1	SACS Collapse Parvinder Jhita Engineering Dynamics, Inc.
2	Engineering Dynamics, Inc. EDI was established in 1973 by three engineers who had worked on NASA’s Apollo (man on the moon) Project. Converted aerospace analytical techniques and computer software into a single integrated Structural Analysis Computer System (SACS) and made it available to private industry on a commercial basis.
3	Engineering Dynamics, Inc. EDI has now a client base of over 150 companies world wide including most major Oil Companies, Certification Agents, Fabrication & Installation Companies and International Design Firms.
4	SACS Collapse Collapse was developed 10 years ago in light of an aging offshore market in the Gulf of Mexico requiring re-assessment and additional emphasis on safety design following Hurricane Andrew. Original Developer : John Fowler (President, EDI) Dr. David Garland (Vice President, EDI) Dr. Parvinder Jhita Mr. Gavin Fury Current Developer : Dr. Jeremy Greenough
5	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 with minimum changes (requires no new modeling).
6	Collapse Capabilities Collapse accounts for geometric and material non-linear behavior. Includes non-linear elasto-plastic pile/soil foundation behavior. Collapse accounts for member global and local buckling. Includes joint flexibility, joint plasticity and joint failure.
7	Collapse Capability (Continued) Collapse includes user defined strain hardening. Accounts for residual stress resulting from unloading. Collapse includes user defined non-linear spring elements. Accounts for non-prismatic elements automatically. Has the capability of sequential load stacking with user controlled load incrementation, includes both loading and unloading.
8	Collapse Capabilities (Continued) Load cases in Collapse may contain loading and/or specified displacements. User defined ductility limits can be included to account for member fracture. Collapse has full ship impact/dropped object capabilities including automatic unloading for post impact analysis.
9	Beam Elements Collapse allows for hinge formation at any point along member length (not limited to member ends and center - this pre defines the failure mechanism). Divide member into sub-elements (maximum of 20, default is 8) and monitor the stress on each sub-element.
10	Beam Elements (continued) Predict 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.
11	Beam Elements Member Cross Section Sub-Areas for different cross sections
12	Plate Elements Allow plasticity to occur gradually through the plate thickness. Sub-divide the plate thickness into sub layers (5).
13	Elements Currently not Supported in Collapse Shell Elements Solid Elements Membrane Plates, Shear Plates, Corrugated Plates, Stiffened Plates Stiffened Cylinder/Box sections, Launch Runner, Double Angle, Dented Tube sections
14	Yield Criterion Collapse uses Von Mises-Hencky yield Criterion used to determine the onset of plasticity.
15	Yield Criterion - Members
16	Yield Criterion Similarly for plates, if the stress levels exceed the Von Mises-Hencky yield surface at a particular sub thickness then the whole thickness layer is assume to be in a plastic state.
17	Stress-Strain Relationship Material Properties User defined bilinear stress strain profile which includes strain hardening
18	Collapse Analysis Procedure Apply an incremental load to the structure
19	Collapse Analysis Procedure (a) Calculate the internal load at each end of each sub-element for each member
20	Collapse Analysis Procedure (b) Calculate the axial and shear stress at each sub-area.
21	Collapse Analysis Procedure (c) Calculate the plasticity by the amount of strain exceeding Von-Mises stress envelope. Retain this strain for each sub area throughout the load sequence for subsequent loading or unloading.
22	Collapse Analysis Procedure (d) Use plastic stresses to compute self-equilibrating plastic forces on each sub-area.
23	Collapse Analysis Procedure Plastic Force
24	Collapse Analysis Procedure (e) Add plastic forces to the global load vector and iterate until the member end, sub-element end deflections and rotations have converged.
25	Collapse Analysis Procedure (f) Apply the next load increment and repeat the procedure
26	Load Increment Size Collapse allows user controlled variable load increment sizes through loading history. Large steps in linear region Small steps in non-linear region (effective linearization)
27	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 collapse
28	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
29	Unloading Plastic Strain Retained for each sub area/segment
30	Global Buckling Collapse can predict elastic buckling including full elasto-plastic behavior.
31	Local Buckling Three methods available to predict local buckling API LRFD Marshall, Gates et el API Bulletin 2U A moment free hinge is inserted at the location of a local buckling point – axial capacity retained
32	Local Buckling API LRFD Check stress in each sub-area to initiate local buckling
33	Local Buckling API LRFD Collapse: for D/t <60 assume no local buckling. Failure will result from plastic hinge formation.
34	Local Buckling – Marshall, Gates et el Lower bound critical strain criteria
35	Local Buckling – API Bulletin 2U Local Buckling assumed for D/t < 134
36	Local Buckling Comparison
37	Joint Flexibility Joint Flexibility – Distortion of chord cross section due to forces in the brace and chord. Particularly Important for old structures where joint cans were not used. Collapse has two methods implemented to predict joint flexibility. These being: (1) Fessler’s Approach (2) MSL Approach
38	Joint Flexibility -Fessler
39	Joint Flexibility - Fessler Linear behavior ( no material/geometric non-linear behavior). Uncoupled – each brace is independent of forces in other braces or chord. Method developed in 1986 gives reasonable answers for elastic joint flexibility.
40	Joint Flexibility - MSL MSL developed continuous functions depending upon 3 parameters.
41	Joint Flexibility - MSL Method accounts for interaction with chord load. Based upon joint classification (K,Y, X joints). Accounts for interaction coupling between internal loads and moments. Apply ductility limits to predict tensile joint failure.
42	Joint Failure Three approaches implemented for joint failure: API LRFD MSL NORSOK
43	Joint Failure – API LRFD Joint failure determined by API LRFD punching shear criterion
44	Joint Failure - MSL Based upon MSL Joint Flexibility approach. Joint assumed to fail when load has reached within 1% of its capacity.
45	Joint Failure - NORSOK
46	Joint Flexibility Validation Compare the Fessler and MSL SACS implementation approaches with detailed finite element models. Validation Examples : T and a K joint
47	Joint Flexibility Validation - T Joint SACS beam model: T joint under in-plane bending
48	Joint Flexibility Validation- T Joint Finite element Model:
49	Joint Flexibility Validation- T Joint
50	Joint Flexibility Validation- T Joint Collapse Results – FE Model
51	Joint Flexibility Validation- K Joint SACS beam model: K joint with braces under axial loading :
52	MSL Joint Flexibility Validation - K Joint Finite Element Model:
53	Joint Flexibility Validation - K Joint
54	Joint Flexibility Validation - K Joint Collapse Results – FE Model
55	General Considerations Iteration Tolerance Levels Deflection Tolerance : 0.01 cm (Default) for large structures this can be increased to 0.1cm Rotation Tolerance : 0.001 RAD (Default) Member Deflection Tolerance: 0.01 cm (Default)
56	General Considerations Use the continuation option to continue if the maximum number of iterations have been exceeded. Avoid small elements to avoid member convergence problems. Avoid very slender elements to reduce the number of iterations per load increment. Avoid member end releases which may produce a mechanism.
57	General Considerations Collapse allows up to six load sequences to be defined – each load sequence is analyzed as an independent analysis. However Collapse will currently only record the last load sequence for Collapse View.
58	General Considerations Collapse run time can be decreased by: Modeling parts of the structure which have little contribution to overall stiffness of the structure (boat landings) as dummy structures. Parts of the structure whose elasto-plastic behavior is not important should be kept elastic. Pre-combining loads where ever possible. Include strain hardening to improve solution convergence.
59	Benchmark Assessment Current Collapse Benchmark Assessments (1) EQE International, Inc. – Independent Benchmark Study. (2) BOMEL Joint Industry Tubular Frames Project Phase III
60	Benchmark Assessment – EQE International, Inc. Original Benchmark did not include Collapse. Collapse was later added by EQE. 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.
61	Benchmark Assessment – EQE International, Inc. 13 Participants in Benchmark Study Software used: ASADS CAP EDP KARMA Micro SAS RASOS SAFJAC StruCAD 3D USFOS Original Benchmark described in OTC7779 paper.
62	Benchmark Assessment – EQE International, Inc. Benchmark model –existing platform installed in 1970 in the Gulf of Mexico - 4 legs - 157ft (48m) water depth - 30ft (9m) distance between legs at WP - 4 conductors - 355ft (108m) piles
63	Benchmark Assessment – EQE International, Inc. 1998 – EQE International, Inc. conducted an independent study to include SACS Collapse into the original Benchmark Assessment conducted by PMB Engineering Assessment conducted by loading platforms in three directions – (1) 225 degrees from true north. (2) 270 degrees from true north. (3) 315 degrees from true north.
64	Benchmark Assessment – EQE International, Inc. 225^o
65	Benchmark Assessment – EQE International, Inc 270^o
66	Benchmark Assessment – EQE International, Inc. 315^o
67	Benchmark Assessment – EQE International, Inc.
68	Benchmark Assessment – EQE International, Inc Results 270^o
69	Benchmark Assessment – EQE International, Inc. Results 315 degrees
70	Benchmark Assessment – EQE International, Inc Independent Assessment Conclusion: “ The results indicate Collapse provides a good estimate of platform ultimate capacity compared to other nonlinear codes. This Benchmark has been a standard of comparison for pushover analysis and Collapse has been shown to match the standard.”
71	Benchmark Assessment – BOMEL Engineering Consultants. 13 Participants in Benchmark Study Software used: ABAQUS APCA ASAS NL Offshore DYNA PALS RONJA COLLAPSE SAFJAC USFOS
72	Benchmark Assessment – BOMEL Engineering Consultants. Objectives of Benchmark To enable industry practice in ultimate strength analysis to benefit from the results of the benchmark exercise. To demonstrate industry capability and assurance in the application of nonlinear analysis techniques in the ultimate strength assessment of offshore structures.
73	BOMEL Frames Project Test Frame 6m wide 12m long 12 m high
74	Benchmark Assessment – BOMEL Engineering Consultants. Support Rig with non-linear supports
75	Benchmark Assessment – BOMEL Engineering Consultants. Load Case 1
76	Benchmark Assessment – BOMEL Engineering Consultants. Load Case 2
77	Benchmark Assessment – BOMEL Engineering Consultants. Load Case 3
78	Benchmark Assessment – BOMEL Engineering Consultants.
79	Benchmark Assessment – BOMEL Engineering Consultants.
80	Benchmark Assessment – BOMEL Engineering Consultants.
81	Benchmark Assessment – BOMEL Engineering Consultants. Joint Failure - Compression
82	Benchmark Assessment – BOMEL Engineering Consultants. Joint Failure - Compression
83	Benchmark Assessment – BOMEL Engineering Consultants. Member buckling
84	Benchmark Assessment – BOMEL Engineering Consultants. Member buckling
85	New Feature – Ship Impact Analysis Previously Ship Impact Calculations carried out in Collapse View. - this required multiple runs to attain maximum impact energy and subsequent unloading. New approach conducts the ship impact scenario within Collapse with automatic unloading once the maximum impact energy has been absorbed. DnV ship indentation curves are built into Collapse.
86	New Feature – Ship Impact Analysis IMPACT line added to Collapse to define impact load case, point of impact, impact energy, ship indentation curve and automatic unloading. ENERGY line added to automatically calculate the kinetic energy of a moving object SHPIND line added for a user defined ship load-deflection indentation curve
87	New Feature - Blast wall analysis (1) Use Dynamic Response to apply blast load profile to the structure at discrete time steps.
88	New Feature - Blast wall analysis (2) Dynamic Response will generate a structural output file containing time history of equivalent static loads (including dynamic and static components). (3) Dynamic Response will also output a load sequence file for Collapse.
89	New Feature - Blast wall analysis (4) Run Collapse for non-linear elasto-plastic analysis using equivalent static loading.
90	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
91	Future Developments Quasi-Newton Solution Methods Arc-Length Methods Mixed Load Control Dynamic Analysis DB Functionality
92	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