Lecture 05-Fixed Structures and Multi-Body Interaction2

Lecture 5: Aqwa Fixed Structures and Multi-Body Interaction

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Introduction to Hydrodynamic Analysis with ANSYS Aqwa

Aqwa Multi-Body Interaction In the previous lecture the concept of a multi-body problem was introduced. In that case there was no hydrodynamic interaction between the bodies. We will now consider the situation where there are two large structures adjacent to each other such that hydrodynamic interaction needs to be considered.

Multi-Body Problems Aqwa can be used to solve problems that involve multiple bodies with up to 20 structures with full hydrodynamic interaction. The Response Amplitude Operators (RAOs) for each of the hydrodynamically interacting structures will be different from those that would have resulted if each of these structures were on its own. The RAOs are not a physical property of a structure but depend on the radiation and diffraction forces. The radiation as well as the diffraction forces change in the case of hydrodynamic interaction and therefore the RAOs of the structures in question will also change.

Multi-Body Interaction As an example consider two barges alongside each other The top results are when the two barges are considered as noninteracting. RAOs are the same

The bottom results are when the two barges are interacting. RAOs are now different, and the wave pattern is modified

2nd order not captured by 1st order terms appear so the asumptions of linerarity are lost

Multi-Body Problems When analyzing multi-body problems it is often necessary to consider the effects of standing wave formation between the structures, when waves can build up to a point where they exceed the breaking wave limit due to the absence of viscous flow effects This is mitigated by including an external lid between the structures. This can also be used for enclosed areas, such as moon-pools The lid consists of special diffraction elements at the water surface which are assigned a Structure Type of Abstract Geometry, and an Abstract Type of External Lid. A Lid Damping Factor is assigned (normally between 0 and 0.2) and a characteristic Gap is given (normally spacing between structures)

Multi-Body Problems Plots below show a typical wave surface elevation for a beam sea crossing two parallel barges The left plot shows the case with no external lid The right plot shows the situation where an external lid has been included

Multi-Body Interaction For the Multi-Body workshop we are going to consider a ship moored alongside a pier. This will introduce some other features •

Fixed structures

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Fenders

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Multiple simultaneous seastates

Fixed Structures Some special considerations have to be given to structures that are rigidly fixed in space since the radiation effects have to be eliminated •

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In the Hydrodynamic Diffraction model a structure can be fixed by setting the Structure Fixity to Structure is Fixed in Place for the structure concerned The Structure is Fixed in Place setting only operates for the Hydrodynamic Diffraction analysis; for any subsequent Hydrodynamic Response analysis a rigid connection has to be manually established between the structure and a fixed point important

Fixed Structures If the fixed structure is to be placed on the seabed, an additional consideration is the modelling of the bottom of the structure. The seabed acts as a boundary condition for the radiation/diffraction computation. If a diffraction element for the structu re is placed in contact with this an invalid condition exists This is overcome by either removing the body (face) of the structure that is in contact with the seabed, or setting the body as being non-diffracting

Fenders In other workshops, connection types of moorings and joint articulations have been introduced. We will now introduce a third type of connection, fenders (or compression elements) only way to model contact in Aqwa

Fenders There are three types of fender •

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Fixed Omni-Directional – The fender is connected to one of the bodies (or ground) and contact with the second body may be anywhere on the surface of the fender (assumed spherical) Fixed Directional  – The fender is connected to one of the bodies (or ground) and contact with the second body is assumed to act in a given direction Floating – Not connected to either body. Contact with either body acts in a given direction. Note that the fender is located vertically at the still water level

Fenders Fender data consists of

Connectivity Type Action Contact Points using previously defined connection points

Fender Properties •

Damping Coefficient

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Friction Coefficient

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Size

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Non-linear stiffness coefficients

Multiple Seastates Ocean waves with different frequencies and directions are very difficult to model mathematically A single irregular wave definition will create a long crested wave form. This does not necessarily lead to a conservative result. The interaction effect between waves from different directions may be important.

Aqwa can make use of multiple directional (short crested) irregular waves.

Multiple Seastates To define multiple irregular waves we use the Irregular Wave Group to bring together the constituent irregular wave definitions Wave spectral groups can include one or more of the following Single spectrum Main spectrum + cross swell (as shown here) Imported wave elevation time history • • •

Note that the following options available in the core Aqwa solver are not available in Workbench 2-D carpet spectrum Spread seas • •

Workshop 5.1  – ANSYS Aqwa Articulations Workshop 5.1  – Ship and Pier Hydrodynamic Interaction Goal: •

Create a combined ship and pier hydrodynamic interaction model.