A pipe assembly is a ROD element along with a set of elements which connect the ROD to a set of bodies. The connecting elements can be either DAVIT or ROLLER elements. Here, DAVIT elements are lines which connect the pipe, while ROLLERs are one-sided constraints which keep the ROD on a pipe-lay route. Such an assembly is defined by the command

     PIPE, ~PIPE_CLASS, EL(1), ........, EL(i), -OPTIONS

and the available options are:

     -PIPE_TENSION, TLOWER, TUPPER

Here, ~PIPE_CLASS is the name of a ROD class which will be used to define the properties of the pipe, and EL(i) are the connector element names of elements which have been previously defined. The connector elements must be either DAVIT or ROLLER elements, and these types of elements cannot be mixed. MOSES assumes that pipe is laid in the negative X global direction, so that all of the connectors must lie along the global X axis when the assembly is defined. The pipe itself is given a name &PIPE and the assembly has a name of &PIPE/ASSEMBLY.

The use of the -PIPE_TENSION option depends on the type of connection. For an assembly of ROLLER elements, TLOWER defines the smallest allowable tensioner value (bforce) and TUPPER the largest allowable tension (bforce). With a ROLLER assembly, the last roller defined is the tensioner. With a DAVIT assembly, the value of TLOWER (bforce) is the nominal initial tension in the pipe where it connects to the first davit, and TUPPER is not used.

A special situation occurs when one has only a single roller. In this case, MOSES will treat the pipe assembly as a vertical pipe with a tensioner, so that one can simulate a riser. Here, the bottom of the rod is placed directly below the roller connection so that the pipe will have a tension of TUPPER.

When initializing a pipe assembly other than a vertical one, it is assumed that the pipe behaves as a catenary. (Thus, the use of the word nominal as regards the initial tension.) MOSES uses this assumption as initial estimates for the configuration of the assembly, and then iterates a solution to the nonlinear problem. A stinger is initially tensioned so that either the slope of the pipe matches the slope of the last two rollers, or all of the pipe is off of the bottom. The assembly problem is particularly difficult with davits. Here, the assembly is initialized by using only the length of the first davit and the nominal pipe tension. The lengths of the other davit lines are then computed to conform to this estimate.

Once an assembly has been initialized, the values can be altered with the internal command:

     &CONNECTOR},  :CONN_SEL  -OPTIONS

where :CONN_SEL selects the elements to be altered, and the available options are:

     -PIPE_TENSION, TLOWER, TUPPER

     -DAV_LENGTH, NEWLEN,

     -MOVE_ROLLER, DX, DY, DZ,

     -LOC_ROLLER, X, Y, Z,

     -A_STIFF, STADGX, STADGY, STADGZ

     -TOP_MOMENT, YES/NO

     -ST_ADDITION, INONUM, STADGX, STADGY, STADGZ

Unless otherwise specified, :CONN_SEL should be &PIPE. The -PIPE_TENSION option sets the lower and upper bounds for the tensioner tension, TLOWER and TUPPER (bforce).

The -DAV_LENGTH option changes the length of the davit elements selected by :CONN_SEL to be NEWLEN (feet or meters). If :DELEM selects the first davit element, then the entire assembly will be re-initialized and all of the lengths will be changed as when the assembly was defined. If the first element is not altered, then the selected lengths will be changed and a new equilibrium configuration of the pipe will be computed.

The two options -MOVE_ROLLER and -LOC_ROLLER both result in the assembly being completely re-initialized. The first of these options moves rollers which match :CONN_SEL from their current location by an amount DX, DY, and DZ (feet or meters). The second simply defines the new location of the roller to be X, Y, and Z, (feet or meters), in the stinger body system.

The last three options: -A_STIFF, -ST_ADDITION, and -TOP_MOMENT operate on the pipe exactly as they do for a simple rod connector, and were discussed above.