Accidental loading

© 2011 Bentley Systems, Incorporated

Accidental Loading

Design Philosophy

Accidents will happen because they are accidents.
Design philosophy is to prevent an accident developing
into a catastrophe.
• Design to:

– Maintain usability of escape ways

– Maintain integrity of shelter areas

– Maintain global load bearing capacity

– Protection of the environment

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Accidental Loading Design

Some typical accidental events on offshore structures are :

• Ship Impact

• Dropped Object

• Blast & Fire Loading

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Accidental events generally involve large plastic strains.

To analyze and design against accidental events requires
software tools capable of predicting:

• Dynamic inertial loading *

• Geometric non- linearity

• Material non- linearity

* API RP 2FB recommends the use of dynamic analysis for blast loading

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Dynamic Inertial Loading

SACS DYNAMIC RESPONSE MODULE

• Allows for linear, quadratic, or cubic interpolation for the time history
input.
• Variable time step integration procedure.
• Time history plots including modal responses, overturning moments,
base shear, etc.
• Generation of equivalent static loads.
• Generation of incremental loads for Elasto/Plastic analysis

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Geometric and Material Nonlinearities

• SACS COLLAPSE MODULE
• Gradual development of a plastic hinge through the member cross
section
• Development of plastic hinges anywhere along the length of the
member
• Local Buckling
• Joint Flexibility
• Joint Failure
• Member Rupture
• Pile Plasticity
• User defined strain hardening

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Beam Elements

• Collapse allows for hinge formation at any point along member length by
sub dividing the member into sub-elements (maximum of 20, default is 8)
and monitor the stress level at each sub-element.

• Not restricted to hinge formation at member end an center – this pre
defines the failure mechanism

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Beam Elements

• Collapse predicts the gradual development of plastic hinge
through a member cross section by:

Dividing the cross-section into sub-areas and monitoring the stress
levels in each sub-area.

By default tubular cross sections are divided
into 12 sub-areas.


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Beam Elements

- Member Cross Section Sub-Areas for different cross sections


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Plate Elements

• Collapse allows plasticity to occur gradually through the plate thickness.

• Sub-divide the plate thickness into sub-layers (5).


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Yield Criterion

• Collapse uses Von Mises-Hencky yield Criterion to determine the onset
of plasticity.


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Local Buckling

Three methods available to predict local buckling

(1) API LRFD
(2) Marshall, Gates et el
(3) API Bulletin 2U

A moment free hinge is inserted at the location
of a local buckling point – axial capacity retained


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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 (linear)

(2) MSL Approach (non-linear)


Dynamic-Nonlinear Analysis

SACINP
(Model File)

DYNPAC
(Modal Analysis)

CLPINP Mode File
(partial) Mass File DYRINP

DYNAMIC RESPONSE
(Force-Time History Analysis)

CLPINA DYROCI
(full)

COLLAPSE
(Non-Linear Analysis)

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Ship Impact

Impact Design Criterion:
• Low Energy (Operational Impact)
Jacket bracing designed to survive operational impact (partial yielding at point of
impact).

• High Energy (Accidental Impact)
Jacket legs designed to survive accidental impact.
Face and leg Joints to survive accidental impact loading.
Jacket bracing allowed to fail – Structure designed to survive loss of brace
member.
allowed).


Ship Impact

Total Impact (Kinetic) Energy:

E = ½ a m V2
m = vessel mass
a = added mass coefficient (1.4 – broadside and 1.1 for bow/stern )
V = vessel velocity

Gulf of Mexico : m=1000 metric tons V= 0.5 m/s (operational)


Ship Impact

Kinetic Energy Absorbed Through:

• Localized plastic deformation (denting)

• Overall elasto plastic deformation of member

• Fendering devices (if fitted)

• Global deformation of platform

• Deformation of the ship itself


Ship Impact

Mesh the impacted member to account for local denting.


Ship Impact

Mesh joint to account for local indentation effects


Ship Impact

To account for energy absorbed by ship deformation, use DNV ship
indentation curves for 5000 ton vessel impacting a 1.5 m cylinder.


Ship Impact

Dynamic Response Input

Added
Mass
Coefficient.

Mass Velocity Direction Distance Impact Joint


Ship Impact

Dynamic Response Results

ship

structure


Ship Impact

Collapse Results


Dropped Object

Dropped Object Analysis

Certain locations such as crane loading and drilling areas
are subject to dropped objects.

The platform should survive the initial impact from a dropped
object and meet the post-impact criteria to survive a one
year environmental load in addition to normal operating
conditions.

Dropped object analysis also required to determine safe lift
heights for platform modification/repair to avoid production
shutdown which can be costly.


Dropped Object

Total Impact Energy:

E = mgh

m = mass of object
g = gravitational acceleration
h = height from which the object is dropped


Dropped Object

Dynamic Response Input

Mass Initial Height Impact
velocity joint


Dropped Object

Dynamic Response/Collapse Results

IMPACT JOINT DISPLACEMENT



Collapse Results


Blast Analysis

Primary objectives for blast resistant design are:

• Personnel safety
• Controlled Shutdown
• Financial Considerations
• Environmental considerations

API RP 2FB specifications requires a Ductility Level
Blast (DLB) design for low probability, high
consequence extreme events.

A DLB design requires a dynamic analysis to
accounts for inertia loading and a large
deflection analysis to account for geometric and
material non-linear effects.

A DLB design is required for temporary refuge,
safe muster areas and escape routes..


Blast Analysis

Blast analysis requires definition
of Blast Wave::

Two Types of Blast Waves

Shock Wave
1. Sudden pressure rise.
- Explosions from materials
in liquid or solid form
- Extremely energetic vapor Idealized profile
cloud explosion

Pressure Wave
2. Gradual pressure rise

“Design of Blast Resistant Buildings in Petrochemical Facilities”

Blast Analysis

Dynamic Response Input:

Time Load
SACS
Factor
Load case


Blast Analysis

Dynamic Response Results:


Parvinder Jhita
Product Manager - SACS
Bentley Systems Inc
2113 38th Street
Kenner
LA 70065
Telephone (504) 443 5481
Parvinder.Jhita@Bentley.com


Accidental loading

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