A pipeline system consists of the flexible pipe and all associated ancillary components up to the connecting components at battery limits. The global system design deals with aspects related to the overall flexible pipe system — not cross-section or component design. Key system design issues to address are specified in ISO 13628-11:2007 (API 17B) and include general system design requirements, flowline design requirements, and interfaces involving different contractors and suppliers. Unlike unbonded flexible risers that operate in tension, flexible pipe laying on the seabed will be subject to compressive forces and global buckling as for rigid pipelines.
The global flexible pipeline system design will use relevant design codes such as ISO 13628-11:2007 and API 17B / 17J. Yet, the terminology and what such design encompasses can be interpreted differently amongst the community of flexible designers, installers and operators. Failure modes, for example, are not perceived in the same way. While axial compressive force may not be allowed for a riser design, it is a normal flexible pipe response to expansion when pressurized on the seabed. Pull-in and connection forces and moments may be perceived as maximum loads by installers and subsea equipment suppliers but the pressure-induced hydrotest and operational loads may exceed these and overstress the piping leading to a HISC failure.
The system functional requirements for flexible pipelines are similar to rigid lines but when it comes to a flowline laid on the seabed, exposed to pressure and temperature, the methodology for establishing accurate predictable global behaviour is still under development in the industry.
To perform the global in-place design of a flexible flowline, a detailed understanding of the effects of pressure and temperature on the unbonded flexible construction with respect to its bending stiffness and expansion and the limiting criteria stipulated by the manufacturer is needed. The flexible’s behaviour can be simulated using a 3D FE model capturing the effects of seabed surface variations, interaction with soil, and entire load history from installation, to flooding and hydrotest to filling with product at a range of pressures and temperatures including start-up shut-down cycles. Whereas general requirements are stipulated in API 17, specific guidelines applicable to global buckling design are found in DNV-RP-F110 for rigid pipelines. This predictive analysis methodology using risk-based design criteria provides the basis for a robust design that can accommodate large variations in installation tolerances, uneven seabed, varying soils, etc., while providing all concerned with a sound understanding of the flexible behavior and interface loads throughout its design life.