6 Insulation Techniques
6.1 General
Deepwater offshore oil and gas fields are economically developed using subsea completions, where produced fluid may travel a quite long distance, before reaching process facilities on the floating production system.
During transport, the produced fluid could cool down to the ambient seawater temperature as low as 4°C or less. Studies and experiences have shown that produced fluid at such a low temperature could cause unacceptable emulsion, hydrate, and paraffin wax deposition problems.
Field proven solutions to above problems are discussed below:
Emulsion: Installation of crude heaters and/or injection of demulsifying agents on the floater could be used to breakdown emulsions, but these alternatives required space, weight and cost on an already congested floating production system.
Hydrates: Once again, establishing facilities for continuous injection of methanol or glycol to depress hydrate formation temperatures add burden to the floating production system and increase operating costs.
Wax: same impact as above if injection of wax suppressants is implemented.
An insulated Flowline (Sealine + riser) approach could overcome these problems by keeping the produced fluid temperature above a required temperature for the different operation modes: e.g. 40°C for production mode and 13°C for well testing.
6.2 Selection of an insulation material
The first step in engineering an insulated riser or Sealine is to find an appropriate insulation material. Properties and requirements to be considered included thermal conductivity, weight (density), compressive strength, availability, manufacturing technique, attachment methods etc.
In order to select the type, and estimate the amount required, a computer program is used to calculate temperature drops in a riser or Sealine with a single phased fluid. A variety of cases are then calculated for various Flowline lengths, production rates, water cuts, etc. – assuming insulated coatings ranging from different types of insulation material.
The selected material should provide sufficient insulation value to meet design criteria, retain its insulation properties in wet environment, resist to installation loads and finally have sufficient compressive strength to keep it from being crushed under hydrostatic pressure encountered in deepwater field development.
6.3 Insulation material for steel riser and hybrid riser tower
For the growing activities in deeper waters the options are limited to the use of solid materials, special engineered polymer composites and epoxy syntactic with hollow glass or silicate microspheres.
Such constructions may be supplied with a thickness of 100mm and a thermal conductivity k=0.1W/m°C, corresponding to a heat transfer U-value of 1.5W/m2°C.
The most promising thermal insulation material in deepwater applications are syntactic foams which fall into two groups as described below:
Pure syntactic foam composed of base polymer as initial constituent with a specific gravity around 1.0 hence the material is almost neutrally buoyant. The density of the polymer is reduced by including large numbers of small hollow glass spheres known as microspheres. The microspheres typically have a diameter of between 100 and 150 microns. Their presence can result in a reduction of the specific gravity to between 0.5 and 0.6. This material is well adapted to rigid steel riser.
Composite syntactic foam where a third component known as macrospheres is added to further reduce the material density. Macrospheres are typically hollow thermoplastic spheres with a nominal external diameter of 50mm. Inclusion of the macrospheres can reduce the syntactic foam specific gravity to between 0.3 and 0.4. This thermal insulation material is well adapted to hybrid riser.
The main thermal insulation materials that are normally considered for use with wet insulated steel riser are detailed below in Table 6.1, “– Thermal insulation material for steel and hybrid riser”:
Table 6.1 - – Thermal insulation material for steel and hybrid riser
Material | Max Water Depth | Thermal conductivity | Density |
(m) | (W/m/°K) | (kg/m3) | |
Syntactic Polyurethane + Glass micro-spheres (ISOTUB, BALMORAL, BPCL) | 1800 | 0.13 | 830 |
Syntactic Polypropylene (ISOTUB) | 900 - 1800 | 0.16 | 710 |
Syntactic Foam = epoxy resin + microspheres + macrospheres (CRP) | 3000 | 0.12 | 500 |
Test Syntactic Polyurethane (Joint venture project) | 2750 | 0.1 | 700 |
Syntactic Tape (also used for flexible lines) | 1000 | 0.11 | 640 |
Multi-layer Polypropylene | 950 - 1070 | 0.17 | 750 |
Insulating Elastomer | 1000 | 0.12 | |
Thermoplastic Rubber | No limit (R&D) | 0.16 | 1029 |
6.4 Insulation material for flexible riser
It should be stressed that the thermal insulation properties of a typical flexible production riser are extremely good in comparison with those of a wet insulated rigid pipeline, due to its multiple plastic layers.
However, in some instances, where the temperature loss along the riser must be kept to a minimum, the thermal insulation of a typical flexible riser is not sufficient.
Several methods are available in order to increase the thermal insulation properties of a flexible riser. The main methods used at present are:
Increasing the thickness or changing the material of the thermoplastic layers (double internal thermoplastic sheath, double external thermoplastic sheath).
Using a special thermal insulation design based on coiling Cofoam material around the pipe. Cofoam (about 1500 kg/m3) is an extruded semi-rigid polyvinyl chloride (PVC) foam (see Figure 6.1, “Thermally insulated flexible pipe”).
Using tape wound on the pipe and composed of hollow glass microspheres, in the size range of 100-200 microns, fibreglass macrospheres 0.124-0.5 inches in diameter and an epoxy, polypropylene, or polyester resin binder.
It should be noted that the Coflon (thermoplastic material used in flexible riser to cope with high temperature produced fluid) has a lower thermal conductivity than Polyamide which in turn has a lower thermal conductivity than high density polyethylene. The thermal conductivity coefficient of these thermoplastics is very low:
K Coflon = 0.16 Kcal/m.h.°C at 34°C and 0.14 Kcal/m.h.°C at 104°C
K Cofoam (Carizite) = 0.13 Kcal/m.h.°C at 70°C
K Polyamide 11 = 0.288 Kcal/m.h°C (between 50 and 100°C)
K High density polyethylene = 0.35 Kcal/m.h.°C at 20°C
For 6-8 inch ID flexible riser, a typical heat transfer coefficient U-value of 1.5-2W/m²K can be achieved with "Carazide" (or Cofoam) insulation material.
Please refer to chapter “Insulation techniques” in document for further information on thermal insulation material applied to deepwater Flowlines.
