This paper focuses on the global analysis of a free hanging cateruuy flexible riser connected to a turret moored FPSO (floating production storage and oftloading) vessel. By global analysis is meant that riser response in terms of minimum radius of curvature and minimum effective tension at touch down point region and maximum effective tension at top will be of primary interest. Loads taken into account are weight, buoyancy, current, vessel static offset and vessel first order motions. The influence of current, vessel static offset, riser structural damping and riser departure angle with regard to vertical at top on riser global response are investigated. Introduction A free hanging cateruuy flexible riser connected to a FPSO may be subjected to negative effective tension and consequently to high curvatures and this has been a concern for the oil industryl. The magnitude of the resulting axial tension depends on various parameters, such as vessel static offset (including second order motions), current intensity and direction, riser departure angle with regard to vertical at top, hydrodynamic damping and vessel first order motions. However, it is likely that the most important of these excitations is the vertical motion (displacement and acceleration) imposed by the vessel at the wave frequencies. In a VLCC (very large crude carrier) being converted to a FPSO to be installed at 780 meters water depth at Campos Basin, offshore Brazil, the distance between vessel center of gravity and the internal turret axis located at bow can be as much as 140 meters. The combined effect of heave and pitch of the vessel during extreme environmental events can produce turret vertical motions in the order of 10 meters for displacement and 2 mJs2 for acceleration (single amplitude). These prescribed motions may cause the riser to undergo compression and exceed the allowable limit for curvature. Turret vertical motions Environmental action and vessel draft. Although the turret provides a weathervane capability to the FPSO, the angle of the wave propagation direction with regard to the vessel longitudinal axis at equilibrium is one aspect which deserves a more thorough investigation in order to develop the load cases for the riser analysis. For this particular FPSO, directional stability is attained passively and no thruster assistance is required. If current, wave and wind are collinear vessel equilibrium is reached at head seas. However, if the environmental actions are not collinear, vessel equilibrium may be such that the wave heading is different from head seas, for instance quartering seas. The consequence of this fact is that turret motions are amplified as the wave incidence departs from 180 degrees (head seas), see Fig. 1. For the environmental conditions at Campos Basin it is assumed that at equilibrium the angle of wave direction with regard to thevessel longitudinal axis lies in the 135-225 degrees range, where 180 degrees corresponds to head seas. The influence of vessel draft on turret vertical motions can also be seen in Fig. 1, where the higher turret vertical displacements are associated to the 21.6 meters vessel draft.

In the last years, the most of offshore oil and gas reserves discoveries in Brazil are placed in ultra-deep water depths. Petroleum production from these offshore fields needs developments with novel solutions in terms of necessary technologies and economical viability. The use of vertical rigid risers such as top tensioned risers (TTR) and others like combined systems as self standing hybrid risers and steel catenary risers for ultra-deep waters have shown viable from both, technical and economical aspects. However, there are needs for detailed studies on their dynamic behavior in order to improve, particularly, the understanding of influence of the environment as wave and current, and floating platform oscillations at the riser top. The present work presents studies on vertical top tensioned riser dynamic behavior through time domain simulations of its displacements and respective, bending moments and stresses. Influences of the vortex induced vibrations (VIV) and waves on the riser service life reduction are analyzed. Maximum and minimum envelops for displacements and stresses along riser length are shown.

Petrobras is going deeper and some fields are feasible only if the production platforms are installed in ultra deep waters, close to the wells. In one case in Campos Basin, for example, the platform will be installed at 1255 m water depth and the solution to allow the production is based on the evolution of flexible lines for high pressure and high loads. For the oil export line, however, a steel rigid line was chosen, due to the large diameter. Some analyses were performed in order to make it feasible the installation of an 18-inch SCR export oil line. Due to fatigue loads, the free hanging catenary configuration did not match with API RP 2RD recommendations, so Petrobras decided to change the original shape to Lazy Wave in order to reduce the top loads and increase the fatigue life. But, some configurations are not feasible to install. High angles close to the installation vessel or high stresses close to the flotation segments are some problems that must be solved. This article will present the steps made to reach a configuration that will make feasible the installation of this riser, in parallel to the static and dynamic analysis, to maintain the stresses in an allowable level, in accordance with API RP 2RD.

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When in shallow waters, not only the risers, but also the structures and equipment are submitted to different conditions from the ones related to deepwater applications. OGX has developed offshore applications in shallow waters in Campos Basin, Brazil, using a FPSO with Lazy S riser configuration, based on the Midwater Arch systems (MWA). MWA systems are feasible due to OGX application scenario, but they present some disadvantages, such as: high compliance of the buoyant section to the FPSO, large static offset (common issue in shallow waters applications), which makes the MWA carry the risers that are clamped at the top; high manufacturing and installation costs, associated to the high weight of the structure, which includes large and heavy buoys; limitation regarding transportation, sometimes requiring heavy duty trucks, and consequently, more expensive ones. These disadvantages could be avoided by using another type of support structure, but it depends on the application conditions. Aiming to optimize the Lazy S configuration for new applications in shallow waters, a viability study of a most simplified concept of support, fixed and less compliant, was carried out considering as a standard scenario the Waimea field (under development), located in Campos Basin, Brazil. As a result of this study, OGX and Wood Group Kenny developed the conceptual project of an innovative design of Riser Support Structure (RSS). Therefore, this paper addresses the technical challenges that were faced during the design of this new concept of Riser Support Structure for shallow waters in offshore applications, including issues regarding the required structural safe response and aspects comprising installation and some decommissioning considerations. Regarding the design, this paper discusses the structural analyses performed to validate the RSS, which include VIV and Finite Element Analyses, presenting its main results, and the critical issues encountered during these analyses. They include issues such as: Overstress due to combined loads; stress concentration in important structural components; and stress concentration due to impact load (issue recognized during dynamic analysis to simulate the pile driving operation).