High-Strength Aluminun Alloys for Deepwater Riser Applications
Authors: Gelfgat M. , Grebtsov N. , Podrazhansky A. , Vygodsky B. , Tikhonov V. , Shaposhnikov V. , Chizhikov V. 
Published: October 2008

This paper introduces the experience in development of drilling riser made of aluminum alloys (ADR). The first commercial ADR string was qualified for 2000 m operational depth. The submerged weight of the aluminum alloys for riser manufacturing is only 80-85% greater than the weight of water. The development of ADR was made possible by many years experience in operating of high-strength aluminum alloy drillpipes and different marine systems in Russia. Number of tests was conducted to study the alloy chosen for the manufacturing of the main riser tube. The samples were tested for static and fatigue strength in different media, impact elasticity and fracture toughness in seawater. The test agenda also involved metallographic examination, corrosion and wear resistance study. The full-scale ADR prototypes were tested for tensile strength, fatigue life and internal pressure. Strong consideration was given to assembling of the extruded tubes and flanges into a 75' long ADR joint. The finite-element analysis was used to provide a thorough strength evaluation of the riser and riser components. The experience in the ADR development shows that high-strength aluminum alloys and available manufacturing technology can be successfully applied in the development of ultra-deep water drilling risers with a variety of coupling designs, high-pressure riser, production risers and other offshore oil and gas systems.

Introduction

The world's hydrocarbon reserves are basically concentrated in the subsea areas. The depth of offshore exploration has been increasing in the past thirty years. The water depth of 1,000 m was overcome in an exploratory well drilled in 1975 but only in 1995 in a production well. Today the deepwater riser drilling record is set at a water depth of 2,956 m, while it is expected that the number of oil fields to be developed in ultra deep waters ranging from 1,500 to 3,000 m will increase. Currently, oil and gas production depths range from 1,000 to 1,500 m with the record depth of 7,208 ft (2,197 m). The qualified fields are found at 10,000 ft water depth and the production systems for 9,000 ft (2,743m) have been designed. [1, 2].

Increased depths required specific equipment for drilling, development and operation of fields. The current technologies provide the use of metal tubular structures (risers) for deepwater applications for all stages of field development. Deepwater risers have extraordinary stringent requirements for the strength, reliability and service life, that is why they are basically fabricated of steel tubes. However, the major limitation to the use of steel is its relatively high density or its low specific strength, i.e. the ratio of strength to the specific weight (syield/?) has the value being rather low as compared with other known structural materials. This characteristic places limitations on the potential riser extension. In the early of 1990s the researchers focused on titanium alloys and composites as structural materials in developing of the production risers design. Today the production prototypes of such risers are being tested [3].