*2.3.4.1. Introduction*

**Figure 14.** Concept and principles of the open BOP control system.

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Proprietary manufacturers of subsea hose bundle strive to provide a product which has a low volumetric expansion characteristic (VEC). This ensures that API closing times are not exceeded for ram type and annular type preventers. In the electro-hydraulic control system, the single greatest contributor to lengthening response times is the hydraulic pilot pressure build time and transport time.

#### *2.3.4.2. Pressure characteristics of the control fluid*

The fluid parameters that govern the transmission time of a hydraulic signal through a thermoplastic tube are:


The values may change, but this is usually associated with significant changes in the ambient operating temperatures. An extreme example is the difference in control fluid parameters in tropical climates. As opposed to climates in far northerly and southerly latitudes, there will be no monoethyleneglycol (MEG) added to the control fluid medium since the seawater temperature at the mudline is significantly above freezing point. (This is applicable for the relatively shallow water depths in which this type of BOP control system is used, and the

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We can say that the density and viscosity of the fluid will remain close at their optimum

One of the basic concerns in regard to using thermoplastic hose to transport hydraulic fluids to great depths is differential pressure across the tube wall. At great depths, the external pressure may be sufficient to collapse the hose. The pressure at which collapse takes place is

dependent upon the hose construction and the nominal diameter of the hose [4, 7].

previous statement is not true for ultra-deep water: >6000 feet.)

values in this operational water depth.

**Figure 16.** The Koomey Shaffer 64 line retrievable control pod.

*2.3.4.3. Factors influencing time response*

**Figure 15.** The 42 line Koomey control pod.

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**Figure 16.** The Koomey Shaffer 64 line retrievable control pod.

• The density of the fluid • The viscosity of the fluid

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• The un-dissolved gas in the fluid • The bulk modulus of the fluid

**Figure 15.** The 42 line Koomey control pod.

The values may change, but this is usually associated with significant changes in the ambient operating temperatures. An extreme example is the difference in control fluid parameters in tropical climates. As opposed to climates in far northerly and southerly latitudes, there will be no monoethyleneglycol (MEG) added to the control fluid medium since the seawater temperature at the mudline is significantly above freezing point. (This is applicable for the relatively shallow water depths in which this type of BOP control system is used, and the previous statement is not true for ultra-deep water: >6000 feet.)

We can say that the density and viscosity of the fluid will remain close at their optimum values in this operational water depth.

#### *2.3.4.3. Factors influencing time response*

One of the basic concerns in regard to using thermoplastic hose to transport hydraulic fluids to great depths is differential pressure across the tube wall. At great depths, the external pressure may be sufficient to collapse the hose. The pressure at which collapse takes place is dependent upon the hose construction and the nominal diameter of the hose [4, 7].

The dominant property of differential pressure in this application arises from the differences between seawater and control fluid densities. Wherever in this type of system, there exists a degree of density difference across the hose tube wall, a chance of invoking hose collapse is possible. For instance, at a depth of 5000 feet, a thermoplastic hose containing a typical mineral oil as the hydraulic medium found in surface stack control systems will experience an overburden of around 220 psi, which is quite sufficient to collapse a hose. Pressures as little 30 psi can cause collapse of hoses with nominal diameters in the range of 3/8–½ in*.*

*API specification 17E: specification for subsea production control umbilicals, states for collapse pressure* [9]:

*"The minimum value of external collapse pressure shall be 150% of the difference in the static head due to hydrostatic pressure at the maximum design depth less the static head at that depth due to the service fluid (hydraulic medium)."*

Further unwanted differential pressure will be generated if the hydraulic lines are not 100% fluid filled. If any entrapped air is present in the tube length, the hydrostatic pressure will dominate and tube collapse will occur. This is easily eradicated by thorough purging and venting of all lines in the subsea umbilical hose bundle. The presence of air, however small, also dramatically increases response times due to the compressibility of gases [3].

The differential problem is overcome by choosing a hydraulic medium which has a specific gravity that is close to seawater.

Seawater has a gravity of ~1.03 and water is 1.00. Providing that the hydraulic medium is water-based with additives that only change the specific gravity to a new value remains close to that of the specific gravity of seawater then the possibility of hose collapse is virtually obviated.

#### *2.3.4.3.1. Viscoelasticity*

Hoses, being composites with polymeric constituents are found to behave in a time dependent viscoelastic manner when applied load is a hydraulic charge as found within a pilot line hose.

The result of the viscoelasticity manifests itself in a pressure decay after initial pressurization. This is not detrimental for the pilot signals in this application since the hydraulic pilot-operated relay valves subsea "fire" and "vent" at pressures well below the nominal pilot pressure of 3000 psi. **Figure 17** shows the pressure decay versus time. The typical time constraints are well beyond time of the hydraulic relay valves "firing" in this control system.

#### *2.3.4.3.2. Hose geometry changes during pressurization*

Extensive laboratory testing has been performed to assess the changes in hose geometry subjected to a step positive change in internal pressure. The changes in geometry were measured using strain gauges, both axially and circumferentially affixed to the outer surface length of the hose under test.

**Figure 17.** Pressure decay in thermoplastic hoses due to viscoelasticity.

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Making the Connection for Well Control on Floaters: Evolving Design Rationales for BOP Control… http://dx.doi.org/10.5772/intechopen.77998 57

**Figure 17.** Pressure decay in thermoplastic hoses due to viscoelasticity.

The dominant property of differential pressure in this application arises from the differences between seawater and control fluid densities. Wherever in this type of system, there exists a degree of density difference across the hose tube wall, a chance of invoking hose collapse is possible. For instance, at a depth of 5000 feet, a thermoplastic hose containing a typical mineral oil as the hydraulic medium found in surface stack control systems will experience an overburden of around 220 psi, which is quite sufficient to collapse a hose. Pressures as little

*API specification 17E: specification for subsea production control umbilicals, states for collapse pres-*

*"The minimum value of external collapse pressure shall be 150% of the difference in the static head due to hydrostatic pressure at the maximum design depth less the static head at that depth due to the service* 

Further unwanted differential pressure will be generated if the hydraulic lines are not 100% fluid filled. If any entrapped air is present in the tube length, the hydrostatic pressure will dominate and tube collapse will occur. This is easily eradicated by thorough purging and venting of all lines in the subsea umbilical hose bundle. The presence of air, however small,

The differential problem is overcome by choosing a hydraulic medium which has a specific

Seawater has a gravity of ~1.03 and water is 1.00. Providing that the hydraulic medium is water-based with additives that only change the specific gravity to a new value remains close to that of the specific gravity of seawater then the possibility of hose collapse is virtually

Hoses, being composites with polymeric constituents are found to behave in a time dependent viscoelastic manner when applied load is a hydraulic charge as found within a pilot line hose. The result of the viscoelasticity manifests itself in a pressure decay after initial pressurization. This is not detrimental for the pilot signals in this application since the hydraulic pilot-operated relay valves subsea "fire" and "vent" at pressures well below the nominal pilot pressure of 3000 psi. **Figure 17** shows the pressure decay versus time. The typical time constraints are well

Extensive laboratory testing has been performed to assess the changes in hose geometry subjected to a step positive change in internal pressure. The changes in geometry were measured using strain gauges, both axially and circumferentially affixed to the outer surface length of

beyond time of the hydraulic relay valves "firing" in this control system.

*2.3.4.3.2. Hose geometry changes during pressurization*

30 psi can cause collapse of hoses with nominal diameters in the range of 3/8–½ in*.*

also dramatically increases response times due to the compressibility of gases [3].

*sure* [9]:

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obviated.

*2.3.4.3.1. Viscoelasticity*

the hose under test.

*fluid (hydraulic medium)."*

gravity that is close to seawater.

The axial gauges measured any bending strains incurred and the circumferential gauges monitored hoop stresses. It was found that the hose length shortened with pressurization and this is explained by the layers of hose braiding attempting to establish a neutral lay angle during the buildup of pressure. The effect is almost instantaneous and remains constant, and hence the axial strain is not responsible for the viscoelastic effect.

Measured hoop strains correlate to observed pressure responses and shown typical viscoelastic behavior. In tandem with strain measurements, volume measurements have been recorded to estimate the variation in wall thickness. Such measurements have been quantified using two equations which account for the bulk modulus and pressure decay following initial viscoelastic expansion of the hose under test.

Results from these tests showed that both the internal and external diameters of the hose increased with pressurization although the OD significantly less than the ID of the hose.

Overall, this indicates that all hydraulic pilot hoses will "accept" more fluid when a pressure signal is initiated from the source and will duly expand in direct correlation with the VEC of the hose: dependent upon construction and materials. At pressure equilibrium (e.g., 3000 psi), the hydraulic pressure peak will transit the length of the hose at approximately the speed of sound.

#### *2.3.4.3.3. Behavioral phenomena of thermoplastic hose*

All hoses that are constructed of material that use a composition of polymers and fibers may be classed as thermoplastic hoses. When subjected to pressure changes internally and externally, they exhibit a viscoelastic time-related response. After an initial pressurization, the pressure decays over a period of time as the hose dilates (see the previous figure) [3].

The extent of the dilation is dependent upon a number of factors such as hose material, construction, age, environment, and so on. A similar effect occurs when the hose is depressurized, this being a time-related contraction effect.

Against logical intuition, hoses bundled together exhibit greater volumetric expansion (VE) than identical hoses pressurized in isolation. There is a mathematical proof for these phenomena but suffice it to say that the reason is simply because there are effects from adjacent bundled hoses which remain pressurized against those vented to zero gauge.

It is known that aging in hoses reduces VE which acts in our favor (in drilling BOP controls) but is considered detrimental in production control systems.

Minimizing the effects of VE promotes faster response times in hydraulically piloted BOP control systems since the pod-mounted relay valves will not "fire" until they have sufficient pressure in the hydraulic pilot signal: normally around 500–700 psi.

The following figures illustrate some of the effects of the volumetric expansion characteristic in thermoplastic hoses (**Figures 18**–**20**).

**Figure 18.** Time response curves from Hydril® empirical testing.

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**Figure 18.** Time response curves from Hydril® empirical testing.

The axial gauges measured any bending strains incurred and the circumferential gauges monitored hoop stresses. It was found that the hose length shortened with pressurization and this is explained by the layers of hose braiding attempting to establish a neutral lay angle during the buildup of pressure. The effect is almost instantaneous and remains constant, and

Measured hoop strains correlate to observed pressure responses and shown typical viscoelastic behavior. In tandem with strain measurements, volume measurements have been recorded to estimate the variation in wall thickness. Such measurements have been quantified using two equations which account for the bulk modulus and pressure decay following initial

Results from these tests showed that both the internal and external diameters of the hose increased with pressurization although the OD significantly less than the ID of the hose.

Overall, this indicates that all hydraulic pilot hoses will "accept" more fluid when a pressure signal is initiated from the source and will duly expand in direct correlation with the VEC of the hose: dependent upon construction and materials. At pressure equilibrium (e.g., 3000 psi), the hydraulic pressure peak will transit the length of the hose at approximately the speed of

All hoses that are constructed of material that use a composition of polymers and fibers may be classed as thermoplastic hoses. When subjected to pressure changes internally and externally, they exhibit a viscoelastic time-related response. After an initial pressurization, the

The extent of the dilation is dependent upon a number of factors such as hose material, construction, age, environment, and so on. A similar effect occurs when the hose is depressur-

Against logical intuition, hoses bundled together exhibit greater volumetric expansion (VE) than identical hoses pressurized in isolation. There is a mathematical proof for these phenomena but suffice it to say that the reason is simply because there are effects from adjacent

It is known that aging in hoses reduces VE which acts in our favor (in drilling BOP controls)

Minimizing the effects of VE promotes faster response times in hydraulically piloted BOP control systems since the pod-mounted relay valves will not "fire" until they have sufficient

The following figures illustrate some of the effects of the volumetric expansion characteristic

pressure decays over a period of time as the hose dilates (see the previous figure) [3].

bundled hoses which remain pressurized against those vented to zero gauge.

but is considered detrimental in production control systems.

pressure in the hydraulic pilot signal: normally around 500–700 psi.

hence the axial strain is not responsible for the viscoelastic effect.

viscoelastic expansion of the hose under test.

*2.3.4.3.3. Behavioral phenomena of thermoplastic hose*

ized, this being a time-related contraction effect.

in thermoplastic hoses (**Figures 18**–**20**).

sound.

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**Figure 19.** Typical response times for 1/4 in. and 1/2 in. Diameter thermoplastic hose.

**Figure 20.** VE curves for high pressure thermoplastic hose.

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**Figure 20.** VE curves for high pressure thermoplastic hose.

**Figure 19.** Typical response times for 1/4 in. and 1/2 in. Diameter thermoplastic hose.

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