Subsea Umbilicals 

Most of the umbilical fabricated to date have used thermoplastic tubes as fluid carriers. Frequently, the electrical conductors are installed separately, but they have been included in some designs, usually termed "electrohydraulic umbilical". Separate control and chemical umbilical were often chosen because of the possible incompatibility between one of the injected chemicals and the hose liner. Such an incompatibility could then result in a chemical attack of the critical element i.e. hydraulic hoses and electric cable insulation with the subsequent loss of control and a total field shutdown. Although chemical compatibility testing was carried out to verify the suitability of the hose liner for transporting the specific chemicals (and alternatives) there was always the risk that an incompatible chemical could be introduced over the field life leading to failure. In addition, the accelerated ageing tests in which the hose liner material is tested with the chemicals at elevated temperatures, although satisfactory, cannot be considered as a fully guaranteed guide to long term deterioration. They are useful in that they will identify most incompatibility problems in a reasonable time.

In the umbilical design, there is a tendency to request duplication of the electrical components, both power and signal. In addition, there is often a requirement to design the umbilical to minimize the cross-sectional area. This may be because of limitations in the I/J tube diameter or handling capacity limitations of transportation or lay vessel reel/carousel.

Power conductors, signal pairs and hydraulic hoses of different diameters and with different elasticity properties are then bundled together using either a planetary machine resulting in a helical lay-up or an alternate clockwise-anticlockwise method known as S-Z. This latter method has a distinct advantage in that it can lay-up longer lengths without joints because of the size limitations of cable drums which can be accommodated on planetary machines.

In addition to the components having different elasticity, the back tensions of each component as they are transferred from the reels or drums to the lay-up machine may be unequal. This residual tension combined with the effects of dynamic forces due to installation vessel motions, the compressive forces due to hydrostatic pressure and potential compression due to the near position in free hanging configuration, may lead to the breaking of electrical cables.

Electro-hydraulic umbilical riser should be designed to withstand dynamic loads induced by FPS. Such umbilical would have a very high degree of radial symmetry and the lay-up angles for all conductor cables and hoses would be high to give the necessary flexibility. They would therefore have a larger diameter and would occupy much more machine time in manufacture than the equivalent static product. As a result, they would be typically 50% more expensive than an umbilical designed for static use.

During the early period of subsea production system implementation (e.g. 80’s), compatibility was not considered to be a major problem, particularly with Polyamide 11, because of its outstanding chemical resistance. Where compatibility evaluations were undertaken, these were performed using dumbbell samples of liner material immersed in the test fluid at elevated temperature (usually 70°C or 100°C). The dumbbell was manufactured nominally 2 mm thick from a sheet of material produced by means of an injection moulding process.

Life prediction was generally based on the half-life principle, whereby the time for the elongation at break to reduce to the 50% level was determined. This prediction that the rate of a chemical reaction doubles for every 10°C rise in temperature, was used to extrapolate the minimum service life at operating temperature.

For example, if a series of tests performed on liner material immersed in a fluid at 70°C shows that it takes 7 months for the elongation at break to reduce to half its original value, the predicted life at 20°C would be extrapolated as shown in Table 6.1, “Prediction life of liner material (50% life improvement for every 10°C reduction)”:

The increasing use of subsea production systems and increasing offset distances and the need to inject well service chemicals, has resulted in a wider range of control fluids and a proliferation of well service fluids. These fluids tend to be mixtures of chemicals contained within a solvent base. At this time it had been observed that one of the hose liner materials exhibited considerable anisotropic features and, it was felt, that perhaps the dumbbell test method may not necessarily be a realistic test for such materials, particularly, with fluids of a "cocktail" nature.

Whilst recognising the simplicity and low cost features of the dumbbell test, a more elaborate yet simple low cost test was employed to address the anisotropic aspects. Instead of immersing dumbbells in the fluid at ambient pressure, samples of extruded tube were immersed and instead of measuring the elongation at break as a function of time, the burst pressure of the tube was monitored with time.

In order to compare the sample immersion testing with pressurised hose testing, the pressure cycling compatibility test method was developed and consists in filling the hoses with control fluids, which would be subject to pressure cycling at elevated temperatures.

The qualification programme for a standard hydrostatic hose test in accordance with [20] for hoses up to 10,000 psi rated working pressure is as follows:

Stability of the service fluid is an important consideration and any instability could give rise to localised incompatibility with a hydraulic line.

Stability testing of the service fluid should be undertaken to qualify a fluid and to highlight any potential problem areas. It is understood that API 17F specification (Ref.[4]), "Subsea Control Systems", will include such tests.

The same consideration needs to be given to well service chemicals where separations of products are not uncommon. Where hydraulic lines are installed in a vertical shape, e.g. J-tube, Floating Production System, the potential for separation is greatly increased. The impact of any fluid modifications, however minor, should be fully evaluated before being introduced into a hydraulic line.

Regardless of the materials of construction of a hydraulic line (polymeric, elastomeric or metallic), compatibility testing should be performed with the actual line design to be used in service, in order to minimise the risk of problems arising after manufacture of the umbilical system.

With thermoplastic polyester, the small volumes of additives (lubricant, biocide, etc.) in the control fluid can have a significant effect on the rate of chemical reaction between the fluids and the polymer.

For polymeric hose liners, the rate of increase in chemical reaction is more severe when compared with the historical methods based on dumbbell and tube samples, accelerated without stress.

For polyamide 11, the material would be expected to withstand chemical compatibility and stress ageing. It has been demonstrated that a service life in excess of 20 years for temperature up to 40°C is possible; there are examples of at least 16 years extended service life experience for typical North Sea operational temperatures.

Polymer mechanical properties are shown below in Table 6.2, “Polymer mechanical properties”:

For methanol, the following Table 6.3, “Permeation rate of methanol in Polyamide 11 and Thermoplastic Polyester (69 bar)” shows the permeation rates which have been measured for Polyamide 11 (Pa11) and Thermoplastic Polyester at atmospheric pressure:

The permeation rate of methanol in XLPE lining material for internal pressures of 344 bar and 69 bar are presented in the following Table 6.4, “Permeation rate of methanol in XLPE”:

Remarks : Comparing the permeation at 4°C and 69 bar, of the XLPE to the Polyamide 11, it can be seen that the permeation rate through the XLPE is 750 times less than that of the Polyamide.

Subsea requirements have changed considerably over the past 20 years. Deeper waters, longer offsets, higher reservoir pressures and temperatures - combined with an increased use of chemicals for paraffin, hydrate and corrosion control - are too demanding for traditional thermoplastic umbilical and have resulted to the development of steel tube umbilical.

Thermoplastic umbilical cannot sustain operating pressure and temperatures beyond a certain level (e.g. 5000psi, 60°C). Response time is slow due to volumetric expansion. There are also problems relating to fluid contamination and chemical permeation, along with the fact that some umbilical are difficult to clean and keep clean due to the ligamers leeching into the conveyed fluid. Additionally, end fittings are swaged onto plastic hose and are not reliable at high pressure or under the abrasiveness of well control chemicals.

A viable alternative to thermoplastic umbilical is steel tube umbilical, which offers many advantages over plastic materials. Response times increase dramatically. Much higher internal pressures at large diameters (1” at 15,000psi, for example) are achievable. Fluid contamination and chemical permeation are drastically reduced.

Steel tube umbilicals are also easily cleaned and they remain clean. The tubing end fittings are welded, ensuring their integrity under pressure and the corrosiveness of well control chemicals.

Water depths of current interest limit the use of thermoplastic tubes, since they tend to collapse. Metal tubes can be sized for collapse and will not meet a limit until well beyond 10,000 feet (3000 m) for the range of diameter considered for umbilical applications. Metal tube umbilical of reasonable size should not be limited by collapse.

The steel tube umbilical design allows the steel tubes to carry the axial and compressive loads, which eliminates the need for a costly and cumbersome armour wire package that can unravel or "birdcage". The steel tube umbilicals are also lighter and have a smaller outside diameter than the equivalent thermoplastic umbilical while maintaining favourable minimum bend radii and displaying better on bottom stability.

Yet, for all the benefits over conventional thermoplastic systems, the steel tube umbilical is dealing with proven technology, and not a "new" product. Steel tube umbilical evolved from readily familiar pipeline, piping and pressure vessel and flowline bundle-type systems. The end fitting hardware, the J-tube hang-off assembly, bullnose, subsea termination, bend restrictors, bend limiters – all are conventional, field proven equipment systems.

Metal tubes offer advantages including: (1) impermeability to methanol, (2) the abilities to withstand high internal and external pressure, (3) robustness of the tubes without armouring, (4) potential reduction in umbilical cross section complexity and, in some cases, (5) better compatibility with the working fluids. However, there are still engineering considerations that must be addressed regarding metal tube umbilical, such as internal and external corrosion and, in the case of dynamic applications, fatigue performance of tubulars and end terminations and plastic straining during umbilical fabrication and handling.

Furthermore, although fabrication can be accomplished using techniques similar to those for thermoplastic umbilical, the equipment needed are different, in some cases requiring extensive modifications or completely new machinery and thus involving high steel tube umbilical procurement cost.

The following considerations should be addressed in the design of dynamic umbilical:

Note: Allowing the electrical conductors to move axially, usually by providing lubricants and selecting proper insulation material, is the key to increase their fatigue life.

For dynamic applications the dynamic behaviour and fatigue life of the umbilical are two of the most important characteristics of the design. There are two critical locations that may be deficient in fatigue life and must be examined carefully: the region near the point where the umbilical contacts the seafloor (the “touchdown point or sag-bend radius”) and the hang-off level at FPS. The analysis that is required to assess fatigue life is both complex and time-consuming.

There are two conditions that create fatigue loading: (1) Vortex Induced Vibrations due to current and (2) FPS motions due to wind, wave and current conditions. However, VIV is a minor source of fatigue comparing to FPS motions.

One critical element in the design of a dynamic umbilical is the bend stiffener. This device is an integral part of the top support and serves to limit extreme stresses, due to static offset of the floating production system or strong currents, and reduce the cyclic stresses which contribute to fatigue.

As part of the material selection, the key requirement is maintaining pressure integrity and internal cleanliness for the expected life of the project. Along with the mechanical requirements, the material must resist external corrosion in seawater and mud as well as internal corrosion due to water-based hydraulic fluids and chemical (e.g. methanol).

The tubes within an umbilical are surrounded by seawater. The key parameters of seawater are temperature, oxygen concentration and pH. Another aspect of exterior environment is cathodic protection. Even though it may not be required for the umbilical material, cathodic protection may be required on other parts of the system. It is difficult to isolate these components from the umbilical. In some cases, cathodic protection not properly designed, can lead to hydrogen embrittlement, the degradation of material properties caused when hydrogen is absorbed.

Besides resisting corrosion on the inside, the umbilical material must not significantly change the hydraulic fluid characteristics and in particular the cleanliness of the hydraulic fluid, which must remain near NAS Class 6, since material particle can foul or plug the control valves in the subsea control system.

To meet these conditions, three alternative tube materials were considered:

Each material is further depicted in the following sections. However, it shall be retained that super duplex satisfactorily meets the different requirements and is currently the most selected material. Lean duplex zinc coated has been used in a number of projects in the Gulf of Mexico. Carbon steel difficultly meets the cleanliness requirements and has only be used in a very few cases. Titanium has never been used for umbilical tubes up to now.