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Line Mesh Generation

Mesh Generation

You have control over the distribution of the elements along the line, via the specification of desired maximum and minimum element lengths, and also the division of the line into several subsections if required. The meshing algorithm automatically generates a finite element discretisation based on the guidelines you provide. Ideally, the aim is to create a mesh which is sufficiently dense to accurately capture structural behaviour, while not being unduly complex and resulting in lengthy simulation times. For example, if defining a steel catenary riser, you would typically require a relatively fine mesh in the touchdown region, while a more coarse mesh would suffice for the portion of riser lying flat on the seabed, or the portion extending upwards through the water column. If your model is likely to experience significant compressive loads, you will need to use a relatively fine mesh in order to ensure that the critical Euler buckling load is not exceeded - refer to Compression and Buckling for further details.

The meshing algorithm also attempts to prevent large changes in relative element length across the finite element mesh by gradually stepping up and down element lengths along the structure, and to avoid over-meshing by using longer elements in the middle of long sections of continuous properties. Intersections between different lines, or between different sections within a single line, are generally the points of most interest and therefore have a more refined mesh. It is desirable to step up the element lengths away from intersection regions and use larger elements towards the centre of homogenous sections. The meshing algorithm ensures that the ratio between the lengths of adjacent elements cannot exceed a certain value. The maximum ratio defaults to 1.5 (Standard option), but may be reduced to 1.25 (Fine option) or 1.1 (Super Fine option). The actual value of the ratio is automatically selected to correctly fill the meshed section with elements.

Referring again to the Steep Wave Riser example, you might specify minimum and maximum element lengths of 1m and 5m, respectively, for each of the three subsections, “Lower Catenary”, “Buoyant Section” and “Upper Catenary”. Focusing on the lower catenary section, the element length at the seabed connection would be in the region of 1m, would increase gradually (subject to the Standard aspect ratio constraint mentioned above) to 5m with increasing elevation above the mudline, before gradually reducing again approaching the buoyant section to a length of 1m. This would lead to a total of 21 elements, with corresponding lengths as presented below:

Sample Element Lengths

Line Locations

One further point to note is that the automatic meshing algorithm allows you to request that nodes be positioned exactly at specific lengths along a line. This is in addition to the guidelines you provide in relation to desired maximum and minimum element lengths. It is not particularly important in the context of the steep wave example, but can be useful if in certain circumstances. For example, if you wish to apply a point load at a certain distance along a line, or to connect another component (e.g. another line) to a line at a specific location, you would typically position a node on the line at the required location (it is also important in the context of pipe-in-pipe models, although the process in that case is automatic and seamless to the user). Meaningful names should be specified, as these names will be used by the program to automatically create relevant node labels.

Refer to Meshing Algorithm for further information on this feature.