Model Tests and Computer Simulations for Njord FPU Gas Module Installation

Recently, two gas modules have been installed on the Njord FPU, on the Norwegian Continental Shelf, to enable gas exports from the Njord Field facilities. The 100 and 200 tonnes modules were placed onto the FPU deck by Saipem’s dynamically positioned S7000 SSCV. Detailed model tests and computer simulations were carried out by MARIN and Saipem UK. The purpose of this analysis was to investigate the limits of the offshore installation's operability. Experiences gained from the actual installation in the field proved to be valuable feedback for the performed analysis.
The model tests were carried out in MARIN's Offshore Basin. Various sea states and wave directions, relevant to the Njord field, were modelled. Detailed scale models of the S7000, the Njord FPU and both modules were constructed at a scale of 1:50. The S7000 was equipped with a DP system, including the controller, Kalman filters and azimuthing thrusters. The installation sequences were modelled in detail by using an automatic control of the crane. Measured signals included motions of the floaters and gas modules, loads in the hoisting arrangement and loads on the supports and guides. The FPU mooring loads and S7000 thruster performance were monitored as well.

Time domain computer simulations were carried out using MARIN's simulation program LIFSIM. The simulation model included both floaters, the crane and hoisting arrangement, the gas modules and the guiding system on the FPU deck. First, model test results were used to optimise the performance and accuracy of the simulation model. The tuning of the LIFSIM program included the calibration of damping levels and the detailed mathematical modelling of the guides and supports. A good agreement between the model test and simulation results was obtained. Secondly, a large number of simulations were performed to investigate the different environmental conditions, as well as different approach and lowering scenarios.

Finally, observations and experiences for the actual installation of the modules, which took place earlier in the summer of 2007, are presented in this paper.