Connecting Parts Connecting Parts

Connecting parts is a rather delicate proposition. Normally, the only elements which do not belong to a part are connectors. Connectors, however, are not designed to carry moments and are not as general as beams. If, however, beams are allowed to span parts, their element system is not properly defined. Two alternatives are provided to circumvent these difficulties. The first is that nodes can have an alias. In other words, a node which has an alias will have the same deflection as the alternative name of the node. Defining a node alias is accomplished via:


     ALIAS, *SLAVE(1), *MASTER(1), ...... *SLAVE(n), *MASTER(n)

When this command is issued, an element will be generated between *SLAVE(i) and *MASTER(i). No checking will be done as to the congruence of the locations of the two nodes, and these elements will not be used during simulations. Instead, their only purpose is to insure that when a stress analysis is performed, the two nodes will have identical deflections. One will normally use this technique to connect two nodes in different parts which have the same location in space.

The second method of connecting parts is with special connectors called part connectors. These elements belong to special parts which have a part type of PCONNECT. These elements may be defined by standard BEAM and PLATE commands, or they may be defined in the MEDIT Menu by commands which are similar to the ones used in defining tiedowns. The additional commands are:


     PCONNECT, ~CLASS, *JN, :SEL(2)
     PCONNECT, DX, DY, DZ, ~CLASS, :SEL(1), :SEL(2)

The first format generates part connectors at a single node in one part to several nodes in the other part, using beams of section ~CLASS. Thus ~CLASS is the name of the class property defining the section properties of the part connector member, *JN is the name of the node to be tied down, and :SEL(2) is a selector for the nodes to which *JN is connected.

The second format generates part connectors at several nodes, where the orientation of each part connector is constant. In effect, DX, DY, and DZ define the far end of a beam element at each node which matches the selector :SEL(1). This far node is then connected by a rigid link to the nearest node matching :SEL(2). Here, ~CLASS is as before, and DX, DY, DZ is the distance measured from the node to the body attachment point, in the second part system (feet or meters).

A special method for connecting two parts for the transportation of a structure on a vessel is provided by MOSES. To utilize this method, one must have a model which consists of a single body with a body type of VESSEL, and there must be a part named JACKET with a part type of JACKET. This allows the part system of the jacket to be different from the vessel, and MOSES will automatically rotate the jacket so that the part systems are the same. Here, one must also establish connections between the parts so that an analysis can be carried out. The connections are of two basic types: launchways, which are continuous beams fastened to the vessel and upon which the jacket rests, and tiedowns, which fix the jacket to the vessel.

To use this feature, one should issue the command:


     TRANS_CON, -LOCJ, XO, YO, ZO,  \

     -JLLEGS, *JS(1), ... *JS(n), \

     -JLLEGP, *JP(1), ... *JP(n), \

     -LWAYP, X1, ZNA, L, ~CLASS, :BPSEL  \

     -LWAYS, X1, ZNA, L, ~CLASS, :BSSEL

The launchways are generated by the program as two continuous beams. Nodes on these beams are automatically generated by MOSES to match up with the nodes on the jacket launch legs and with suitable nodes on the vessel. The jacket launch leg nodes are defined with the -JLLEGS and -JLLEGP options where *JP(i) are nodes on the port launch leg, and *JS(i) are nodes on the starboard launch leg. In both cases, the nodes must be ordered so that a given node is further aft than all of the nodes which precede it.

The jacket location on the vessel is given on the -LOCJ option where: XO, YO, ZO are distances (feet or meters), in the body system, from the vessel origin to the point midway between *JP(1) and *JS(1). With the jacket located on the vessel, the two launchways are defined using the -LWAYP and -LWAYS options. Here, X1 is the vessel coordinate of the beginning of the launchway (feet or meters), ZNA is the vessel coordinate of the launchway neutral axis (feet or meters), L is the length of the launchway (feet or meters), ~CLASS is the name of the class property defining the section properties of the launchway, :BPSEL is a selector for the nodes to be rigidly connected to the port side (LWAYP) launchway, and :BSSEL is a selector for the nodes to be rigidly connected to the starboard side (LWAYS) launchway. Figure 19 shows the effect of the above command.

To connect parts elastically, one employs tiedowns in one of two formats:

     TDOWN, ~CLASS, *JN, :SEL(2)

     TDOWN, DX, DY, DZ, ~CLASS, :SEL(1), :SEL(2)

The first format generates tiedowns at a single jacket node which are connected using beams of section ~CLASS to several vessel nodes. Thus ~CLASS is the name of the class property defining the section properties of the tiedown member, *JN is the name of the jacket node to be tied down, and :SEL2 is a selector for the vessel nodes to which *JN is connected.

The second format generates tiedowns at several jacket nodes, where the orientation of each tiedown is constant. In effect, DX, DY, and DZ define the far end of a beam element at each jacket node which matches the selector :SEL(1). This far node is then connected by a rigid link to the nearest node matching :SEL(2). Here, ~CLASS is as before, and DX, DY, DZ is the distance measured from the jacket node to the body attachment point in the vessel system (feet or meters). The tiedown geometry is illustrated in Figure 20.