sacs basics

SACS Engineering Dynamics, Inc

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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:

Cnoidal Theory

Solitary Wave Theory

Stokes 5th Order Theory

Stream Function 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,

loading.

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

D is the diameter, U is the velocity , is the fluid

density and A is the area.

UACUUD0.5CF MD

Drag force Inertia force

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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

Cd and Cm.

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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

D = Member diameter

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Wave Load


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

t = average thickness of marine growth

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Wave Load


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

coefficient.

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Wave Load


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


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=Umo Tapp/2 (amplitude of oscillation)

S=center to center spacing

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Wave Load


Surface roughness input with the marine growth data (MGROV input line).

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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).

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Wave Load

(4) Default table for clean and fouled members

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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 Cosn 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

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.

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Wind Load

Wind load criteria options available

API

ABS

Australian criteria

Cyclonic or Non-Cyclonic criteria

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Wind Load

API RP2A 21st Edition Criterion API-RP2A 20th Edition Criterion

Gust effects Included Gust effects not included

Where: z = height t = gust duration

Uo = one hour wind speed at reference height of 10 meters (32.8 ft)

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Wind Load

API RP2A 21st Edition Criterion verses API-RP2A 20th Edition

Criterion

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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

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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

= 0.9 0.16 for 1 min average wind

= 0.125 for 1 hour average wind

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Wind Load Wind Load on Members

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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

is the angle between the direction of the wind and the axis of the

member (or plane of surface)

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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.

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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

projections together.

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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.

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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.

Wind shield zones are defined by specifying the bottom and top elevation of

the zone. Elevations are defined using global z elevation.

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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

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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.

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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

tension can be defined on the MEMB2 input line.

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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.

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Special Elements User defined load-deflection elements:

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

linear load deflection characteristics.

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

When all three translational and/or rotational degrees of freedom are fixed,

the support joint coordinate system may be redefined using two reference

-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

perpendicular to the local X axis.

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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

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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.

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Post - Processing

Member Design

API-WSD

API-LRFD

Norsok

Eurocode

Danish

British

Canadian

Linear Global (Section 17)

Joint Design

API-WSD

API-LRFD

Norsok

Danish

Canadian

MSL

Linear Global (Section 17)

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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 K-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.

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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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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.

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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.

the reduction factor Cm for combined axial compression and bending unity check,

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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.

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Post - Processing 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.

Joint capacities dependant of joint classification (i.e. K, X and Y)

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Basic Capacity of joints without overlap is given by:

Strength Factor Qu varies with the joint and load type (Table 4.3-1 API RP2A

21st Edition Supplement 2)

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Chord Load Factor Qf

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

FS = factor of safety

Pc and Mc are axial

and bending moment

resultants in chord

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Joints with Thickened Cans

Lc is the chord length. Pa dependent upon chord length (BRCOVR)

where : is the thickened can reduction factor

Tn is nominal chord member thickness

Tc is the chord can thickness

(Pa)c is axial allowable based upon

chord geometric and material properties

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Strength Check Interaction Ratio

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API assumes compression capacity is

limited by brace. Joint Can assumes Qu for

compression is the same as for tension.

Grouted Joints

The Qf 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

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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

classification

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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.

.

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PSI - Capabilities

Piles can be modeled as tubular or H

sections.

P- 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.

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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

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.

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

The jacket structure is initially reduced to a super element at each pile head.

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PSI Solution Process

Iterative Solution Procedures

Stiffness Table Approximation

(5%)

Fine Tune Solution

SOLUTION

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Pile Head Axial Force vs. Axial Deflection

Fax(d)

d

Actual Solution


Solution Objectives

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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

User-specified with the TABR line

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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

Axial Adhesion Model

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

Alternative Method to refine TABR data

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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


5 0.000626 0.000001


Excerpt from PSI listing file

Stiffness Table

Approximation

Fine Tune Solution

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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

Fully constrain the pile heads and run SACS

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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)

Scour Depth Guidelines

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Solution Objectives (lateral)

Fz(dz, )

dz

Actual Solution


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

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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.

1. Effective thickness based upon moment of inertia of composite section

sacs basics

Documents

wave velocity

wave loading

wave height

wave motion

spacing edi wave load

wave loads

wave theories

reduced wave