PIPELINE SYSTEMS 

Wet insulation systems are flowlines or pipelines that have insulation coatings applied directly to the pipe or corrosion coating. The insulation is not protected by a steel jacket pipe as with either pipe-in-pipe or bundled systems. The essence of a non-jacketed insulation is that it must be able to withstand the hydrostatic pressure of the subsea environment, the crushing loads during installation and retain its insulation properties in wet environment for the predicted field life.

Wet insulation and corrosion protection systems have been used in the North Sea for more than 15 years and were initially based on the use of bitumen and polyethylene elastomers. Good experience has later encouraged the supplier industry to develop numerous options of solid and foamed polymers/elastomers material that can be tailor-made for most applications and demands in shallow water.

For the growing activities in deeper waters (>300m) the options are, however, reduced to the use of solid materials, special engineered polymer composites and epoxy syntactic with hollow glass or silicate microspheres, that can sustain a water depth of more than 1000m and a temperature of 135°C.

The pipeline wet coating is then composed either of external anti-corrosion coating or combination of anti-corrosion and insulation coatings.

Anti-corrosion coatings are covered by DNVGL recommended practice RP-F106.

Common systems used include:

The primary insulation materials used are polymeric coatings. There are various options for thermal insulation coatings although generally they fall into three categories:

Solid coatings in polyurethane (PU) or polypropylene (PP) are of low relative thermal efficiency compared to the syntactic or foamed coatings so are generally used either as field joint coatings, water or thermal barrier layers in multi-layer foam systems.

Syntactic coatings consist of microspheres within a polyurethane or polypropylene matrix. The microspheres are either plastic or, for deep water applications, glass. Syntactic coatings have a greatly improved thermal performance compared with solid coatings, whilst retaining significant strength. Syntactic coatings can be applied in thick layers up to the order of 150mm although typically they would be in the range 20-50mm.

Generally they are reelable although the polymer specification must be carefully selected to avoid cracking of the coating.

Foam coatings in PU or PP are made by blowing the polymers with CO2, N2 or water. The density of the foam can be controlled and this determines the “wall thickness” of the foam bubbles. The thermal conductivity of foams is proportional to the density: high density = high conductivity, low insulation performance, greater strength.

Most insulation requirements tend to be in the range between 2 and 5 W/m²K. Typical pipe coating systems for this insulation duty are:

It is worth mentioning that SPU uses polymer beads, hence the lower water depth capability. Foamed PP systems are capable of greater water depths when used with a base syntactic or solid layer.

These would be applied over a corrosion protection coating comprising:

Note that the above depth and temperature limitations may not both be attainable together. For example it may be possible to get polyurethane foam to achieve either 150 m or 100°C rating, but not both at the same time.

Some of the limitations in the above can be overcome by using composite systems. For example, by applying a layer of solid polypropylene between the pipe and a layer of polypropylene foam, the temperature in the foam is reduced and the system can be used up to temperatures of 140°C.

There are two main concerns with wet insulated pipeline in deepwater applications:

Coating suppliers for deepwater applications are, e.g. SOCOTHERM Group and BREDERO SHAW (Thermotite).

The Figure 5.1, “5Layer Pipeline Insulation coating” shows the Thermotite® 5-layer System build up. The 3-layer anti-corrosion coating is applied by a side or cross head extrusion process and the quality tested and approved, prior to the application of the thermal insulation layers (2-layer; PP Foam and PP outer shield).

Figure 5.1 - 5Layer Pipeline Insulation coating

Field joint coatings are covered by DNV recommended practice RP-F102.

Pipeline joints are generally factory coated with a cut-back of typically 150mm to 250mm from each end to allow for welding of joints into the pipeline string. This means that a field joint coating needs to be applied following welding. On an S-lay or J-lay barge this coating needs to be applied as part of the continuous lay process, and therefore requires a rapid setting system. For reel lay, the field coating process is less time critical as, for most joints; it is performed during the fabrication of the pipe stalks.

Field joints for insulation coatings are typically solid or syntactic polyurethane although foam half-shells can be included at the field joint if necessary. In reeling applications, special consideration of bonding and joint stiffness at the field joint is required to ensure the integrity of the field joint coating and bending of the pipe itself. In reeling applications the field jointing process typically consists of:

A summary of the specific limitations of polymer coatings is presented below:

While conventional polymer based coatings are the most common form of insulation for subsea pipelines they have a limited operational envelope in terms of thermal conductivity, temperature and water depth. To meet production requirements beyond that provided by conventional pipe coatings a pipe in pipe system is often proposed.

Pipe in pipe is generally specified where a low OHTC value is required. A low OHTC value is generally considered to be below 2.0W/m²K.

A pipe in pipe consists of a product carrying flowline that is installed within a carrier pipe. The space between the flowline and carrier pipe is known as the annulus. The annulus provides a dry environment that generally exists close to atmospheric pressure. This annulus allows insulation materials (e.g. polymeric PU foam, mineral wool, Porextherm Fumed Silica, Aspen Aerogel) that can be classified as non-marine to be used on the pipeline as they are not subjected to either moisture or hydrostatic pressure.

The attractiveness of pipe-in-pipe flowlines for deepwater is because they are simple to fabricate, use low cost proven materials, and because a high strength protective steel jacket offers nearly unlimited depth capabilities. However, the deepwater versions of these systems can be cost prohibitive for small or marginally economic fields.

The technical performance of the insulation materials should be considered in parallel with a costing of the insulation material. The overall cost of the insulation is not just a function of the material cost. The following factors should be considered when selecting an insulation material.

Mineral wool insulation is a significantly cheaper insulation material than both Porextherm Fumed Silica and Aspen Aerogel, it is also likely to be much easier to apply to the pipeline.

However to achieve comparable OHTC values it will be necessary to have a larger OD carrier pipe. This will be more expensive in terms of steel quantity, welding time and it may result in a greater number of vessel trips. In cases such as these the higher more expensive insulation materials may result in a lower overall project cost. As an example Table 5.2.1 describes 3 systems that provide an OHTC (Uvalue) value between 0.7 and 0.9W/m²K.

The example case has been selected to show that improving insulation material can result in a reduction in carrier pipe size for a set of similar OHTC values.

Figure 5.2 - Pipe in Pipe 3D View (Typical)

For reel-lay installation, for a long deepwater pipeline requiring multiple trips to a field remote from the spool-base the Aerogel option is likely to be a more attractive proposal as the cost of insulation material will be small in relation to the fabrication costs and vessel transit times. Conversely for a short shallow water pipeline requiring a single pipelay trip the Porextherm or Mineral wool options become more attractive. The selection of the insulation material should therefore consider the overall effect that this will have on the pipeline design and alternatives should always be considered.

Polyurethane foam (PUF) insulation is one of the most common pipe in pipe insulation materials. PUF has been used successfully for both S-lay and J-Lay projects. However use with reel lay has been limited due to concerns over potential damage to the insulation during reeling.

The main thermal insulation materials that are normally considered for use with pipe in pipe are detailed below in Table 5.2, “Thermal insulation material for pipe in pipe system”:

Drawing a vacuum in the annulus of a pipe in pipe is considered to have potential for providing insulation. There is no material cost associated with this system and a very small annulus gap is feasible allowing the outer pipe diameter to be reduced. This system is currently available, and commonly used, as insulated drill pipe. For drilling applications single joints of vacuum sealed pipe in pipe are supplied. Reflective (low emissivity) aluminium foil or Mylar is commonly included with vacuum insulation systems as the radiation component of heat loss can also be minimised. This will also be applicable for pipe in pipe where multiple layers of reflective foil will reduce the OHTC value. The disadvantage of this system is the practicality of achieving and maintaining a pressure low enough to provide a satisfactory level of insulation (typically below 0.1mbar). The ability to apply and maintain such low pressures over long pipeline lengths is unknown and presents the most significant challenge with vacuum systems.

Technip is evaluating the development of the LNG transfer from a platform to a remote offloading station. This offloading station is primarily a Single Point Mooring (SPM) system, which will ensure two functions:

The SPM can be a buoy (catenary anchor leg mooring type) or a small platform (steel jacket type) fitted in both cases with a turntable enabling the ship (moored by hawsers fastened to her bow) to freely turn around and remain head wind. A cryogenic flowline will connect the SPM to the platform, which can either be a LNG receiving terminal or a production plant located in shallow water.

This cryogenic line must feature an insulation performance similar to that required for large diameter LNG loading lines on land based LNG facilities (terminal or liquefaction plant). However, the conventional insulation system used on those lines (and also LNG carriers) i.e. large thickness of polyurethane foam (up to 160 mm) protected by a plastic vapor barrier is not suitable for such a subsea application. In this case, a robust casing must protect the thermal insulation, ensuring:

The pipe-in-pipe technology is particularly suited to meet the above basic requirements and has therefore been retained for this new application. The main components of the cryogenic pipe in pipe are given as follows:

Technip has performed a comprehensive qualification test programme for the developed cryogenic PIP technology. Tests have been performed on full scale test pipe (26 Invar x 32 S/S, 10 m long) using liquid nitrogen, with a detailed engineering approach.

The objectives of the test programme are the followings:

Figure 5.5 - Prototype as tested with Invar inner pipe (Technip)