Abstract

In a corrosive downhole environment, the service life of a completion can beseverely reduced if suitable precautions are not taken. Pipe made of corrosion resistant alloys (C.R.A.) is used to minimize corrosion. Recently, steel manufactures have developed modified corrosion resistant alloys (C.R.A.), specifically derivatives of Cr13, that are characterized by a black smooth scale surface on the outside of the pipe.

When these so called "Super" or "Hyper" alloys were first used, occasional slipping was reported by field personnel. Fortunately no string was lost and noinjuries occurred, however, precious rig time was lost in resuming safe operations. These slippages occurred with various inserts and with all types of slip-type elevators or spiders.

An investigation carried out by the University of Hannover revealed that the surface scale on the pipe had a hardness of over 60 Rc. Even inserts specifically designed for C.R.A. pipe that use special gripping elements, suchas the Micro-Grip™ system have a hardness that is only slightly higher than this surface scale. Using conventional wedge type spider and elevator systems can lead to slippage under certain load conditions, if no special precautions are taken.

Based on these results and on the operating principles of wedge type spiders and elevators, a new Compact Spider/Elevator was designed. This device can be either air or hydraulically operated and puts an additional downward force ontothe slip, as much as 40 tons for the hydraulic version, so that a firm grip isassured. In addition to conventional steel inserts, the device can be equipped with so called Micro-Grip™ inserts, a gripping system that distributes the required load equally onto a large number of small peaks. This minimizes the indentation of each single peak and thus, as previous research and fielde xperience has shown, preserves the surface integrity of the pipe with regard to corrosion [2, 3, 4]. The device can be run flush-mounted, further enhancingsafety on the rig-floor.

1 Introduction

In a corrosive downhole environment, the service life of a completion can beseverely reduced if adequate precautions are not taken. This is especially true of tubular material made of corrosion resistant alloys (C.R.A.), which is often used to minimize corrosion. Due to the great variety of downhole conditions within a production well such as temperature, dissolved and undissolved gases and their depth-dependent partial pressures, an optimal corrosion protection system requires comprehensive planning. Analysis of the downhole conditions leads to the selection of the appropriate material to assure the best lifetimeunder the corrosive environment present.

The pipe manufacturers have reacted to the special requirements of theoperating companies. The main component used to improve the corrosion resistance of an alloy is the addition of chromium. C.R.A. alloys with up to 13% percent chrome content are commonly used and occasionally the content of chrome is higher to cope with special corrosion conditions.

Conventional spider and elevator inserts cause die marks that can promote corrosion. A gripping system named Micro-Grip distributes the required load equally onto a large number of small peaks, minimizing the indentation of eachsingle peak.

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