Defining Load Cases Defining Load Cases

The loads which will be generated by MOSES are determined by the LCASE command. This command has different forms to generate different loads. One can input as many of these commands as desired, and each one will define at least one load case. If two of these commands are used which specify the same load case name, then the results of both of them will be combined. Also, one can change the current process name when defining load cases in order to perform a single analysis for several different physical situations.

In general the form of this command is:


     LCASE,  -OPTION, DATA

The next option is used for analyzing fixed structures subjected to an environment. It uses the options:



     -STATIC, ENV_NAME, LCNAME, CHECK, PERIODIC

This will produce a set of loads which result from the environment, ENV_NAME, and the resulting load case name will be LCNAME. Here, the system will be assumed to be fixed in space, and the loads which yield some maximum will be generated as the load case. The CHECK value defines what maximum will be checked. If it is OVERTURN the maximum overturning moment about the mudline will be used as a criteria; if it is SHEAR, it will be maximum shear. The actual operation of the program depends upon the type of wave specified. If a periodic wave was specified, then a search algorithm will be used which is substantially more efficient than simply "passing the wave" through the structure. If one wants to override this algorithm for some reason, he should set PERIODIC to PERIODIC. If PERIODIC is specified as NO. MOSES will then act as if a non-periodic wave had been specified. For non-periodic waves, the program will simply compute the forces on the structure for the times defined with the -TIME option of the &ENV command and pick the time which creates the maximum.

Two options of LCASE are available for use with frequency domain situations.



     -RAO

     -TIME, ENV_NAME, CASE(1), T(1), ... CASE(i), T(i)

The -RAO form is used to perform an analysis in the frequency domain. It will generate a load case corresponding to the mean position of the structure and two "RAO" load cases for each heading and each period at which response operators were computed. The names of these generated cases depend upon how many LCASE commands have been issued. Basically, the names are FRQMEAN(i) for the mean, and RAO(i)NUMBX for the ones corresponding to the response operators. Here, (i) is equal A for the first, equal to B for the second set, etc. The values NUMB are numbers corresponding to the headings and frequencies. Both the frequencies and headings are in sorted numerical order, and the values for NUMB are 1, 2, ...., N for frequency 1 and headings 1 through N. For frequency 2, they are N+1, N+2, etc. The final part of the name, X, is used to denote the real and imaginary part. Here, I denotes the imaginary part, and R denotes the real part. The results from the structural solution for the RAO load cases are deflections and element loads resulting from a regular sea of given period and heading and a height of 1 database unit. In MOSES, the database unit is an inch regardless of the current values set with &DIMEN. Thus, if one wishes to interpret the results directly, he should use the CASES -COMBINE command to scale them to more meaningful units.

The -TIME option is used to perform a snap-shot analysis of the frequency domain results. Here, ENV_NAME, is the name of a seastate which was previously defined by a &ENV command, CASE(i) is the name which the user wishes to give to the case, and T(i) is the time at which the loads will be combined to produce the "snap-shot".

The next option for LCASE is used to perform a solution for selected times during a process. This is similar to the situation discussed above with two major exceptions: Here, a process must have been defined previously with a TDOM or LAUNCH command, within the Static_Process Menu, or by issuing some &EVENT_STORE commands. Also, here, the forces in the connections are explicitly taken into account, since MOSES will generate a set of loads which will sum to the resultant of the rigid constraint loads (i.e. MOSES will inertia relieve these load cases). The form of the command is:



     -PROCESS, CASE(1), T(1), ... CASE(i), T(i)

When LCASE is issued with the -PROCESS option, MOSES will generate a load case for each event in the process which was specified. Here again, CASE(i) is the name which the user wishes to give to the case, and T(i) is the event of the process for which the load case will be computed.

The final option for LCASE is used for generating a solution for a launch simulation as above but also approximating the loads that the launch barge applies to the jacket. The form is:


     -LAUNCH, QMID, QBEG, TBLEN  CASE(1), T(1), ... CASE(i), T(i)

The -LAUNCH option is used only for generating load cases during the launch of a jacket where it provides an approximate way of subjecting the launch legs of the jacket to a distributed load. Caution: one should never use -LAUNCH in conjunction with BODSOLVE. With this option, a S_PART command specifying only the jacket should be issued, and some restraints should be selected to completely define the system. The precise distribution generated depends on whether or not the jacket has tipped off the barge. Before tipping, a single distribution will be generated. After tipping, the distribution will be composed of two trapezoidal distributions, each TBLEN (feet or meters) long, which are symmetric about the tiltpin. The relative intensities at the pin and at the ends of each distribution are governed by two parameters, QBEG and QMID. Here, QMID is the relative load intensity under the tiltpin (percent), QBEG is the relative load intensity at the ends of the tiltbeam (percent), and TBLEN is one-half the tiltbeam length (feet or meters). Notice that QMID + QBEG should equal 100, and that they only describe one half of the loading on the tiltbeam. This load condition is illustrated in Figure 27. For a uniform loading, values of 50 and 50 should be used for QMID and QBEG, respectively. For a trapezoidal loading with twice the load at the pin compared to the ends, values of 66 and 34 should be used.

If either SSOLVE -NONLINEAR or BODSOLVE is used for solving a launch, MOSES will create connections modeling the launchway at each load case. The type of restraint supplied in the area of the tiltbeam is dependent on the use of the -BEAM option of the LLEG command. This option defines a bending stiffness and length for the beam. Before tipping, all jacket nodes between the aft end of the tiltbeam and the bow of the barge will be restrained. After tipping, only jacket nodes between the two ends of the tiltbeam will be restrained. In cases where this criteria does not yield at least two nodes, the node furtherest forward yet aft of the pin and the node furtherest aft yet forward of the pin will be added to the others. The selected nodes are then connected with compression only springs to the closest barge nodes. For nodes in contact with the barge proper, a nominal "stiff" spring is used. For those in contact with the tiltbeam, this stiff spring is put in series with the bending stiffness of the tiltbeam at the proper location. In some cases, the compression only springs will be allowed to carry tension! This happens when less than two nodes remain during the iterative process. The stiffness of the restraints and the number of restraints in the tiltbeam area are further explained in Figure 28

and Figure 29.

The final option of LCASE is :


     -DEAD, CNAME, INT(1), ..  INT(6)

which instructs MOSES to create the load case named CNAME by multiplying the inertia of the system by various intensities: i.e. the values are the six components of the accelerations which act on the structure (G's and rad./sec**2). Normally, however, one will use one of the other forms of the LCASE command.