Flow assurance including thermal insulation are critical elements in the design and operation of flowlines and risers in deep waters due to a combination of low temperatures, high pressures and economic drivers for high availability. The stringent requirements put new challenges on insulation systems and the paper will discuss a suitable insulation system that can meet these requirements.
Over the past ten years, thermal insulation of subsea flowlines and risers has become increasingly important. With the advent of multi-phase flow in flowlines and risers from subsea completions, possibilities of wax and hydrate formation prevailed. Thermal insulation is used to prevent hydrate and wax formation during shutdowns and to maintain the fluid temperature inside the flowlines for easier fluid separation topsides or onshore1.
For single pipe flowlines and risers, the mechanical loads as well as the thermal insulation requirements normally increase with deeper waters. Hence, the traditional thermal insulation foam technology used in shallow waters and the associated design and test methodology may not be applicable to deep water projects.
Polymer foams change mechanical and thermal properties as a function of foam density. Higher density normally means better mechanical properties and reduced density improves insulation capacity. For deep water thermal designs, this could lead to build up of excessively thick coatings that may cause manufacturing concerns as well as reducing installation vessel capacity. In addition, excessive coating thickness may reduce seabed stability for the flowline and increase drag on a steel catenary riser (SCR).
The paper will describe the development and qualification related to a novel deep water thermal insulation system for single pipe flowlines and risers based on polypropylene (PP). Included items are materials development, designmethodology and test methods, qualification tests and a brief description of the first installation of this system in the Gulf of Mexico. Figure 1 shows a typical build-up of such a thermal insulation system.
Figure 1. Thermal Insulation System for Single Flowline. (Available in full paper)
The development program defined a set of performance criteria for the insulation coating. These requirements are shown in Table 1. Relevant aspects related to the different load scenarios for installation and operation were defined in the functional requirements. Reeling produces the highest stress and strain in the coating during installation, especially below 0°C and this was the selected installation method for qualification.
As described in the introduction, the foaming process of polymers generally lead to a trade-off between mechanical properties and thermal insulation properties. The increased hydrostatic head associated with deeper waters calls for higher compressive strength of the PP-foam. Higher compressive strength also improves creep characteristics and can generally be attributed to higher polymer stiffness and the final foam structure.
The new generation PP materials are based on a unique balance between stiffness, toughness and good long-term creep resistance. This heterophasic PP material is a highly crystalline material with a finely distributed and dispersed ethylene-polypropylene rubber phase.