Although flexible flowlines had been used earlier elsewhere with varying levels of success, the advent of their use in the North Sea occurred in the mid-1970s
Early instances of their utilization in the North Sea occurred in Mobil's Beryl Field (1975), Shell's Brent Field (1976), Beryl (1978), Hamilton's Argyl Field (1979), Beryl (1979) and Chevron's Ninian Field (1979)
These applications were almost invariably concerned with the transport of oil from subsea development wells to a central surface installation for manifolding, processing and export ashore.
At about the same time flexible flowlines were employed extensively m the development of other offshore oil-producing areas, nota6lyTWest-Africa, Brazil, the Middle East and, to a lesser extent, in the Gulf of Mexico.
In certain circumstances the use of flexible flowlines presents useful technical and economic advantages over the use of steel, or rigid, flowline In broad terms, these advantages become significant when connections have to be made over relatively short distances, and where the presence of surface installations, or subsea equipment, restricts the access of the laying vessel. The virtue of flexibility is a considerable asset when several flowlines have to be accommodated and connected in close proximity to each other, e g at subsea templates and manifolding centres.
The technical benefits may be summarized as follows
the ability to accommodate misalignments,
it may be installed from small, manoeuvrable lay-vessels,
the ease of installation in restricted areas,
the ability to curve around obstructions,
the ability to conform to an irregular sea bed,
the ease of recovery for repair on the surface
It is difficult to separate completely the economic from the technical implications, as one is frequently interdependent on the other However, the principal economic advantages of flexible flowline are derived from the greater ease and simplicity of its installation and connection compared with rigid steel flowline
In general terms, the economc benefits include
the lower day-rate for laying vessel,
the faster laying speeds attainable,
the reduced time required to connect ends,
that span rectification/stabilization are seldom required,
the flowline may be recovered and used again elsewhere
Situations frequently arise in which two lengths of rigid line have to be connected, and in which there is a limited degree of control over the relative location of the ends of these lines The effects of thermal expansion may have to be accommodated, or the connection may have to be effected across an area of unstable or u-regular sea bed
Rigid spool pieces are costly, requiring accurate on-site measurement by divers of the relative location and orientation of the flanges to be connected, followed by manufacture and test of sections of rigid line to bridge the measured gap.