Moss Maritime, a part of the Saipem group is one of two consortia fighting to secure a huge FPSO contract. The use of the Nauticus Software has been essential for an effective and time limited structural design process.

Moss Maritime has used the Nauticus FPSO Software package on the 1 billion- barrel AKPO deep-water field in the Gulf of Guinea. The FPSO is permanently moored in about 1,300 m of water depth, has an oil storage capacity of 2.0 Mbbl and a total operational displacement of 404,000m3 .

The Akpo FPSO will be designed and built according to Bureau Veritas (BV) Rules. However, due to the considerable advantage of automatic calculation of wave loads in Wadam, the superelement technique in Sestra, and the ABC facility, the Nauticus Software has been chosen as the preferred analysis tool.

The design process of the Hull Primary Structure of the FPSO with Nauticus has been performed in two steps:

• First, the cargo area was designed using of a 3 cargo holds Finite Element (FE) model where the hydrodynamic loads were calculated according to the Rules
• Then the superelements used for the cargo FE model were used in a global FE analysis giving a detailed validation of the ULS and FLS capacity of the entire hull. This approach is using direct calculation of wave loads by Wadam.

The Automatic Buckling Check in Nauticus was extensively used during the design process. The following analysis tools were necessary for using the ABC on the Akpo FPSO FE model:

• Retrieving of stresses from a specified superelement in a superelement hierarchy
• Retrieving of stresses from combined result cases
• Covering of transverse structures, such as web frames and horizontal stringers
• Worst case – scan of load cases

The ability to retrieve stresses from a specified superelement in a superelement hierarchy is a very useful tool in order to make the post-processing more efficient. Also with respect to changes, it was much more efficient to re-run parts of the model than the complete model consisting of one single superelement only. For the Akpo project the global FE model should also be used as basis for detail analysis using Submod, where it is preferable to utilise the superelement technique.

In order to utilise the engineers and the number of software licenses, the use of the superelement technique was a very efficient way of creating the very large and detailed FE model.

In most cases when working with large FE models, real load conditions are created by combining single result cases. The tool for retrieving stresses from combined result cases is mandatory for cargo hold analyses, where the results from the analysis of local and global load effects are merged into one single database and the single loads are combined to form design load combinations. This option has also been utilised during the global FE analysis.

One of the main concerns during the design of the Akpo hull structure was the buckling capacity of the transverse web frames. At an early stage of the project it was necessary to have a very quick and efficient tool for buckling evaluations of large areas of the hull structure. Since the revised ABC also covers transverse structures, the amount of manual buckling checks has been reduced considerably.

The opportunity to export the results to Microsoft Excel spreadsheets together with the scan facility enables easier post processing of large amounts of results.

All these new capabilities make the Nauticus Software a very powerful and useful tool for the design process, and for projects at an early stage. For the Akpo Structural design team the new capabilities in Nauticus ABC were necessary in order to achieve the results within the schedule.

Author: Jan A. Korsnes, Moss Maritime