In order to avoid that the loads generated by the floating units movements be transferred through flexible risers to subsea equipment, significantly additional flow lines length have been used to absorb those loads by line/soil friction. Due to the high costs of these lines, Petrobras has developed a new anchor system concept to fix the risers on the seabed, in lieu of increasing flow lines length. This anchor system, called "Torpedo", consists of a pipe pile filled with scrap chain and concrete, close ended with a cone tip, installed by free fall from a vessel, and connected to the risers with a special system operated by ROV. Bundle of three risers have been anchored by each pile with holding power of 1400 kN. Taking advantage of the experience in this pile design and installation to anchor risers, larger free fall piles with holding power up to 7000 kN have been designed and already certified to anchor floating units. This paper aims to present an overview of the design and installation procedures, cost estimation, field and model tests results and applications of the Topedo Anchor System. Introduction and Background Flexible risers are anchored on sea bed by means of drag anchors or plates, gravity structures and piles which may be installed by driving, suction or drilled and cemented in. Because of the high costs of these anchoring method for deep water, since 1996 Petrobras has developed the technology of free fall piles to be installed in soft clay to anchor flexible risers and floating structures. To evaluate the costs and performance of this solution, in house softwares have been developed and full scale in situ tests have been performed in Campos Basin (see Fig. 1). Model tests (see Fig 2), with different weights and shapes were made in laboratory, in order to optimise the torpedoes design. Torpedo Anchor Characteristcs The torpedoes designed to anchor flexible risers is a 30" OD pipe pile, twelve meters long, filled with scrap chain and concrete (see Fig. 3). It has a dry weight of 240 kN and a holding capacity of 1400 kN. The pile/mooring line connection has an internal padeye underneath a guide that allow the anchor load to be applied in any direction. With these characteristcs, it has been used to anchor a bundle of three risers up to 12" of diameter in 1300m water depth. Torpedoes Anchors with that same 30" OD tube twelve meters long, ballasted with heavier material and provided with four vertical fins (0.45 m × 9 m), are being fabricated to anchor MODU's (Fig. 4). To anchor Floating Production Systems (FPS) with a required holding capacity of 7500 kN, torpedoes designed with 42" OD tube, fifteen meters long, dry weight of 950 kN, and provided with four vertical fins (0.90 m × 10 m), are being manufacturated.

Petrobras developed a new kind of anchoring device known as Torpedo. This is a steel pile of appropriate weight and shape that is launched in a free fall procedure to be used as fixed anchoring point by any type of floating unit. There are two Torpedoes, T-43 and T-98 weighing 43 and 98 metric tons respectively. On October 2002 T-43 was tested offshore Brazil in Campos Basin. The successful results approved and certified by Bureau Veritas, and the need for a feasible anchoring system for new Petrobras Units in deep water fields of Campos Basin led to the development of a Torpedo with High Holding Power. Petrobras FPSO P-50, a VLCC that is being converted with a spread-mooring configuration will be installed in Albacora Leste field in the second semester of 2004. Its mooring analysis showed that the required holding power for the mooring system would be very high. Drag embedment anchors option would require four big Anchor Handling Vessels for anchor tensioning operations at 1400 m water depth. For this purpose T-98 was designed and its field tests were completed in April 2003. This paper discusses T-98 design, building, tests and ABS certification for FPSO P-50.

This paper presents a numerical based study on the undrained load capacity of a typical torpedo anchor embedded in a purely cohesive isotropic soil using a three-dimensional nonlinear finite element model. In this model, the soil is simulated with solid elements capable of representing its nonlinear physical behavior and the large deformations involved. The torpedo anchor is also modeled with solid elements, and its geometry is represented in detail. Moreover, the anchor-soil interaction is addressed with contact finite elements that allow relative sliding with friction between the surfaces in contact. A number of analyses are conducted in order to understand the response of this type of anchor when different soil undrained shear strengths, load directions, and number and width of flukes are considered. The results obtained indicate two different failure mechanisms: The first one involves significant plastic deformation before collapse and, consequently, mobilizes a great amount of soil; the second is associated with the development of a limited shear zone near the edge of the anchor and mobilizes a small amount of soil. The total contact area of the anchor seems to be an important parameter in the determination of its load capacity, and, consequently, the increase in the undrained shear strength and the number of flukes and/or their width significantly increases the load capacity of the anchor.

Usage of FPSO in shallow water always presents challenges to keep flexible risers’ stresses at the TDP (TouchDown Point) below acceptable values. Depending on the water depth, the catenary configuration can impose excessive high stresses at the TDP and for this reason, configurations like Lazy S, Steep S, Lazy Wave, Steep Wave, Pliant Wave and other complex configurations should be used. Choosing the type of configuration is always dependent on, among others, local environmental conditions, FPSO’s behavior and supports, the water depth, the number of risers, bottom arrangement of them, possibility of line clashes and mainly the cost of the solution. Depending on the number of risers to be supported, construction, installation and keeping good reliability of structural solutions generate considerable increases in the overall investment and maintenance costs. The paper presents two innovative fixed submerged supports of flexible risers, in lazy S configuration, and their installation procedures, that manage to lower construction and installation costs and also decrease inspection tolls due to increased reliability of the structure in comparison to floating buoy solutions.

The paper describes the development of a soil investigation equipment, consisted of a piezocone installed at the tip of a torpedo-pile. The new equipment, named torpedo-piezocone, is able to measure cone resistance (qc), sleeve friction (fs), pore-pressure at the cone face (u1) and cone shoulder (u2) as well as cone temperature during free-fall and some time after final stop. Velocity, as well as displacement (depth) are obtained from accelerometer data, as in the case of the torpedo-pile. The various steps to develop the equipment are presented, from the requirements of the transducers until the calibration procedures in the laboratory. The first tests performed onshore are also presented. In general, very good results have been obtained.