Spun Steel Pipes For The Offshore Industry
Authors: Royer Alain , Dumas Bernard , Gantois Michel 
Published: October 2008

During the last few years, many efforts have been spent throughout the world to develop weldable steels with a high yield strength and with transition temperature below minus 40° C, for manufacture of pipes and offshore platform components having to withstand very severe climatic conditions.

However, many parts either for sea-line pipes such as "buckle" or "crack arrestors", or for structures may require the use of wall tubular products with high mechanical properties. Such heavy-wall pipes may be produced by centrifugal casting.

After a brief description of the centrifugal casting technique and its advantages, we introduce the grades developed for heavy-wall pipes with yield strengths up to 100,000 psi. The mechanical properties, Battelle, fatigue, static bending, C.O.D., weldability, corrosion tests, etc... of Centrishore II (R) are given and compared to other material.

Possible Offshore applications and other potential applications of parts produced by centrifugal casting are described.

Introduction

Two Mn-Mo steels have been developed for medium-wall pipes (e ? 35 mm) to be used under very severe climatic conditions: an acicular ferritic steel, a pearlite reduced steel produced by controlled rolling techniques (1) (2) (3). More alloyed chemical composition and heat-treatments are needed to produce heavy-wall pipes. Then production of such pipes are more difficult and sometimes impossible.

Observations made on controlled-rolled Mn-Mo steel led to a better understanding of the influence of metallurgical structures and chemical composition on steel characteristics. Similar metallurgical structures can only be reached via other routes, for example centrifugal-casting of steel associated with heat-treatment lead to the production of heavy-wall pipes with high strength and suitable transition temperature.

We aim to develop by centrifugal-casting, low-alloyed steel pipes with the following characteristics:High yield strength.Good toughness at low temperature.Excellent weldability at the mill and in the field.Compatible with the Oil Industry Specifications.

Principle Of Centrifugal-Casting Of Steel

Centrifugal-casting means rotating the mold while filling it with a liquid metal. There exists many types of casting machines in Industry but the most widely used is described in Fig. 1.

The whole technique is based on the utilization of both centrifugal and gravitational forces but other parameters such as metal inertia, internal frictions, viscosity, pressures on the mold wall are of the utmost importance.

The centrifugal force which appears once the mold is rotating around its axis radially to the rotating axis is used to regularly fill the metal in the mold cavity.

According to the nature, size, weight or number of pipes to be manufacture, the mold can be either in metal or sand.

The usual rotational speed employed results in forces of 100 g. which allow a homogeneous structure in the mold.

Once the molten metal is poured in the mold, a pressure gradient takes place through the thickness of pipe to be manufactured due to the centrifugal acceleration.