Insulation for deepwater project

Paul Boughton

Discovered in 1999, the Thunder Horse Field development in the Gulf of Mexico is now home to the largest production drilling quarters (PDQ) semi-submersible platform in the world.

It weighs more than 50000 tonnes, houses 229 people and is designed to produce 250000bb/d of oil and 200mmcf/d of gas (Fig.1). The platform is operated by BP, which has a 75 per cent stake in the field, with the remaining share being owned by ExxonMobil. It is located 150 miles south east of New Orleans in 2200 metres of water.

Because of the depth at which it operates, the platform’s flowlines are exposed to extreme external pressure and low temperatures. Combined with a flowing oil temperature of 132°C, this presented a huge thermal insulation challenge. In such deep water, normal foam insulation solutions are not feasible as the water pressure causes foam to collapse and lose its thermal properties.

The stringent requirements imposed on the thermal insulation system by deep water and high temperatures led Thermotite, a division of Bredero Shaw and a leading manufacturer of thermal insulation systems for offshore pipelines, to execute a rigorous development and qualification program capable of demonstrating the suitability of polypropylene based materials in these extreme environments. Through collaboration with Borealis, this qualification programme concluded by selecting novel Borcoat materials. The multi-layer insulation concept based on Borcoat polypropylene materials is a claimed breakthrough with respect to material selection for insulation of high temperature, deepwater flowlines and risers. It gives wide flexibility which will allow modification to match the needs of similar future projects.

The first of the seven-layer system is a fusion-bonded epoxy primer, the remaining six layers are made up of a range of Borealis Borcoat products. The combination is new in its entirety and some grades have been specifically developed for the Thunder Horse application (Fig.2).

To one of these new Borcoat products, Thermotite adds glass spheres during the extrusion process to create a syntactic polymer. The syntactic polymer layer acts as a thermal barrier and as thermal insulation on the risers. Also novel is the combination of two Borcoat grades that are mixed together with a blowing agent to create high strength foam, which further increases flowline insulation and resistance to extreme sea pressure. This combination of syntactic and blown foam is known as the Thermotite seven-layer system.

Through the combination of existing and newly developed products, Borcoat enabled the design of the multi-layer solution to meet the necessary strength and stiffness requirements, as well as the stable thermal insulation performance required during operation of the flowlines.

Thermotite has installed a state of the art facility for the application of anti-corrosion coatings on linepipe. The plant can handle pipe diameters ranging from 1-24 inch and applies fusion bonded epoxy (FBE) standalone or polyolefin (PP/PE) systems.

The pre-treatment facility includes fresh water washing, double independent wheelabrators, inspection and dressing of steel surface and a chromate wash. The pipe surface is preheated by induction coil to a minimum of 3°C above dew point before the blasting process. Surfaces are cleaned to a minimum blast standard of Sa 2.5. The surface is brought to coating temperature (200–240°C) by use induction heat.

The epoxy powder is applied to the induction-heated pipes with electrostatic guns, covering the entire pipe circumference. The epoxy powder builds a homogenous and continuous film. Immediately after FBE application, the pipes are quenched by water prior to quality control (QC) inspection and further handling.

Where multi-layer coatings are specified – and three is more usual than the seven required for the Thunder Horse project – a modified polyolefin copolymer adhesive is applied to the FBE during the gel period of the FBE. The adhesive layer reacts and bonds to the partly cured FBE, and forms a transition layer to the complete coating system.

The outer protective layers of polyolefin are immediately applied to the adhesive surface, ensuring 100percent adhesion. The adhesive and polyolefin are applied by a side extrusion method.

The coated pipe is then water quenched prior to QC inspection and further handling. Cut-back preparation is done using tools and techniques designed not to damage the line pipe steel. An FBE tail protrudes from below the polyolefin coating, in the cut back area, to allow for efficient field joint coating.

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