SACS Hands‑On: Model, Run and Check a Simple Offshore Jacket Frame (Beginner Workflow)

Introduction

This hands-on tutorial walks a junior engineer through a simple, practical workflow in Bentley SACS to model an offshore jacket frame, run modal and linear static analyses, and check the most important outputs for early-stage design and verification. The goal is not to cover code-level design checks, but to give you confidence in setting up a clean model, identifying common issues, and producing outputs suitable for an early engineering review.

Key Takeaways

• How to create a clear jacket geometry in SACS (nodes, members, levels).
• Where to define materials, sections and added mass for hydrodynamic effects.
• How to set supports, apply basic environmental loads, run modal and static analyses, and inspect key outputs.
• Common beginner mistakes and quick debugging techniques.

1. What you will learn and why it matters

Clear goals for this tutorial:

• Build a simple four‑legged jacket model with primary bracing and a small deck column.
• Apply self‑weight and simple environmental equivalents (wind and current) as practical first checks.
• Run a modal analysis and linear static cases, then check mode shapes, global displacements and member forces.

Why SACS?

SACS is widely used for offshore steel structures. Junior engineers commonly need to create simple jacket models for concept checks, FE verification and preparing inputs for detailed design. This workflow covers those typical tasks in a beginner‑friendly way.

Before You Start: Minimum SACS Knowledge Required

Before beginning this SACS jacket model tutorial, it helps to understand a few basic concepts.

You should be familiar with:

• Nodes and members
• Structural supports and boundary conditions
• Basic steel section properties
• Load cases and load combinations
• Fundamental concepts of modal analysis

Do not worry if you are not yet comfortable with offshore hydrodynamic loading. This tutorial focuses on the structural modeling workflow rather than advanced wave loading techniques.

2. Project preparation and file setup

Folder and naming

Create a project folder with a simple naming convention. Example:

• /Projects/JACKET_CONCEPT_001/
• Model files: jacket_v01.sac, jacket_v01_input.txt, jacket_v01_report.pdf

Units and template

Choose consistent units before modeling. For this tutorial use SI: meters (m), kilonewtons (kN), seconds (s). Document units in the model notes and SACS header so reviewers know what to expect. Start either with a blank model or a minimal template that only contains your preferred unit settings—avoid legacy templates that may contain unexpected definitions.

3. Create geometry: nodes, members and levels

Plan the grid and elevations

Sketch a plan and elevation first. For a simple jacket use a 4×4 plan with legs at the corners of a square 10 m x 10 m and nodes at elevations: mudline (−30 m), knee (−10 m), deck (0 m), and top of deck column (+5 m).

Enter nodes

Enter nodes in a clear order (e.g., clockwise around the plan at each level). This helps later when creating members and reduces mistakes.

Create members

Connect nodes to create leg members and bracing. Example member types:

• Legs: continuous members from mudline to deck column.
• Main bracing: diagonal members between leg nodes (knee to mudline etc.).
• Deck beams/columns: short members at the deck level.

Name or number members consistently (LEG01, LEG02, BRAC01, DECK01). Consistent naming helps when filtering results later.

4. Assign materials, sections and mass properties

Materials

Define a material for steel: example E = 210000 MPa, density = 7850 kg/m3, yield strength 355 MPa (or API grade as required). Add a concrete grade if the deck or grouted members are modelled.

Sections

Assign pipe sections to members. Example: leg pipes OD = 0.6 m, thickness = 0.02 m; bracings OD = 0.3 m, thickness = 0.012 m. In SACS, define section properties (area, Ixx, Iyy, J) and check orientation for open sections.

Added mass and buoyancy

For early hydrodynamic checks include simple added mass for members or nodal added mass for submerged sections. For example, add a uniform added mass coefficient in the global mass table (e.g., ρ_added = 500 kg/m per submerged member) or use SACS hydrodynamic item entries if you are familiar with them. This will affect modal periods and static inertia loads.

5. Supports, connections and boundary conditions

Apply supports at foundation

At mudline nodes set supports according to the assumed foundation: fixed (all DOF blocked) for pinned piles, or prescribed spring stiffness if you want more realistic conditions. For beginners, start with fixed supports and note this assumption in your report.

Model connection stiffness

If some joints are idealized as pinned or include stiffeners, apply releases or rigid links as needed. In SACS, you can set member end releases for axial/shear/bending. Use rigid links sparingly and document where they were applied.

Connectivity checks

Use the SACS topology or connectivity tools to highlight unconnected nodes or members with zero stiffness. Visual inspection of the wireframe in several views also helps find “floating” members.

6. Define loads and loadcases (practical beginner set)

Self-weight

Always include self-weight as a default loadcase in SACS. Verify units: SACS often expects mass units consistent with your environment. For SI, self-weight will usually be applied automatically from member density. Check the gravity direction and magnitude (g = 9.81 m/s²).

Simple environmental loads

For early checks, use simple static equivalents:

• Wind: uniform lateral pressure applied to projected area. Example: 0.5 kN/m² applied as equivalent nodal or line loads on deck and columns.
• Current: horizontal line loads on submerged members or distribution per projected area — e.g., 1.0 kN/m on submerged leg length.
• Wave: for beginners, use an equivalent horizontal load or add to the static combination; full Morison or diffraction loads need more advanced setup.

Loadcase naming

Use clear names: SELFWT, WIND_X, CURRENT_Y, MODAL, STAT_COMB1. For modal analysis create a separate case (MODAL) and for static create combinations (e.g., ULS_EQ = SELFWT + WIND_X + CURRENT_Y).

7. Run analyses and inspect results

Modal analysis first

Run a modal analysis (eigenvalue) to obtain natural periods and mode shapes. Typical checks:

• First two or three modes should be rigid‑body or global bending/torsion modes—very low frequencies may indicate insufficient support or disconnected nodes.
• Compare the first natural period qualitatively with expectations (a simple jacket might have a first period of a few tenths to a few seconds depending on mass and stiffness).

Linear static analysis

Run linear static cases for each loadcase and the combinations. Inspect global displacements (max translation at deck), member axial forces, shear and bending moments. Export results for critical members (legs and primary bracing) to tabular form for review.

Key outputs to export

• Mode shapes (plot first 6 modes as diagrams or animations).
• Maximum nodal displacements and their location.
• Member axial force and bending moment envelopes—identify members with the largest demands.

How to Interpret Mode Shapes Correctly

Many junior engineers can run a modal analysis but struggle to interpret the results.

When reviewing mode shapes:

• Check whether the first mode represents global frame bending.
• Verify that no isolated members move independently.
• Confirm that torsional modes appear reasonable.
• Compare mode shapes with the expected structural behavior.

If a mode shape appears unusual, review support conditions, member connectivity, and mass definitions before proceeding with design checks.

8. Quick checks, troubleshooting and reporting

Common checks and mistakes

• Unit mismatches: double‑check SACS units and gravity setting; wrong units cause unrealistic loads/displacements.
• Disconnected nodes or duplicate nodes at same coordinates: cause zero stiffness or artificial mechanisms.
• Zero‑stiffness members or unintended releases: verify section properties and end releases.
• Rigid links used excessively: can mask realistic deformation patterns—use only where justified.

How to debug

If you see unexpected large displacements or rigid modes, isolate the problem region:

1. Run modal analysis with supports temporarily modified (e.g., fix all nodes) to see whether the issue is connectivity or boundary conditions.
2. Simplify the model—remove secondary bracing and run a reduced model to check primary load paths.
3. Check member IDs and connectivity tables to find disconnected elements.

Prepare a short results summary

For senior review include a 1–2 page summary with:

• Model assumptions and units.
• Screenshot of the wireframe and mode shape thumbnails.
• Table of first natural periods, maximum deck displacements for each loadcase, and axial/bending maxima for critical members.
• List of known limitations and next steps (detailed hydrodynamic loading, fatigue checks, connection modelling).

Practical example: small checklist with numbers

Using the example dimensions above (10 m plan, 30 m height):

• Material: steel E=210 GPa, density 7850 kg/m3.
• Leg section: 600×20 mm pipe; bracing: 300×12 mm pipe.
• Gravity: 9.81 m/s2; verify SACS uses consistent mass units so self‑weight appears as ~0.785 t/m for a 600×20 pipe (verify from section area).
• Loadcases: SELFWT, WIND_X = 0.5 kN/m2 equivalent, CURRENT_Y = 1.0 kN/m equivalent.
• Modal: expect first bending period roughly 0.5–2.0 s depending on added/subtracted mass—if you get 0.01 s you likely have a units issue or a very stiff model.

Beginner SACS Jacket Modeling Checklist

Before submitting your model for review, confirm the following:

✓ Units are consistent throughout the model

✓ Material properties are verified

✓ Section properties are assigned correctly

✓ Supports are applied properly

✓ No disconnected nodes exist

✓ Self-weight is included

✓ Modal analysis completed successfully

✓ Static load cases converge without warnings

✓ Maximum displacements reviewed

✓ Critical member forces exported

This simple checklist can prevent many common modeling errors during early-stage offshore structural analysis.

Conclusion

This tutorial gives a practical entry path for junior engineers to model a basic jacket frame in SACS, run modal and static analyses, and perform the essential checks needed for early‑stage verification. Keep assumptions simple, document units and boundary conditions, and use short, named loadcases. When unexpected results appear, isolate, simplify and re-run. The workflow here is intended to prepare you for more complex tasks: hydrodynamic analyses, fatigue and code checks once you are comfortable with the basics.

Sources

• Bentley SACS documentation and user manual (Bentley Systems).
• DNVGL-ST-N001 and API RP 2A for offshore structural guidance (for background reference).
• Practical modeling guides and training notes from industry workshops (consult your company SACS templates and standards).