Liwan 3-1: How Deepwater Sediments From South China Sea Compare With Gulf of Guinea Sediments

Abstract

Abstract The first deep water geotechnical survey in South China Sea has been performed at Liwan 3–1 Field in Pearl River Mouth Bassin. The investigation comprised Piezocone Penetrometer Tests (PCPT), T-bar, insitu vane tests, pore pressure dissipation tests, jumbo piston cores, box cores, along with advanced laboratory testing. The in-situ and laboratory data results obtained have been used together with the geophysical data (AUV data) to provide the soil design parameters used for the design of the deepest PLEM and Manifold foundations (i.e. between 1300 and 1500m WD). The high quality and variety of in-situ and laboratory tests performed enable a sound comparison with other deep water sediments. Most of the soil properties of the Liwan 3–1 clays are in the range of those encountered in the Gulf of Guinea (Colliat et al. 2010). The use of scanning electron microscopy (SEM) revealed highly structured clay what is in accordance with the relatively high sensitivity of about 5 to 9 obtained from T-bar tests after 30 cycles. However some particularities arise in their mechanical behavior. The shear strength anisotropy (i.e. answer under different modes of shearing) is for example not significant. It can also be noted that the increase of shear strength with time or with the rate of shearing (i.e. respectively the thixotropy and rate effects) are less pronounced. Introduction Located about 300 kilometres southeast of Hong Kong, Liwan field is a major offshore gas discovery. The field contains contingent reserves of between 4 trillion and 6 trillion cubic feet of gas based on the Liwan 3-1-1 discovery well drilled in mid-2006 in Block 29/26. An AUV survey took place in 2009-2010 across five pipeline routes and two sites in the Liwan area. Some gravity cores were also taken at that time. An extensive geotechnical site investigation took then place in 2011 to investigate the deep water structure locations (Liwan 3–1 PLEM, manifolds and SDH), shallower locations (LH 29–1 manifold and SDA) and pipeline routes. This survey also included specific geohazard locations aiming at quantify the susceptibility of soil slope instability along the Liwan 3-1 to CEP pipeline route. The Liwan 3–1 field is located on the lower continental slope of the South China Sea (Figure 1a), in water depths comprised between 1300 and 1500m. Seabed gradients in the vicinity of the proposed seabed structures can be significant. The PLEM Location itself is located on a slope of 2.6°, on the east wall of a submarine canyon (Figure 1b). This large submarine canyon runs from north to south, crossing the LW 3–1 area, and is located between the West Manifold Location and the PLEM Location. A detailed engineering geolocical model together with a slope stability assessment and a debris flow model were developped along the canyon but will not be discussed in this paper. The present paper focuses on the deep water sediments encountered at the Liwan 3–1 structure locations. The soil investigation strategy, the soil physical and geotechnical properties are discussed and compared to the experience gained for deep water projects in the Gulf of Guinea and particularly to the state of the art paper presented at the ISFOG 2010 Conference in Perth (Colliat et al. 2010). The first Chinese deep water field offers many similitudes to other deepwater fields when looking at the shallow sediment properties. Some particularities were however highlighted by the laboratory test results. The impact of the observed differences on future foundation design and installation are discussed in the present paper.