Project Specific Vessel Motion Based Abandonment Criteria

Abstract

When harsh weather conditions are expected during an offshore pipeline operation, the decision to start the abandonment procedure has to be taken based on pipeline integrity limits. The pipeline integrity which is analysed by a maximum design sea state does not correspond to the situation offshore, because sea states always occur in a different combination of parameters and therefore difficult to predict. Consequently abandonment decisions based on sea states to prevent the pipeline from buckling are difficult to take. Presently the Allseas’ pipelay vessels are running on-board software (SMD) what combines sea state predictions from multiple sources including metocean data, Wavex™ information and data from Motion Reference Units (MRU) which measure the actual vessel motions real-time. The SMD software can predict the significant wave height, zero crossing periods and approach directions. Furthermore the exact vessel motions can be predicted in the coming hours with fair accuracy which suggests an opportunity to develop vessel motion based abandonment criteria. With the use of OrcaFlex by Orcina, pipeline installation is modelled for three case situations which differ in water depth and pipe properties. By performing time domain simulations using the Finite Element Method these cases are analysed to find the dominant vessel motions concerning pipeline integrity in a way such that some degrees of freedom can be discarded. Two different types of simulations are distinguished: The first type uses a large sum of short simulations with the model excited by theoretical predefined harmonic vessel motions. These simulations without wave effects are used to determine the direct relationship between vessel motions and pipeline integrity. The second method uses long time simulations with the model excited by stochastic design sea states. These simulations are used to determine which vessel motions and structural responses are expected. For both methods amplitude peaks of the time histories between the quantities are compared to develop and evaluate vessel motion based criteria for the different case situations. For specific deep water operations where the pipeline is leaving the stinger almost vertically (departure angle of about 80°) and the tensioners are modelled on the brakes, the maximum bending moment which occurs in touchdown area is found to be well correlated with bottom tension and vertical velocity of the stinger tip. For deep water operations with an intermediate departure angle of about 45° and compensating tensioners, the maximum bending moment which occurs near the stinger tip is found to be correlated with the top tension and the vertical acceleration of the stinger tip, since the pipeline weight acts in the same direction. For increasing amplitudes and frequencies of the vertical stinger tip motions the correlation with bending moment is affected due to geometric nonlinearities. For deep water operations this happens for unlikely excitations. However, in particular shallow water operations which result into small departure angles it is found that the natural frequencies which occur near the vessel motion peak frequencies result into large dynamic effects and increased geometric nonlinear behaviour. These geometric nonlinearities affect the correlation between stinger tip motions and the pipeline structural response in such way that the found vessel motion based criteria are not applicable. Concluded is that for specific deep water projects with large departure angles, vessel motion based criteria based on vertical stinger tip motions show promising results. It is recommended to further study the applicability of the proposed vessel motion based criteria by analysing it with data generated by SMD during an actual pipelay operation.