Volume XI - Summer 2020 | mscsoftware.com | 27

Structures

Hyundai Heavy Industries saves construction

period and costs significantly for offshore

structures using MSC Nastran and Patran

By Ul-Nyeon Kim, Jun-Tae Jang, Hyundai Maritime Research Institute, Hyundai Heavy Industries Co., Ltd.,

Bhoomi Gadhia, Product Marketing Manager, MSC Software

Volume XI - Summer 2020 | mscsoftware.com | 27

28 | Engineering Reality Magazine

The design of offshore structures like Floating Production Storage, Offloading unit (FPSO) and semi-submergible rigs shown in Fig.1, focuses on maintenance because they are not re-docked

periodically and do not seek shelter in extreme offshore conditions. As such, their design load is designed to withstand 100-year cycles of wind and waves, which are longer than those of ships (20~25 years). So, some strict specifications and requirements are reinforced for the safety of

the structural members so that safer offshore structures can be built.

Based on specifications and requirements, it is extremely important to verify the structural integrity of a region with numerous penetration-holes in offshore structures such as semi-submersible rig and FPSO. In order to effectively check the yielding and buckling strength of plate members with penetration-holes, a screening analysis program was developed using the FE analysis tool, MSC Nastran, to generate a fine meshed model using theoretical and analysis methods. When a hole appears in the plate structure members, the flow of stress is altered such that concentrations of stress form near the hole. Stress concentrations are of concern during both preliminary and detailed design and need to be addressed from the perspective of strength.

In order to configure the geometrical shape, very fine meshed Finite Element analysis is needed as the most accurate method. However, this method is practically impossible to apply for the strength verifications for all perforated plates. Hyundai Heavy Industries laid out a screening analysis method which was introduced to reduce analysis tasks prior to detailed Finite Element analysis. This method was applied to not only the peak stress calculation combined stress concentration factor with nominal stress but also nominal equivalent stress calculation considering cutout effects. The areas investigated by very fine meshed analysis were to be chosen through screening analysis without any reinforcements for penetration-holes. If screening analysis results did not satisfy the acceptance criteria, direct FE analysis method as the 2nd step approach were applied with one of the coarse meshed models considering holes or with the very fine meshed model considering the hole shape and size. In order to effectively perform the local fine meshed analysis, an automated model generating program was developed based on MSC Nastran and Patran which are Finite Element analysis software.

Figure 1 : offshore structure

Buckling strength was also evaluated by Common Structure Rule (CSR) adopted by IACS as the stress obtained from very fine meshed FE analysis. Due to development of the screening analysis program and automatic finite element analysis, we were able to reduce the design periods and structural analysis costs.

Firstly, to the concerns about the development of the procedure and program for efficiently evaluating the local strength of ships and offshore structures with many penetration-holes. To quickly perform strength evaluation, a screening analysis program was developed, and an analysis automation system was implemented in MSC Nastran/Patran a universal finite element structural analysis program, so that a fine meshed structural analysis can be easily performed in consideration of penetration-holes. If there are penetration-holes that abruptly change their shapes in the plate structures, the flow of the internal force resisting the external force becomes uneven, leading to stress concentration. Due to that fear, we perform excessive structural reinforcement around the many penetration-holes or use the fine meshed model to perform a finite element analysis, and demand structural stability analysis review. In response to this, shipbuilders attempt to calculate according to the design standard or relevant classification regulations, but there exists a lot of difficulty in correct calculation in consideration of the local stress concentration. So, a finite element analysis in consideration of the shape of the penetration-hole was performed. Accordingly, as structural strength analysis takes a lot of time, construction could be delayed. The strength evaluation program and procedure regarding penetration-holes, which was created through this study, were pre-approved by the classification society, and applied to semi-submergible rigs and FPSO construction, thereby greatly reducing the design period and the cost of structural analysis.

Using MSC Nastran and Patran, a strength evaluation automation system was implemented for penetration holes. The source program for evaluating and analyzing strength was written in the Fortran language and PCL programming language.

As an example, the program used the semi-submersible rig that Hyundai Heavy Industries designed and built (See Fig. 2).

Fig. 3 illustrates penetration-hole information recognized in the finite element model for the vertical column and pontoon with regard to the hull of the semi-submergible rig. About 3,000 small and mid-size penetration-holes were detected in the vertical column and about 5,400 in the pontoon.

The result of the screening analysis shows that in the vertical column, 16% (477 locations) of the plate members with penetration-hole installed, and in the pontoon, 2.4% (130 locations) of the plate members with penetration-holes installed had insufficient yielding strength. For this, a local fine meshed analysis was conducted, and 10 locations had insufficient strength. So, the dimensions of the plate members were changed, and additional stiffeners were installed.

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In Fig. 4, with regard to perforated plates that had insufficient yielding strength, the notch stress extracted from the screening analysis was compared with the peak stress obtained from the local very fine meshed analysis. As the notch stress was about 1.7~2.1 times greater than the peak stress, it was confirmed that a safety factor is secured in the screening. Accordingly, if the strength evaluation criteria are satisfied in the screening analysis, it is deemed that all the results of fine-meshed analysis satisfy the criteria. It is now possible to control the accuracy of strength evaluation by determining the allowable stress criteria of the screening analysis for individual offshore structure constructions based on consultation with the client/classification society.

Conclusion

In this study we focused on implementation of an automation system for the screening analysis and local fine meshed analysis to quickly handle the yielding strength and buckling strength of ships/offshore structures with numerous penetration-holes.

The strength evaluation program and procedure for penetration-holes were pre-approved by the classification society and applied to the construction of the semi-submergible rig and FPSO to reduce the design/construction period by 4~8 months and the cost of structural strength analysis by hundreds of millions of Korean won.

Figure 2 : Semi-submersible rig

Figure 3 : FE model of the hull structure

Figure 4 : Comparison of the notch and peak stress

The strength deterioration and stress concentration of the hull girder in areas with many penetration holes are installed in close proximity need further research attention to come up with a practical design guideline and ensure appropriate placement of penetration-holes and reinforcements.

References

1. A Study on the Strength Evaluation Method of Plate Structures with Penetration-holes Kim, Ul-Nyeon;Jang, Jun-Tae

2. Journal of the Society of Naval Architects of Korea. 2017. Dec, 54(6): 476-484

3. DOI : http://dx.doi.org/10.3744/SNAK.2017.54.6.476