SMALL SCALE MODEL TESTS ON SUBGOUGE SOIL DEFORMATIONS

摘要

There are several challenges related to Arctic offshore oil and gas field developments. Among these is the design of pipelines transporting hydrocarbons in the field or to shore. Special considerations must be carried out to take into account the Arctic conditions. One of the most critical concerns for pipelines in shallow waters is the possible damage due to ice ridge gouging. The ice ridge keels can in some areas of the Arctic be as deep as up to 30 meters, ref. Gudmestad et al., (1999), and may damage the pipeline even if that is trenched below the mudline. Laboratory small scale tests executed to establish qualitative figures of the soil and buried pipe behavior below the gouge are reported in this paper. The tests were conducted at 1g and aimed to visualize the effects of gouging on the soil below and the buried pipe segment. A box (115 × 50 × 40 cm) filled with different types of soils (sand and silty sand) was used for the tests. The model ice keel was made of a steel plate hanging from a wooden frame and mounted on top of the box such that only horizontal translation was allowed. For several of the tests, pipe segments were embedded into the soil. The performed tests indicated the presence of subgouge soil deformations and their dependence of several gouge-related parameters. It was observed that the pipes experienced cyclic movements, being first dragged forward-downwards as the model keel approached the pipeline and then re-bounding when the keel passed over. The tests showed that the maximum pipe displacement decreases exponentially with the pipe burial depth. Naturally formed soil mound in front of the keel during gouging showed to influence vertical pipeline displacement. Furthermore, horizontal pipe movements were larger at lower attack angles of the ice keel. The authors are by no means the first to ponder the subject of subgouge soil deformations and pipeline response. Many researchers have reported their studies before; see e.g. Woodworth-Lynas et al. (1996), Nixon et al. (1996), Yang and Poorooshasb (1997), Phillips et al. (2005) and Konuk et al. (2006). The authors are also fully aware of the challenges related to scaled testing of the soil-structure and do not assume the obtained results to be scaled up. Nevertheless, the reader may find some observations to be of interest while evaluating the present (disputed) approach to the problem.