Introduction

This blog written by Dr. Matthew Knight (member of the UK technical team) focuses on using Implicit Dynamics for some offshore applications like pipe laying and pipe pull-in simulations, impact analysis, and riser dynamics.

Pipe Laying Procedure

The traditional lay procedure for offshore pipelines on an uneven seabed typically involves modelling an initially straight pipeline in contact with a flat rigid surface while subjected to gravity and external pressure. The rigid surface is then lowered through a 3D seabed profile, allowing the pipe to settle in contact with the real geometry. Once laid, the pipe is normally subjected to an internal pressurise and thermal loading, which can cause the pipe to locally buckle. This method provides additional support to the pipe during the lay procedure, making the problem easier to solve using the general static procedure. It does not account for the effects of lay tension; however it can provide an indication of on-bottom roughness and free span potential.

-          Initial Configuration

-          Final Configuration

A potential alternative is to use the implicit dynamic procedure during the pipe-laying process. With this method the pipeline is slowly dropped onto the seabed through the application of gravity. A separate dummy rigid surface with contact is not required, kinetic energy is quickly absorbed and run times can be up to four times faster (problem dependent). In addition, the implicit dynamic and general static procedures can be combined within a single analysis.

The implicit dynamic procedure introduces inertia and damping into the model which aids overall convergence and generally simplifies the contact.

Implicit dynamics has three options, depending on the level of damping required.

*DYNAMIC, APPLICATION=TRANSIENT FIDELITY or MODERATE DISSIPATION or QUASI-STATIC

TRANSIENT FIDELITY has minimum damping and is used when the problem is truly dynamic, MODERATE DISSIPATION has moderate damping and is needed for problems involving contact and QUASI-STATIC has the highest level of damping and is suitable for quasi-static analyses. For this application the QUASI-STATIC can be used.

The following ‘real-life’ example has been solved using both approaches and the results are very comparable, however the total run time decreases by a factor of four.

-          Traditional approach (General Static Procedure)

-          New approach (Implicit Dynamic Procedure, using QUASI-STATIC option)

Final note:

To model the interaction between the pipe and the seabed, softened contact is typically used where the contact stiffness is lowered to match the stiffness of the seabed. This can lead to relatively high levels of contact penetration, which in turn can cause convergence difficulties as Abaqus will automatically abandon an increment if the penetration is greater than a predefined value. To overcome this issue adjust the HCRIT parameter on the contact pair definition, as follows:

*CONTACT PAIR, HCRIT=X

HCRIT is the distance by which a point on the slave surface must penetrate the master surface before Abaqus/Standard abandons the current increment and tries again with a smaller increment. The default value of HCRIT is half of the length of a characteristic element face on the slave surface. This parameter does not apply to contact pairs that use the finite-sliding, surface-to-surface contact formulation.

Abaqus References

• ‘Implicit dynamic analysis using direct integration,’ Section 6.3.2 of the Abaqus Analysis User's Guide