When to Use Flexible Pipe

Non-bonded flexible pipe, made to API 17 J specification, is a spooled product and typically competes with steel pipe in the offshore oil and gas sector for infield pipelines. Typical uses are oil and gas production, water injection, gas injection/gas lift, and chemical injection. Flexible pipe is not a natural choice for export pipelines because export pipelines are typically large diameter and long length. Flexible pipe is specified by the internal diameter; the typical maximum ID is currently 16-inches. Because of volume constraints on the reels, flexible pipe costs tend to become exponential above 8”ID (nominal 10” steel pipe equivalent).

Economics for Flexible Pipe

When considering flexible pipe economics, one must always include the installation cost in the evaluation. Rigid steel pipe is less expensive per foot than flexible pipe, but the installed cost of the steel pipe in certain scenarios is actually greater. For comparison to typical deepwater steel pipe as-installed costs, the rule of thumb is 70:30 steel pipe installation / pipe material cost vs. 30:70 for installation / flexible pipe costs.

Rigid steel pipe may be reeled for faster installation compared to welding individual 40 ft. joints together offshore. But there is a lot of preparation work involved. The initial welding is done onshore without vessel cost, then fully spooled together at the shore base with a lower vessel rate, and the installation occurs faster because of the continuous length. For short distances, however, fixed costs work against the overall as-installed cost of reeled steel. These costs include: weld testing, spoolbase set-up, PLETs and jumpers at each end, and initiation/hold-back piles at the beginning of the lay.

 

Some scenarios whereby a flexible could be a better choice over a steel pipeline are:

  • Short tieback distances: PLETs, jumpers not required
  • Corrosion resistant alloy requirements: stainless steel is standard for flexible pipe
  • High dynamic motion/fatigue requirements: lazy wave SCR v free hanging catenary flexible riser
  • Jumpers for steel pipe flowlines: metrology precision is less critical
  • Limitations in a brownfield development: congested seabed, smaller departure angle, reduced top tension

Short Tieback Distances

Typically, flexible pipe is delivered on large spools or “reels”, 28ft-35ft in diameter with 400 tonne weight capacity. Several installation vessels have the capacity to load out with 4 reels in one trip or transpool from reels to a carousel at the shore base. These carousels can usually take 6-10 reels worth of flexible pipe, depending on the OD and weight of the pipe. With these numbers in mind, flexible pipe is cost competitive with standard reeled steel pipe up to the tieback length that would cause a second mobilization of flexible pipe reels. For a 5”ID flexible pipe, this is about 5 miles.

Corrosion Resistant Alloy Requirements

When corrosion resistant alloy is required at the pipe bore, steel pipe solutions will look to Inconel inlay or mechanically lined pipe, sometimes stainless steel or duplex pipe. Even for low levels of CO2, if the temperature and/or pressure are high, CRA pipe may be required. The cost increase for the steel pipe solution is significant and weld qualification can be a schedule/ cost risk. The flexible pipe typically uses 316 stainless steel as its base carcass layer whether CRA is specified or not, so cost adder for flexible pipe is nil. If higher spec CRA is required, like duplex, the total cost impact for the flexible pipe is typically less than 5%. So, if CRA materials are required, consider the flexible pipe even for longer lengths, especially up to a nominal 8” steel pipe equivalent (roughly 6” ID flexible pipe).

Tricia Hill
July 2017

Tricia Hill is a Georgia Tech graduate with a Bachelors of Mechanical Engineering degree. Tricia has 20 years of experience in the subsea oil and gas industry with substantial experience working at one of the major flexible pipe manufacturers, GE Wellstream, in flexible pipe design, analysis, testing, and commercial operations from 1997 to 2015.