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What was new in Version 4.2

Technical enhancements

Frequency domain dynamic analysis : In addition to the standard time-domain approach based on Newmarks implicit integration scheme, dynamic analyses can now be carried out in the frequency domain. Linearization of drag forces exerted along the lines allows to re-write the equations of motion in the frequency domain, which can be solved at a fraction of cost of the traditional time-domain form. This alternative approach is widely used for structures such as drilling risers and has proven to give reliable results for many other structures. The frequency domain analysis feature included in the GUI opens new horizons when it comes to screening studies for extreme analyses or fatigue analyses based on Rain-Flow counting methods, or complex coupled analyses with export lines and CALM buoys for instance. Any dynamic analysis defined in the GUI can be run in the frequency domain and each model components is compatible with this technique.

Pipe/soil interaction and suction effect : Pipe/soil interaction models are of primary importance to properly estimate the fatigue life or the maximum stress in the touch-down area. Experience shows that the apparent soil stiffness varies for permanent and cyclic loads. The normal reaction force model has therefore been improved to account for different soil stiffness values in static and dynamic conditions. Both static and dynamic stiffness values can be either imposed or automatically derived from pipe and soil characteristics using published models such as DNV Guidelines. Suction and trenching effects may also be included in the dynamic analyses to better predict the additional bending stress caused by this phenomenon.

Time-dependant internal pressure and temperature loads : Dynamic internal pressure, temperature and internal fluid density profiles may now be defined along any line. True tension and axial stress including thermal elongation effects will be directly impacted by the pressure and temperature variations. Creating time-dependant internal fluid contents with the GUI allows for instance to define cyclic P&T loads and thereby simulate lateral buckling or pipeline walking arising from consecutive shut-down and restart operations. Slugs may also be modeled this way through dynamic pressure and internal fluid density profiles. A more complex slug model is also available into the solver (see *INFLOW) but most of our tests showed that the driving physical parameters were properly caught by apparent weight and pressure variations only.

Non linear bending stiffeners : Bending stiffeners used to avoid over-bending at flexible risers connections were commonly modelled as equivalent elements which properties were automatically derived from the bend stiffener shape and material characteristics. These equivalent bend stiffeners elements were however restricted to linear material laws and contact between the pipe and the stiffener was not actually modelled. The interaction between the flexible pipe and the inside of the bend stiffener may now be accurately handled through contact elements. Non-linear elastic relationships between elongation and tensile stress as can be observed for polyurethane materials can now be directly specified from the bend-stiffener definition window, which leads to more realistic prediction of the pipe curvature inside the stiffener.

Definition of springs : Springs were previously available in the solver only and in the GUI as boundary conditions. These components can now be defined within the GUI to serve as links between any pair of model components present in the model. Springs can be given both linear or multi-linear stiffness laws and initial length at zero tension.

Definition of pulleys : A new pulley element has been added into the GUI and the solver, which allows to deflect tension loads. This element consists in a master node that can be attached to any existing object, and two cable strands linked to other lines or vessels. Pulleys can for instance be combined with the new simple winch component to simulate S-lay operations.

Simplified winch model : The simple winch model allows to perform dynamic installation analyses of risers and pipelines. It may also be used to simulate lay down of heavy equipment. This new winch component can be seen as a stiff spring between two nodes with time-dependant reference length, and no cable is modelled by default. The simple winch component is available within the GUI from the spring object definition window. Pay-out or pay-in rates can be either constant or user-defined over the time. A more complex winch model is also available into the solver (see *LAYING) but is not accessible through the GUI yet.

Quick data access: The quick data access is a new module of the GUI that displays most of the model data in tabular forms. This provides a very convenient way to check or modify these data. For instance, users may now edit all the line segment types, connections between objects, analyses calculation parameters, from the same window. These tables can be exported or imported from Excel.

Improved restart facility : The restart option was solely based on the nodes positions. This has been improved so that the restart actually behaves as a replay of the former calculation until the new calculation begins. This ensures full compatibility of external loads like incremental offsets, currents and wave phases between the initial calculation and the new one.

User-defined line section shapes : Line sections could be given different default shapes when setting-up the model such as catenary, straight, upper and lower arcs. There is now an option to specify user-defined section shapes with all nodes coordinates entered in a dedicated ASCII file. These user-defined shapes will allow to arrange the line along known trajectories like pipeline routes on the seabed, or complex shapes with initial bends. This option may be combined with initial bends to create rigid spools for which the unstressed configuration is not straight. Rigid spools installation and in-place analysis including seabed contact and thermal loads can now be efficiently performed.

New formulation for hydrodynamic damping on CALM buoys : The CALM buoy object edit window in the GUI has been enhanced to ease the definition of hydrodynamic damping. The new formulation is based on stacks of cylinders placed on the buoy body and skirt circumference which properties can be specified from the GUI. The definition of this hydrodynamic damping is based on the second phase of the CALM buoy JIP and allows to combine the advantages of the absolute and relative flow velocities formulations to better estimate the motions of CALM buoys within coupled dynamic analyses.

Trelline OOL modelling capability : The Trelline is an export line system composed of bounded flexible hoses for which no simple relationship exists to derive the stress in the different layers from the tension, bending moment, internal and external pressure. A new hose type has been included in the line segment types for which the stress in several components can be interpolated from pre-existing proprietary Excel databases, depending on the tension, bending moment and pressure differential. This particular hose type allows for example to calculate the fatigue damage of Trelline export lines directly within the GUI.

Hydrodynamic database files viewer : The contents of the hydrodynamic database files including RAOs amplitude and phase can now be plotted in the GUI, which allows simple comparison between several hydrodynamic databases and data checks. The hydrodynamic database files viewer is available from HDB Viewer icon in the main tools bar and from the vessel properties definition window.

Non linear flexjoints : Flexjoint elements used to model end connections of rigid pipes used to be given either a constant stiffness or multi-linear elastic stiffness values. These components can now also follow non-linear elasto-plastic like behavior laws, with variable stiffness according to the angle amplitude. Associated parameters may be specified in the new FLEJOINT and FLEXPROADD keywords. Resulting hystereric effects can therefore be simulated, which provides more realistic stress in the pipe welds close to the flexjoint connection.

Import of external flow computed with CFD programs : This new feature has been designed for situations where the flow acting on a pipe is significantly impacted by other structures located in its vicinity, for instance close to the hull of a vessel or in the wake of a towed fish. In these cases, the standard approach based on undisturbed current and wave flows would fail to properly estimate the Morison forces along the pipe. This can be now corrected by calculating the flow using an external CFD computer program and then importing all associated results in an ASCII file using the new *FLOWCFD keyword.

Van Der Pol oscillator model for VIV prediction : VIV prediction models based on the modal decomposition approach are not appropriate to calculate transient VIV phenomena and in-line vibrations. A new model using Van Der Pol oscillators is now integrated both in the solver and the GUI to overcome these limits, and may be used for standard rigid pipe as well as Spar platforms to estimate vortex induced by the low frequency motion of such floating vessels.

Advanced line split feature : Until now, any line section could be split in two sections only by introducing one connection point at a specified curvilinear abscissa. Defining lines having many connection points obliged to repeat the split operation several times. The advanced line split feature now allows to split any section in a predefined number of sections which will be given common properties. This is particularly useful to generate lines presenting repetitive patterns with connection nodes in-between, such as rigid pipe towed close to the sea water surface with tens of buoys connected to the pipe with cables, or riser tower core pipe with several guiding devices distributed along the whole length.

Export of modes shape files : Once a modal analysis is completed, mode shapes files can now be exported at the standard .MDS file format. This provides an interface to SHEAR7 for VIV analysis with this program. These files may be exported at the post-processing stage with the GUI and PODEEP, either in interactive or batch processing mode. Modal shapes and curvatures graphs may also be plotted with the GUI and exported to Excel.

Gaussian wave spectrum : A Gaussian wave spectrum type has been added to the standard wave spectra that can be used when running irregular wave dynamic analyses. This new spectrum type can be used in particular to describe certain swell components observed offshore West-Africa. These spectra are available from the waves object definition window and are fully compatible with the spectral fatigue method post-processing for steel risers and mooring legs.

User-defined units systems : In addition to both metric and imperial units systems already available in the GUI, user-defined units systems can be created with the new units system wizard. This window allows to define any type of units system based on the metric or imperial system by specifying conversion tables. The new units will be automatically reported into the GUI and graphs.

Definition of batch post-processing instructions files : The ASCII instructions files used to run post-processing tasks in batch mode had to be created by-hand when using the previous version of the GUI. These commands files may now be automatically generated by the GUI provided that the associated post-processing requests have been defined first in the Predefined results folder of your analysis. This new feature ensures that the proper syntax is used in the instruction files and avoid users to key-in all the instructions in the file. These post-processing instruction files may then be modified on demand by-hand if necessary.

Access to zoom box : In addition to the zoom button located in the tool bar, the zoom box function can now be accessed by holding the SHIFT key and left mouse control button and dragging the mouse over the 3D View window.

Definition of contact zones : Contact-related numerical parameters were previously all equal along the whole line length when defined with the GUI. The density of contact elements could in particular not change along the line to match with varying pipe elements segmentation. Contact properties may now be different from one pipe segment to another, which allows to adapt the density of contact elements to the pipe segmentation.

Matrix profile storage with constant stiffness : On rare instances, the matrix profile storage scheme used by default within the Newton-Raphson algorithm may fail to converge due to undetermined variables. The workaround consisted in using the more robust matrix band profile storage with constant stiffness, which however resulted in significantly increased CPU. A new matrix profile storage scheme with constant stiffness has therefore been added, which combines the efficiency of the previous matrix profile storage scheme and the robustness of matrix band storage with constant stiffness.

Improved clamp connection : Clamp connections may be defined at line ends by specifying the departure angle through its azimuth and elevation. This determined the departure direction only and the solver used to set-up the local reference frame attached to the cross section independently at the initial stages of the calculation. On rare instances, this method could induce some fictitious torsion in the line. An improved initialization technique for initialization of departure angles which avoids such problems has therefore been developed and can now be activated from a checkbox placed aside the edit boxes used to input the azimuth and elevation angle. Selecting one method or the other makes however no difference at all for the vast majority of cases.

Enhanced browsing capability from one object to another : Drop down lists buttons have been added in the definition windows for every objects. These drop down lists can be used to switch from one object to another without closing the edition window and re-opening another one.

Search and replace functions in .LOG and .LIS files : Both search and replace functions have been added to look for keywords or particular entries in the .LOG and .LIS files. These functions are available in the GUI from the window used to run the analyses in interactive mode. In addition to these functions, a drop-down list has been included in the same window, which allows to access the different chapters in the .LOG files with a single click.

Hydrostatic stiffness of partially submerged elements : The stiffness matrix of beam and bar elements is used to find the converged solution. There is a specific stiffness on an element crossing the free surface due to the difference between weight in air and weight in water. A specific development takes into account such stiffness when the pipe is not vertical (floating hose type of calculations). In most applications where the elements are vertical, the stiffness is not important compared to the tension in the pipe. One specific application is the use of flexible pipes for flotation. In that case, the pipe is vertical and the tension can be small. In order to make convergence easier a new *BUOY keyword has been created. A group of elements can be designed as buoy and in that case, a stiffness due to the weight difference is introduced on all elements of the group.

Matrix profile storage scheme for modal analyses : Profile matrix storage scheme is customarily used to speed-up the calculation time but was restricted for static, time-domain and frequency dynamic analyses. The profile matrix storage technique is now also available for modal analyses which results in significant decrease the CPU time required to compute the natural frequencies and associated modal shapes for complex risers systems, such as risers including several pipes and multiple connections.

Corrected problems

Display of reference frames : The reference frame corresponding to built-in angle at line-ends was sometimes incorrectly displayed in the 3D View window. This has been corrected and the frame now displays correctly in all the cases.

Display of reference frames : The reference frame corresponding to fairlead points attached to buoy components was incorrectly displayed in the 3D View window. This has been corrected and the frame now displays correctly in all the cases.

Batch-processing of envelopes : Batch processing of variables envelopes was systematically performed on the whole duration of the dynamic simulation, even though initial time and final time were specified in corresponding the post-processing instructions file. This problem is now corrected and the initial and final times are correctly taken into account when computing envelopes.

Wake effect : The change in current velocity induced by the wake effect was not properly considered along the upstream riser. This problem is now corrected.

Contact diameter of flexible pipe segments : The diameter used to handle contact and perform clashing analyses with flexible pipe segment types was erroneously set as the outer diameter defined in the stress post-processing panel. This is now corrected and the contact diameter value considered in the analyses is that of the buoyancy diameter. Should the buoyancy diameter be set to zero, then the hydraulic diameter would be used.