One problem that new engineer facing when doing modeling (including me when I started this job many years ago) is messed up the linear dependencies connections. Why linear dependencies are so important in modeling? It is because linear dependencies will tell the computer which element will act as resistant and provide stiffness to structure, and which element will act as load attractions element and do not provide stiffness to structure. Take conductor and riser for example. We all know that conductor and riser shouldn’t provide any stiffness to the structure. Thus, we put conductor and riser elements as “slave” and tie it to structural nodes. This will make main structure as a “master”. Hence, we will have conductor and riser as load attractions to the structure without providing any false support to the main structural model.  Take a look on how we specify linear dependencies in a model at picture below:

There are 6 degree of freedom Fx, Fy, Fz, Rx, Ry and Rz. Take a closer look at the picture above and you will notice that we put 1 for Fx and Fy and 0 for the rest. This binary code is the only language that our computer understands. It is simply 1 = yes and 0 = no. In modeling, we put 1 to Fx and Fy mean that we fixed the slave node at this direction. 0 at Fz, Rx, Ry and Rz mean we let slave to be tight to master at this direction. We also can put 1 for all 6 degree of freedom, this will set slave node to behave totally like its master. However, I only put restriction to Fx and Fy, and let other 4 degree of freedom free to capture the real configuration of offshore structure. Most conductors and risers are restricted at lateral x and y directions by its frame, but the frame provide less support for z directions and rotational movement.

For pile and leg connections, things get different. This is because of global and local coordinate arrangement.  We need to treat pile and leg in a special way when we putting linear dependencies. Have a look at picture below:

The global coordinate is referring to the whole structure while local coordinate refer to only one element. This two coordinate is totally different. So, when we want to set boundary condition of pile ad leg for linear dependencies we restricted Fy and Fz and let loose Fx, Rx, Ry and Rz. Fy and Fz in local coordinate is actually Fx and Fy in global coordinate. We still locking Fx and Fy and release Fz, Rx, Ry and Rz in global coordinate.