Abstract

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.

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