Physical Characteristics and Impact of Mass Transport Complexes on Deepwater Jetted Conductors and Suction Anchor Piles

Abstract

Abstract Recognition of the importance of sedimentary deposits derived from mass transport complexes (MTCs) has increased significantly as offshore energy industry exploitation and development activities extend into deeper water. This understanding is in part due to improved seismic data. It certainly is the case in the foundation zone, defined in this study as the upper 100 m below the seafloor. The purpose of this study is to assess the physical characteristics of MTCs within the foundation zone and relate these characteristics to installation performance of jetted conductors and design considerations for suction anchor piles. The evaluation of physical characteristics of these features is based on a review of published scientific literature and results of geotechnical soil borings. Three case studies, involving either jetted conductors or suction anchor piles, are evaluated to understand the potential condition that may be encountered while penetrating MTCs in the foundation zone. The important conclusions that emerge from this review of previous data and the examination of three case studies are threefold. First, MTCs appear to be more consolidated, when compared to unfailed, conformable deposits. Second, the identification and characterization of MTCs is vital to assess the performance of jetted conductors and suction anchor piles. Third, presence of MTCs should be considered in the design, in order to minimize costly operational delays. Introduction As the offshore energy industry continues to extend into deeper water, the chance of encountering and ability to recognize a depositional environment that consists of redeposited (failed) sediments has increased significantly. These features, herein termed mass transport complexes (MTCs), have been recognized on the seafloor for several decades (e.g., Walker and Massingill, 1970; Jacobi, 1976; Kenyon, 1987), but remained obscure in part due to confusing terminology (Weimer and Shipp, 2004, this volume). Recently, several new studies have documented that MTCs can contribute a large percentage (occasionally greater than 50%) of the nearsurface sedimentary volume on the continental slopes in various basins worldwide (e.g., Brami et al., 2000; McGilvery and Cook, 2003; Lee et al., 2004, this volume; Newton et al., 2004, this volume). Due in part to the high-resolution seismic data in the nearsurface section, details of MTCs are much better imaged in the upper 100 m (328 ft) below the seafloor, called the foundation zone in this study. Jetted conductors for deepwater wells and suction anchor piles for rig and production platform moorings are commonly installed in this interval. A jetted conductor functions as the foundation for a deepwater well and typically penetrates 40 to 80 m (131 to 262 ft) below the seafloor. The jetting operation entails attaching the bottom-hole assembly of the next casing interval (to be drilled) inside the conductor casing with a quick-disconnect tool, and washing the casing into the formation with seawater pumped through the drill string. Suction anchor piles in mooring systems are usually large-diameter, steel cylinders that typically penetrate 18 to 40 m (60 to 131 ft) into the formation. Embedment is achieved by a combination of self-weight penetration and internal underpressure, applied using a subsea pump connected to the pile top.