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Frequently asked questions

Below are some of the most frequently asked questions. In case you don't find the answer to your problem and need some help, please check see the Getting help section.

How can I check if the rotations imposed to my floater are correct ?

The pitch, roll and yaw are of course properly imposed to any floater using RAO or other prescribed displacements, though at this time it is not dead simple to check through the graphical interface. The main reason is DeepLines outputs pseudo-rotations instead of actual angles. Pseudo-rotations are used within DeepLines finite beam element formulation to deal as accurately as possible with non-commutative 3D large rotations. Basically, pseudo-rotations allow to define the rotation matrix, which in turn is used to get the local coordinate system from the global one - more details are available in the Theory Manual.

If only one rotation were imposed to the floater, pseudo-rotations would match the actual one, that is RAO amplitude times the wave elevation. For the general case with multiple rotations, one way to check the floater motion is to define a vertical rigid bar clamped at the floater COG, and run the simulation again. Once the simulation is finished, you can get the vertical angle at the bar clamped end, which equals that of the floater local vertical axis.

Can I run some post-processing in batch ?

Post-processing can be performed in batch mode using the Batch Processing user interface. Batch post-processing capabilities include extraction of any type of outputs such as detailed plots and results statistics associated with series of analyses, Zones Study post-processing, fatigue damage analysis and lines clearance checks.

What do I do if the GUI suddenly closes as I open my .DSS file ?

In a particular situation and depending on your hardware configuration, the GUI could close suddenly when you open your .DSS file and move the mouse over the 3D view window. The trouble is basically due to possible incompatibility between DeepLines and some graphics drivers. This problem has been fixed for most of the hardware configurations. Should it persists however, then here is a simple workaround that prevents such troubles to occur. Open the Display properties window from the Windows settings and press the Advanced button on the Parameters tab. Then, in the Troubleshooting tab, set Hardware acceleration to zero. Finally, re-open your .DSS file. Note that this operation is perfectly safe and has no effect at all on the processor performance.

An error message displays in the GUI when running my analysis. What's next ?

More detailed information about the errors may be found in the listing file (.LIS) associated to your analysis. This file is placed in the sub-folder that contains all the results for your analysis. Use Windows explorer to open this file with a text editor such as NotePad or WordPad and search for 'error' occurrences. Note that error messages sometimes display below the 'No error found' statement.

What is the maximum acceptable aspect ratio for two adjacent beam elements ?

It is recommended that the aspect ratio of two adjacent beam elements does not exceed about 1.5. Suppose that the first beam element at the top riser is 1m long. Then to ensure maximum numerical accuracy, the second element length shall not exceed 1.5m. Creating a finite elements mesh suitable to your analysis is not always an easy task, and should therefore be done step by step. Using needlessly fine meshes is obviously CPU time consuming. Therefore, start first with a relatively coarse mesh and run a simple static analysis to identify high loading areas that will require refined meshing. Where needed, refine the mesh adequately, and generate intermediate transition zones so that the maximum aspect ratio does not exceed the acceptable limit.

Why is the top tension altered in the top element when using structural damping ? Is there a workaround ?

On special instances, the top tension may be altered when using structural damping in your line. This is basically due to a boundary effect within the algorithm which computes the internal efforts induced by damping. This may occur in case the motion of the node is directly imposed - for instance for a node directly linked with a support vessel. The simplest workaround is to use stiff spring connections rather than pins, or stiff flexjoints rather than clamps. This will add one intermediate node (having the same coordinates than the top node) between the line end and the support vessel, which will prevent from any boundary effect and altered top tension.

Can I automate post-processing using in-house made macros ?

Yes. Most of the time, post-processing is run from the GUI, which allows you to export all your results into Excel spreadsheets by a single click with the mouse. Then you can create any Excel macro you like for further data processing. More complicated, you can also run PoDeep post-processor from Excel macros, which means the elimination to ask for the result wanted via the GUI, but can drive all the process from Excel. Doing so would require generating adequate PoDeep input files and read PoDeep results files using Excel. This allows to automate repetitive post-processing tasks in a very efficient manner and save valuable time.

How does DeepLines get RAOs out from irregular waves runs ?

DeepLines can produce RAO of motion, tension or stress from both regular and irregular waves runs. This latter facility is of particular interest when performing coupled analyses with irregular waves, as a single run based on a representative sea-state is enough to derive the complete RAO. Suppose that you wish to get the RAO of tension at a mooring line connection point. The time history of wave elevation and tension is firstly used to calculate both related correlation functions. Then DeepLines computes the co-spectrum Stw and the wave elevation auto-spectrum Sww using a discrete Fourier transform. The linear complex transfer function is actually the ratio of these spectra. Finally, complex values are then interpreted in terms of RAO amplitude and phase.

Is a license required to open .DSS files for demonstration on a laptop computer ?

Yes. Opening a simulation file (.DSS) requires a license with the key DEEP_IHM key to be installed on your laptop or available through your local network. Should you need to post-process results from the binary results files, then you must have the DEEP_POST key activated as well. More information about license files may be found here.

What do I need to run a coupled hydrodynamic analysis ?

Simple uncoupled analyses are based on imposed motion at the floaters COG, either through RAO or more complex trajectories. Coupled analyses require that the floaters properties and hydrodynamic forces acting on it are input to DeepLines solver, as a function of wave incidence and frequency. The coupled analysis facility is part of DeepLines basic installation, so there is no need to purchase any optional module. In fact the only thing you really need is an hydrodynamic database .HDB text file with special formatting. Such a file can be created by various marine hydrodynamic software packages, such as Principia Diodore for instance.

What does the apparent weight includes exactly ?

The apparent weight parameter is a convenient way to impose the weight per unit length for your pipe. When this option is activated, the external force due to gravity in the right hand side term of the equation of motion is directly set to the value you have specified. The apparent weight must include the contribution from the mass of pipe in water empty, plus that of the internal fluid when relevant : when it is used, the value must be modified depending on the internal fluid density.

What is the additional mass parameter placed in the rigid pipe type dialog ?

This parameter can be used to include the contribution any additional layer such as coating, insulation, etc... to the inertia and weight. In other words, this is the mass of the non-steel part in the pipe.

How does the restart facility actually works ?

The restart facility allows to run an analysis based on initial nodes positions obtained during a previous quasi-static or dynamic analysis. This facility should be thought of as 'Read initial position of nodes from...' rather than a real restart option which would also keep the environmental loadings unchanged for instance. Using the restart feature require that the number of nodes and number of elements are identical for the initial analysis that will serve as a starting point and the new analysis.

How can I model simple spring elements ?

Spring elements are normally available to connect any primary point in a line with another object - such as a floater. When necessary, springs may be modelled and included to your model by using simple bar elements with adequate properties. The axial stiffness (EA) of the bar must be defined as EA = K*L where K is the spring stiffness and L its length. The bar must also be given null mass and apparent weights.

Which diameter is used to compute the buoyancy force for Cable/Chain segment types ?

No buoyancy diameter can be directly input through the GUI for Cable/Chain segment types. The buoyancy force is automatically computed from the seawater density and the volume based on the hydraulic diameter. It is worth pointing out that for such structures, you may easily use the apparent weight since no internal fluid at all is considered with such lines.

How does the FE segmentation affect the natural modes ?

Natural modes shapes and frequencies are sensitive to the number of finite elements in the line. For instance, a minimum of three elements between two consecutive vibration nodes are necessary to properly capture a bending mode. Therefore, the higher the mode number, the more refined your segmentation must be. You can check the impact of different segmentations by plotting the curves that give the modal frequency as a function of the mode number.

When looking at the clearance between two lines, DeepLines reports negative values. How is this possible ?

Though the expected values should always be positive, this situation may occur when asking for the absolute gaps between two lines, while taking the external diameter into account. The absolute gap between mean lines always remains positive, whereas the gap between external diameters is computed as follows :

Gap_DiameterAccounted = Gap_MeanLines - (Diam1 + Diam2)/2

Therefore negative or zero values for the gap between lines with external diameter accounted indicate that the two lines would actually clash.

The diameters used in the above formula are the buoyancy diameters input from the segment type window.

Is a special formatting required when inputing the RAOs data from the GUI ?

Yes. You may enter RAO data for different headings, but must ensure that the wave periods are similar for all the headings. This means the same number of lines in the grid and the same values in the first columns. This format was not required within previous versions of DeepLines, and has become compulsory since the systematic use of Hydrodynamic database files.

Are the damping coefficients for the floater defined in the .HDB file ?

No. When defining a coupled analysis, all the damping coefficient (linear and quadratic 6x6 matrices) must be defined separately using the Floater's motion tab in the GUI. Alternatively, artificial beams can also be attached to the floater so as to simulate the viscous effects - appropriate drag coefficients must however be specified. In that case, the relative fluid/structure velocity will be considered.