EuroWire March 2018

Technical Article

The greatest difficulty in obtaining higher strengths is to not lose ductility (often attributed by high amounts of cold work), which will ultimately create fatigue failures. The following controls are essential in providing galvanised steel wire with ideal properties: • Electro-magnetic stirring and efficient rod cooling to avoid degradation and unwanted microstructures • Rod patenting to attain higher strengths through a fine pearlitic microstructure • Increased inter-stand cooling and an optimised die configuration to avoid strain ageing In addition, the high temperature galvanising process can drastically reduce ductility. Therefore, the design and placement of the galvanising operation becomes a challenge that has to be overcome. working, moderate levels of drawing soap residue that remain on the wire can prevent wire adhesion with the cable. This introduces a new obstacle because increased amounts of soap residue reduce wire heat (which reduces strain ageing). A balance between these operations is essential to produce a high strength wire with good ductility. From rod chemistry selection and cooling, through to heat-treating and cold work, every step is critical to obtain ultimate performance. ▼ ▼ Figure 5 : Steel chemistry and rod cooling, rod patenting (normalising) and cold work (high-speed drawing) Additionally, during cold

▲ ▲ Figure 3a, 3b, 3c : Examples of umbilical and subsea cables

▲ ▲ Figure 4 : Cable failures

Problems with cables can fall into many areas, such as: • Incidents during transportation or installation • Design flaws • Incorrect usage • Fatigue failure • Manufacturing problems • Damage from shark attacks and marine growth • Accidental damage Mechanical failure is usually attributed to excessive axial tension or torque, or excessive bending. In the steel layers, issues can include separation and instability due to birdcaging, mechanical degradation due to wear and fatigue of the steel, or exposure to adverse external agents such as pressure and temperature. In the steel tubes, failure can also occur due to excessive external pressure or hose/ tube bursting due to excessive internal pressure. Deep ocean cables require high strengths to restrain elongation and maintain long life cycles. As subsea depths become greater, the strength requirements become harder to attain. The cable strength is attained by using galvanised steel wire. Thicker wire can add more strength, but it also adds weight. Therefore, the greatest benefit is achieved through higher ultimate tensile strengths (UTS). The main contributing factors to UTS are: • Steel chemistry and rod cooling • Rod patenting (normalising) • Cold work (high-speed drawing) Many of these issues are often interrelated.

platforms, subsea equipment and shore. This layer can have multiple layers of steel armour around the cable; this armour layer can be constructed from either round or slat strip. Figure 3b shows a typical example of a subsea steel tube umbilical used for hydraulic control and chemical injection lines for wellhead control systems. Figure 3c shows a typical example of a subsea umbilical being an assembly of hydraulic hoses, which can also include electrical cables or optical fibres, used to control subsea structures from offshore platforms or a floating vessel. The need for technological advances has largely been driven by a combination of issues: the rising demand for oil and gas; environmental concerns regarding the need for increased reliability and safety in all subsea equipment relating to oil and gas production; and a need to extract oil and gas from more difficult areas of the oceans, usually with deeper water. Deep water requirements are increasing the demand for the steel layers to be capable of withstanding of the more dynamic applications the cables have to be subjected to. This, in turn, leads to the demand of the steel materials to have higher fatigue resistance and higher strength, in order that specific umbilicals and power cables can be smaller, leaner, lightweight designs. At the same time the safety factors must be kept in high order so that the cable’s guaranteed life, which is in the order of 20 years, is not put at risk.

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March 2018