Project Component - Units, General & Constants
All of the PipeLay inputs are in meaningful units that are familiar to the installation engineer. All data entry dialogs and component views display the relevant units required for any particular input (depending on the unit system in use), so that you can be sure that you have input your data correctly. The required units for each input are also listed in the corresponding component data input article.
The unit system used by PipeLay is selected on the Units drop-down list on the Project component, either Metric or Imperial.
Clicking on General displays the following dialog.
Here you specify general project information, as the name of the dialog implies. This information is used in the headers of various analysis reports, which are created as part of postprocessing.
The Constants dialog is used to specify values for different project constants. The format of the dialog you get by clicking on the Constants button depends on whether Units is set to Metric or Imperial. For example, this is the Constants dialog for metric units.
The Constants dialog allows you to specify values for the following:
•Constants such as the acceleration due to gravity (g), the mass density of seawater (ρw), and the kinematic viscosity of seawater (μ). There are appropriate default values for these, but you can of course override these.
•Options relating to the lengths of elements in the finite element mesh. These options are discussed in the ‘Mesh Settings’ section below.
•Inputs related to the basic operation of the PipeLay contact modelling algorithm, namely Contact Ramp and Contact Power. A full description of these inputs can be found in ‘Supports’. The Contact Ramp value defaults to 0.1m or 0.328ft while the Contact Power defaults to 1.5.
•A drop-down list called Optimum Contact Stiffness, which is used to select the calculation procedure for the determination of default optimum contact stiffness for supports. You can choose between Largest (default) and Individual. A full description of the significance of these options can be found in ‘Supports’.
•An input called Pivoting Bed Factor which is used to scale the bending stiffness of elements in contact with supports that adopt the Pivoting Bed contact modelling option. Again this input is described in full in ‘Supports’. Note that the default value is 1.2.
•A drop down list to specify whether element drag (used by default) or buoyancy diameters are to be used during the computation of the added mass and inertia terms in Morison's equation. For pipe sections both diameters are typically very similar, however for stinger sections and structures there can be noticeable differences between the diameters, in which case what diameter actually gets used in Morison’s equation is important. Note that on a strict theoretical level the buoyancy diameter is the more appropriate for Morison’s equation.
•A drop down list to specify whether to introduce non-linear stiffness properties to the static analysis solution before quasi-static analysis steps are performed (the default being No). Note that doing this is only beneficial in certain unique cases such as for floating stinger models where the flex joints for modelling the stinger hinges have non-linear moment-angle curves that are intended to apply a pre-load moment to the stinger (to mimic the presence of stinger buoyancy).
When you run an Analysis component in PipeLay an internal meshing algorithm automatically generates a finite element discretisation based on the specified model configuration. The objective is to create a mesh which is sufficiently dense to accurately capture structural behaviour without being unduly complex, leading to lengthy simulation times. In order to accurately simulate the interaction between the pipeline and the stinger, the mesh generated by PipeLay in the stinger region is relatively fine. Conversely, the mesh density to model the behaviour of the sagbend region may be relatively coarse, as it is typically a long section of pipeline with smaller levels of loading than in the stinger region. The meshing algorithm also attempts to prevent large changes in relative element length across the finite element mesh by gradually stepping up and down element lengths along the pipeline, and to avoid over-meshing by using longer elements in the middle of long sections of continuous properties.
Intersections between components are generally the points of most interest and therefore have a more refined mesh. It is desirable to step up the element lengths away from intersection regions and use larger elements in the centre of homogenous sections. The ratio between the lengths of adjacent elements in a section cannot exceed approximately 1.5. The actual value of the ratio is selected by the program to correctly fill the meshed section with elements. The Maximum Element Length value determines the size of the largest elements generated by the program. It defaults to 10m or 30ft, but you can override it via the Constants dialog.
The surface contact algorithm in PipeLay works by checking the position of nodes at each solution step for contact with the surfaces modelling the rollerboxes etc. Because the contact algorithm is based on monitoring the position of nodes, it is important that the finite element mesh in sections of the pipeline that may come into contact with rollerboxes is sufficiently fine to properly model contact. You have the option of ensuring a refined mesh in the stinger region, by explicitly specifying an appropriate Maximum Stinger Element Length. The meshing algorithm cannot generate an element longer than the value you specify. The default is 2m or 6ft.