Apparatus for Measuring Pipe-Soil Interaction Behavior Using Shallow ‘Pipe-like’ Penetrometers

Abstract

Reliable characterisation of surficial marine sediments is essential to ensure safe and economical design of subsea infrastructure for offshore energy facilities (e.g. seabed cables, pipelines and shallow foundations). Conventional in-situ testing methods (e.g. CPT or T-Bar) require careful interpretation to account for the effects of shallow embedment, while laboratory tests are affected by sampling induced disturbances, the impact of which can be significant at the low stress levels relevant to the design of subsea infrastructure. This paper describes two novel box-core sized shallow penetrometers-the hemiball and toroid-which mimic the shape of subsea pipelines and have been designed to reliably measure the strength, consolidation and frictional properties of surficial offshore sediments. The development and specification of the actuator used to operate these probes, is also described. Another major benefit of these penetrometers, which are intended to be used offshore for on-deck testing aboard a survey vessel, is their capability to generate effective stress interpretations of the soil behaviour, since pore pressures transducers are installed and monitored throughout testing. The results of a first laboratory proof test are presented to illustrate the potential of this novel sensor concept.

Shallow penetrometer tests: theoretical and experimental modelling of the rotation stage

“…Equation 11was used to define the scaling factor, β, for the hemiball. For the toroid, a value of 0.725 was determined (Schneider et al, 2019a), which is in agreement with numerical SSFE solutions reported by Yan (2014). This value is similar to that derived for the hemiball invert location (Figure 3a).…”

“…Additionally, conventional direct shear box tests (without an interface) were performed to check whether the surface finish of the penetrometer interface was sufficiently rough that the full friction angle of the kaolin was mobilised (Schneider et al, 2019a). These tests confirmed that the friction mobilised by the roughened Delrin plastic inserts is practically identical to the results derived from the standard shear box tests, showing that the penetrometer interface can be treated as 'fully rough' (i.e.…”

“…It is also necessary to rotate by a sufficient distance to reach sliding failure prior to pore pressure dissipation, in order that the undrained interface strength can be identified before consolidation occurs. Toroid experiments published by Schneider et al (2019a) showed that a non-dimensional circumferential displacement, dx/D (at the effective radius reff), of about 0.02 is required for failure. The time for significant (e.g.…”

“…A detailed description of the box-core scale shallow penetrometers is given in the companion paper (Schneider et al, 2019b) and in Schneider et al (2019a). The toroid has a lever arm to diameter ratio, L/D, of 2, which is sufficiently large to eliminate interference across the penetrometer section (Yan et al, 2011).…”

“…The bearing and sliding response is therefore the same as for a straight pipe of the same diameter. The devices are actuated using a 2 degree-of-freedom electro-mechanical actuator described in Schneider et al (2019a).…”