1	SACS Collapse Parvinder Jhita Engineering Dynamics, Inc.
2	SACS Collapse Collapse is a Large Deflection, Elasto-Plastic Non-Linear finite element software tool for structural analysis. Fully Integrated into the SACS suite of programs. Same input as standard SACS analysis and does not require any additional modeling - accounts for non-prismatic elements automatically.
3	Collapse Capabilities Collapse accounts for geometric and material non-linear behavior Includes non-linear elasto-plastic pile/soil foundation behavior Accounts for both overall and local member buckling Joint flexibility, joint plasticity and joint failure are included
4	Collapse Capability (Continued) Collapse includes user defined strain hardening User defined non-linear spring elements can also be included Collapse accounts for residual stress’s resulting from elasto-plastic unloading
5	Collapse Capabilities (Continued) User defined ductility limits can be included to account for member fracture Collapse has full ship impact/dropped object analysis capabilities
6	Beam Elements – Plastic Hinge Formation Collapse allows for hinge formation at any point along the member length by subdividing each member into sub-elements and monitoring the stress in each sub-element ( hinge formation not limited to member ends and center - this pre defines the failure mechanism). By default, each member is divided into 8 sub elements. Collapse allows for a maximum of 20 sub-elements per member.
7	Beam Elements – Plastic Hinge Formation Collapse allows the gradual development of plastic hinge through member cross section. Divide cross-section into sub areas and monitor the stress level in each sub-area. - Tubular cross sections divided into 12 sub areas.
8	Beam Elements Member Cross Section Sub-Areas for different cross sections
9	Plate Elements Collapse allows plasticity to occur gradually through the plate thickness Sub-divide the plate thickness into sub layers (5) and monitor stress levels in each sub-thickness
10	Yield Criterion Collapse uses Von Mises-Hencky yield Criterion used to determine the onset of plasticity
11	Yield Criterion - Members
12	Yield Criterion Similarly for plates, if the stress exceeds the Von Mises yield surface at a particular sub thickness then the whole sub-thickness layer across the plate element is assumed to be plastic.
13	Material Properties Material properties include bilinear stress strain profile including user defined strain hardening
14	Collapse Analysis Procedure Typical Collapse analysis procedure begins with the application a load increment to the structure
15	Collapse Analysis Procedure The next step is to calculate the internal load at each end of each sub-element for each member
16	Collapse Analysis Procedure The axial and shear stress is then calculated at each sub-area at each end of every sub-element.
17	Collapse Analysis Procedure Calculate the plastic strain. Retain the plastic strain for each sub area throughout the load sequence for subsequent loading or unloading
18	Collapse Analysis Procedure Use elastic stresses to compute self-equilibrating plastic forces on each sub-area.
19	Collapse Analysis Procedure Add plastic forces to the global load vector and iterate until the deflections and rotations have converged at each member end and also the sub-element ends.
20	Collapse Analysis Procedure Update the stiffness matrix, apply the next load increment and repeat the procedure
21	Load Increment Size Collapse allows user controlled load increment sizes through loading history. Large increments in linear region. Small increments in non-linear region.
22	Limit Points Global Limit Point indicating overall structural failure. Collapse solution will diverge and stop when load is increased beyond the global limit point indicating structural failure.
23	Limit Points Local Limit Point indicating local structural failure Collapse solution will jump to the next stable configuration when load is increased beyond the local limit point
24	Overall Buckling Collapse can predict elastic member buckling and subsequent elasto-plastic behavior.
25	Results Output Results Output in 3 Formats Tabulated Format Graphical Format Pictorial Format
26	Results Output Tabulated Format & Graphing Formats: Collapse includes extensive user defined reporting and graphing capabilities.
27	Results Output Pictorial Format Includes: Deflected shape plots Color coded plasticity plots - Color indicates percentage of cross-section which is plastic.
28	Results Output Pictorial Format Color coded soil axial capacity plots - color indicates percentage of axial soil capacity utilized Special symbols to depict local buckling and joint failure events.
29	Local Buckling Collapse has three methods available to predict local buckling API LRFD API Bulletin 2U Marshall, Gates et el A moment free hinge is inserted at the location of local buckling point – axial capacity retained
30	Local Buckling Comparison
31	Joint Flexibility Joint Flexibility – Distortion of chord cross section due to forces in the braces and chord. Particularly Important for old structures where joint cans are not used. Collapse has two methods implemented to predict joint flexibility. (1) Fessler’s Approach (2) MSL Approach
32	Joint Flexibility - Fessler Method developed in 1986. Predicts linear behavior and does not include material and geometric non-linearity's. This is an uncoupled approach in that the behavior of each brace is independent of forces in other braces or chord.
33	Joint Flexibility - MSL This is fully coupled approach in that the behavior of each brace is depends on forces in other braces and the chord. The approach is based upon joint classification (K,Y, X joints).
34	Joint Failure Collapse has three approaches implemented for joint failure: API LRFD NORSOK MSL
35	Joint Flexibility Validation The two joint flexibility approaches in Collapse (Fessler’s and MSL) were validated through comparison with detailed finite element models for a T and K joint.
36	Joint Flexibility Validation - T Joint Collapse beam model for a T joint with lateral in-plane brace loading.
37	Joint Flexibility Validation- T Joint Finite Element Model
38	Joint Flexibility Validation- T Joint
39	Joint Flexibility Validation- T Joint Collapse Results – FE Model
40	Joint Flexibility Validation- K Joint Collapse beam model of a K joint with braces under axial loading
41	MSL Joint Flexibility Validation - K Joint Finite Element Model
42	Joint Flexibility Validation - K Joint
43	Joint Flexibility Validation - K Joint Collapse Results – FE Model
44	Benchmark Assessment Current Collapse Benchmark Assessments (1) Independent Study by EQE International, Inc. in which EQE added Collapse results into an earlier benchmark. (2) Joint Industry Tubular Frames Project headed by BOMEL in the UK.
45	Benchmark Assessment – EQE International, Inc. Original Benchmark headed by PMB Engineering Inc., 1994. Joint Industry Project to determine the variability in ultimate strength analysis results using draft Section 17 guidelines.
46	Benchmark Assessment – EQE International, Inc. 13 Participants in Benchmark Study 9 Software packages used: ASADS CAP EDP KARMA Micro SAS RASOS SAFJAC StruCAD 3D USFOS Original Benchmark described in OTC7779 paper.
47	Benchmark Assessment – EQE International, Inc. Benchmark model: Existing 4 legged Gulf of Mexico platform installed in 157 ft of water with 355 ft piles.
48	Benchmark Assessment – EQE International, Inc. 1998 – EQE conducted an independent study to include SACS Collapse into the original Benchmark Assessment. Assessment conducted by loading platforms in three different directions: (1) 225 degrees from true north. (2) 270 degrees from true north. (3) 315 degrees from true north.
49	Benchmark Assessment – EQE International, Inc. 225^o
50	Benchmark Assessment – EQE International, Inc 270^o
51	Benchmark Assessment – EQE International, Inc. 315^o
52	Benchmark Assessment – EQE International, Inc.
53	Benchmark Assessment – EQE International, Inc Results 270⁰
54	Benchmark Assessment – EQE International, Inc. Results 315⁰
55	Benchmark Assessment – BOMEL Engineering Consultants. 13 Participants in Benchmark Study 9 Software used ABAQUS APCA ASAS NL Offshore DYNA PALS RONJA COLLAPSE SAFJAC USFOS
56	BOMEL Frames Project Test Frame 6m wide 12m long 12 m high
57	Benchmark Assessment – BOMEL Engineering Consultants. Support Rig with non-linear supports
58	Benchmark Assessment – BOMEL Engineering Consultants. Load Case 1
59	Benchmark Assessment – BOMEL Engineering Consultants. Load Case 2
60	Benchmark Assessment – BOMEL Engineering Consultants. Load Case 3
61	Benchmark Assessment – BOMEL Engineering Consultants.
62	Benchmark Assessment – BOMEL Engineering Consultants.
63	Benchmark Assessment – BOMEL Engineering Consultants.
64	Benchmark Assessment – BOMEL Engineering Consultants. Joint Failure - Compression
65	Benchmark Assessment – BOMEL Engineering Consultants. Joint Failure - Compression
66	Benchmark Assessment – BOMEL Engineering Consultants. Member buckling
67	Benchmark Assessment – BOMEL Engineering Consultants. Member buckling
68	Recent New Feature Add to Collapse– Ship Impact Analysis Ship impact analysis with automatic unloading. Local indentation energy calculations as per API. DnV ship indentation curves are inbuilt.
69	New Feature added to Collapse - Blast analysis Blast analysis feature added which contains both static and dynamic components of the structural response
70	Future Developments Arc-Length Methods to enhance prediction of post buckling events. Full Dynamic Collapse Analysis Capability. Include Elasto-Plastic behavior for corrugated plates.
71	Engineering Dynamics, Inc. EDI Engineering Dynamics, Inc. 2113 38^th Street, Kenner, LA 70065 USA Phone: (504) 443 5481 Fax: (504) 443 6120 Email: sacs@edi-nola.com Web: http://www.sacs-edi.com
72	References API RP2A LRFD First Edition, July 1, 1993 “Inelastic Dynamic Analysis of Tubular Offshore Structures”, Peter W. Marshall, William E. Gates, Dames and Moore and Stavros Anagnostopoulos, Shell Development Co. Bulletin on Stability Design of Cylindrical Shells, API Bulletin 2U (BULL 2U), Second Edition, October 2000 “Parametric Equations for the Flexibility of Single Brace Tubular Joints in Offshore Structures” Fessler, H., Mockford, P.B. and Webster, J.J. Proc. Inst. Civ. Eng., Part 2, 81, December 1986. “JIP- Assessment Criteria, Reliability and Reserve Strength of Tubular Joints (Phase II)”, Final Report, July 2000, MSL Engineering Ltd. “SACS Collapse Benchmark Assessment”, EQE International, October 1998 DESIGN OF STEEL STRUCTURES, NORSOK STANDARD, N-004, Rev.1, December 1998. Joint Industry Tubular Frames Project – Phase III, Benchmark Conclusions, Bomel Limited, December 1999 “Modification to and Applications of the Guidelines for Assessment of Existing Platforms Contained in Section 17.0 of API RP 2A”, OTC7779, K.A. Digre, F.J. Puskar, R.K. Aggarwal, J.T. Irck, W.F. Krieger and C. Petrauskas, May 1995