Motion analysis of suspended jacket transport with the Heerema SSCV Sleipnir

Abstract

This research explores the development of an accurate yet fast methodology to use measured vessel motions for a fatigue damage calculation in the cranes during suspended transport. First, the kinematics, definitions and particulars of the system of interest – the vessel including crane and suspended jacket - are provided. Then, literature research is done on the main two problems with any transport at sea: the inconvenient draft problem, and the forward speed problem. To remove the inaccuracies introduced by both problems, the novel TF A2B Method is introduced. The method can be used to find the required force transmissibility functions at location of the cranes that are most prone to fatigue: the welds at the crane boom pivots. It is mathematically proven that the TF A2B Method can be used and with validation tests set up for unrestrained suspended transport it is shown that the required RAO’s can be obtained from in-house modeling software Liftdyn. Overall, its concluded that the TF A2B Method works in principle, removing the need to consider inconvenient draft and forward speed.

The hypotheses proposed in this research are further validated by the proposed validation tests in combination with a synthetic data model created in software tool Liftdyn. The Synthetic Data Model (SDM) uses a Jonswap Spectrum to obtain the motions at the Motion Reference Unit (MRU, ‘A’) and the Control Point (‘B’) at the modelled SSCV Sleipnir, which is assumed as a rigid body. After successful validation, the SDM model is then used to calculate the motions at the Jacket Sensor (‘B’). Further validation of the found responses is done and it is confirmed that the location ‘B’ can in fact be located on another rigid body as the MRU, which in this research was the location of the jacket sensor. It is also shown
that the transmissibility functions (TFs) between A and B are almost equal for different draft, resulting in that the TFs between A and B are not significantly dependent on the hydrodynamic properties of the modelled system. Therefore, it is concluded that the TF A2B Method can correctly find motions at location B by using the motions of location A in combination with the corresponding TFs between A and B.

The TF A2B Method is then applied to calculate the jacket motions using measured vessel motions. Data obtained from the X suspended transport were processed and applied using the Measured Data Model. The calculated jacket motions are compared to the measured jacket motions during the X suspended transport to the accuracy of the TF A2B Method with real data. An approximate 70% accuracy match with the measured suspended jacket motions, Roll and Pitch during the X Suspended Jacket transport was found. It is noted that not all data was found to be suitable for use. Suggestions for improving this have been made and are expected to further improve accuracy of the match. The validated Liftdyn model of the X transport is then used to model the suspended transport of X. From the Liftdyn model Force RAO’s can be obtained at the selected fatigue location of the cranes. A Fatigue Data Model using the TF A2B Method is proposed to find the stress cycles and thereby the fatigue damage during suspended transport.

Concluding, the TF A2B Method developed in this research shows positive results overall. The method can in principle be used to translate vessel measurements during suspended transport into fatigue life consumption of the crane. Further research is needed to improve the Measured Data Model, for which the use of a Surge and Sway motion sensor at the MRU is recommended. Furthermore, smaller vessel motions and more data samples should be used to further validate the accuracy of the method which is required to validate the Liftdyn model of the transport. Finally, to verify the results of the Fatigue Data Model, it is suggested to install strain gauges at the welds of the crane boom pivots for validation.