Surface Roughness Analysis of Subsea Cables/Umbilicals

Abstract

Surface roughness is an important factor influencing subsea cable/umbilical–soil interaction. The cable/umbilical goes through several steps before being laid on the seabed, including production, spooling, unspooling, and installation on the seabed. Yet, there is no standard method for assessing the outer sheath roughness, whether extruded or roving, of subsea cables/umbilicals, and outer sheath roughness has not been measured in many cable/umbilical–soil test datasets. The lack of a universally agreed method for assessing and preparing surface roughness stems from the diverse applications of cables/umbilicals, each of which is subject to varying environmental conditions and operational requirements. Such diversity complicates the establishment of a single standard. The objective of this paper is to present the measurements used to determine the surface roughness of the extruded outer and roving outer sheath of subsea cables/umbilicals. The surface roughness of the outer sheath of subsea cables/umbilicals is required for the soil interface direct shear tests, and the corresponding results are essential for determining the friction factors of the cable/umbilical–soil interaction on the seabed.

1. Introduction

Subsea cables/umbilicals have played a vital role in the oil and gas industry [1,2], as they are required to deliver hydraulic or electrical control signals to subsea control devices such as pilot valves, solenoid valves, or relays. Subsea cables/umbilicals have also been used successfully in providing power to subsea actuators or motors, either hydraulic or electric, to convey the injection chemicals to subsea trees or manifolds. Furthermore, subsea cables/umbilicals can be used to monitor well annulus pressure. Moreover, subsea power cables/umbilicals are a critical element for the transmission of electric power from offshore wind farms to onshore customers [3,4,5,6]. Such cables/umbilicals are critical for transmitting power and data in subsea environments, making their interaction with the seabed a key factor in their performance and reliability.

Surface roughness plays a crucial role in the performance and reliability of subsea cables/umbilicals. It directly impacts cable/umbilical–soil interaction, influencing lateral and axial resistance on the seabed, which are critical parameters for operational stability during hydrodynamic events. Despite its importance, the surface roughness of outer sheaths has been largely underexplored, leading to gaps in optimizing the design and deployment of subsea cables/umbilicals. This study aims to bridge this gap by systematically measuring and analysing surface roughness using advanced techniques, thereby providing valuable insights into the oil and gas industry and renewable energy applications.

As described in [1,2,7], subsea cables/umbilicals are usually covered with an outer sheath that provides overall protection to the cable/umbilical components to secure the armouring during transit and installation, and to assist in distributing the handling and installation loads experienced by the cable/umbilical components. Another function of the outer sheath is to enable easy cable/umbilical identification and visibility by utilizing colour. Subsea cables/umbilicals may be covered with an outer sheath of polypropylene rovings, as shown in Figure 1. The outer layer of rovings is applied over the outer armour layer. To guarantee positive locations during handling and deployment of the cables/umbilicals, the rovings yarns are anchored to the armour wires using bitumen applied immediately before the application of the polypropylene rovings [1,2,7].

The extruded outer sheath, as illustrated in Figure 2, is a seamless construction achieved through advanced extrusion techniques. This sheath is a solid extrusion of polyethylene, designed to tighten diametrical control during the extrusion process, ensuring uniform wall thickness along the entire length of the cables or umbilicals. It provides critical protection against abrasion and environmental factors and ensures mechanical integrity during subsea operations by safeguarding the underlying components from damage caused by hydrodynamic forces, soil interaction, and lateral movement along the seabed. The uniform thickness of the extruded outer sheath contributes to its durability, while the material’s properties, such as high Young’s modulus and abrasion resistance, make it suitable for diverse subsea environments. The extruded outer sheath is typically made of high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE) [1,2,7]. LLDPE is preferred by most umbilical manufacturers as it is easy to extrude and handle, whereas HDPE is difficult to extrude but is recommended when the stiffness of the cables/umbilicals needs to be increased.

The surface roughness of the outer sheath has a strong influence on the cable/umbilical–soil interaction and interface shear resistance. However, no specific standards or data have been published concerning the surface roughness of the outer sheaths of subsea cables/umbilicals. In addition, cable/umbilical manufacturers do not have any methods for measuring the surface roughness of the outer sheath, whether extruded or roving. The feedback obtained from all cable/umbilical manufacturers is that they have not previously measured or taken any initiatives to measure the outer sheath surface roughness. For subsea pipelines, DNV-RP-F114 [8] recommends that pipe–soil interface roughness needs to be considered when establishing the geotechnical model for assessing the pipe–soil interaction parameters for the design of subsea pipelines. DNV-RP-F114 [8] also recommends that the interface material used in the specialized pipe–soil interaction interface testing should have surface roughness characteristics representative of the planned pipe coating. Therefore, DNV-RP-F114 [8] recommends that the surface roughness of the planned pipe coating should be established or estimated when planning the interface testing using appropriate measures. More importantly, consistent measures should be used to characterize the surface roughness for both pipeline coating and interface material used in the interface testing to ensure that the surface roughness of the interface material used in the interface testing is representative of the planned pipeline coating. For measuring pipeline roughness, DNV-RP-F106 [9] refers to ISO 8503-4 [10] for the roughness measurement of steel using the stylus instrument technique. ISO 8503-4 [10] specifies the stylus instrument and describes the procedure for calibrating ISO surface profile comparators conforming to the requirements of ISO 8503-1 [11]. It is also applicable to the determination of the surface profile, within the range of 20 to 200 μm, of essentially planar blast-cleaned steel. The determination can be carried out on a representative section of the blast-cleaned surface or, if direct observation of the surface is not feasible, on a replica of the surface.

ISO 8503-5:2003 [12] describes a field method for measuring the surface profile produced by any of the abrasive blast-cleaning procedures given in ISO 8504-2 [13]. The method uses a replica tape and a suitable gauge for measuring, on-site, the roughness of a surface before the application of paint or another protective coating. The method is applicable within the range of profiles cited for a given grade (or thickness) of replica tape. The commercial grades currently available permit the measurement of average peak-to-valley profiles of 20 to 115 μm. The method is valid for surfaces that have been cleaned with either metallic or non-metallic abrasives.

ANSI/ASME B46.1 [14] defines roughness parameters and is therefore a relevant reference but is concerned with the geometric irregularities of surfaces. It defines surface texture and its constituents, including roughness, waviness, and lay. It also defines parameters for specifying surface texture. The terms and ratings in this standard relate to surfaces produced by such means as abrading, casting, coating, cutting, etching, plastic deformation, sintering, wear, erosion, etc.

The purpose of the current paper is to provide measured surface roughness values of the outer sheath of subsea cables/umbilicals comprising HDPE, LLDPE, or yarn/roving. The measurements given in the paper can be used during future cable/umbilical–soil interface shear resistance tests, which can be carried out using replica (i.e., making rubber replicas at desired intervals) or maybe by machining grooves on a polypropylene plate.

2. Effect of Surface Roughness on Cable/Umbilical–Soil Interface Shear Resistance

One of the factors that affect seabed resistance against cable/umbilical lateral and axial displacements on the seabed (e.g., during a design hydrodynamic event) is the interface shear resistance at the cable/umbilical–soil interface. The interface shear resistance at the cable/umbilical–soil interface depends on the soil properties (e.g., particle size, particle angularity, and particle strength), normal effective stress at the cable/umbilical–soil interface, and surface roughness and hardness of the cable/umbilical–soil interface. Figure 3 and Figure 4 show published results [15,16] for typical examples of the effect of interface surface roughness (in terms of the average surface roughness, R_a, or normalized surface roughness, R_n = R_a/D₅₀, where D₅₀ is the median particle size of the soil) on the undrained interface strength ratio (i.e., the ratio of undrained interface shear strength to the consolidation normal effective stress, s_u-res-int/σ′_n0) for normally consolidated clay and on the residual drained interface strength ratio (i.e., the ratio of drained interface shear strength to normal effective stress, τ_res/σ′_n0 = tanδ or μ_res) for both clay and sand. It should be mentioned that the data presented in Figure 3 and Figure 4 are scattered because they were compiled from a wide range of sources. However, these sources provided the foundation for establishing correlations between the surface roughness and the interface shear strength.

As shown in Figure 3a, despite the wide scatter of the data, it can be observed that the undrained interface strength ratio for normally consolidated clay increases slightly with increasing R_a. However, as illustrated in Figure 3b, the drained interface strength ratio of clay increases significantly, by up to ~5 times (from ~0.2 to ~1), when the R_a increases from 1 mm to 30 mm. The drained clay properties used a measured friction angle (δ′) directly derived from critical state shear tests, ensuring alignment with field conditions. A similar observation can also be drawn from Figure 4, indicating that the drained interface strength ratio for silica and carbonate sands increases significantly when R_n increases. As shown in Figure 4, the drained interface strength ratio for silica sand increases from ~0.15 to ~0.7 (~4 times) when R_n increases from less than 0.0001 to greater than 0.01, while the drained interface strength ratio for carbonate sand increases from ~0.2 to ~1 (~5 times) when R_n increases from less than 0.001 to greater than 0.1. The data presented confirm that the interface surface roughness has a significant influence on the interface shear resistance, particularly for the drained interface shear resistance. In addition, the differences in the drained interface strength ratio for silica and carbonate sand observed in Figure 4 highlight that the interface shear resistance is also strongly influenced by soil composition.

It should be noted that the variability in the data presented above arises from differences in soil mineralogy, angularity, and testing protocols. In other words, the scatter of the data could be due to the variability in soil properties (e.g., particle angularity and mineralogy), interface characteristics (e.g., material and hardness), and testing conditions (e.g., equipment, procedure, or experimental errors). Since the variability/uncertainty in interface shear resistance will be propagated to the variability/uncertainty in the calculated seabed resistances for cable/umbilical design, it is recommended that high-quality project-specific low-stress interface shear testing be conducted using (i) site-specific soil; (ii) interface material that is representative of the cable/umbilical outer coating (e.g., hardness and surface roughness); and (iii) low normal stresses (e.g., 5 to 20 kPa) that cover the range of expected normal stresses at the cable/umbilical–soil interface to reduce variability/uncertainty in the calculated seabed resistances for optimized and safe cable/umbilical design. A similar guideline is also recommended in DNV-RP-F114 [8] for pipeline design. If the surface roughness of cable/umbilicals and its variability can be quantified, low-stress interface shear testing using the representative project-specific interface materials and site-specific soils could be designed to derive project- and site-specific correlations between surface roughness and interface shear resistance (such as those shown in Figure 3 and Figure 4). The derived correlations could then be used to define the design interface shear resistance ranges with reduced variability for each soil type zone (i.e., zone with similar soil characteristics) identified along the cable/umbilical routes for optimized and safe cable/umbilical design.

3. Surface Roughness Measurement Methods

This section briefly describes currently available techniques for measuring surface roughness. Based on the limitations of each technique, the interferometric technique was used in the current work to measure the roughness of the outer sheath. In this study, interferometry measurements were conducted using an Olympus DSX1000 Digital Microscope (Olympus Corporation, Tokyo, Japan), which provided high-resolution 3D analysis following advanced interferometric principles. No laser-based systems or stylus instrument techniques were used in this work.

3.1. Mechanical Technique (Stylus)

The stylus technique is a mechanical two-dimensional (2D) method based on dragging a sharp needle along the required surface while recording the height z (x). ISO 8503-4 provides a comprehensive description of the stylus instrument technique for surface roughness measurement. The apparatus itself is small, easily portable, inexpensive, and fast. Unlike other techniques, this technique requires minimal training. Typically, the stylus has a maximum (y) range of less than 1 mm; other systems are far below 1 mm, while R_a is often limited to less than 100 μm. Advanced stylus systems are available that can yield three-dimensional (3D) data. However, such advanced systems are larger, slower, and require extensive training to operate.

3.2. Laser-Based Technique

In principle, laser triangulation may be utilized to determine the surface roughness. The systems used for determining the scattering angle of a reflected laser can also provide the roughness values of surfaces. Laser systems are more expensive than a stylus and certain surface reflectivity is also required. There are many advantages to adopting laser-based systems, including faster measurement and not coming in contact with the sample (non-contact measurement). Some systems are developed for installation in production lines, providing continuous quality control. However, the large laser spot size used in laser triangulation results in a lateral resolution of around 300 μm or larger. The scatter-angle systems have a typical upper R_a limit of 2 μm.

3.3. Interferometric Technique

Many non-contact optical techniques determine the topography of a sample using light reflected from the surface to interfere with a reference beam. Such techniques yield detailed 3D data, z (x,y). Some optical systems do not use interferometry (e.g., chromatic aberration to determine the height) but optical instruments tend to be larger and more expensive than stylus systems and are best suited for laboratory environments with minimal movement. Such systems are also sensitive to vibrations. Interferometric systems can measure a (z) range of up to 8 mm with accuracy and precision in the range of 10 nm. Laterally, features as small as 0.5 μm can be measured over distances of several millimetres. Interferometry is distinguished by its superior precision and non-contact measurement capabilities, essential for capturing complex topographies of cable/umbilical sheaths. Compared to mechanical and laser-based techniques, interferometry offers high-resolution 3D imaging and is less prone to surface damage during measurement, making it ideal for evaluating anisotropic surfaces such as extruded HDPE or LLDPE.

4. Description of Roughness Measurements

The samples used in this study were obtained from one leading manufacturer of subsea cables/umbilicals. Each sample was processed to standard specifications and included HDPE, LLDPE, and yarn/roving materials. Measurement steps involved visual inspection for damage, imaging at three arbitrary locations (5 mm, 25 mm, and 45 mm), and subsequent analysis using the Olympus DSX1000 Digital Microscope under dark field illumination. Curved samples were analysed across three distinct strings, ensuring comprehensive coverage of surface variability. The Ash Omni Digital Microscope was employed in this study for preliminary surface inspections, offering high-resolution 2D imaging capabilities. This provided an overview of surface conditions before detailed 3D surface roughness measurements were conducted using the Olympus DSX1000 Digital Microscope, based on interferometric principles.

Before embarking on the measurement of surface roughness, the surfaces of the samples were visually inspected for any signs of damage. Following the inspection, the samples were imaged using the Ash Omni Digital Microscope to provide an overview of their surface condition. Images across the sample surface were then taken in three different areas of size 920 × 920 μm, using the Olympus DSX1000 Digital Microscope in dark field illumination mode. Figure 5 shows the coating samples tested for surface roughness in this work. For HDPE and LLDPE samples, the areas of inspection were selected arbitrarily at 5 mm, 25 mm, and 45 mm from the top of each sample, as highlighted in Figure 6, which also details the orientation and inspection areas for HDPE and LLDPE samples, providing a schematic representation of the sampling process. It should be noted that the orientation of samples was kept consistent throughout this paper. For the curved samples, images were taken in three different areas of size 1860 × 1860 μm, across three different strings per sample and the strings were numbered from the top of the sample down, starting from String 1, as shown in Figure 7. The fibres of every string each constitute an individual cylindrical surface and therefore the measurement of the surface roughness parameters using the same method as the HDPE and LLDPE samples was not suitable for this type of material. Instead, and to obtain additional information, profiles in the (y) direction were taken for each of these areas and the roughness parameters were calculated using the same methodology as the other samples. Additionally, images were taken of size 13.150 × 13.150 mm. The temperature of the lab during the measurements was 24 °C.

Images for the samples were taken at different locations, and 3D models were then constructed from these images and tilt-corrected using built-in software. After the tilt correction, the roughness parameters were calculated in both (x) and (y) directions for each sample area, using the built-in software. Figure 8, Figure 9 and Figure 10 present 3D reconstructions of sample surfaces, highlighting key topographical features. Figure 8 and Figure 9 illustrate typical examples of the 3D images taken at 45 mm from the top of the sample for HDPE and LLDPE, respectively, whereas Figure 10 shows a typical example of the 3D image for the curved samples taken from String 3. For HDPE (Figure 8), smoother regions dominate the central area, suggesting lower Ra values. In contrast, LLDPE (Figure 9) exhibits more pronounced peaks, indicating higher Ra values near the edges. Curved samples (Figure 10) display significant variability across strings, emphasizing the challenges in measuring fibrous materials. These observations suggest a strong dependency of roughness parameters on manufacturing processes and material composition.

5. Roughness Parameters

The ANSI/ASME B46.1 standard provides definitions for roughness parameters, including R_a and R_q, which were used in this study. The most common roughness parameters that describe the height differences of a surface in various ways are as follows:

R_a: Arithmetic average deviation from the mean surface height, which is widely used due to its simplicity and intuitive understanding.

R_q: Root mean square of height deviations from the mean height, which is more sensitive to larger peaks and valleys compared to R_a.

R_t: Maximum height difference in a given measurement, representing the largest peak-to-valley distance.

R_z: Average of the 5, 25, or 45 largest peak-to-valley distances within a given measurement area.

The above parameters complement each other by capturing different aspects of surface texture. For instance, R_a and R_q are effective for comparing overall roughness, providing a general measure of the surface’s texture, while R_t and R_z highlight extreme topographical variations, which are important for understanding surface irregularities. Additionally, Fourier transforms can be applied to analyse surface patterns, enabling the identification of dominant wavelengths and periodicities, thereby offering a more holistic evaluation of surface roughness.

The surfaces to be measured may differ significantly from each other. There are no indications that any one parameter is better suited to describe the surfaces at hand. Thus, it is suggested to use the most common parameter, R_a and R_q, defined by the ANSI/ASME B46.1 standard as follows (see Figure 11):

(1)$R_a={\displaystyle \frac{1}{n}}{\displaystyle \sum _{i=1}^n\left|z_i-z^{-}\right|}$

(2)$R_q=\sqrt{{\displaystyle \frac{1}{n}}{\displaystyle \sum _{i=1}^n{\left(\left|z_i-z^{-}\right|\right)}^2}}$

where: $n$ is the number of measured points; $z_i$ is the height of the ith point; and $z^{-}$ is the mean height of all points. The R_a and R_q values are useful for comparing the topography of umbilical or cable outer sheaths/yarns produced with the same method and pattern over time. However, for umbilical or cable outer sheaths/yarns with different patterns or methods of surface generation, one should also have a parameter describing the lateral dimensions of the surface (e.g., the main repeat distance of the pattern in the axial direction). A Fourier transform of the 2D surface profile can then be used to extract the dominant wavelength of the pattern (in the measured direction) and the degree of periodicity. Such an analysis can be automated and is fast once the desired output parameters have been decided. In addition, the evaluation length (Ir) is the segment of the surface profile over which Ra and Rq are calculated. Selecting an appropriate Ir ensures consistent and representative measurements of surface roughness.

Table 1 and Table 2 present the roughness parameters for the HDPE and LLDPE samples, at three different locations per sample, in the (x) and (y) directions, respectively, whereas Table 3 shows the results of the roughness parameters for the curved samples in the (y) direction. On the other hand, Figure 12, Figure 13, Figure 14 and Figure 15 show the graphical representation of the obtained results for the HDPE and LLDPE samples in the (x) and (y) directions, respectively, whereas Figure 16 and Figure 17 show the graphical results of the curved samples in the (y) direction.

It can be seen from the results given in Table 1 and Table 2 and Figure 12, Figure 13, Figure 14 and Figure 15 that all graphs only have three data points, which do not give a curve trend. In future work, increased reliability of measurements for the HDPE and LLDPE samples would be ideal with more measurements per sample (i.e., more than three areas per sample or more samples from each batch to measure). Also, the interpretation of the first graph without context and extra data for reliability suggests that roughness decreases at the centre of the samples for HDPE and LLDPE. It can also be seen from the results given in Table 3 and Figure 16 and Figure 17 for the Curve 1 and Curve 2 samples that there is little to be interpreted from these results. The roughness measurement for the yarn or roving was quite challenging due to its fibrous nature.

6. Conclusions

The surface roughness measurements provided in this study can assist in improving the accuracy of interface shear resistance predictions and optimizing the design of subsea cables and umbilicals. Such measurements can also be utilized to create rubber replicas for offline 3D analysis and serve as a reference for future studies. By providing surface roughness data for both roved and extruded outer sheaths, this study establishes a foundational understanding of surface roughness in subsea cables/umbilicals, paving the way for future work to refine testing methods and standardize measurements across diverse applications. The current study yielded the following important results:

For the surface roughness measurements to yield useful outer sheath roughness results, the measured area must be representative of the outer sheath. Coating roughness modifies the frictional responses. The residual resistance is influenced by sliding at the cable/umbilical–soil interface. The results of friction factors would be different if the model cable/umbilical had a rougher or smoother surface. It is important to ensure that the outer sheath roughness of the cable/umbilical is adequately captured in any model tests. The nature of the outer sheath requires the use of interferometric techniques such as the digital microscope used in this paper. A limitation of such point-by-point measurement systems is that they require scanning the probe laterally to obtain 3D images. To fully capture the topography of patterned outer sheaths (i.e., roved and extruded), 3D measurements are necessary instead of 2D profiles. Since the topography of the outer sheath is not isotropic (e.g., contains stripes in any direction), a single measurement is not sufficient. This was rectified by introducing a second parameter in the mathematical model to take the pattern shape into account and distinguish different patterns with the same roughness. Thus, for anisotropic or periodic patterns, a single parameter is insufficient to properly describe the topography in the model if multiple patterns occur on the umbilical or cable to be installed. In future work, to keep routine analyses simple and inexpensive, it is recommended to investigate if it is possible to use 2D measurements but make imprints (replicas) of the same surfaces for subsequent 3D analysis if more information is required. Such replicas will also provide an archive of the surface quality.

Author Contributions

Conceptualization, A.R., H.E.L. and M.A.S.; methodology A.R., H.E.L. and M.A.S.; formal analysis, A.R., H.E.L. and M.A.S.; investigation A.R., H.E.L. and M.A.S.; resources, A.R., H.E.L. and M.A.S.; writing—original draft preparation, A.R., H.E.L. and M.A.S.; writing—draft review, A.R., H.E.L. and M.A.S.; data curation, A.R., H.E.L. and M.A.S.; visualization A.R., H.E.L. and M.A.S.; supervision, A.R., H.E.L. and M.A.S.; project administration, A.R., H.E.L. and M.A.S. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

Author Han Eng Low is employed by the Fugro Australia. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Example of subsea cable/umbilical covered with yarn/roving outer sheath.

Figure 2. Example of subsea cable/umbilical covered with an extruded outer sheath.

Figure 3. Shear strength ratio vs. average surface roughness (Ra) for (a) undrained conditions and (b) drained conditions [15].

Figure 4. Drained residual shear strength ratio versus normalized surface roughness (Rn) for (a) silica sand and (b) carbonate sand [16].

Figure 5. Examples of coating samples tested in the current work (the scale bar provides a reference for the size of observed features).

Figure 6. Schematic of sample area selection for HDPE and LLDPE samples, indicating inspection points at 5 mm, 25 mm, and 45 mm from the top of each sample (these locations were chosen to capture variability in surface roughness across the sample length).

Figure 7. Schematic of sample area selection for curved samples, highlighting inspection points at String 3, String 6, and String 9 (these locations were chosen to ensure comprehensive coverage of surface roughness variability across the fibrous structure of the samples).

Figure 8. Typical example of a 3D image of an HDPE sample taken at 45 mm from the top of the sample.

Figure 9. Typical example of a 3D image of an LLDPE sample taken at 45 mm from the top of the sample.

Figure 10. Typical example of a 3D image of a curved sample taken from String 3.

Figure 11. Definition of the surface roughness parameter Ra.

Figure 12. Graphical representation of Ra values in the (x) direction for HDPE and LLDPE samples measured at 5 mm, 25 mm, and 45 mm from the top (the y-axis annotations are consistent with the roughness parameter definitions provided in the text).

Figure 13. Graphical representation of Rq values in the (x) direction for HDPE and LLDPE samples measured at 5 mm, 25 mm, and 45 mm from the top (the y-axis annotations are consistent with the roughness parameter definitions provided in the text).

Figure 14. Graphical representation of Ra values in the (y) direction for HDPE and LLDPE samples measured at 5 mm, 25 mm, and 45 mm from the top (the y-axis annotations are consistent with the roughness parameter definitions provided in the text).

Figure 15. Graphical representation of Rq values in the (y) direction for HDPE and LLDPE samples measured at 5 mm, 25 mm, and 45 mm from the top (the y-axis annotations are consistent with the roughness parameter definitions provided in the text).

Figure 16. Graphical representation of Ra values in the (y) direction for curved samples measured at String 3, String 6, and String 9 (the y-axis annotations are consistent with the roughness parameter definitions provided in the text).

Figure 17. Graphical representation of Rq values in the (y) direction for curved samples measured at String 3, String 6, and String 9 (the y-axis annotations are consistent with the roughness parameter definitions provided in the text).

Table 1

Roughness parameters in the (x) direction at 3 different locations for the HDPE and LLDPE samples.

Sample	Ra (μm)			Rq (μm)
	5 mm	25 mm	45 mm	5 mm	25 mm	45 mm
HDPE1	0.154	0.091	0.21	0.204	0.114	0.264
HDPE2	0.341	0.195	0.225	0.437	0.241	0.296
LLDPE1	0.27	0.204	0.222	0.349	0.246	0.271
LLDPE2	0.11	0.142	0.23	0.136	0.174	0.28

Table 2

Roughness parameters in the (y) direction at 3 different locations for the HDPE and LLDPE samples.

Sample	Ra (μm)			Rq (μm)
	5 mm	25 mm	45 mm	5 mm	25 mm	45 mm
HDPE1	0.36	0.262	0.183	0.456	0.334	0.227
HDPE2	0.45	0.319	0.135	0.575	0.385	0.168
LLDPE1	0.666	0.313	0.222	0.808	0.381	0.263
LLDPE2	0.191	0.215 1	0.192	0.236	0.267 1	0.249

Note: ¹-Roughness measurements taken for sample LLDPE2 at 25 mm included a deep pit at the centre of the sample at this location. For this data point, the roughness parameters were calculated off-centre to avoid the anomalous pit, ensuring the results were not skewed. Roughness parameters for the centre of the image, including the deep pit, are as follows: R_a = 0.594 μm and R_q = 0.707 μm.

Table 3

Roughness parameters in the (y) direction at 3 different locations for the curved samples.

Sample	Ra (μm)			Rq (μm)
	String 3	String 6	String 9	String 3	String 6	String 9
Curve1	15.079	8.841	6.318	20.862	14.375	8.232
Curve2	4.54	6.215	4.236	5.874	9.049	5.622

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11. ISO 8503-1 Preparation of Steel Substrates Before Application of Paints and Related Products—Surface Roughness Characteristics of BLAST-CLEANED STEEL SUBSTRATES PART 1: Specifications and Definitions for ISO Surface Profile Comparators for the Assessment of Abrasive Blast-Cleaned Surfaces; ISO: Geneva, Switzerland, 2017.

12. ISO 8503-5 Preparation of Steel Substrates Before Application of Paints and Related Products—Surface Roughness Characteristics of Blast-Cleaned Steel Substrates—Part 5: Replica Tape Method for the Determination of the Surface Profile; ISO: Geneva, Switzerland, 2003.

13. ISO 8504-2 Preparation of Steel Substrates Before Application of Paints and Related Products—Surface Preparation Methods—Part 2: Abrasive Blast-Cleaning; ISO: Geneva, Switzerland, 2019.

14. ANSI/ASME B46. 1-2002 Surface Texture (Surface Roughness, Waviness, and Lay); American Society of Mechanical Engineers: New York, NY, USA, 2002.

15. Westgate, Z.; White, D.; Savazzi, M. Experience with Interface Shear Box Testing for Axial Pipe-Soil Interaction Assessment on Soft Clay. Proceedings of the Offshore Technology Conference; Houston, TX, USA, 30 April–3 May 2018; OTC-28671-MS.

16. Westgate, Z.; Argiolas, R.; Wallerand, R.; Ballard, J.-C. Experience with Interface Shear Box Testing for Pipe-Soil Interaction Assessment on Sand. Proceedings of the Offshore Technology Conference; Houston, TX, USA, 16–19 August 2021; OTC-31268-MS.