Design Considerations for the Configuration of a Production Riser for Deepsea Mining of Polymetallic Nodules

摘要

The technical feasibility of deepsea mining operations of polymetallic nodules in 5,000m+ water depth has been demonstrated through a number of pilot tests done in the later 1970s and early 1980s. Since then, there have been significant technological advances, especially those in the offshore deepwater oil and gas exploration and production. Market demands for rare-earth minerals and specialty metals, such as cobalt which is essential for a wide variety of high-tech and industrial applications have seen a rapid growth, and the trend is projected to accelerate. The existence of these mineral resources in large abundance in many parts of the world's deep oceans are well known, such as the Clarion-Clipperton Zone (CCZ) and southeastern regions of the Pacific Ocean. All those factors have contributed to the renewed interest in commercial-scale deepsea mining development projects. A key component in a deepsea mining system is the production riser used to transport the production stream, usually in the form of multi-phase slurry, from the seabed level to a production vessel at the surface. For this reason, the riser is also known as VTS (Vertical Transport System). Unlike a normal production riser for oil and gas field development where the riser is "anchored" to the seabed, a production riser for deepsea polymetallic nodule mining operations is suspended and free-hanging off, and is under tow by, a production vessel floating on the surface during normal mining operations. To fulfil other functional requirements, the riser also needs to carry other pipes bundled to it as well as the umbilicals for system monitoring and seabed mining operations. This paper presents an outline description of the mining operations and a production riser design using airlift proposed for a prospective deepsea mining project. The principal design drivers and major considerations in the production riser configuration development for commercial deepsea mining of polymetallic nodules are the primary focus of the discussion presented in this paper. Critical load cases to be considered for the preliminary engineering design assessment are summarized.