Defining a Pipe or Riser Assembly Defining a Pipe or Riser Assembly

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.