Model Summary |
 
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Model Summary |
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The riser in this example is a model of a buoyantly tensioned spar production riser. The riser is oil filled and of single-casing type, and is situated in 3520 ft of water. A stress joint is located at the riser base and between this and the vessel keel are 42 bare riser joints. A keel joint is located at the point of entry of the riser into the vessel moonpool, with tapered stress joints above and below the keel joint. A riser guide, which is attached to the spar, is located around the keel joint at its midpoint. Above the upper keel stress joint are a further 4 bare riser joints. This section of the riser is followed by a section comprising 3 riser joints fitted with air cans. The riser joints with air cans are each separated by a single bare riser joint. Hull-mounted air can guides, which are located towards the bottom of each of the air cans, provide lateral support. Finally a spacer joint is located above the top air can.
The riser guides are modelled using zero-gap guides which provide lateral support. A zero-gap guide may be thought of as a cylindrical sheath positioned around a section of riser, with the internal diameter of the sheath equal to the external diameter of the riser. So there is a contact clearance of zero between the guide and the riser. The zero-gap contact algorithm works by checking the position of nodes at each solution step for contact with zero-gap supports. If a node comes into contact with a zero-gap guide, appropriate boundary conditions are applied at the node in local axes which are both perpendicular to the longitudinal guide axis. Unlike flat guide surfaces, reaction forces are not considered when releasing the restraints. Rather, the node is free to move in the axial direction (subject to frictional restraints) through the zero-gap guide, and whenever the node leaves the contact region, the boundary conditions are removed.
Hydrodynamic loading within the spar moonpool is modelled by assigning moonpool hydrodynamic coefficients to elements of the structure that may be subjected to hydrodynamic loading within the vessel moonpool. The program checks at each solution time whether each integration point on a given element is within the volume enclosed by the vessel moonpool at that particular time, or if it is in the so-called transition region, or if it is completely outside the influence of the vessel moonpool. If it is within the volume enclosed by the moonpool, then the water particle velocities and accelerations used in calculating the Morison’s Eq. force at the integration point are calculated from the vessel motions. If the integration point is outside the influence of the moonpool, the water particle velocities and accelerations are calculated from the ambient wave field. If the integration point is within the transition region, then the water particle velocities and accelerations are interpolated linearly from those in the moonpool and those in the wave field. In this way, the program automatically accounts for elements that may move in and out of the vessel moonpool during the course of an analysis.