COMPUTERS AND STRUCTURES, INC., MAY 2011 AUTOMATIC WAVE LOADS TECHNICAL NOTE WAVE LOAD OVERVIEW

Transcription

1 COMPUTERS AND STRUCTURES, INC., MAY 2011 AUTOMATIC WAVE LOADS TECHNICAL NOTE WAVE LOAD OVERVIEW This Technical Note presents a brief overview of the automatic wave loading features in the program. The automatic wave load calculations in the program are based on the requirements presented in the American Petroleum Institute 2000 reference (API 2000). Define Wave Loads The automatic wave load is a special type of load pattern. It generates loads on the structure resulting from waves, current flow, buoyancy, and wind. The automatic wave load is defined using the Define menu > Load Patterns command in SAP2000 or the Loads > Load Patterns command in CSiBridge to open the Define Load Patterns form. In the form the load type is set to Wave and the Auto Lateral Load is set to something other than None. After this load has been added to the list, click the Modify Lateral Load Pattern button to display a form for defining the wave loading parameters. A wave load can be specified to act in any direction. For a given wave direction, any number of crest positions can be considered in a single load pattern. Wave velocity and acceleration fields can be automatically generated using Airy (linear), Stokes or Cnoidal wave theories or, alternatively, they can be directly input by the user. When using wave theories, the user specifies a wave height, wave period (as viewed by a stationary observer), and a storm water depth. Current velocity and direction, marine growth, drag coefficients and inertia (mass) coefficients can all be specified as a function of water depth. The current direction can differ from the wave direction. Wind load, acting in any direction, can also be specified as a part of the wave load. The wind load acts on the portion of the structure that is above the wave surface. Define Wave Loads Page 1 of 6

2 Several wave loading parameters can be overwritten on an object-by-object basis. The wave parameters are overwritten by selecting the desired objects and clicking the Assign menu > Frame Loads > Auto Wave Loading Parameters command in SAP2000 or the Advanced > Assign Loads > Frames > Auto Wave Loading Parameters command in CSiBridge. Refer to Defining Wave Loads for more detailed information. Calculate Wave Load Values The wave velocity and acceleration fields are user-defined or they are calculated from the storm water depth, wave height and apparent wave period using Airy (linear), Stokes, or Cnoidal wave theory. The Airy and Cnoidal theories are documented in Hallam, Heaf and Wootton The Stokes theory is documented in Skjelbreia and Hendrickson If the wave load is calculated from theory, the apparent wave period used in the calculation is determined from the actual wave period and the free-stream current velocity (not reduced by blockage) in the direction of the wave. The horizontal components of the wave velocity and acceleration fields are multiplied by a wave kinematics factor that is intended to account for direction spreading and irregularity of the wave profile. Current velocity multiplied times the current blockage factor is vectorially combined with the wave velocity field that has been adjusted by the wave kinematics factor. The wave force acting on the member is calculated using Morison s equation. The size of a member (area and volume) used to calculate the wave load force is based on the section assignment, the specified marine growth and any overwrite of the section dimension or marine growth. The wave loads are applied as distributed loads on the portion of the structure that is below the wave surface and above the mud line. Buoyant forces are calculated using a rational method. They consist of projected vertical upward distributed loads acting along all members that are not vertical and concentrated forces at each end of each frame object that is submerged. Calculate Wave Load Values Page 2 of 6

3 The wave wind load acting on a member is calculated using a specified onehour mean wind speed, a shape coefficient (which can be overwritten), the mass density of air, and the member area. The member area used to calculate the wave wind force is based on the section assignment, the specified marine growth and any overwrite of the section dimension, marine growth or ice thickness. If both marine growth and ice thickness are specified at a location, only the ice thickness is considered. The wave wind loads are applied as distributed loads on the portion of the structure that is above the wave surface. Refer to Calculation of Wave Load Values for more detailed information. View Wave Load Input Data Numerous tools are provided to review wave load input data. Wave Plot While defining the wave load parameters you can display a wave plot using the current parameters. This plot displays the wave particle velocities, accelerations and pressures in many different formats. Contour plots and horizontal and vertical section cut plots through the wave can be displayed. Wave particle velocities, accelerations and pressures can be shown at the mouse pointer location. Refer to Wave Display Plots and Tables for more detailed information. Wave Table While defining the wave load parameters you can display a wave table using the current parameters. This table displays the wave particle velocities, accelerations and pressures at points throughout the wave. The data shown in this table is also available in the model definition tables discussed later in this Technical Note. If you want to copy or print the data in this table, do so from the model definition tables. Refer to Wave Display Plots and Tables for more detailed information. On Screen Display of Wave Loads After the wave load has been defined using the Load Patterns form, the resulting generated loads applied to the model can be displayed on the screen. Click the Display menu > Show Load Assigns > Frame command in SAP2000 or the Home > Display > More > Show Load Assignments > Frame/Cable/Tendon command in CSiBridge to bring up the Show Frame View Wave Load Input Data Page 3 of 6

4 Loads form. Select a load pattern that has an automatic wave load assigned to it and select the Span Wave Loads load type option. When the wave loads are displayed on the screen, left and right arrow buttons appear on the right side of the status bar. Click those arrow buttons to step through the wave crest positions of the wave load. A Start Animation button on the right-hand side of the status bar causes the program to automatically step through the different wave crest positions. The speed of the animation is controlled by the slide bar to the left of the Start Animation button. The wave load forces displayed in this plot are only those automatically generated by the program. Any additional loads that the user may have assigned to the same load pattern are not included in this plot. Typically we do not recommend that you assign other loads to a load pattern that includes automatically generated loads. Create Videos of Wave Loads After the wave load has been defined using the Load Patterns form, a video can be created of the resulting generated loads applied to the model. Click the File menu > Create Video > Create Multi-Step Animation Video command in SAP2000 or the Orb > Pictures > Create Multi-Step Animation Video command in CSiBridge to bring up the Multi-Step Video File Creation form. Select the Multi-Step Load plot type, select a wave load pattern, set the other video parameters as desired and click the OK button to create the video AVI file. Wave Load Data in Model Definition Tables Wave load data in the model definition tables can be displayed, printed, interactively edited, exported and imported. Any of the following commands give you access to the model definition tables in SAP2000, File menu > Print Tables Display menu > Show Tables Edit menu > Interactive Database Editing File menu > Export > SAP2000 MS Access Database.mdb File File menu > Export > SAP2000 MS Excel Spreadsheet.xls File File menu > Export > SAP2000.s2k Text File File menu > Import > SAP2000 MS Access Database.mdb File View Wave Load Input Data Page 4 of 6

5 File menu > Import > SAP2000 MS Excel Spreadsheet.xls File File menu > Import > SAP2000.s2k Text File and in CSiBridge. Orb > Print > Print Tables Home > Display > Show Tables button Advanced > Edit > More > Interactive Database Editing Orb > Export > Access Orb > Export > Excel Orb > Export > Text Orb > Import > Access Orb > Import > Excel Orb > Import > Text Each of those commands, except the import commands, brings up a form where you can select the tables to be displayed. The forms have different titles for the various commands but otherwise look essentially the same. You can select check boxes to indicate the tables you want printed, displayed or exported. The check boxes that relate to wave loading are the Auto Wave Loads check box in the Loads area of the form (on the Definitions tab) and the Wave Overwrites check box in the Frame/Cable Assigns area of the form (on the Assignments tab). Analyze using Wave Loads When a load pattern containing an automatic wave load is specified in a load case, the characteristics of the load depend on the type of load case. Multi-Step Static Load Case For multi-step static load cases, each step (wave crest position) of the wave load corresponds to a step in the multi-step static load case. Time History Load Case For modal and direct time history cases, the wave load is applied as a sequential series of overlapping triangular pulse loads. Each pulse load corresponds to a step (wave crest position) of the wave load. The duration of each half-pulse load is equal to the apparent wave period (as seen by an observer moving with the current, if any) divided by the number of wave crest positions specified for the wave load. Analyze using Wave Loads Page 5 of 6

6 The maximum value of pulse load n is equal to the full wave load value calculated for step n (wave crest position n). Pulse load n starts from zero at the time pulse load n 1 is at a maximum and returns to zero at a time that pulse load n+1 is at a maximum. The figure to the right shows pulses associated with steps n 1, n and n+1. Step n-1 Step n Step n+1 For time history load cases with wave loads, the function specified for the load case is ignored by the program. Ritz-Type Modal Load Case If a wave load is specified as a load for a ritz-type modal load case, loads (starting vectors) are created for each step (crest position) of the wave load. Other Load Cases For load cases that are not modal (ritz), multi-step static or time history load cases, the wave load is not considered. Viewing Wave Load Analysis Output Data Wave load analysis output is viewed in the same ways as all other analysis output in the program. References American Petroleum Institute (API) Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms Working Stress Design; API Recommended Practice 2A-WSD (RP 2A-WSD). Twenty-First Edition. December. Hallam M.G., N.J. Heaf, and L.R. Wootton Dynamics of Marine Structures: Methods of calculating the dynamic response of fixed structures subject to wave and current action, Report UR 8, Second Edition. CIRIA Underwater Engineering Group. London. October. Skjelbreia, L., and J. Hendrickson Fifth Order Gravity Wave Theory. Seventh Conference on Coastal Engineering. ASCE Vol. 1, Chapter 10. References Page 6 of 6