Finite element analysis and experimental testing of lifting capacity for GRP cover.
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Structures made of glass reinforced plastic (GRP), often abbreviated GRP cover, are used to protect subsea equipment in the oil and gas industry. These structures need to be lifted during fabrication, transportation and during the installation. The present work investigates the lifting capacity of the GRP covers. The aim is to achieve a more accurate analytical estimate of the lifting capacity, with a desire to replace the simplified hand calculations used today. In order to investigate the lifting capacity, a lifting point (reinforced lifting holes made of GRP) is studied with both finite element analysis and experimental tests in a hydraulic tensile bench. Three different test setups, Case 1, Case 2 and Case 3 are investigated. Case 1 is a representation of a lift through the splash zone during installation, where the GRP cover is in an upright vertical position. Case 2 is an approximation of a horizontal lift with the lifting point located on top of the cover, causing the lifting point to encounter out-of-plane loads. Case 3 is a representation of a horizontal four-point lift, with the lifting point placed on the side walls. The experimental tests were performed in collaboration with Highcomp AS at Westcon Løfteteknikk in Haugesund. The finite element analysis represents the test setup in the hydraulic tensile bench in order to achieve comparable data. Finite element analysis can reduce cost and time compared with physical experiments. For Case 1, with in-plane loads representing the lifting through the splash zone, the results achieved were within a 2% error margin. For the out-of-plane situation (Case 2) with solid elements, the results were within a 20% error margin. The results from the in-plane and out-of-plane scenarios were used in a capacity evaluation to create a graph which represents all results and takes into account the out-of-plane angle from the lifting slings. This graph is easy to use in design and provides good results. Based on the comparison of the results, it was concluded that the Puck criterion with gradual degradation provides the most accurate estimate for the capacity and that use of this criterion can replace simplified hand calculation and reduce the number of physical experiments in the future. Based on these findings, an improvement study was conducted for a 30 mm laminate with Puck criterion and gradual degradation with a new lay-up consisting of fibers at [0, 90, +45, -45] degrees, compared to the one used today [0, 90]. The results of the new study showed an increase of over 28% for the capacity in Case 1 and an increase of over 16% compared with Case 2. These are interesting results, leading to the conclusion that by introducing the fiber directions + 45 degrees, one can enhance the capacity of the material by about 20%, and it is easy and efficient to implement in a new design of the lifting point used in GRP covers.
Master's thesis in Structural engineering