Integrated Simulation Method Based on Multibody Dynamics for Production Design Verification in Ships and Offshore Structures

Abstract

It is the most important to verify the safety of the production design before the real operation. However, the verification which depends on the experience of the production engineer or the rule and regulation cannot be clearly proven or results in overestimation. Therefore, the verification based on dynamic analysis is widely adopted. However, it is impossible for existing programs to support some mechanical equipment such as the equalizer and SPMT (Self-Propelled Modular Transporter). Therefore, this study analyzes the requirements that are essential to simulate the lifting and erection operation in ships and offshore structures and proposes the integrated simulation framework based on multibody dynamics. The proposed framework is composed of five layers such as simulation core layer for solving the equations of motion, interface layer for data communication, simulation components layer including constraints, forces and collision, equipment layer, and service layer. This study develops a dedicated and differentiated program for dynamic analysis in ships and offshore structures, named SyMAP (SyDLabs Multibody Analysis Program).<br/> The proposed simulation framework integrates several modules based on various theoretical backgrounds. First of all, the equations of motion are based on multibody dynamics. Among the several formulations, we adopt the DELE (Discrete Euler-Lagrange Equation) to achieve the robustness during numerical integration. Furthermore, we formulate the equations of motion of the 1D frame element and 2D shell element based on ANCF (Absolute Nodal Coordinate Formulation). Kinematic constraints including joints and constraint-based wire rope between the rigid bodies, and between the rigid and flexible bodies are also derived. Especially, an equalizer which distributes the tension of wire ropes between the load and equipment equally is modeled based on the real mechanism by using the constraint-based wire rope.<br/> Meanwhile, we also deal with special issues in collision detection and response. Because the shape exports from the ship CAD system contains unenclosed meshes, we propose the position difference method which checks an intersection using the line segment made by the two vertices or the trigonal prism consisting of the two triangular meshes at time t0 and t1. Furthermore, BVH (Bounding Volume Hierarchy) and exclusion boxes were adopted to increase the performance. For collision response, non-interpenetration constraint method between a vertex and a plane is derived. This method is applicable when two bodies collide at the multiple points, and it does not compulsively violate the kinematic constraint because the collision force was also solved together when the equations of motion were solved numerically. Moreover, the collision force could be determined automatically, reflecting material properties such as restitution and softness.<br/> This study proposes the modeling of the mechanical parts of the SPMT taking into consideration the axle compensation mechanism to maintain the level of the platform when the SPMT drives over an uneven roadway by lifting up and down the wheel. As external forces, hydrodynamic force, wind force, current force, and mooring force are also explained.<br/> For the verification, comparison of the benchmarking tests of multibody systems and the examples of commercial multibody software DAFUL is conducted. The analytic solutions and the simulation results are compared in case of the flexible multibody dynamics. To verify the characteristics of the motion due to the hydrodynamic forces, the motion of the floating barge is compared with RAO given by WADAM, OrcaFlex, and SIMA. For the validation, the simulation results are compared with the data collected in the real operations.<br/> Finally, we provide four representative applications such as block lifting using equalizers, LPG tank erection considering a collision, thin plate block lifting considering deformation, and block offloading using SPMT, which have not been solved before. We conclude that the problems issued in ships and offshore structures are solved by the proposed or adopted methods. We convince that the developed program based on the proposed integrated simulation framework is able to cover all of the operations in ships and offshore structures.