Centrifuge modelling of an on-bottom pipeline under equivalent wave and current loading

doi:10.1016/j.apor.2012.10.009

Applied Ocean Research

Volume 40, March 2013, Pages 14-25

https://doi.org/10.1016/j.apor.2012.10.009 Get rights and content

Abstract

Pipelines are the main element in transporting hydrocarbons from their extraction sites to on-shore or floating facilities, with preference now given to pipelines laid directly on the seabed due to their fast and economic installation. However, these pipelines are exposed and must be stable under all environmental conditions, and therefore, their design for on-bottom stability is of critical importance. Although accurate prediction of the pipe–soil interaction behaviour under hydrodynamic loads from waves and currents is of major concern, limited physical testing of pipes subjected to these cyclic loading conditions has occurred. Tests have concentrated on simpler load combinations in order to develop pipe–soil friction factors or the key parameters in plasticity models that described pipe–soil behaviour. In this paper, results from geotechnical centrifuge experiments of a model pipe on calcareous sand soil collected from offshore on the North West Shelf of Australia are presented. A sophisticated load control scheme allowed complex paths characteristic of hydrodynamic loads to be applied during the testing. Furthermore, pipe testing could be extended to relatively large horizontal movements of up to 5 pipe diameter. The results of the centrifuge testing programme provide improved understanding of the pipe–soil interaction under complex hydrodynamic load paths. They have also been used to assess a state-of-the-art plasticity model describing pipe–soil interaction on calcareous sands.

Highlights

▸ Results from geotechnical centrifuge tests that applied hydrodynamic storm loading on an unburied pipeline are presented. ▸ Results provided for relatively large horizontal movements of up to 5 pipe diameter. ▸ Description of the pipe–soil behaviour, including the build-up of lateral berms, under realistic cyclic load paths. ▸ Verification of a state-of-the-art plasticity model describing pipe–soil interaction on calcareous sands presented.

Section snippets

1. Introduction

Offshore pipelines laid directly on the sea floor are one of the main elements in the development of an offshore oil or gas field. They are used to transport hydrocarbons to on-shore processing units or, in some projects, to connect the well heads with the transporting FPSO or tanker facilities. Operators and regulators require that pipelines remain in position and fully operational during their service life. Therefore, accurate simulation and design of the pipeline under hydrodynamic loading

2.1 Experimental facilities used

The tests were conducted in the beam centrifuge at the University of Western Australia. The centrifuge is an Acutronic Model 661 geotechnical centrifuge that has a swinging platform radius of 1.8 m and is rated at 40 g-tonnes [17]. The platform supports standard rectangular ‘strongboxes’, which have plan dimensions of 650 mm × 390 mm and are 325 mm deep. A headroom of 900 mm above the strongbox allows the equipment to be mounted to perform ‘in-flight’ events such as the robotic manipulation of

3. Experimental results

Table 2 shows the loading details and the number of cyclic loads used for the inclined loads tests 1–5, the regular load test and the Irregular load test until the maximum horizontal displacement was reached. The proportion in the number of cycles represents the distance into a cycle before the maximum horizontal displacement was reached and the test concluded.

The test results for Inclined test number 1 are shown in Fig. 9. Fig. 9a verifies that the load control scheme was correctly implemented

4. Introduction to the force-resultant pipe–soil model

In order to assess its suitability, the calcareous sand pipe–soil model developed by Zhang [9], Zhang et al. [10,11] and improved by Tian and Cassidy [27,28], and known as the UWAPIPE model, was used to numerically retrospectively simulate the pipe centrifuge tests. The use of force-resultant models in pipeline analysis offers the advantages of numerical efficiency and direct incorporation into structural finite element programmes. Based on the plasticity theory, force-resultant models directly

5. Estimation of the numerical model parameters

The majority of the model parameters were assumed to be consistent with those of Zhang [9], though confirmation of the bounding surface shape and assessment of the vertical stiffness for this experimental soil sample was conducted.

Four centrifuge sideswipe tests were performed to track the UWAPIPE bounding surface of the soil sample used. These were performed in the same box as the inclined, regular and irregular load tests already presented. The tests were conducted according to the overloaded

6. Retrospective numerical simulations of the centrifuge tests

The centrifuge tests were retrospectively simulated using the UWAPIPE bubble model [27,28]. The same input load paths used to control the experimental programme (as shown in Fig. 5, Fig. 6, Fig. 7, Fig. 8) were used as input to the numerical analyses. The default UWAPIPE model parameter values (shown in Table 3) were used in these simulations, except for the test-specific parameters listed in Table 4 and as described in the previous section.

Fig. 19, Fig. 20, Fig. 21 show three illustrative

7. Conclusions

Pipe centrifuge testing was performed to investigate the pipe–soil interaction under cyclic loading of complex paths characteristic of real hydrodynamics on an offshore pipeline. The results presented highlight the different stages of the pipe–soil interaction behaviour during the centrifuge testing. The main findings of the pipe centrifuge tests are summarised as: (i) the pipe gained more penetration under the cyclic loads, where the horizontal displacement was almost less than 10% of the pipe

Acknowledgments

The work described here forms part of the activities of the Centre for Offshore Foundation Systems (COFS), the UWA Ocean Institute, the Australian Research Council Centre of Excellence for Geotechnical Science and Engineering, the Australian-China Natural Gas Technological Partnership Fund and The Lloyd's Register Educational Trust, an independent charity working to achieve advances in transportation, science, engineering and technology education, training and research worldwide for the benefit

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