This paper discusses the utilisation of steel tubes in subsea control and injection umbilical. It examines what has happened in the past, where the industry is at the present time, and the future direction in respect of these product types. It also contrast differences in design requirements between thermoplastic hose and stell tube components. The contents of the paper are provided to assist the reader to appreciate and comprehend the difficulties encountered by the umbilical manufactures and operators in successfully utilising this product type within subsea hydrocarbon production systems.
It is not the intention of this document to prejudice specific tube materials and/or the companies and processes associated in their manufacture
Umbilicals form a critical link in a subsea production system and, despite the talk of umbilical-less control systems, it seems likely this vital linkage will be a part of the industry for the foreseeable future with both thermoplastic and steel tube components or combination thereof. It is in the industry's interests that whichever component type/combination is chosen, that the product is fully understood, properly designed and well manufactured to provide the Operators with a lasting, trouble free product
Subsea production systems have been utilised for almost 40 years and in that period of trine many hundreds of subsea wells have been installed worldwide. It is unusual, even now, with this large number of wells to find two systems which are alike The process of technical evolution, coupled with the design engineer s desire to continually innovate, has resulted in the constant redesign and customisation of subsea production systems.
In the pioneering era of subsea production, wells were remotely controlled using umbilicals incorporating thermoplastic hoses bundled together and contained within an extruded thermoplastic sheath. Seabed stability and tensile strength was achieved by means of a wire rope tied to the umbilical during installation. Such products were prone to damage and the wire rope was replaced by the two layer wire armour concept which provides all-round mechanical protection to the internal functional components.
During the mid 1980's, with relatively short tie-back distances, director piloted hydraulic control systems were a design consideration for subsea production systems. As a consequence some interest was shown in using steel tubes for control lines where response tune was considered to be an important was consideration. Some umbilical were supplied containing steel tubes, mainly for the Brazilian market.
However such products had limited success due to the following factors
The umbilical were supplied as a 'flatpack constriction? whereby the steel tubes are laid parallel to each other embedded in a thermoplastic polymer extrusion. Such a product with minimal functional component protection, present installation difficulties due to its uni-directional bending characteristic and, additionally effects with sea currents.
Steel tubes were constructed in stainless Steel (grade 316 and were prone to corrosion.
Outside of Brazil, steel tubes in umbilical service started in the early 1990's and in recent years have found increasing usage, with the trend to deepwater subsea production systems, higher operating pressures and the requirement to withstand very high seabed hydrostatic pressures.