To keep the conveyed fluid warm, the pipeline should be heated by active or passive methods. The active heating methods include, electric heat tracing wires wrapped around the pipeline, circulating hot water through the annulus of pipe-in-pipe, etc. The passive heating method is insulation coating, burial, covering, etc.
Glass syntactic polyurethane (GSPU), PU foam, and syntactic foam commonly are the commonly used subsea insulation materials (see Figure 8.2.1). Although these
insulation materials are covered (jacketed) with HDPE, they are compressed due to hydrostatic head and migrated by water as time passes, so it is called a “wet insulation”.
Figure 8.2.1 GSPU (left) and Syntactic Foam Insulation (right)
OHTC or U value is used to represent the system‟s insulation capability. Lower U value prvides higher insulation performance. Heat loss can occur by three processes:
conduction, convention, and radiation. Conduction is a heat transfer through a solid by contact, and convection is a heat transfer due to a moving fluid. Radiation is a heat exchange between two surfaces (heat is radiated to the surrounding cooler surfaces).
Good insulation can be achieved by minimizing the above heat loss processes.
Conduction is dependent on material size and thermal conductivity. Convective heat transfer (film) coefficient can be obtained from internal and external fluid Reynold‟s and Prandtl numbers.
The OHTC or U value can be obtained using the formula below:
h1 = internal surface convective heat transfer coefficient hm = external surface convective heat transfer coefficient r = radius to each component surface
K = thermal conductivity of each component
For example, the U value for a 6.625” OD x 0.684” WT pipe with a 1” GSPU coating is:
Pipe r1 = 2.6285” r2 = 3.3125” K1 = 30 Btu/hr-ft-oF GSPU r2 = 3.3125” r3 = 4.3125” K2 = 0.096 Btu/hr-ft-oF Neglect FBE corrosion coating and HDPE outer jacket and assume h1 & h3 = 1,000 Btu/hr-ft2-oF.
8.3 Pipe-in-Pipe
Another pipe insulation method is pipe-in-pipe (PIP) which an inner pipe is covered by a larger outer pipe (Figure 8.3.1). The annuls between inner pipe and outer pipe are filled with insulation materials including: micro-porous silica (Aerogel), polyurethane foam (PUF), Wacker/Porextherm, Mineral wool, etc.
Figure 8.3.1 PIP
Aerogel
Microporous silica with a pore size of 10-9m.
Best U value 0.0139 W/m-oK at 50oC.
The density is 0.11 SG.
Developed for the reeling process and many track records exist.
Requires centralizers with a spacing of every 2m or so.
Cheaper than Wacker/Porextherm product.
PUF
2nd cheapest form of insulation.
2nd poorest U-value (0.029 W/m-oK at 50oC) of all insulation materials but used extensively for S/J-lay projects, normally without centralizers.
Densities are in the range of 0.07 - 0.12 SG.
Use with reel-lay has been limited due to potential damage (compression and crack) during reeling.
Wacker/Porextherm
Fumed microporous silica with a pore size of 10-6m. Wacker is purchased by Porextherm.
Most expensive thermal insulation product.
Good U-value (0.0195 W/m-oK at 50oC).
Standard density is 0.19 SG.
Developed for the reeling process and many track records exist.
Requires centralizers with a spacing of every 2m or so.
Mineral Wool
Cheapest form of insulation.
Poorest U-value (0.037 – 0.045 W/m-oK at 50oC) of all insulation materials but used extensively in the North Sea.
Densities are in the range of 0.1 - 0.12 SG.
Not good for low U value unless combined with other method such as heat tracing.
PIP system requires bulkheads, water stops, and centralizers, depending on fabrication methods. The end bulkhead is designed to connect the inner pipe to the outer pipe, at each pipeline termination (see Figure 8.3.2). Intermediate bulkheads may require for reeled PIP to allow top tension to be transferred between the outer pipe and the inner pipe, at intervals of approximately 1 km. During installation, the tensioner holds the outer pipe only, so the inner pipe tends to fall down by its dead weight and may result in buckling at sag bend area near seabed, if no intermediate bulkheads exist.
Figure 8.3.2 End Bulkhead
Bulkhead Flange
Outer pipe Inner pipe
Water stops (see Figure 8.3.3) are installed to limit the pipeline length damaged in the event that the annulus is flooded by pipeline failure or puncture. Considering low fabrication cost and low heat loss, it is recommended to install one or two water stops per each stalk length. The stalk length varies, due to spool base size and pulling capacity, typically between 500 m to 1,500 m. It should be noted that the water stops are not a design code requirement but they are recommended for deepwater project where recovery of the flooded pipeline is challenging.
EPDM (ethylene propylene diene monomer) rubber, Viton (a brand of synthetic rubber), and silicone rubber have been used for the water stop material. The axial compression for the water stops is provided by using an interlocking clamp arrangement which will provide the radial expansion of the ring against the pipe walls.
Centralizers or spacers (see Figure 8.3.3) are polymeric rings clamped on the inner pipe for reeled PIP:
to protect insulation‟s abrasion damage during insertion of the inner pipe into the outer pipe
to protect insulation‟s crushing due to bending load while reeling
to protect insulation‟s crushing due to thermal bucking during operation
The centralizer works as a “heat sink” due to its high thermal conductivity (~0.3 W/m-oK , 10 to 20 times higher than insulation materials). Therefore, reducing the number of centralizers by increasing the centralizer spacing (2 m typical), or centralizer-less design can reduce both the material and fabrication/installation costs.
Figure 8.3.3 Water Stop Seal (left) [1] and Centralizer (right) [2]
Outer Pipe Inner Pipe
Insulation
Net Gap Centralizer
Annulus Gap
For the reeled PIP, the annulus gap needs to be sufficient to put insulation material, centralizer, and clearance gap to account for the weld beads, welding misalignment, pipe manufacturing tolerances, etc. The annulus gap should be in the range of 30 to 40 mm and the net gap (between insulation and outer pipe ID) should be 15 mm or higher (see Figure 8.3.4). The maximum reeled PIP that has been installed by Technip is 12.2”
x 17” PIP for Dalia Project.
Figure 8.3.4 Reeled PIP with Centralizers
The PIP can be used for cold products such as LPG (liquefied petroleum gas) and LNG (liquefied natural gas) to keep the product as cold as possible. For example, LNG flows at -256°F (-160°C), and the LNG pipelines need to be kept below a certain temperature and above a certain pressure to prevent vapor generation. The LNG is commonly transported from ship carrier (LNG tanker) to onshore facility via thick insulated pipelines installed on a jetty. Dredging may be required along the ship channel to facilitate vessel access to the jetty. To control the pipeline contraction due to cold product temperature, frequent expansion loops are also required.
Recently, many subsea LNG pipelines are under development. The advantages of subsea LNG pipelines include; increase security due to pipeline buried under the seabed, low cost of jetty construction and dredging, no expansion loops, no insulation coating damage, and sound control of thermal cyclic fatigue, etc. Some challenges of subsea cryogenic LNG pipelines are; effective insulation system (vaccum, Nanogel, Aerogel, IzoFlex, etc.) and special cryogenic materials for pipe, forgings, and welding consumables. Either 36% nickel alloy (Invar) or 9% nickel alloy is typically used for the inner pipe of the cryogenic LNG pipelines [3]. A triple PIP (pipe-in-pipe-in-pipe) system is introduced by ITP (InTerPipe) to transport LNG through subsea [7].