First Hyperstatic-Riser Joint Field Tested for Deep Offshore Drilling

Abstract

Summary The riser is one of the key elements for deep offshore drilling. However, the total weight of the system increases rapidly with the water depth in the same way as the required tensioning capacity. Static loads and fatigue may become critical and threaten riser integrity. Furthermore, the natural period of the riser in the disconnected mode may also reach the wave-energy window and may seem able to put the riser into a dangerous resonant state. A new technology developed by the L'Institut Français du Pétrole (IFP) for integrating riser joints, presented in the paper, reduces the weight of the system by up to 30% and lowers the natural period significantly. It consists of joining together all the pipes constituting the riser (main pipe and peripheral lines) in such a manner as to share the axial tension between all of them. This so-called hyperstatic-working mode, which provides an axial-load sharing between all riser lines, may be used jointly with hybrids (steel/composite) previously developed for kill and choke lines (Guesnon et al. 2002). After the first step of riser analysis and technical studies through extensive finite-element analyses (FEAs) and computer-assisted design (CAD), a field test was decided upon to obtain information on the capacity of the technology to work properly in the operational context of a drilling rig and its capacity to maintain the hyperstatic-working mode of the riser joint in that context. The Pride Angola drillship was chosen for the test. This paper presents the basis of the technology, describes the three phases of the field test, and comments on the first results obtained from the rig. To conclude, the technology is then put in the context of deep-water drilling, and its benefits are discussed for conventional, slender, and dual-gradient risers.