DEFORMATION ANALYSIS OF OFFSHORE PLATFORM USING GPS TECHNIQUE AND ITS APPLICATION IN STRUCTURAL INTEGRITY ASSESSMENT

One of a major problem with offshore platform is the occurrence of deformation
which can have serious and potentially fatal consequences. The implementing of a
deformation monitoring system to maintain regular surveillance of the stability is a
means to address both human safety and company profitability. The approach
developed in this study uses a precise relative Global Positioning System (GPS)
which is advantageous for deformation monitoring in terms of long-baseline data as
offshore platforms are located hundreds of kilometres from shore. This research
focused on customizing GPS data processing of offshore platform deformation and its
implementation for structural integrity assessments. A preliminary investigation was
performed on simulated GPS network to ensure tool reliability, processing method
feasibility and enhanced precision of the processed data. Additionally, preventative
steps were taken on the network simulation to ensure that the technique was capable
of detecting any deformation. Commercial software was found to be inadequate for
long-baseline processing and was substituted with GAMIT/GLOBK scientific
software, capable of processing GPS data for offshore platforms. This case study
refers a Jacket-type offshore platform using secondary three epochs GPS data to
analyze deformation. The results of data processing revealed deformation magnitude
in the form of three dimensional displacement, dx, dy and dz which was then used to
assess platform’s structural integrity, focusing on four points of the main pile located
on the upper deck. The structural integrity assessment identified that translation and
rotation of all structural joints was influenced by any displacements of restrained
joints. These translations and rotations increase almost nearly proportional to the
increased displacement value. In the simulation epoch of 10 years, the greatest value
displacement of North is approximately 18-26 cm, East is around 6-18 cm and Up is
about 15-50 cm. These values are assumed as linear function of the displacement of
two month epochs. The great effect occurs on the upper deck with the value of U1 =
±6 cm (point 67), U2 = ±30 cm (point 68), U3 = ±60 cm (point 78), R1 = ±3 radian
ix
(point 80), R2 = ±0.5 radian (point 67) and R3 = ±1 radian (point 84). The greatest
effect arises at the translation in the direction of Z. In the seabed, the achievement
value of R1 = ±5 radian (point 13), R2 = ±0.3 radian (point 14), R3 = ±0.1 radian
(point 14) with no translation effect of in the directions of X, Y and Z. The occurring
translations and rotations in the structural joints contribute to the stability of the
platform, confirming deformation monitoring to be a viable technique in structural
integrity assessment. The deformation analysis indicated coordinate differences
among the three epoch observations, however, a significant test did not categorise
these as a significant displacement. To conclude, a precise GPS relative positioning
technique was found to be a reliable approach for offshore platform monitoring
deformation, enabling precise detection to a few millimeters. This level of precision
could be increased with implementation of processing and observational strategies.