Sacs Basics

SACS

Wave Load

•

For the design of offshore structures, the waves are characterized as

regular waves with reasonable accuracy.

•

Several wave theories are available for the purpose of determining

the wave loads:



Airy‟s Linear Theory



Cnoidal Theory



Solitary Wave Theory



Stokes 5th Order Theory

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Wave Load

•

The wave theory to be used is selected based on the water depth and wave height

.

•

Wave loading on a member is categorized into Drag , Inertia, Diffraction, Slamming and Vortex Shedding Induced load

•

If the member size is small < (1/5) x Wavelength,

Morison‟s equation can be used to calculate the wave loading.

•

Morison‟s Equation:

Where : Cd is the coefficient of drag, Cm is coefficient of mass

D is the diameter, U is the velocity , r is the fluid density and A is the area.

U A ρ C U U D ρ 0.5C F _D      _M    

Wave Load

•

Various options available for defining coefficient of drag Cd and coefficient of mass Cm

•

(1) API Defaults

smooth Cd=0.65 Cm=1.6 rough Cd=1.05 Cm=1.0 Note: Cd & Cm are constant for all diameters

•

(2) Wake Encounter Effects

As a wave moves past a vertical cylinder a wake is produced. The turbulence produced by the wake impinges on the cylinder again due to the circular motion of the water particles in a wave motion. The amount of turbulence affects

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Wave Load

•

(2) Wake Encounter Effects (Continued)

Members within 15 degrees of vertical subject to wake encounter effects.

Use Keulegan-Carpenter number K to calculate Cd and Cm.

K = Umo. Tapp/D

Umo = Max horizontal velocity (containing inline

current effects) at MWL under crest. Tapp = Apparent wave period

Wave Load

•

(2) Wake Encounter Effects (Continued)

Use relative surface roughness „e‟ to find the drag coefficient for steady flow Cds from Figure C.2.3.1-4.

e=k/Deff

where:

k = average peak to valley height along the surface of the marine growth Deff = Dc + 2t

Dc = Diameter of clean tube

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Wave Load

•

(2) Wake Encounter Effects (Continued)

Use the Keulegan-Carpenter together with the steady flow drag coefficient Cds and figures C.2.3.1-5 and C.2.3.1-6 to find the drag

Wave Load

•

(2) Wake Encounter Effects (Continued)

Similarly use figures C.2.3.1-7 and C.2.3.1-8 to find the mass coefficient Cm.

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Wave Load

•

(2) Wake Encounter Effects (Continued) Shielding Factor

Closely spaced members such as conductors may have a reduced wave loading due to shielding. The amount of shielding depends upon the centerline spacing and the wave velocity and period.

The shielding factor may be different for each wave direction. Both Cd Cm are multiplied by the shielding factor.

Use shielding factor as follows:

A/S > 2.5 Use figure C2.3.1-9 0.5 < A/S < 2.5 Linear Interpolation A/S < 0.5 No shielding

A=U_mo T_app/2p (amplitude of oscillation) S=center to center spacing

Wave Load

•

(2) Wake Encounter Effects (Continued)

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Wave Load

•

(3 ) User defined coefficients of mass and drag

Input diameter verses coefficients of drag and mass. The program will linearly interpolate for intermediate sizes (CDM input line).

Wave Load

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Wave Load

Wave Kinematics Factor

Directional spreading of waves produces peak forces that are smaller than those of unidirectional seas.

The wave kinematics factor is given by :

where n is the exponent of the Cosnq _{spreading function at spectral peak}

frequency.

API Recommendations:

Kinematics Factor = 0.88 (hurricanes)

Kinematics Factor = 0.95 to 1.0 (extra-tropical storms)

• Note the Kinematics Factor multiplies the horizontal velocity and acceleration value of the wave.

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Current Load

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Current Profile

User defined current profile defined from mudline upwards. Current Stretching options include:

- constant - linear -nonlinear

User defined current blockage. Blockage calculated automatically using a reference elevation.

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Wind Load

•

Wind loads are calculated on all members above the mean water level as per API-RP2A guidelines.

•

Typically a wind load for a 5-sec gust, is considered for global loading on the decks.

•

For shallow water fixed platforms (i.e. jacket type structures) wind loads contribute less than 10% of the total load

.

Wind Load

•

Wind load criteria options available

API

ABS

Australian criteria

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Wind Load

API –RP2A 21st _{Edition Criterion API-RP2A 20}th _{Edition Criterion}

Gust effects Included Gust effects not included

Where: z = height t = gust duration

Wind Load

API –RP2A 21st _{Edition Criterion verses API-RP2A 20}th _Edition

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Wind Load

ABS Criterion Shape Coefficient Cs Beams 1.5 Cylinders 0.5 Sides of buildings 1.5 Overall Projected Areas 1.0

Wind Load

ABS 2000 Criterion Where: P = pressure z = height Cs = shape factor Ch = height coefficient

Vz = wind velocity at height z

Vref = wind velocity at reference height of 10m Zref = reference height of 10m

b = 0.9 – 0.16 for 1 min average wind

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Wind Load

Wind Load

•

Wind Load on Inclined Areas/Members

Where : p is pressure

A is the total area exposed to wind load in the direction of wind

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Wind Load

• Wind Areas

Wind areas or are defined to account for the wind loading on un-modeled items such as derricks, buildings, mechanical equipment, flare booms, etc.

A wind area is designated by a two character area identifier and consists of one or more surfaces defined using AREA input lines.

The orientation of the surface is specified either by entering the projections of it on planes normal to the global axis or by specifying the area along with the azimuth and elevation angles.

Wind Load

• Wind Areas

If more then one projected plane is specified for the same area identifier then the resultant area is used.

It is recommended that if an object has projected areas in two or three planes that two separate wind areas be defined rather than specifying two

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Wind Load

• Wind Areas

The surface shape may be designated as flat or round together with a corresponding shape factor.

The wind force components are calculated by multiplying the calculated wind pressure by the shape factor and the projected areas. The wind force is assumed to act at the specified centroid of the surface.

Wind Load

Wind Areas

The wind load is distributed over the specified number of joints.

If more than one joint distribution is specified, the program assumes that these joints are connected to a rigid body to which the wind force is applied. The load is distributed to each joint by assuming the rigid body is supported at each joint by three translational and three rotational springs.

The stiffness of the translational springs is unity while that of the rotational springs is 0.01 in the unit system the problem is defined.

Wind Shield Zones

By default, members located above the water surface receive wind loading. The program allows the specification of wind shield zones where members do not receive wind loading.

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Special Elements

SACS Special Elements

:

Wishbone Elements

Gap Elements : Compression Only Element Tension Only Element

No Load Element

User defined Load Deflection Element Friction Element

Special Elements

Wishbone Element:

Wishbone Element is a factious element connecting two coincident joints used to model special boundary conditions between

connecting structures.

6 inch

direction of offset

For example : Pile inside leg, conductor guide.

two coincident joints

member end release at one joint

10 0 1 1 1 x y z rx ry yz 1

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Special Elements

Compression only elements:

Compression only element can be used to model supports during load out where loss of contact may occur between the structure and the support due to uneven fabrication yard surface or motion of barge. Initial gap spacing can also be defined on the MEMB2 input line.

Special Elements

Tension only elements:

Tension only elements/ Cable elements can be used to model slings for a lift analysis in conjunction with moment member end releases. Pre

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Special Elements

No Load elements:

No load elements can be used to model tie downs for the pre

transportation analysis phase. The no load switch can then be turned off for the transportation analysis and the results from the two can

then be combined directly. Same model can be used for both analysis. No load elements can also be used for loadout analysis to model loss of support.

Special Elements

User defined load-deflection elements:

User defined load deflection elements can be used to define non-linear load deflection characteristics.

Many uses: Contact problems, suction pile behavior…etc

P

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Special Elements

Spring Elements

Any or all degrees of freedom of a joint may be designated as a translation or rotation elastic spring provided that the degree of freedom is designated as fixed (i.e. „1‟) on the respective joint description line.

When all three translational and/or rotational degrees of freedom are fixed, the support joint coordinate system may be redefined using two reference joints on the „Joint Elastic Support‟ line. The support joint local X-axis is

defined by the support joint and the first reference joint. The local XZ plane is defined by the support joint and the reference joints with the local Z-axis

Special Elements

Dented Members

Accounts for local indentation and overall deformation. The local dent and the overall deformation are in the direction of member local z direction. The

length of the dent is the length of the member or length of segment. The local z can be orientated in any direction using a chord angle or a reference joint.

Code check in accordance to modified API equations to account for reduction in cross section and overall deformation as per J.T. Loh paper “Ultimate

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Special Elements

Super elements:

A super element can be defined to be a portion of the structure which has been modeled and reduced down to a set of boundary joints in terms of a reduced stiffness matrix and reduced loads – also known as sub-structuring. Super elements can be useful where: The model is too large for analysis, where portions of the structure are repeated or for linearization of the

foundation.

Method:

The structure is broken down into two portions. The boundary elements on the substructure are defined by boundary conditions of 222222.

The same joints exist on the master model with no special boundary conditions.

The substructure is reduced using the Superelement module.

The super element is imported into the master model during analysis via the super element tab under the analysis options.

Post - Processing

•

Member Design

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API-WSD

–

API-LRFD

–

Norsok

–

Eurocode

–

Danish

–

British

–

Canadian

–

Linear Global (Section 17)

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Joint Design

–

API-WSD

–

API-LRFD

–

Norsok

–

Danish

–

Canadian

–

MSL

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Post - Processing

•

Element Code Check

K-Factors / Effective Buckling Lengths

• K-factors or effective buckling length, but not both, may be specified for

buckling about the local Y and Z axes. K-factors are specified on the pertinent GRUP line in columns but may be overridden on the MEMBER line in

columns.

• When factors are used, the effective buckling length is calculated as the K-factor multiplied by the actual member length. When effective lengths are specified on the MEMBER line, then the effective buckling length is

determined by multiplying the K factor from the GRUP line with the effective length value on the MEMBER line.

Post - Processing

•

Element Code Check

X Brace K-Factors

For X bracing the K factor for compression elements is 0.9 when one pair of members framing into the joint must be in tension if the joint is not braced out of plane.

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Post - Processing

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Element Code Check

K Brace K-Factors

For K bracing the K factor for compression elements is 0.8 when one pair of members framing into the joint must be in tension if the joint is not braced out of plane.

Post - Processing

•

Element Code Check Reduction Factor Cm

Cm can be based upon a constant value of 0.85, based upon end moments or axial load calculations or set to 1.0. The various options are defined on the GRUP line on column 47.

Alternatively enter „M‟ in column 34 of the OPTIONS line to exclude the value of the reduction factor Cm for combined axial compression and bending unity check, or enter „C‟ to globally set the value of Cb to 1.0

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Post - Processing

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Element Code Check Cb

The value for Cb for members with Compact or Non-compact Sections with

Unbraced length greater than Lb can be taken as 1.0 (default) or based upon end moment calculations as shown below by entering B in column 33 of the OPTIONS line.

Post - Processing

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Joint Can

API RP2A 21st _{Edition Supplement 2 guidelines implemented.}

Joints checked against API specified validity ranges.

Where validity ranges have been infringed, Joint Can will report the lesser capacity based upon actual geometry or the limiting dimension.

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Post - Processing

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Joint Can

Basic Capacity of joints without overlap is given by:

Strength Factor Q_u varies with the joint and load type (Table 4.3-1 API RP2A 21st _{Edition Supplement 2)}

Post - Processing

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Joint Can

Chord Load Factor Q_f

Values for C₁, C₂ and C₃ vary by joint type (Table 4.3-2)

FS = factor of safety P_c and M_c are axial and bending moment resultants in chord

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Post - Processing

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Joint Can

Joints with Thickened Cans

Lc is the chord length. Pa dependent upon chord length (BRCOVR) where : is the thickened can reduction factor

T_n is nominal chord member thickness T_c is the chord can thickness

(P_a)_c is axial allowable based upon chord geometric and material properties

Post - Processing

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Joint Can

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Post - Processing

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Joint Can

API assumes compression capacity is

limited by brace. Joint Can assumes Qu for compression is the same as for tension. Grouted Joints

The Q_f calculation for double skinned joints is based upon the chord thickness T With load sharing between the chord and inner tube accounted for.

Implementation to Overlapped Joints Currently under consideration

Ovalization failure capacity estimated by using effective formulation. T=chord thickness, Tp = Inner tube thickness

Post - Processing

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Joint Can

Mixed Class Joints

For mixed class joints the axial term in the interaction equation can be based upon either interpolation or ratio calculations.

Interpolation

Ratio

In which k, x and y are proportions of the

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PSI - Capabilities

Foundations can be modeled using two approaches:

(1) Adhesion (API + User defined) (2) P-Y, T-Z data (API + User

defined)

Adhesion – Linear (surface friction) P-Y, T-Z – Nonlinear load deflection curves.

PSI - Capabilities

Piles can be modeled as tubular or H sections.

P-d Effects accounted for.

Finite Difference approach used Mudslide condition simulation capabilities.

Creates equivalent linearzied foundation super-elements to be used by dynamic analyses in lieu of pile stubs.

Creates foundation solution file containing pile stresses to be used for fatigue analysis.

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PSI - Capabilities

• The Pile and Pile3D programs, which are sub-programs of PSI, may be executed alone to calculate the

behavior of a single pile. In addition to the features outlined above, the

• Pile program has the following features:

• Determines an equivalent pile stub that yields the same deflections and rotations as the soil/pile system.

PSI - Modeling

• Pile head Joint

• The interface joints between the linear structure and the nonlinear foundation must be designated in the SACS model by specifying the support condition „PILEHD‟ on the appropriate JOINT input line. NOTE: The „PILEHD‟ support condition represents fully fixed condition in lieu of a PSI analysis.

• Pile Local Coordinate System

The pile default local coordinate system is defined with the local X axis

pointing upward from the pile head joint along the pile axis defined by the pile batter joint or batter coordinates. By default, the local Y and Z axis orientations are load case dependent. For each load case, the local Y axis is automatically oriented such that it coincides with the direction of maximum pilehead

deflection.

The orientation of the local Y and Z axes may be overridden by the user by specifying the rotation angle about the local X axis in columns 51-56 on the PILE line

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PSI - Modeling

• Specifying Elevations for Soil Resistance Curves

• Within a soil stratum, the PSI program connects the input P-Y or T-Z points with straight lines to fully define the pile/soil interaction curve for arbitrary

displacements in that stratum. At depths between specified soil strata, PSI has the ability to linearly interpolate between curves or to use a constant T-Z

curve. Interpolation between different strata may be achieved by omitting the bottom of strata location.

PSI Solution Procedure (P-Y, T-Z)

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PSI Solution Process

Iterative Solution Procedures

Stiffness Table Approximation

(5%)

Fine Tune Solution

Pile Head Axial Force vs. Axial Deflection

F_ax(d)

d

Actual Solution

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Stiffness Table Approximation

• Approximate model of the pile head behavior

• Pile head forces are sampled for a range of points

• Linear interpolation between the points

• Reduction of computation time

• Improved chance of solution for highly non-linear problems

• Automatically generated (internal) … OR …

TABR lines

Excerpt from PSI Listing

File

*********************** TABR CARD IMAGES *******************

TABR AXIAL DF .0250 .10 PL1 SOL1 TABR DEFLECTN 0. 2.0 5.0 PL1 SOL1 TABR ROTATION -.01500. .0150 PL1 SOL1 TABR TORSION 0. 100.0 PL1 SOL1 TABR AXIAL DF -.0557.44430 PL2 SOL2 TABR DEFLECTN 0. 2.0 5.0 PL2 SOL2 TABR ROTATION -.01500. .0150 PL2 SOL2 TABR TORSION 0. 100.0 PL2 SOL2

TABR AXIAL LD -7500. 0. 7500. PL3 SOL3

cm/in rad kips-ft/kN-m cm/in kN/kips

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User-specified TABR lines

• PSI Listing File

Cut and paste into PSI Input File Manually refine using Datagen

• Single Pile Analyses (Pile, Pile3D) Generate SPA Data

Additional refinement as needed

Starting

Points

Non - Convergence

PSI Convergence Tolerances

Iterative Solution Procedures

Stiffness Table

Approximation

Fine Tune Solution

SOLUTION

Force Tol. (0.5%) Deflection Tol. (5%) Rotation Tol (0.0001) Deflection Tol (0.001)

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Convergence Report

** ITERATION DATA FOR LOAD CASE XXXX **

ITERATION RMS DEFLECTION RMS ROTATION

1 0.039673 0.000027

2 0.001083 0.000003

3 0.000070 0.000000

MAXIMUM PILEHEAD FORCE DIFFERENCE= 7.53085 % 4 0.022679 0.000026

MAXIMUM PILEHEAD FORCE DIFFERENCE= 7.67680 % 5 0.000626 0.000001

MAXIMUM PILEHEAD FORCE DIFFERENCE= 0.35047 %

Excerpt from PSI listing file

• Stiffness Table

Approximation

Trouble Shooting – A checklist

•

Review convergence report

•

If necessary, use TABR lines

•

Check tolerances and controls

•

Review soil data

•

Investigate each pile using Single Pile Analysis

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Future Developments

• Shallow Foundations Spud-can Foundations

Soil Plasticity Models (Collapse only)

• API RP 2A-WSD /21 Supplement 3

CPT Methods (loose soils, dense silt)

Solution Objectives (lateral)

F_z(d_z,θ)

θ Actual Solution

Stiffness Table Approximation

Lateral Pile Head Force vs.

Lateral Deflection and Rotation

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PSI/Pile Module

•

PSI Utilities

Plot Soil Data Plot Pile Capacity Plot Pile Load

Grouted Joints – Joint Can

The following technique is used to determine the internal loads of a leg.

1.The internal moment in the leg is determined by ratio the moment of inertia of the combined section to that of the leg only.

2. Similarly, the axial load in the leg is based upon the ratio of the combined section area to that of the leg only.

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Grouted Joints – Joint Can/Fatigue

The following methods are available for determining the effective thickness of a leg for joint can and fatigue analysis.

Grouted Joints – Joint Can/Fatigue

The following methods are available for determining the effective thickness of a leg for joint can and fatigue analysis.

2. Effective thickness based upon moment of inertia of the walls instead of composite section.

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Grouted Joints – Joint Can/Fatigue

The following methods are available for determining the effective thickness of a leg for joint can analysis.

Grouted Joints – Fatigue

The following methods are available for determining the effective thickness of a leg for fatigue analysis.

4. API RP2A 21st _{Edition Supplement 2 recommendations.}

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Ring Stiffened Joints – Fatigue

SCF‟s factored using Smedley and Fisher ring stiffened formulation.

The RING input line may be used to define the ring dimensions and the ring group.

The ring stiffened connection input line CONRST may be used to apply the rings to a particular brace and the location of the ring.

Modal Extraction : DYNPAC

• Some of the main features and capabilities of the DYNPAC MODULE are:

• Determines Natural Frequencies and modes of vibration.

• Accounts for structural mass and fluid added mass automatically

• Supports lumped or consistent mass generation

• Determines modal mass participation to allow determination

of number of modes required for subsequent forced dynamic analysis.

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Modal Extraction : DYNPAC

Analysis Procedure:

Linearize Foundation (Pile Superelement)

- identify load cases for pile linearization, load cases dependent upon type of analysis.

- include dead load

- run PSI to generate Pile superelement. Modal Analysis

- specify retained degrees of freedom.

- Identify load cases to be converted to mass. - check cumulative mass participation factors.

- check natural frequency and period (dynamic response low if period less that 2 seconds)

Load Path Dependent SCF’s

For any tubular connection, all braces that lie in a plane with the Chord or within 15 degrees of that plane are considered in the calculation of load path SCF‟s

The chord member is selected on the following hierarchy: 1. Largest diameter

2. Largest wall thickness 3. Highest Yield stress

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Load Path Dependent SCF’s

Joint Classification

KT-connection: Axial load in middle brace opposes axial load from outside brace. For a KT connection the load to be transferred is taken as the smallest value of:

1) Center brace axial load

2) Twice the axial load component normal to the chord.

The KT percentage for each brace is ratio of the transferred KT normal axial load component and the original axial load value.

The remaining axial loads are then considered for K joint load transfer.

Load Path Dependent SCF’s

Joint Classification

For a K joint the axial load component normal to the chord is balanced by the axial load component normal to the chord in other braces (on the same side of the

chord).

The brace with the smallest normal axial load component is considered first with the brace containing the largest opposing normal axial load component.

The balanced load is subtracted from the opposing brace and the process is repeated until all K joints are identified.

Any X joint load paths are considered next for braces on opposite sides of the chord. The largest opposing normal axial force is considered first. The balanced load is subtracted from opposing brace and the process is repeated until all X joints are identified. Braces with remaining unbalanced axial loads are treated T/Y joints.

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Load Path Dependent SCF’s

SCF Determination

The load path dependent SCF is calculated as a weighted average of the applicable KT, K, X and TY joints as:

SCF = R_KT*SCF_KT+ R_K*SCF_K + R_X*SCF_X + R_TY*SCF_TY

Importing FE Joint Mesh

Joint Meshing

Two approaches are available for importing meshed joints into a SACS „stick‟ model.

1. FEMGV

Precede can generate a FEMGV batch file once a joint has been isolated by

inserting a joint on the braces and chord members to define the portion of the joint that needs to be meshed. Precede will require the joint name, the number of

elements around the circumference of a brace with the smallest diameter and also the element type.

The batch file can then be subsequently read into FEMGV and the mesh is generated automatically.

FEMGV can generate a FEMGV neutral file which can be read back into Precede and the mesh can be incorperated into the rest of the model by tools provided in Precede.

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Importing FE Joint Mesh

Joint Meshing (Continued)

2. SACS Mesh Joint Utility

Very simple to use. Provide joint name to mesh, the mesh intensity ( limits 0.5 – 2, mesh intensity 1 = approx 28 nodes around the circumference of the smallest

brace) and the model file name.

The mesh utility will automatically mesh the joint and output a OCI file containing the „stick‟ model with the joint mesh incorporated.

FEMGV - Mesh Joint Utility

FEMGV allows the user to control the length of brace/chord to be meshed. Also gives choice of element types. Cannot mesh joints with overlapped braces.

Mesh Joint Utility allows the meshing of overlapped joints. No user control over the length of brace/chord member to be meshed. Meshing restricted to triangular palte elements (this is not a disadvantage).

Fatigue Analysis

•

Fatigue analysis is required due to the cyclic loading imposed

on the Jacket tubular joints by wave

loads.

•

Fatigue analysis could be of two types:



Deterministic Fatigue



Spectral Dynamic Fatigue

•

SCF for tubular joints are based on

parametric equations available for the

class of joint under consideration.

•

Number of cycles to failure N

_i

is

obtained from the S-N curve.

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Fatigue Analysis

•

In Deterministic Fatigue, discrete number of waves are used to characterize the total fatigue environment

•

Partial Damage from the sea state =

•

Damage is accumulated over all sea states (Miners Law):

• Deterministic analysis has been done for many years and has proven to be a reliable approach for dynamically insensitive structures, and for situations where all fatigue waves

are of sufficiently long wave periods to avoid peaks and valleys of the structures transfer function.

• Very sensitive to the waves chosen for the analysis

i i

N

n

...

N

n

N

n

N

n

N

n

3 3 2 2 1 1 i i

























i

D

Fatigue Analysis

• The spectral fatigue approach utilizes wave spectra and transfer functions, thus allowing the relationship of the ratio of

structural response to wave height as a function of wave frequency to be developed for the wave frequency range. Therefore spectral fatigue accounts for the actual distribution of energy over the entire wave frequency range.

•

In Dynamic Spectral Fatigue , Spectrum used to define the fatigue environment are:

•

JONSWAP

•

Ochi-Hubble

•

Pierson-Moskowitz

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Fatigue Analysis

•Fatigue program features are as below

• Includes a wide range Stress Concentration Factor (SCF) theories and allows user defined input.

• Automatic redesign of chords or braces may be done to determine required joint can or brace stub thickness

• API, AWS and NPD fatigue failure (S-N) curves are built into the program. Also allows user defined

input.

• Generates output for the Interactive Fatigue Module for Interactive redesign.

J182 J186-J182 TUB 45.7 1.0 K BRC 3.74 5.03 4.76 4.74 4.84 0.26334 B 94.93 J182 JF2-J182 TUB 55.9 2.5 K CHD 3.74 2.31 1.88 1.88 1.99 3.70E-03 T 6,763.51 Chord Len.(m) Out of Plane Service Life Stress Concentration Factors

OD (cm) WT (cm) Joint Type Member Type Original Dimensions

Joint Member Type ID Axial Inplane Damage Location

(Crown)

Axial (Saddle)

Dynamic Spectral Fatigue

Analysis Procedure:

Linearize Foundation

-choose load cases for developing foundation superelement

Modal Analysis to generate mass and mode files

- check cumulative mass participation factors

Run Wave Response to generate Transfer Function for each direction.

- use waves of constant steepness to generate transfer function. - avoid waves under 1 foot ( 30cm )

- check transfer function for overturning moment and base shear. - solve for equivalent static loads.

Run Fatigue

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Earthquake Analysis

Two approaches available:

1)Response Spectrum:

A response spectrum depicts the maximum response to a ground motion of a series of single degree of freedom oscillators having different natural periods but the same amount of internal damping.

2) Time History

Earthquake Analysis

Analysis Method:

1. All support points are assumed to be moving with the ground.

2. Each mode of vibration is assumed to act as a single degree of freedom. 3. Solve equations of motion for each mode.

4. The response from each mode for each direction (X, Y and Z) is combined using the SRSS (Square Root of the Sum of Squares) method to obtain the multi

directional response. The SRSS approach is used on the assumption that the responses from different directions are uncoupled

5. The response for each mode in each direction is also combined using the CQC (Completer Quadratic Combination) method. For the cases where there is sufficient modal separation in different directions the CQC method devolves into the SRSS approach.

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Earthquake Analysis

Analysis Method:

6. The dynamic response program creates a common solution file containing end forces, stresses, reactions and displacements. Because these results are obtained by combining modal results using RMS techniques, end forces, stresses…etc. have no sign associated with them and are taken as all positive values.

7. The dynamic response generates two sets of load cases for both the member check and the joint check.

8. The seismic results are then combined with the results from a static analysis. This is followed by element code check and joint can check.

Earthquake Analysis

Damping:

Damping effects are important and for structure immersed in fluid the damping is a nonlinear effect since damping is a function of the amplitude of response. Three options for damping available.

1.Linear modal damping. (API recommends overall modal damping of 5% (SDAMP line)

2. User defined amplitude to be used in fluid damping calculation (FDAMP line). 3. Program will calculate through iterative technique as follows (FDAMP line): (a)Calculate the response based upon an assumed amplitude.

(b) Calculate equivalent fluid damping based upon this response.

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Earthquake Analysis

Strength Requirements:

Zone Z 1 2 3 4 5

G 0.005 0.10 0.20 0.25 0.40

G is the ratio of the effective horizontal ground acceleration to gravitational acceleration.

Using the response spectrum, the ordinates of the spectrum should be multiplied by the factor G for the zone in which the platform is located.

The resulting spectrum should be applied equally along both principal horizontal axis and one half in the Vertical direction. All three spectra should be applied simultaneously and responses combined using CQC

Earthquake Analysis

Strength Requirements:

The strength requirements are intended ensure that no significant structural damage can occur due to a strength level earthquake.

For strength level earth quake both the member check and joint check allowables may be increased by 70 percent.

Tubular Joints

Joints for the primary structural members should be sized for either the tensile yield load or the compressive buckling load of the brace.

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Earthquake Analysis

Strength Requirements:

Tubular Joints – calculation of allowables.

The punching shear stress allowable, v_pa is :

The factor Q_f is given by :

In which the factor A is computed as :

Where f_AX, f_IPB and f_OPB are stress in the chord due to twice the strength level seismic loads in combination with gravity, buoyancy, hydrostatic pressure or or the full capacity of the chord away from the joint can – whichever is the less.

Earthquake Analysis

Strength Requirements:

Tubular Joints – calculation of unity check

For combined axial and bending stresses in the brace the following interaction equation should be satisfied:

For earthquake analysis the terms corresponding to bending are ignored since we are checking against the tensile yield loads or the compressive buckling load of the brace.

Joint can requirements for a earthquake analysis can be activated by specifying the EQK option on the JCNOPT line and also by specifying 2.0 for the joint

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Earthquake Analysis

Ductility Requirements

Rare Intense Earthquake:

In seismically active areas, rare intensive earthquake motion may involve inelastic action and structural damage may occur. The ductility requirements are intended to ensure that the structure and the foundation have enough reserve capacity to

prevent collapse in the event of a rare intense earthquake.

Equivalent Static Loads:

The Dynamic response module can output equivalent static loads corresponding to The modal responses being combined to generate the highest amount of base

Earthquake Analysis

Design Criteria:

Equivalent Static Loads:

The Dynamic response module can output equivalent static loads corresponding to the modal responses being combined to generate the highest amount of base

shear or overturning moment in 20 directions (every 18 degrees).

For rare intense earthquakes the equivalent static loads can be used to design the foundations and also conduct an elasto-plastic analysis of the structure to design against failure.

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Earthquake Analysis

Design Criteria:

Low-level Earthquake:

For areas where the ground acceleration is less than 0.05g no earthquake analysis is required. For areas where the ground acceleration is between 0.05g – 0.1g a low level earthquake analysis is required. The joint check requirements for a low level earthquake are the same as those for an in-place analysis.

The joint can requirements for a low-level earthquake analysis can be activated by specifying LLEW option for API working stress design , LLEL for API LRFD design on the JCNOPT line and also by using the DLOAD load line in the Joint can input file to identify the dead load case used in static analysis.

Spectral Wind Fatigue

Method:

1.Conduct modal extraction analysis – determine mode shapes and natural frequencies.

2.

2. Each mode of vibration is assumed to act as a single degree of freedom. 3.Determine Mechanical Transfer function H(f) for each mode.

3. 3. 3. 3. 3. 3.

where K_i is the generalized stiffness matrix, f_n is the natural frequency and c is percent damping.

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Spectral Wind Fatigue

Method (continued):

4. Determine

s

_RMS

response for each mode.

where S

_i

(f) is the Harris spectrum given by:

where L

_H

is the reference length (1800m), k is the surface roughness

coefficient (0.0025) and v

₁₀

is the wind speed at 10m reference height.

Spectral Wind Fatigue

Method (continued):

5. The response for each mode is combined to obtain the total response using the CQC (Completer Quadratic Combination) method.

where I and k refer to the ith _{and the k}th _{mode and P}

ik is the modal correlation

coefficient.

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Spectral Wind Fatigue

Method (continued):

6. Wind velocities are assumed to conform to the Weibull distribution.

Wind velocities are selected to define the Weibull distribution time slices and velocity ranges for integration limits to calculate fraction of occurrence.

Non-Linear Analysis : Collapse

• Salient Features of Collapse Module are

• Linear and non-linear material behavior

• Includes member Global / Local buckling including 8 or more hinge points per member

• Includes tubular joint flexibility, joint plasticity and joint failure due to excessive strain

• Includes strain hardening and residual stress

• Creates analysis results file which is read by Collapse View Program which shows failure progression and the

gradual plastification and collapse mechanism graphically

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Pushover Analysis

• Pushover Analysis conducted to determine the reserve strength ratio of a jacket

structure.

Loading applied to the structure in sequence. Apply all gravity loads first.

Apply environmental storm loading. Increase magnitude of environmental

loading until the structure fails.

RSR = Base Shear at 100% storm Load

Base Shear at Failure

Other approaches define failure with 100, 500, 1000, 5000,…year storms First Failure

 Ship Impact Analysis is carried out to determine the Reserve Strength in the structure after the collision.

 Vessel mass, added mass coefficient and velocity at the time of collision is specified on ENERGY line.

 Impact load case and impact joint defined on IMPACT input line.

 Run Collapse – impact load will be applied until all

kinetic energy is absorbed. The structure will then automatically unload and the post impact loading can be applied.

 SACS can optionally account for vessel

deformation. Local indentation energy can be accounted for by either meshing the impacted

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Dynamic Response

Force Driven Analysis

Force time history, Periodic and Engine vibration analyses are supported.

The main capabilities and features for force driven analysis are detailed below:

Force Time History

1. Linear, quadratic, or cubic interpolation available for the time history input.

2. Automatic load case selection based on overturning moment, base shear, joint displacement, etc.

3. Variable time step integration procedure.

4. Time history plots including modal responses, overturning moments, base shear, etc.

Blast Analysis

 Large Deflection, Elasto-Plastic Non- Linear Finite Element Analysis is performed in SACS for Blast loads.

 Blast Resistant Design to minimize the risk to people and facilities from the hazards of

accidental explosions.

 Dynamic Response Module can be used to apply blast load profile to the structure at discrete time steps.

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Blast Analysis (continued)

 Dynamic Response will generate a structural output file containing incremental loading (including dynamic and static components).

 Dynamic Response will also output a load sequence file for Collapse.

 Run Collapse for non-linear elasto-plastic analysis using incremental loading.

 Collapse shows plasticity over time with each load step representing a time increment.

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Dynamic Ship Impact

 Use Dynamic Response module to determine dynamic structural response due to impact.

 Set analysis option to SHIP on the DROPT line.

 Define vessel mass, velocity, direction of motion and the impact joint on the SHIP input line.

 Define the time history source as SHIP on the time history load line THLOD.

 Prepare Collapse input with control parameters and load sequence for dead loads.

 Run dynamic response.

 Dynamic will output Collapse load sequence and incremental loading containing dynamic and static components of the structural

response

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Dynamic Response

Earthquake/Base Driven Analysis

Both spectral earthquake and time history earthquake analyses are supported. Some of the seismic analysis capabilities follow:

Spectral Earthquake

1. API response spectra are built into the program. 2. Supports user defined response spectra.

3. Spectral motion can be described as acceleration, velocity, or displacement. 4. Modal combinations using linear, SRSS, peak plus SRSS, or CQC methods. 5. Ability to use a different response spectrum for each direction.

Dynamic Response

Earthquake/Base Driven Analysis

Time History Earthquake

1. Includes earthquake time history libraries. 2. User defined input time histories.

3. Linear, quadratic, or cubic interpolation available for the time history input. 4. Variable time step integration procedure.

5. Automatic load case selection based on overturning moment, base shear, etc. 6. Graphical representation of output variables.

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Dynamic Response

Engine/Compressor Vibration

1.Supports mechanical unbalanced forces and gas torques in addition to reciprocating loads.

2. Linear and/or nonlinear interpolation of forces between running speeds. 3. User can select specific joints to monitor or monitor all joints.

4. Allows user defined phasing of forces and moments within a load case. 5. Can automatically combine maximum response of various load cases. 6. Generates plots of input data versus time for any load case.

Dynamic Response

Spectral Wind Analysis

The wind spectral fatigue and extreme wind analyses are supported. Some of the spectral wind analysis capabilities are as follows:

Extreme Wind

1. Determines dynamic amplification factors automatically.

2. Generates common solution file containing internal loads, stresses, reactions and displacements multiplied by its own dynamic amplification factor.

3. Includes cross correlation of modal responses using the Complete Quadratic Combination (CQC) modal combination technique.

4. Plots generalized force spectrum and response spectrum for each wind speed. 5. Uses Harris Wind spectrum.

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Dynamic Response

Spectral Wind Analysis

The wind spectral fatigue and extreme wind analyses are supported. Some of the spectral wind analysis capabilities are as follows:

Wind Fatigue

1. Uses Harris Wind spectrum.

2. Optionally creates Fatigue input file automatically. 3. Distributes wind speed utilizing a Weibull distribution. 4. Assumes Rayleigh distribution of RMS stresses.

Dynamic Response

Ice Force Analysis Ice Vibration

The ice vibration analysis capability includes the following features: 1. Automatically includes ice stiffness.

2. Maximum and minimum peak selection. 3. Automatic cycle count for fatigue analyses. 4. Creates fatigue input data automatically.

5. Full plot capabilities including ice forces, modal responses, overturning moments, base shear, etc.

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SACS

Engineering Dynamics

2113 38

th

_Street

Kenner

LA 70065

USA

Telephone: (504) 443 5481

www.sacs-edi.com