Seamless steel tube for use as a steel catenary riser in the touch down zone

Abstract

The present invention describes an upset SCR of novel low carbon chemical composition and microstructure as well as method of manufacturing the same, which achieves higher improvement in the fatigue life as it is integral with the riser pipe section at the Touch Down Zone. The low carbon upset SCR achieves its desired properties by the thermal treatment which it is subjected to. The novel low carbon chemical composition and microstructure comprises in weight per cent, carbon 0.04-0.10, manganese 0.40-0.70, silicon 0.15-0.35, chromium 0.40-0.70, molybdenum 0.40-0.70, nickel 0.10-0.40, nitrogen 0.008 max, aluminum 0.010-0.045, sulfur 0.005 max, phosphorus 0.020 max, titanium 0.003-0.020, niobium 0.020-0.035, vanadium no more than 0.10, copper 0.20 max, tin 0.020 max, and carbon equivalent 0.43 max and PCM no more than 0.23 and having a yield strength of at least of 65000 psi, the ultimate tensile strength of at least 77000 psi and YS / UTS ratio below 0.89 in material representing the pipe body, the transition zone and the upset end.

Description

FIELD OF THE INVENTION[0001]This invention relates to seamless steel tubes for use as steel catenary riser.BACKGROUND OF THE INVENTION[0002]In recent years, the interest in exploiting deeper water off-shore oilfields has increased sensibly. As a consequence, various solutions of marine production systems have been developed. The currently-available solutions are directed generally to semi-floating and floating production systems which are subjected to various movements with respect to seabed, mainly due to marine waves, currents and tides phenomena. The aforementioned systems are complemented by compliant riser systems compatible with mobile surface stations.[0003]Steel Catenary Risers (SCR) represents one of the most outstanding riser systems to be adopted in these challenging situations. Such component is normally subjected to complex spectra of fatigue loading related to both the mobility of the floating platform and to the very large free span of unconstrained line from the seab...

AI Technical Summary

Benefits of technology

[0015]The present invention describes an upset SCR of novel low carbon chemical composition and microstructure which achieves higher improvement in the fatigue life as it is integral with the riser pipe section at the Touch Down Zone. The low carbon upset SCR achieves its desired properties by the thermal treatment which it is subjected to. The novel low carbon chemical composition and microstructure comprises in weight per cent, carbon 0.04-0.10, manganese 0.40-0.70, silicon 0.15-0.35, chromium 0.

Problems solved by technology

Such component is normally subjected to complex spectra of fatigue loading related to both the mobility of the floating platform and to the very large free span of unconstrained line from the seabed to surface.

The application of SCRs is challenged by, in some cases, the high fatigue damage at the Touch Down Zone (TDZ) from a combination of field specific parameters such as riser size, fluid characteristics, vessel motions, metocean parameters, soil conditions, and water depth.

In this zone, the riser experiences the highest level of cumulative fatigue damage.

The severity of fatigue loading in the TDZ is further complicated by the presence of continuous impacts of the portion of the line when entering in contact with the ground.

In other words, constant motion by the topside floating vessel results in cyclic pounding for the riser against the sea-floor that, if not properly designed, can result in fatigue failure.

In addition to the riser motion, other factors that can increase the severity of the TDZ fatigue include large pipe diameter, deep water depth, high currents, and sour service (corrosion degradation).

In most of threaded cases, though, the increase in fatigue life has been limited to a factor between 2 to 3.

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