Development Of Light-Weight Flexible Riser Pipe

Abstract

Abstract Light-weight flexible pipe is needed for deep-water offshore oil fields, but there is a limit to the depth at which conventional flexible riser pipe can be used. In this paper we first describe the concept of the limits of applicability for conventional flexible riser pipe and discuss trends in and methods for making it lighter-weight. Then, because the use of fiber reinforced plastic (FRP) for the tensile reinforcing layer is considered the most effective and practical way to lower the weight of pipe, we discuss the conditions and characteristics for FRP for use in the reinforcing layer of FRP riser pipe, as well as the design, manufacturing, and evaluation of FRP riser pipe. Introduction As shown in Figure 1, flexible riser pipe consists of an interlocked conduit, an inner plastic pipe, hoop stress reinforcement, tensile reinforcing layer, and an external sheath.1')2)3) Such pipe has come to be widely used in offshore oil fields, where it has become an essential item. As deeper offshore oil fields are developed, this pipe is being used at greater depths. By 1992 oil fields laying 850 m underwater are under developments) and the future is expected to bring the development of offshore oil fields at even greater water depths. Weight is an important factor in floating production systems, and research and development is being carried out in order to create lighter-weight TLP tethers and rigid risers.4) The use of flexible riser pipe in such deep-water oil fields causes problems of increased laying tension and increased side wall pressure due to the weight of the itself. This has led to the development of methods for installation at great depth5)6and the development of lighter-weight flexible riser pipe.7) Study is underway for replacing the reinforcing layer of light-weight flexible riser pipe with aluminum or a composite material (FRP). Meanwhile, the use of stronger reinforcing steel and the use of FRP are being considered to solve this problem, and this paper discusses the use of FRP. Key problems The depth at which flexible riser pipe can be used is limited by the allowable tension and the laying side wall pressure, which depend on the weight of the pipe itself. As shown in Figure 1, flexible riser pipe has a structure in which a plastic pipe is reinforced by multiple layers of metal. The greater the depth at which the pipe is to be used, the greater its weight must be, and therefore the thicker its tensile reinforcing layer must be, but this produces even greater weight, requiring even greater thickness, in a vicious cycle that imposes design limits. To ascertain the characteristics of the tension of flexible riser pipe in the depths of the ocean, we analyzed the fluctuations in tension produced by waves in a free hanging configuration. Here we considered the maximum static movement assumed for a semi submersible platform and the dynamic response for a significant wave height of 18 m and a period of 15 see, while the pipe is subjected to the use pressure.