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.

Flexible risers are complex structures composed of several concentric polymeric and steel armor layers which withstand static and dynamic loads applied by the floating production vessel and by the ocean environment. Determining the axial and torsional stiffness values of such structures is an important task for the global structural analysis, since it provides a probable value that can be used in this analysis to predict the load distribution along the line and permitting, thus, to estimate the expected life of the structure. Although such stiffness values may be provided by the manufacturer, it is quite desirable that they can be estimated by analytical models instead. However, any analytical model proposed for such a task must be checked with well-conducted experimental results in order to be considered as an acceptable analysis tool. The aim of this work is to present the main results involving axial-torsional tests in a 2.5" flexible riser, carried out at the Technological Research Institute of Sa˜o Paulo (IPT). Besides presenting full data concerning the internal structure of the riser, this paper describes the experimental procedures used to perform the tests and the main obtained results (e.g., Force × Displacement and Twisting moment × Displacement curves). Tests involving internal pressure were also performed and the obtained results are also presented in this work. Comparisons between analytically calculated values of the axial and torsional stiffnesses with those obtained experimentally are made and discussed. A brief discussion about the validity of some hypotheses which are usually assumed by analytical models found in the technical literature is made at the end of the work.

This paper describes the installation and related analyses of a subsea production manifold in 620 meters water depth at Campos basin, offshore Brazil. A pre-installed mooring system was used to moor the crane barge which lowered the structure. Time domain and frequency domain computer programs were used to evaluate the response of the system A sensitivity study was conducted in order to assess the added mass, damping, stiffness and excitation forces influences on the system response and the importance of such parameters in the computer model calibration is discussed. Model tests were performed in order to determine the manifold hydrodynamic coefficients to be input to a computer program to evaluate the displacements and forces during the descent of the structure. Offshore measurements were performed on the hoist forces, vessel motions, crane boom tip vertical motions and manifold vertical motions. The anticipated responses are compared with some of these measurements. The method used for lowering the manifold, the mooring system and the manifold orientation and positioning system are described. The design and installation strategy considered to adapt a conventional crane barge for installation beyond its nominal specifications, but considering safety and cost-effectiveness, are also described. Introduction In December 1995 the DL-2 production manifold was installed in 620 meters water depth at the Albacora field on the continental slope at Campos basin, offshore Brazil. The contract for fabrication was granted to a Brazilian company and the operator of the field, Petrobras, undertook the installation of the manifold through its owned 122 by 30.5 by t3.5meters pipe lay and crane barge BGL-1, The structure was first loaded out on 18* of November onto the deck of Petrobras transportation barge BS-5, transported to BGL-1 inshore and then lifted by BGL-1 crane and sea-fastened to the crane barge deck. The manifold was landed on the sea bed on 19th of December. The manifold dimensions are 19 m by 14.5 m by 7.5 m high and it weighs 4120 kN in air, which includes a foundation substructure, the manifold itself and a installation lifting tool. Shallow foundations were adopted and four 40 m2 footings were utilized to provide the required bearing capacity. For details about the functional features of the manifold please see OTC paper 8236. The seabed has a 5 degrees slope and the substructure was built so as to have the mudmats at the same inclination. The manifold had to be landed within +/- 10 degrees in heading, in a target area with 10 meters radius and within +/- 2 degrees in leveling. Should the later not be achieved, the manifold leveling system would be used. In this case anothervessel with drill pipe handling capability would be required. Installation planning The installation alternatives considered for lowering the manifold involved a drilling rig, a dynamically positioned crane vessel and a conventionally moored crane barge.

Flexible risers are complex structures composed of several concentric polymeric and steel armor layers that withstand static and dynamic loads applied by the floating production vessel and by the ocean environment. Determining the response of these structures when subjected to axisymmetric loadings (i.e., any combination of traction, torsion, and internal or external pressures) is an important task for the local structural analysis since it provides probable values for the loading distribution along the layers and, thus, allowing estimating the expected life of a riser using fatigue tools. Although finite element models have been increasingly used to accomplish this task in the last years, the simplicity and the reasonable accuracy provided by analytical models can be seen as reasons that justify their continued use, at least in the initial cycles of the design. However, any analytical model proposed for such a task must be checked with well-conducted experimental results in order to be considered as an acceptable analysis tool. The aims of this article are twofold: (i) to present the main results of experimental tests involving both internal pressure and traction loadings on a 63.5 mm (2.5 in.) flexible riser, carried out at the Institute for Technological Research of Sao Paulo (IPT), which can be used as a means of checking finite element or analytical models proposed by other researchers, and (ii) to compare some results obtained experimentally with those predicted by an analytical model which can also include any combination of axisymmetric loadings. Besides presenting full data concerning the internal structure of the riser, the experimental procedures used to perform the tests and the main results (e.g.. Force x Displacement curves) are also presented. A brief discussion about the validity of some hypotheses that are usually assumed by analytical models found in the technical literature is made.

In August 2001 the MRL-5 production manifold was installed by PETROBRAS in 940 meters water depth at the Marlim field offshore Brazil. The semi-submersible Amethyst, using an 18-5/8” marine riser, deployed it into the location. During the manifold deploying, several in-site measurements of the hook forces (force at the drill line dead end) and the semi-submersible accelerations were done. Both time series for the vertical accelerations and forces were obtained for two positions of the manifold along the water column. The main objective of this paper is to compare the results from the column riser system numerical analysis with the riser axial forces measurements obtained by the monitoring system.