If weather conditions become rough, vessel motions with an increased amplitude are excited due to the presence of waves. The sagbend region of a suspended pipeline catenary may experience high loads due to these motions. When the loads become too high, the pipeline integrity cannot be guaranteed. In such case, the pipeline is lowered to the seabed to prevent buckling and excessive damages to the pipeline. The pipelay operation is to be resumed later on when pipelay conditions have been improved. This process is called Abandonment & Recovery (A&R). It is essential to lay pipe as efficiently as possible to reduce the time spent offshore. This study investigates the influence of the vessel motions with regard to pipeline integrity. This results in two objectives that are intertwined with one another. The first objective is to investigate the influence of vessel motions on the dynamic pipeline behaviour and integrity during installation operations. The second objective is to develop a method that will define a vessel motion based criteria for pipeline Abandonment & Recovery operations with respect to pipeline integrity. Two methods are proposed to create ´statistical prediction´ curves to quickly assess the pipeline integrity. This requires the generation of multiple pipeline installation models during project preparation. These methods are applied to various combinations of water depths and pipe properties. The dynamic behaviour of the pipeline is very sensitive to changes of these parameters. A shallow water and a deep water case are used for the analyses of the results and for the validation of the methods. The pipeline integrity is assessed by predicting the maximum strain, the DNV buckling check and fatigue damage for a pipeline during installation based on stinger tip motions.

During S-lay pipeline installation, tension fluctuations in the suspended pipeline are compensated by the tensioner system on the vessel. The goal of this thesis is to answer the questions: to what extent does the active tension compensation (ATC) influence the pipeline integrity during offshore installation. Secondly, which tensioner model should be used for pipelay analyses that can give accurate results? And lastly, what is the implication on the existing tensioner models (deadband and linear damping with velocity cap) that are currently used in pipelay analyses?

The pipelay operation is modelled in OrcaFlex with 7 case studies from shallow to deep water based on full-scale measurement data measured during offshore installation. For each case study, three types of measurement data are available: vessel motions, tensioner pay-out motion and pipeline top tension. Vessel motions and tensioner motion signals are used as inputs to the OrcaFlex model, whereas the measured tension signal is used for comparison with the output pipeline top tension for model validation.

It is observed that the ATC becomes more important in deeper water as the reduction in maximum von Mises strain in the sagbend and accumulated fatigue damage become more significant with increasing water depth. The maximum von Mises strain is found to be in between 0% and 2.2% lower from the model with uncompensated tension. Moreover, the reduction in accumulated fatigue damage is more significant as the allowable standby time increases between 3.2% and 19.5%.

Subsequently, different tensioner models are included in the simulation. The tensioner model computes the pay-out motion based on the pipeline top tension deviation from a set tension at each time step. The initial model with specified tensioner motion is then used as a benchmark for the other tensioner models. It is found that the existing tensioner models are not conservative as the maximum von Mises strain and accumulated fatigue damage are underestimated, with the exception of the accumulated fatigue damage in shallow water cases.

As an alternative to the existing tensioner model, a tensioner model with a PI controller is introduced to represent the PI controller of the tensioners systems on the vessel. This PI controller model scripted in Python is implemented as an external function to the OrcaFlex model.

The controller gains are tuned proportional to the logged controller gains from the vessel so that the tensioner model produces the pay-out signal similar to the measured pay-out. Based on the pipeline integrity check, the tensioner model with PI controller is valid and can be used for a more accurate investigation of the pipeline integrity in all cases.