Stinger - Articulated S-Lay
In the Stinger component, the topmost drop-down box, Type, allows you to choose an Articulated S-Lay stinger. An Articulated S-Lay stinger is comprised of multiple individual stinger sections which are linked by hinges where the stinger itself will be part of the model and its motions will be part of the solution. Its specification is similar to that of a Hinged Rigid S-Lay stinger, with some exceptions. Firstly, you are required to define the characteristics of the hinges between the stinger sections and between the stinger and the vessel. Secondly, any structural and hydrodynamic properties specified for the individual Stinger Section components are taken into account by PipeLay in making up the stinger finite element model. Each individual section of the stinger is defined using a Stinger Section component, and each hinge is characterised by a Flex Joint component. Note that the order in which the flex joints and stinger sections are added to the stinger is important, they are assumed to be specified in order from the vessel end to stinger tip. Thirdly, there is an option to specify the stinger sections/hinges undeformed orientation. Moment-Angle curves for any flex joints along the stinger are relative to this undeformed orientation and so it should be kept in mind when defining the Flex Joint components. Finally, there is an option to allow PipeLay to automatically optimise the stinger sections buoyancy in order to maintain a set of static section angles.
Four dialogs on the Stinger component are enabled for an Articulated S-Lay stinger, namely Stinger Sections, Section Deformation, Buoyancy and Hitch Rotation. Specification of a stinger hitch rotation is optional, and is again relevant only if you want to rotate the whole stinger, and all its associated sections and hinges, about the hitch point; the operation here is the same as for other S-lay stingers.
The Stinger Sections dialog for an articulated stinger is shown in the 'Articulated S-Lay Stinger Sections' figure below.
Articulated S-Lay Stinger Sections
The drop-down lists in the first and last columns list all of the Stinger Section and Flex Joint components, respectively, currently defined in the project. (You must add stinger sections and flex joints in pairs.) The flex joint is between this section and the previous one – that is, the one just above it in the dialog. If this is the first section (the topmost row), the flex joint is between the stinger and the vessel. If no Flex Joint is specified here, PipeLay will create a default rigid connection between the section in question and the previous section on the stinger. The orientation of each section is specified using the Section Orientation column. This may be specified as an absolute angle with respect to the horizontal plane or as an incremental angle relative to the previous section; which is determined by the Orientation Type drop-down list. This has two options, Incremental and Absolute, with Incremental the default.
When a Flex Joint is specified to use a non-linear moment-angle curve for rotational stiffness then it is worth noting that a downward rotation of the joint generally corresponds to a negative angle on the curve, whereas an upward rotation corresponds to a positive angle. This convention may influence how you define the moment-angle curve for the Flex Joint.
The Section Deformation dialog is shown in the 'Articulated Stinger - Section Deformation' figure below. This allows you to specify the Undeformed Orientation for stinger sections/hinges as being either aligned with the Horizontal plane or with the Specified Orientations under the Stinger Sections dialog. Selecting Horizontal means that the origin point (zero rotation point) for the moment-angle curve of any flex joint along the stinger corresponds to the flex joint being perfectly aligned with the horizontal plane. On the other hand, selecting Specified Orientation means that the origin for the flex joint moment-angle curve corresponds to the flex joint being aligned with the individual Section Orientation specified under the Stinger Sections dialog. The latter option means that if you change the Section Orientation at any point then you may also have to change the flex joint curve so as maintain the same overall global rotational response, which is controlled in reality by hinge rotation stops etc. Clearly the Horizontal option does not encounter such an issue as the curve origin is always fixed regardless of the Section Orientation.
Articulated Stinger - Section Deformation
The Buoyancy dialog is shown in the 'Articulated Stinger - Buoyancy' figure below. Selecting Yes in the Optimise Buoyancy drop-down allows PipeLay to automatically optimise the buoyancy of each stinger section so as to statically maintain the section orientations specified in the Stinger Sections dialog. This optimisation process is carried out during a static analysis of any model containing the stinger in the following manner.
Articulated Stinger - Buoyancy
Firstly, an initial static analysis is performed where all the flex joints along the stinger have automatically been assigned a rigid stiffness. Once this initial analysis step is completed the bending moment in the furthermost flex joint from the vessel is determined. A buoyancy force required to counter this moment is then calculated and distributed to the elements on the stinger sections outside/below the flex joint; by appropriately changing the buoyancy diameter of the elements. If the stinger sections in question already have initial buoyancy values specified, this is accounted for in the calculation and their buoyancy is either increased or decreased accordingly. Also, if a particular set of sections is partially submerged, which could potentially be the case with sections close to the vessel, then a submerged length is calculated for the sections and this is used in the buoyancy calculation rather than a full length.
After completing the buoyancy calculation for the stinger sections, the selected flex joint is reverted back to its original user specified stiffness and a quasi-static analysis is performed to allow the flex joint, and associated stinger sections, to reach static equilibrium with the newly optimised buoyancy values applied. Once this analysis is completed the static orientation of the sections is compared with the user specified input orientation and the comparison is then displayed on the Progress tab of the Analysis component, in addition to the new buoyancy values. Any difference in the compared orientation values should ideally be small if the buoyancy optimisation process has been successful.
Next, the algorithm moves up the articulated stinger to the next flex joint, which is still rigid from the first step, and it repeats the buoyancy calculation for the new stinger sections before performing another quasi static analysis etc. This process is repeated until all flex joints are assessed, and reverted to their original stiffness, with the final end solution providing a set of optimised buoyancy values for all stinger sections. The usual analysis sequence can then proceed as normal thereafter.