Defining a Sling Assembly - Ultramarine.com Defining a Sling Assembly

A sling assembly is an abstraction of the lines which a lift vessel can apply to another body. Here, the point at the boom where the connection begins is called the boom point. This point is connected to the hook by a length of line, and in turn the hook is connected to several points on the body by other lines. In MOSES, the hook can be connected to the body with up to four elements. This set of lines is called a tip-hook set, and is referred to by a name given to the hook. Several different tip-hook sets can be defined connecting different bodies.

Each tip-hook set is defined by a command: 


     ASSEMBLY T-H_DEFINITION, NAME, BHE, EL(1), ........, EL(4), -OPTIONS

and the available options are: 


     -INITIAL,

     -ORIENT,

     -VERTICAL

     -DEACTIVATE

Here, NAME is the name you wish to give the tip-hook set, BHE is the name of the element connecting the boom point to the hook, and EL(i) are the names of the elements connecting the hook to a body. All of these elements must have been previously defined with CONNECTOR commands, must have a class category of SLING, and must have a single node. Additionally, all of the nodes of the EL(i) must belong to the same body.

The -INITIAL option instructs MOSES to "initialize" the set. If this option is specified and this is the first tip-hook set defined, the body will be moved so that the hook point is directly below the boom point. If the option is selected and this is not the first tip-hook set, then the length of the tip-hook elements will be changed so that there is no sag in the lines.

The -DEACTIVATE option instructs MOSES to to deactivate all previously defined tip-hook sets.

The -ORIENT option can be used to alter the definition of the body system of the body to which the EL(i) are connected. With this option, the order of the sling nodes is used to define the local body system. The origin of this system is defined as the midpoint of the vector connecting the first two sling nodes. The local Y axis is in the direction of the first node toward the second, the local Z axis is from the second node to the third, and the local X axis is given by the right hand rule.

During static processing, the configuration of the body is defined by the height of the hook attachment point above the waterplane and two angles. These two angles are called pitch and roll. The two angles are defined by vectors which can be defined via a &DEFAULT command. The -VERTICAL option can be used to automatically define these three quantities. Here, pitch is the angle which the normal to the plane formed by the sling nodes makes with the waterplane, roll is the rotation of the line defined by the first two sling nodes, and height is measured to the midpoint of the line connecting the first two nodes.

Once the sling is defined, the lengths of the tip-hook elements can be changed with &INSTATE -SL_SET. MOSES will place the system in the specified configuration and compute the lengths of each tip-hook element so that there is no slack in the assembly. Also, the &CONNECTOR command can be used to change the length of the boom element and deactivate the tip-hook set. The name to use here is the, NAME, the name given the set.