With advances in marginal field development, subsea completions, and production step-out programmes, the need for small diameter high pressure flowlines has continued to increase. To meet this demand a wide spectrum of projects specific unbonded layered flexible pipes have been engineered using state-of-the-art computer-aided design and materials selection technology. This paper highlights the circumstances in which such products offer technical and economic advantages over traditional steel pipe solutions.
The quayside cost of flexible pipe has hitherto generally been several times the quayside cost of corresponding rigid steel pipe. The use of flexible pipe has therefore been confined to applications where there was an obvious inherent technical advantage, for example dynamic risers and subsea jumpers, or where there was an economic advantage associated with the shorter mobilization time, lower dayrate, and faster laying speed of the flexible pipe deployment vessel - it is the installed cost on the seabed that is of interest to the operator
The scope for using flexible pipe is widening rapidly, however, as a result of the increasing number of subsea well tie-backs involving aggressive process fluids and difficult environmental conditions and as a result of the increasing importance of extending the installation season in the interests of achieving early production The expanding demand is being met by advances in materials technology and a re-thinking of the traditional criteria for the use of flexibles Competitive flexibles for unsophisticated applications are also now available.
This paper does not specifically address dynamic risers or subsea jumpers (though many of the considerations addressed apply equally in those cases) since the need for flexibles in those applications is usually obvious Attention is centred rather on subsea flowlines and on the opportunities which taylored flexibles have to offer in this context
Reference is made to authors who have previously addressed the uses of flexible pipe Among these are Dubois 1980, Strader 1982, Colquhoun 1982, and Nielsen 1983
Flexible pipe is likely to offer technical and economic advantages over rigid steel pipe in the following situations
Isolated flowlines of limited length where installation mobilization costs are predominant, Colquhoun 1988,
Flowlines of all lengths in situations where the need for early production necessitates short weather-window off-season installation,
Flowlines in extended well-test situations or other limited operational life situations which favour the flexible, which can be recovered and redeployed,
Flowlines in deep water, where the flexible is much more easily and quickly installed than the rigid pipe, Strader & Nielsen 1982,
Flowlines of all lengths with sweet low-pressure process conditions ("sweet & low") which enable cheaper materials to be used,
Flowlines of all lengths in which the aggressiveness of the process fluid would require a rigid pipe to be made of expensive stainless steel or would require a significant capital and operational expenditure on a corrosion inhibition system, Hill 1989 (OTC 6 1 14) and Hill 1989 (Corrosion 89),
Flowlines of all lengths in which the production fluid leaving the wellhead is at an elevated temperature (typically above 800C) which would result in wax dropout or hydrate formation due to falling temperature along the line in the absence of thermal insulation
