Pipeline Thermal Insulation

As oil/gas production fields move into deeper water there is a growing need for thermal management to prevent the build up of hydrate and wax formations in subsea systems. The thermal management strategy is chosen depending on the required U-value, cool-down time, temperature range and water depth.

External Insulation Coating System

Figure 1 shows a typical multi-layer coating system which combines the foams with good thermal insulating properties and PP shield with creep resistance. These coatings vary in thickness from 25mm up to 100mm or more. Typically thicknesses over about 65mm are applied in multiple layers.

Figure 1. Typical multi-layers Insulation System

Coating systems are usually limited by a combination of operating conditions including: temperature, water depth and water absorption. Combinations of temperature and hydrostatic pressure can cause creep and water absorption, with resultant compression of the coating and a continuing reduction in insulating properties throughout the design life. These issues need to be accounted for during design.

Insulation Material
Table 1. External Insulation Systems (Click to enlarge)

Table 1 summarizes the insulation types, limitations and characteristics of the most commonly applied and new systems being offered by suppliers. The systems have progressed significantly in the past 10 years on issues such as: thickness limits, water depth limitations, impact resistance, operating temperature limits, creep with resultant loss of properties, suitability for reel lay installation with high strain loading, life expectancy and resultant U-values and submerged weight.

Brief descriptions of external insulation systems for subsea systems are given below:

• Polypropylene (PP); polyolefin system with relatively low thermal efficiency. Applied in 3-4 layers, can be used in conjunction with direct heating systems. Specific heat capacity 2000 J/(kg-K);
• Polypropylene-Syntactic PPF (SPP); controlled formation of gas bubbles are used to reduce the thermal conductivity, however as ocean depth and temperature increase, rates of compression and material creep increase. Generally not used on its own;
• Polypropylene-Reinforced Foam Combination (RPPF); a combination of the above two systems incorporating a FBE layer, PPF and an outer layer of PP to minimize creep and water absorption;
• Polyurethane (PU); a polyolefin system with relatively low thermal efficiency; has a relatively high water absorption as the temperature increases over 50 ^C. Commonly applied with modified stiffness properties to field joints;
• Polyurethane-Syntactic (SPU); one of the most commonly applied systems over recent years; offers good insulation properties at water depths less than 100 m. Specific heat capacity, 1500 J/(kg-K);
• Polyurethane-Glass Syntactic (GSPU); similar to SPU but incorporates glass providing greater creep resistance; specific heat capacity 1700 J/(kgK).
• Phenolic Syntactic (PhS), Epoxy Syntactic (SEP) & Epoxy Syntactic with Mini- 

  Spheres (MSEP); based on epoxy and phenolic materials which offer improved 

  performance at higher temperatures and pressures. These materials are generally applied by trowel, pre-cast or poured into moulds. For example SEP was used on the 6-mile King development by pouring the material under a vacuum into polyethylene sleeves. Specific heat capacity 1240 (J/kgK).

PP and PU are the main components of insulation coating systems. The syntactic insulations, incorporating spheres are used to improve insulation strength for hydrostatic pressure.

Figure 2. Thermal Insulation
http://www.f-e-t.com/images/uploads/sea-sleeve-for-field-joints-on-concrete-weight-coated-cwc-pipelines.gif

Structural Issues

Installation and operation loads:

Coating systems for installation through reeling need careful selection and testing, as some systems have experienced cracking particularly at field joints where there is a natural discontinuity in the coating. This can lead to strain localization and pipe buckling. Laying and operation of sub-sea pipelines requires the load transfer through the coating to the steel pipe and from the steel pipe to the coating. The coating should have a sufficient shear load capacity to hold the steel pipe during the laying process. Thermal fluctuations in the operation process lead to expansion and contraction of the pipe. The thermal insulation coating is required to follow such changes without being detached or cracked. These requirements have resulted in most thermal insulation systems being based on bonded geometries, comprised of several layers or cast-in shells.

Hydrostatic loads:

Deepwater flowlines are subject to significant hydrostatic loading due to the high water depths. The thermal insulation system must be designed to withstand the large hydrostatic loads. The layer of foamed thermal insulation is the weakest member of the insulation system and therefore the structural response of the insulation system will depend largely on this layer. Elastic deformation of the system leads to a reduction in volume of the insulation system, and an increase in density and thermal conductivity of the foam layer, which leads to an increase in the U-value of the system. In order to reduce the elastic deformation of the system stiffer materials are required. The stiffness of the foams increases with increasing density. Therefore to reduce elastic deformation of the system, higher density foams are required.

Creep:

Thermal insulation operating in large water depths for long periods (e.g. 20/30 years) can be subject to significant creep loading. Creep of thermal insulation can lead to some damaging effects, such as structural instability leading to collapse, and loss of thermal performance. Creep of polymer foams leads to a reduction in volume or densification of the foam. The increased density of the material leads to an increased thermal conductivity and therefore, the effectiveness of the insulation system is reduced. To design effective thermal insulation systems for deepwater applications the creep response of the insulating foam must be known.

The factors affecting the creep response of polymer foams include temperature, density and stress level. The combination of high temperatures and high stresses can accelerate the creep process. High density foams are desirable for systems subject to large hydrostatic loads for long time periods.

The design of thermal insulation systems for deepwater applications is complex and involves a number of thermal and structural issues. These thermal and structural issues can sometimes conflict. In order to design effective thermal insulation systems, the thermal and structural issues involved must be carefully considered to achieve a balance. For long distance tie-backs the need for a substantial thickness of insulation will obviously have an impact on the installation method due to the increase in pipe outside diameter and the pipe field-jointing process. As most insulation systems are buoyant, the submerged weight of the pipe will decrease. It may be necessary to increase pipe wall thickness to achieve a submerged weight suitable for installation and on-bottom stability.

Source:Bai, Yong and Bai, Qiang. Subsea Pipelines And Risers. USA: Elsevier Inc. 2005

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