Design analysis of 3,500 tonne double boom offshore crane

China’s Dalian Huarui Heavy Industry Group Co. (DHHI) commissioned Casper, Phillips & Associates (CP&A) to carry out a structural design review on a 3,500-tonne capacity double boom offshore crane.

DHHI recently completed manufacture of the offshore crane and each boom which sit side-by-side at the front of the vessel and can lift 1,750 tonnes. They measure over 128m and will be used both singularly and in pairs. The height of the crane mast, meanwhile, reaches 40.5m.

DHHI’s crane can handle cargo as large as 32m-high, with a 50m by 50m footprint, over water or on land. Similar cranes, which will typically have a captain and a crane operator, have been used to erect dockside or quayside cranes and bridges, and in offshore applications, such as wind farms or oil rigs.

CP&A analysed the crane and reported the reaction forces at the crane boom pin connections so that a foundation could be properly designed. Similarly, it reported the reactions of the mast ends attached to the barge deck under various load cases, extracting information for ship designers, so they could design for the required strength. There were a lot of different load combinations and scenarios that needed to be considered, such as different boom angles, wind, and wave motions, said CPA, and the lattice boom required many connection checks.

Richard Phillips, mechanical engineer at CP&A, commented: “The analysis was rather complex; we had to use a special pipe-to-pipe connection post-processor that we created in-house. Pre-processing is the creation of the mathematical model of the crane structure [defining structural beams, developing loads and load cases]. FEA processing is when you take the math model and solve it to get the forces in each structural member [beams and columns]. Post-processing is when you take the forces in each structural member end and check to see if the structural members and connections have adequate strength and resilience.”

CP&A’s stress program takes the forces from the finite element analysis and automatically checks stresses of each member based on the specified design code. CP&A’s connection program takes the forces from the stress calculation results checks stresses of every pipe end connections according to the structural welding code AWS D1.1 for every single load combination.

Project execution required collaboration between Chinese and Pacific Time zones, with Mike Zhang, the chief mechanical engineer at CP&A, able to communicate in fluent Mandarin where necessary. Phillips said that Zhang was the lead engineer on the project, which was especially useful when certain norms, guidance, metrics, and phraseology were used in dialog.

CP&A said it frequently works with Chinese companies and that it has a long-running relationship with DHHI, notably collaborating with it on the crane base anti-seismic isolation system (BASIS) – a seismic isolation system built on nonlinear time history analysis (NLTHA). BASIS can protect a crane’s main structure from damage, even in contingency level earthquakes.