At the time of installation, the Gemini umbilical at 28.1 miles was the longest umbilical containing both electrical and hydraulic elements. A super duplex tube construction was selected. The importance of adequate and comprehensive quality control of the tube strings and the installation stresses in deepwater are emphasized.
The reliability of thermoplastic umbilical has been under scrutiny for a least a decade. In 1990 a report was issued in the UK entitled Umbilical Performance and Reliability following an initiative two years earlier by number of major operators in the North Sea. The development of steel tube umbilicals provided an alternative construction. As installed water depths increased, the attraction of steel tubes increased due to their superior collapse resistance. In addition, the methanol "permeability" problem (the transport mechanism involves permeability and diffusion) is eliminated.
Steel tube construction presents a new and different set of challenges to ensure a reliable product. At the time of the Gemini project initiation in January of 1998, industry standards in the form of API 17E and API 17I were predominantly concerned with thermoplastic construction. Super duplex steel tube construction was experiencing major problems by a number of different operators. However the excellent mechanical and corrosion resistant properties of super duplex led the Gemini project team to specify this type of construction.
In April of 1999 the Gemini umbilical installation was successfully completed in water depths ranging from 3400 to 300 feet. This paper focuses on two aspects associated with the design, testing and installation of the project, which for the purposes of umbilical tube manufacture are currently inadequately covered by existing specifications, namely:
Installation loads and their effect on wall thickness for deepwater installations
ASTM G48A testing on super duplex welds
A description of the method adopted for the guidelineless deep to shallow installation of the umbilical with an attendant drilling rig is also described.
The general method adopted to determine wall thickness for umbilical tubes is based on ASME B31.3 chapter IX and ASME A789. Umbilical tube stresses should then be checked for installation loads when the construction will be subjected to:
Tensile stress from umbilical tension
Hoop stress from internal pressure
Bending stress from the squeeze effect of a tensioned helix
Bending stress from umbilical bending over the deployment structure (e.g. wheel)
The bending forces induced over a laying wheel and from caterpillar clamping force are very difficult to determine analytically. No satisfactory design methodology exists. The approach taken on Gemini was to perform a verification test on the completed umbilical. Although this establishes confidence, the test occurs at very late stage in the project and a failure at this stage would have resulted in delayed production. It is essential in the early design phase to develop conservative methodology to confirm that installation forces are not the determining factor for wall thickness. As water depth increases, tensile stresses increase proportionally.
