• weight
• optimization
• subsea
• christmas tree
• apdl
• ansys

The purpose of this work is to develop an APDL tool, suitable in the conceptual and preliminary design phases of a Subsea Christmas Tree (XT in the following), which has to perform preliminary structural assessments and weight optimization of the structure’s frame.
The tool has been developed as a sequence of organized APDL macros with a layout to grant general applicability to the tool.
This circumstance implies that different structure can be processed with the tool without any changes in its structure and in its core functionalities.
All of the customization operations required for the application of the tool on a specific structure result simple and quick to carry out, in particular if compared with the time needed for the definition of a tool for the preliminary assessment and weight optimization dedicated for a specific structure.
The tool needs, as input, simplified representations of frames, and performs linear elastic structural assessments on the resultant FE model.
All of the choices regarding the properties of the geometrical model and the type of analysis performed by the tool are in line with its intended purposes.
The definition of the simplification procedure needed for the building up of the geometrical model of the frame has been carried out as integral part of the work.
Once defined, the tool has been tested on two different XT frames.
For each of these, detailed structural assessment results are already available. These ones are referred to frame models made of solid elements and have been taken as reference.
The first frame has been represented using a geometrical model made of lines and areas. The resultant FE model contains both beam and shell elements.
Structural assessment results have been compared with those ones coming from the reference model; this operation is called benchmark phase.
Benchmark phase highlights that the frame representation used is too much approximate.
Accordingly, the second frame has been represented with a geometrical model made of areas only, which generates a FE model containing only shell elements.
With this simplification strategy, the benchmark phase better validates the model.
Despite the discrepancies highlighted for the first simplification strategy adopted, the weight reduction procedure has been applied on both frames.
Weight reduction obtained is about 25% of the original weight for the first frame, and about 16% for the second one.
Thanks to the optimization restraining, weight reduction results are conservative.
This implies that the frame redesign which follows the indications suggested by the tool should satisfy every structural requirement in final detailed design verification phase.
As future developments, the tool can be applied on structures different from XT frames, such as manifold frames; on XT frame analysis, the introduction of spools allows the assessment of structural interactions which are not considering with these models.
Moreover, implementation of a more effective and efficient optimization criteria will make the developed tool more suitable for the purpose of the weight optimization.