Centrifuge Modeling of Glide Block and Out-runner Block Impact on Submarine Pipelines

Abstract

Abstract This paper presents the results of a series of physical experiments to quantify the drag force on a submarine pipeline caused by a glide block or an out-runner block impact normal to the pipe axis. The experiments were carried out in a geotechnical centrifuge at C-CORE under submerged conditions at a centrifugal force of 30 times the Earth's gravity (i.e. N = 30) and simulated steady and uniform impact velocities ranging between 0.1 and 1.3 m/s with the soil blocks being approximately 5 m in height in prototype scale. The soil blocks were made of kaolin clay consolidated to have undrained shear strengths ranging between about 4 and 6 kPa. The diameter of the model pipes were 6.35 and 9.5 mm corresponding to about 0.19 and 0.29 m in prototype terms. The shear strain rates, defined as the ratio of impact velocity to pipe diameter, in the centrifuge model are N times higher than that in the prototype. The shear rates simulated ranged from about 10 to 136 reciprocal seconds. The paper presents a method for estimating block impact drag force on submarine pipelines based on the results of the centrifuge experiments. Introduction A submarine pipeline is a system of connected sections of pipe that usually transports crude oil or refined hydrocarbons. The pipe is laid on or buried in the seafloor. It typically ranges from 0.1 m to 1.0 m in diameter. The total length of a pipeline is dictated by the distances between the production platform(s) and the onshore or offshore destination(s) and by the route which poses the least risk in terms of offshore geohazards. Submarine landslides and the associated mass movement can potentially have devastating consequences on seafloor installations such as pipelines, flow lines, well systems, cables, etc. Submarine landslides occur frequently on both passive and active continental margins and slopes, releasing sediment volumes that may travel distances as long as hundreds of kilometres on gentle slopes (0.5 to 3°) over the course of less than an hour to several days [1]. The movement of landslide and the released sediment volumes in general terms are so called ‘density flows’. From the initiation to deposition, density flows undergo complex processes that depend on many factors such as the composition, strength characteristics and properties, terrain topography, etc. Geohazards in an offshore oil and gas perspective can be due to local and/or regional site and soil conditions having the potential to develop into failure events causing loss of life and damage to the environment or field installations. Triggering of these events can be caused by natural geological processes or by man's activities, as outlined in a recent state of-the-art review [2]. Research on understanding the mechanisms behind and the risks posed by submarine slides has intensified in the past decade [e.g. 3, 4-10], mainly because of the increasing number of deep-water petroleum fields that have been discovered and in some cases developed. Production from offshore fields in areas with earlier sliding activity is ongoing in the Norwegian margin, Gulf of Mexico, offshore Brazil, the Caspian Sea and West Africa [11]. Estimating magnitude of the drag forces on pipelines caused by density flow impact is an important design consideration in offshore engineering. For buried pipelines in cohesive soils in slowly moving unstable slopes, the available methods seem to provide more or less similar estimates for the drag force normal to the pipe axis. However, this is not the case for estimates of the drag force parallel to the pipe axis [2]. In cohesive soils, the magnitude of the drag force is a function of the rate at which the soil is sheared during interaction with the pipe. Recent works by Zakeri et al. [1, 12-14] provide a method for estimating drag forces caused by clay-rich debris flow (fully remoulded and fluidized density flow) impacting a pipeline normal to its axis. Later, the work was extended to cover all angles of impact [15].