)=\frac{N_{wc}}{N_w}\sum\,W=pA_c\] A suitable piston area can be estimated by assuming a mean pressure \(p=150\,\mathrm {bar}=15000\,\mathrm {kN/m^2}\) . Maximum oil flow and valve characteristics \[q_\mathrm {max}=2K_q=A_c[\displaystyle \frac{(Z_{p_k}-Z_{p_{k-1}})}{dt}]_\mathrm {max}=\displaystyle A_c\frac{N_w}{N_{wc}}[\dot Z_{cr}]_\mathrm {max}\] \[K_q=\displaystyle \frac{A_c}{2}\frac{N_w}{N_{wc}}[\dot Z_{cr}]_\mathrm {max}\] \([\dot Z_{cr}]_{\mathrm {max}}\) = maximum compensated crane velocity Feed-forward gain \[K_f=\frac{A_c}{K_q}=\displaystyle 2\frac{N_{wc}}{N_w}\frac{1}{[\dot Z_{cr}]_\mathrm {max}}\] Time constants for valve and forward feed loop Recommended values: \(\mathrm {T_{fd}=T_q=0.8}\) seconds Feed-back and LP filter parameters These include: \(K_p\) feedback gain \(T_d\) differentiation time constant \(T_{fl}\) low-pass filter time constant It is recommended to adjust these parameters through series of test runs, as the performance will depend on the suspended load and the lifting system. Suggested start values: \(\mathrm {K_p=3,T_d=0,T_{fl}=0.2}\) . A simulation time step longer than \(\mathrm {T_{fl}/5}\) will reduce the compensator performance, and is not recommended. An example of input is given below: BODY COMPONENT ' ============ Coupling point 2 at body TOP COUPLING POINT ' ------------ Coupling point TOP\_CP2 at body TOP ' chcopo type x y z TOP\_CP2 FIXED 0. 0. 0. '------------------------------------------------- MOTION COMPENSATOR 'cmoco HC\_S 'itype 1 'limod factor ampmax ehla 1 1.0 10. 0 'nwire nwirec stroke acyl 2 2 4. .02 ' hckp hctd hckf hctfd hckq hctq tf1 3.0 0.0 .75 0.8 0.0069 .8 0.2 1.5. Tensioner For the couplings Simple wire coupling and Fixed elongation coupling, a TENSIONER may be given. The tensioner is a passive pneumatic hydraulic cylinder, where the supplied pressure holds the mean tension in the pipe, preferrably so that the piston moves around its mean-stroke position. The holding force at the mid-stroke position can be adjusted by adding or removing gas/oil at the pressure side. The input may ge given as shown below: ' =================================== BODY COMPONENTS ' =================================== COUPLING POINT '------------------------------------ Crane winch Txt\_2 'chcopo cpltyp xcpl ycpl zcpl COPO1 FIXED 0. 0. 0. '------------------------------------ COUPLING WINCH '------------------------------------ 'chcowi cwico Winch\_1 NRUN ' waccm wvelm druma druml nrun 0.5 1. 850. 1000. 0 ' TENSIONER ' ftens df\_dt stifft stroke IHLA 3200. 5. 80. 8. 1 ' hla\_name HLA\_TENSI Figure 6. The effect of a tensioner on a characteristics. Station-keeping forces Coupling forces