Severe hurricane activity of 2004 and 2005 has reminded the industry of the importance of integrity management of floating structures. One of the main methods used in assessing the fitness of a floating system after a hurricane event, is to evaluate the actual behavior of the hull, mooring and riser systems versus design assumptions using high and low frequency motions of the platform. As a result, vessels are typically outfitted with inertial six degree-of-freedom inertial navigation systems (INS) and differential global positioning systems (DGPS). Common practice is to use INS for wave frequency measurements whereas DGPS is used for low frequency drift motions.
Combining INS and DGPS signals into a single signal covering the entire frequency range requires the choice of appropriate cut-off frequencies for both instruments and the use of advanced filtering techniques. Both tasks are usually source of much debate.
The objective of this study is to demonstrate that modern DGPS have the same ability as INS to measure floating systems wave frequency motions and provide a redundant set of measurements that users can use to verify the accuracy of the INS data.
Using marine data collected on BP Holstein truss spar during hurricane Katrina and BP Atlantis semi-submersible during a 2006 winter storm, the wave frequency motions extracted from the DGPS signals after correcting for pitch and roll motions show very good agreement with the INS signals in the time domain.
Since both vessels are outfitted with DGPS of varying accuracy, the accuracy of these different systems is discussed. The results are also presented as a function of the sea states to demonstrate the ability of DGPS in the smaller sea states. The results of the study show that the DGPS can be used in the wave frequency motion ranges, which is typically been measured with INS. The results presented in this paper help in better understanding and interpretation of the field data and lead to better practices in the integrity management of offshore platforms.
BP's fleet of deepwater floating offshore installations (FOIs) in the Gulf of Mexico include a tension leg platform (Marlin), three truss spars (Horn Mountain, Mad Dog and Holstein) and three semi-submersibles (Na Kika, Thunder Horse and Atlantis) moored in water depths ranging from 3,250 to 7,070 feet.
In the last five years, BP has led a significant effort to preserve and analyze the data collected by comprehensive marine monitoring systems installed on each of these FOIs. Even though the six degree of freedom motions of these three types of floating systems have been studied extensively during model tests and numerical simulations, a lot of interest is placed on capturing their true motions for several reasons:
Comparison to predictions of fully coupled numerical models.
Calculation of actual fatigue accumulation in the risers (H.H. Cook, 2006)
Post hurricane investigation.