Product(s):	SACS
Version(s):	23.00.
Area:	Collapse

Problem

I want to simulate DNV-RP-C208 clause 4.2 to 4.9 for selection of software for nonlinear material behavior. Does SACS have any option in reference to the clause? 

Solution

SACS Standard Collapse and Collapse Advanced programs support plasticity and geometric nonlinearity. These two are part of the Finite Element formulation that SACS is using. The main difference between Standard Collapse and Collapse Advanced is that Collapse Advanced is based on more complex formulations and utilizes more advanced numerical methods. Hence, we encourage our users to move to Advance Collapse in case of any troubleshooting required in Standard Collapse.

Below is the comparison made by our development team when it comes to simulation with DNV requirement.

FE Software (Section 4.2):

• Nonlinear material behavior: The collapse programs support isotropic hardening with bi-linear and multilinear stress-strain curves.
• Nonlinear geometry: They both support nonlinear geometry, but Collapse Advanced has complete formulation, while Standard Collapse uses simplified formulation. 
• Contact: We don't support full contact simulation. The collapse program only supports analysis with known load - like a point contact (single joint impacted by the dropped object or ship impact).
• Temperature Effects: Collapse Advanced has temperature loads (not thoroughly tested), but it does NOT support material thermal degradation.
• Nonlinear load effects: In Standard Collapse, the loads remain in their global direction defined in the SACS model file (i.e., conservative loads), and the element configuration will change relatively to the loads. Therefore, because of the large deformation, A load initially perpendicular to a member or a plate may not remain perpendicular during the analysis. In Collapse Advanced, the member loads are like Standard Collapse. However, plate pressure applied directly to the plate elements will always remain perpendicular to the plate element regardless of the plate's large deformation. So, Collapse Advanced plate pressures are follower loads (i.e., follow the structure deformation).

Analysis Methods (Section 4.3):

• Nonlinear Dynamic Analysis (Sections 4.3.1, 4.3.2, and 4.3.3): The SACS Collapse program only supports Quasi-dynamic analysis where the inertia loads are calculated using Linear Dynamic analysis. Therefore, we approximately support Sections 4.3.1, 4.3.2, and 4.3.3.
• Nonlinear Static Analysis (Section 4.3.4 and Figure 4-1): Using the Arc-length method, Collapse Advanced supports all features mentioned in Section 4.3.4. Standard Collapse doesn't have the Arc-length method, so it supports analysis until the Limit Point in the left plot in Figure 4-1. 

 

Mesh Density (Section 4.5) and Geometry Modelling (Section 4.6):

• The User is responsible for using SACS modelling tools to follow these sections' recommendations.

 

Material Modelling (Section 4.7):

1. Section 4.7.2: Collapse programs support isotropic hardening, but we don't have kinematic hardening. Collapse Advanced has a better algorithm to solve isotropic hardening.
2. Section 4.7.3: SACS Collapse programs always use engineering strains. 
3. Section 4.7.4: Collapse Advanced checks for strain ductility limit at integration points and adjusts element stiffness and forces if the ductility limit is reached.
4. Section 4.7.5: Standard Collapse has some features for multilinear stress-strain curves. Collapse Advanced fully supports multilinear stress-strain curves with strain hardening or softening for members, plates, and pile elements. 

 

Load Application (Section 4.9)

• Load Sequence: Collapse programs support load sequences, as explained in this section. 
• Conservative Loads vs Follower Loads: see comment above in 4.2. 
• Load Increments: In Standard Collapse, the User is solely responsible for providing load increments and the number of steps. Collapse Advanced provides automatic sub-incrementation, improving the analysis efficiency and accuracy. But this is User’s responsibility for entering meaningful load sequence and load steps.