Flow assurance issues are driving more and more the design of Deep Water Flowlines, and Cool Down Time requirement has a major impact on the global system design for most West African subsea field developments.
A new thermal insulation solution has been developed, named ILS (Liquid solid Insulation), based on the use of phase change materials (PCM). This paper presents the principle of the ILS solution and the main results achieved through the development program, from the theoretical analysis to the qualification tests.
In normal flowing conditions, the PCM liquefied by the oil's heat flux acts as a heat accumulator. During production shutdowns and resulting cool down, the phase change material's crystallization restores partially this stored heat to the flowline. Therefore, such an insulated coating produces a significant thermal inertia to the flowline and the cool down delay before hydrates formation is 2 to 4 times longer with regard to the existing insulation technologies.
Due to the incompressibility, the low density and the low cost of chosen phase change materials, ILS is able to deal with deepwater & ultra deepwater flow assurance requirements and to compete with existing pipe-in-pipe or syntactic thermal insulation.
It is now obvious that construction and exploitation of deep sea and ultra deep sea offshore fields induce specific and novel concerns, with regard to conventional shallow water offshore. One of the more critical problems is flow assurance at such water depths, for both steady state production and shutdown / restart conditions.
The first deep sea flowlines characteristic is their need to be efficiently thermally insulated, in order to preserve a sufficient production temperature on the FPSO topsides, and also to avoid solid deposits on the pipe wall : actually, deep offshore fields whose exploitation is forecast to be profitable are often spread over very large areas, which implies longflowline layout on the sea bottom, in low temperature sea water. Moreover, due to the depressurisation inside high deep offshore risers, the gas phase expansion and the light cuts vaporization produces a severe cool down ofproduced effluents.
Another specific concern about deep sea flowlines is the risk of gas hydrate plug formation, at relatively high temperature under high pressure, especially during production shutdown and restart episodes. The cool down time to reach the hydrate formation temperature in shutdown condition is a major requirement of deep sea flowlines ; it must be sufficient enough to allow the completion of flowline emptying operations, or to start a dedicated heating feature.
In order to fulfill these requirements, the current or emergent technical solutions can be sorted in the following two categories :
Passive thermal insulation : Pipe-in-pipe technology or syntactic foams, ensuring simultaneously the insulation function and the cool down time function.