In the last 5 years, a number of key advances have been made with threaded and coupled (T&C) risers. These advances together with high strength steels have enabled these products to reach parity with the current standard of weld-on connectors. The T&C riser concept demonstrates the same reliability, field proven history and high fatigue performance.
This concept of connectors and engineered materials up to 125ksi, allow for significant weight savings on the riser system and platform construction. To meet the Global Energy needs now, the industry must move to deeper water and higher fatigue. Thus a new generation of connectors, fully optimized for fatigue, has been designed for inner and outer production riser applications.
These connections reach best-in-class fatigue performance and include innovative fatigue compliant sealing systems. The large number of full scale fatigue test results increases confidence to such a level that fatigue trends and influencing parameters now help to refine the qualification programs. This new standard will also allow the industry to look for new generations of heavy wall riser connectors for xHPHT, flow line, and SCR applications.
The purpose of this paper is to highlight the knowledge that has been accumulated in various fatigue enhancement techniques and the continuous improvement of analysis capabilities.
Over the last 10 years, the industry has moved to deeper water, and in the Gulf of Mexico, to higher temperature and higher pressure developments. Dry tree concepts have been prevalant and offer many advantages in terms of drilling and completion costs and long term well maintenance and intervention capability. Those systems have used forged and machined weld-on premium riser connectors in order to achieve the required fatigue performance in loop currents, and to provide metal-to-metal seals both internally and externally. External seals prevent the ingress of sea water and subsequent corrosion and fatigue degradation of the thread profile area. These weld-on connectors required the use of steel in grades of X-80 ksi and less in order to achieve the weld ductility necessary for the riser fatigue design. In many cases, the welds have also been ground "essentially flush" both externally and internally with strict procedural requirements, special equipment and exacting surface finishes in order to meet the performance requirements of long-term operations in areas of significant loop currents. The Horn Mountain Truss Spar set a world record for these type riser systems in 2002 of about 5,450' water depth and a pressure rating of 5,500 psi.
BP in the past has looked at dry trees for HPHT developments in the GoM, but the X-80 riser designs became so heavy they were not considered workable with existing air can technology. BP installed the Holstein Truss Spar in 2005, which was the first dry tree spar application of hydro-pneumatic tensioned risers, albeit with X-80 material and weld-on connectors. The tensioner concept was so successful from an operational standpoint, that air cans have become the secondary option, and tensioners the default/preferred solution. At the same time, BP was looking to the next generation of developments in water depths all the way out to 10,000'. In these extreme water depths, it became apparent, that the X-80 riser design was not cost effective, even with larger hydraulic tensioners. Switching to higher strength steel and a T&C connector design, however, have made the systems much lighter and became an enabling technology for dry trees. Now, dry tree systems in 10,000' water depth were technically feasible and cost effective. HPHT developments also benefited, as now, 15 ksi and greater riser systems could be designed and fabricated.