Concrete Mattress Research Articles

Analysis of the Effect of Using Concrete Mattresses on Subsea Pipelines

Analysis of the Effect of Using Concrete Mattresses on Subsea Pipelines

Experimental analysis of scour around an offshore wind gravity base foundation

Scour processes and corresponding scour protection measures have been extensively studied for vertical cylinders. Several formulations for estimating scour and determining the optimum size of rocks for scour protection are widely available in the literature. However, when studying other types of structures, the geometric variability of the structures, limits the application of available semiempirical formulations. Scour phenomena around complex structures have been investigated experimentally in reduced-scale wave-current basins. However, the number of tests focused on support typologies different from monopile structures is still limited, restricting the availability of semiempirical formulations focused on conceptual design of scour protection solutions or in the estimation of the free scour development.In this work, a better understanding of the scour processes around gravity-based foundations (GBFs) and the performance of different scour protection solutions is gained via experimental results obtained from a large-scale test programme (1:35 test scale), including combined wave and current tests. The experimental tests were divided into two main types: (1) free scour and (2) scour protection tests. The main conclusion was that the triggering of a horseshoe vortex is the main driver of scour processes together with the contraction of flow lines at the foundation contours. Since monopile semiempirical formulations are not suitable for scour prediction, a new semiempirical approach was obtained from the test results. Moreover, two distinct scour mitigation methods were analysed (rock protection and concrete mattresses). Based on the undisturbed mobility parameter, statistical and dynamic limits were defined considering the rock grading tested. Finally, for concrete mattresses, the design driver was identified, and the adaptability of these types of solutions to seabed changes was verified.

A hybrid approach to assess the hydraulic structures rehabilitation work, case study: El-Bagoureya head regulator, Egypt

This research paper presents an integrated approach in dealing with the hydraulic models to investigate the impact of the reinforcement and maintenance work of the hydraulic structures, and presenting a case study of the El Bagoureya Regulator. The reinforcement works for El Bagoureya Regulator include the regulator's stilling basin, which will have to be filled with a reinforced concrete mattress, the extension of the central piers to the end of the winged wall, and finally the construction of a new concrete weir at the end of the winged wall downstream of the regulator. During the rehabilitation works, the flow will therefore pass through the navigation channel of the lock. The aim of the study is therefore to investigate the lock capacity to allow the maximum flows through at constant water levels and to meet the water demand. The study also examines the efficiency of the proposed solution and the impact on the hydraulic condition of the canal. The required protection and dredging both upstream and downstream of the lock were also studied. Therefore, A 2D numerical hydraulic model was integrated with a physical model at an appropriate scale as a near-field model. The results show a good correlation between the two models. The physical model confirmed the results of the numerical model and guaranteed the hydraulic efficiency of the proposed reinforcing structure. The results show that the proposed reinforcement works improved the flow pattern and the distribution of the flow velocity downstream of the pressure regulator. It also confirms that the navigation lock is capable of handling the high discharge during the rehabilitation works while maintaining the same operating water level. Finally, the physical model has ensured the stability of the proposed protection of the bottom upstream, downstream the navigation lock and downstream the regulator at the end of the stilling basin.

Service Life Prediction of Flexible Concrete Mattresses (FCMs) in the Yangtze River

Many scholars have studied the failure mechanism of waterway regulating structures, but there is still a need to improve research on their reliability and service life in the industry. Currently, flexible concrete mattresses (FCMs) are the regulating structure widely used on the Yangtze River Golden Waterway, so it is important to study their service life. The stability function has been built based on the mechanism of stability against sliding and a time-varying reliability model. Experiments have been conducted to analyze the critical failure load of FCMs, determine the influence parameters, and derive a service life calculation formula. Finally, the middle reaches of the Yangtze River were taken as an example for analysis.

Investigations on the Effectiveness of Protection Methods for a Submarine Pipeline Exposed to the Impact of a Falling Anchor

The occurrence of a buried submarine pipeline crossing a channel becoming damaged by the impact of a falling anchor is becoming more common. It is important to analyze the dynamic response of pipelines exposed to such impact and develop effective protection methods to ensure the safe operation of the pipelines exposed to the impact of falling anchors. In this study, different protection methods, including pure rock, concrete mattress + rock, concrete mattress + rock + rubber pad, and compound flexible pad + rock, are physically tested. The strains at the impacting point and along the pipeline were measured with the fiber Bragg grating (FBG) sensors. The effectiveness of the protection methods is analyzed based on the maximum strain and its affected length on the pipeline. Then, a theoretical model is established to analyze the deformation and strain of a pipeline. Through curve-fitting the experimental results, the bearing capacity coefficients for different protection methods are determined. The protection method of compound flexible pad + rock has the best performance to protect the pipeline from the impact of a falling anchor.

Numerical study of Articulated Concrete Mattresses (ACMs) for offshore pipeline protection

In offshore pipeline projects, inevitably long sections of the transportation pipeline network require protection from natural hazards. Under such environmental conditions, concrete mattresses are recognized as a well-proven technology to surpass challenges in offshore pipeline construction, umbilical deployment and seabed protection/stabilization. To have a better understanding of the physical process of Fluid–Structure–Seabed-Interactions (FSSI) with a ACMs–Pipe–Seabed system, a workable numerical model is proposed to simulate wave–current induced soil responses around the system. Based on the proposed model, this paper discusses the momentary seabed liquefaction near the offshore pipeline and the stability of ACMs. Numerical examples show that ACMs have a significant positive effect on preventing seabed liquefaction around the pipeline by pushing the liquefaction zone to both sides of the shaft. Compared with unprotected conditions, the instability of the protective layer is mainly concentrated in its edge area, and the liquefaction depth can be reduced by around half of the pipe diameter. For the double ACMs–Pipe-System, it is found that the stability of the nearby seabed foundation is related to the mattress spacing and interaction angle.

A Method for Measuring Hydrodynamic Force Coefficients Applied to an Articulated Concrete Mattress

A physical model testing method to determine the hydrodynamic force coefficients of an object is proposed and applied to an articulated concrete mattress placed on a flat surface under steady current condition. The test setup, which comprises of a pulley system that is able to pull the concrete mattress in either direction relative to the flow (e.g., with the flow direction or against the flow direction) and one load cell to measure the force required to pull the mattress, is simple and straightforward. Writing the equations of load balance for two different pulling directions allows the force coefficients to be deduced. The novelty and advantages of the method are that it completely removes the difficulties associated with measuring forces on individual concrete blocks and isolating the mattresses from contacting the solid surface, which were common in conventional test methods for measuring hydrodynamic forces on structures founded on a solid surface. A series of flume tests have been conducted to demonstrate the validity of the proposed method. It is expected that the proposed testing method is applicable to a wide range of structures, bed surfaces and flow conditions.

EXPERIMENTAL INVESTIGATION INTO THE EFFECTS OF REDUCED VERTICAL CENTRE OF GRAVITY OF AN ARTICULATED CONCRETE MATTRESS IN CURRENT FLOW

An articulated concrete mattress model has been utilised to investigate the effects of reduced vertical centre of gravity on the stability of a 400 series block. Experimental testing was conducted at the AMC CWC, Beauty Point. To determine the effects that a reduced centre of gravity has on stability, the 3 by 3 articulated concrete mattress model was subject to pure uniform current flow. The subsequent forces were analysed with a six degree of freedom underwater force sensor. In order to gain a range of real world scenarios, the experimental model was tested at six flow angles ranging from -15 degrees through to 60 degrees, at 15 degree increments. Additionally, five fluid velocities starting at 0.6 m/s through to 1.4 m/s, at 0.2 m/s increments were investigated. These results explain how the inversion of a 400 series block increases its hydrodynamic coefficients and subsequently decreases its stability performance in current flow. Ultimately, this study provides experimental information for the installation of 400 series articulated concrete mattresses in the inverted orientation.

Benthic and Fish Interactions With Pipeline Protective Structures in the North Sea

An increasing number of pipelines and associated protective materials in the North Sea are reaching the end of their operational life and require decommissioning. Identifying the optimal decommissioning option from an environmental perspective requires an understanding of ecological interactions; currently there is little knowledge as to species associations with pipelines and associated protective materials. This study utilises industry ROV footage from the North Sea to quantify these interactions. A total of 58 taxa were identified, including 41 benthic taxa and 17 fish taxa. Taxa were grouped into seven groups for analysis including four groups for benthic epifauna: grazers, suspension/filter feeders, decapods, and colonial/encrusting taxa. Fish were organised into three groups: pollock, other fish, and other gadoids. Using zero-inflated generalised linear mixed models, we show that abundances of benthic epifauna and fish vary between types of protective structure (e.g., concrete mattresses, rock dump), depth, levels of fishing effort and proximity to oil and gas platforms. Six taxa groups exhibited higher abundances on concrete mattresses than bare pipelines with benthic epifaunal decapods showing the highest difference at 3.04 (1.83, 4.84, 95% CrI) times higher on mattresses compared to bare pipelines. Six groups were higher in abundance within the 500 m fisheries exclusion zone around platforms, compared to outside of the zone, with other gadoids showing the highest difference at 1.83 times (1.09, 2.89, 95% CrI) times higher inside zones. Five groups decreased in abundance with an increase in fishing effort, with the biggest effect observed on grazers which decreased in abundance by 28% (14 – 40, 95% CrI) per 50 h of fishing. We show that pipelines and protective materials are operating as artificial reefs, and our results suggest that removal of infrastructure could result in the loss of habitat and species.

Renewable energy homes for marine life: Habitat potential of a tidal energy project for benthic megafauna

An increasing number of offshore structures are being deployed worldwide to meet the growing demand for renewable energy. Besides energy production, these structures can also provide new artificial habitats to a diversity of fish and crustacean species. This study characterises how concrete mattresses that stabilise the submarine power cable of a tidal energy test site can increase habitat capacity for benthic megafauna. A five-year monitoring, which relied on both visual counts and video-based surveys by divers, revealed that these mattresses provide a suitable habitat for 5 taxa of large crustaceans and fish. In particular, two commercially valuable species, i.e. the edible crab Cancer pagurus and the European lobster Homarus gammarus, showed a constant occupancy of these artificial habitats throughout the course of the project. The shape and the number of shelters available below individual mattresses largely determine potential for colonisation by mobile megafauna. Local physical characteristics of the implantation site (e.g. substratum type, topography, exposition to current etc.) significantly impact amount and type of shelters provided by the concrete mattresses. Thus, to characterise habitat potential of artificial structures, it is not only essential to consider (i) the design of the structures, but also to (ii) account for their interactions with local environmental conditions when deployed on the seafloor.

Risk Assessment of Subsea Pipeline due to Installation and Operation of Single Point Mooring (SPM)

Offshore exploration facilities located in remote areas require high investment and high operational cost, the assets are classified as vital assets for the companies. The careful study and examination is therefore required to ensure that the risk of the facility is acceptable. Offshore exploration used subsea pipelines that has several risks if the accident occurs. Based on historical data, most of subsea pipelines failures are caused by third party factors (dropped anchor, dragged anchor, and ship sinking). This paper examines risk assessment of subsea pipelines due to installation and operation of SPM is assessed used DNVGL RP F107 standard and numerical simulations. Frequency is assessed by using Bayesian Network methods. Whereas the consequences were assessed by using calculations on the DNVGL RP F107 standard and FEM simulation. The results of frequency and consequences are mapped within a risk matrix according to DNVGL RP F107. Based on the calculation, the frequency of all types of vessels is less than 10-5 (rank 1). While the lowest consequence was rank 3 with a dent/diameter is 12.32% and the highest was rank 5 (>20%) which can damage the subsea pipelines. The risk matrix shows that the risk level are in acceptable and ALARP condition. Acceptable obtained by frequency in rank 1 and the consequence in rank 3 and 4. While ALARP obtained by frequence rank 1 and consequence rank 5. Although it is not mandatory for mitigation, it is recommended to provide the subsea pipeline with external protection. Rock dumping is recommended as external protection for subsea pipelines due to the good ability to protect the subsea pipelines and it has lower cost compared to concrete mattress.

Assessment of slope instability of canal with standard incomat concrete-filled geotextile mattresses lining

Geosynthetics polymeric materials are recently widely used for geotechnical problems of canals specifically seepage losses with cost-effectiveness, low environmental impact, and quantifiable performance. While the geotextile is one of the geosynthetics, incomat filled concrete mattress is the standard and most famous polyethylene (PE) and polyamide (PA) double woven type of geotextiles. Its main functions are as a liner and/or controlling erosion of soil that creates changes of permeability which could affect the stability of the side slopes of canals. Among irrigation canals in Egypt, Ismailia canal (129.5 km length) is considered the worst case of seepage problems and erosions. The Ministry of Water Resources and Irrigation (MWRI) implemented preliminary technical studies to locate the most critical sections along the canal which seepage losses are the maximum. Lining samples (100 m2, area) by standard incomat were executed in 2016 where four sections of lining with a total length of 61 km were defined. This research assesses the effect of applying the geotextile liner on the slope instability of Ismailia canal at the different proposed four locations of lining where the canal geometry and soil properties are variables. SLOPE/W module of GeoStudio finite element software was used to model the simulation after a clear definition of the soil and geotextile physical properties. The factor of safety, Fs of slope stability for the normal condition of the canal (without lining) ranges from 1.68 to 2.23. The lowest Fs (1.421) was obtained for the first part (km 17.5–28.0) of the second lining section of the canal (km 17.5–38.0) where bed width equals 49 m, maximum and minimum water depth equals 5.79 m and 5.05 m where the soil type is fine sand and the side slopes are 2:1. After implementing the lining, the minimum factor of safety was 1.421 for the same previous location with a 15.57% reduction. The maximum reduction in Fs was obtained for the second part (km 75.0–92.5) of the fourth lining section (km 72.0–92.5) while it records 21.61% with a factor of safety 1.491. Those two parts are critically affected after implementing the lining. However, the maximum reduction in Fs was not exceeded 5% for all sections with sandy clay soil where the Fs did not fall below 1.70. While the geotextile is significant in seepage controlling, particularly at locations where the soil type is coarse sand, specifications and requirements of the thickness uniformity of the mattresses should be achieved for the concrete filling process during the construction and measurements of long-term durability could be considered to sustainably function.

SUBSEA PIPELINE PROTECTION DESIGN SUBJECTED TO DROPPED ANCHOR USING CONCRETE MATTRESS

Out of the many potential hazards, certain critical possible hazards need to be assessed, and actions need to be taken to improve the probabilities and consequences to an acceptable degree. One of the major hazards commonly found in subsea pipeline operations is a dropped anchor, especially for pipelines located near jetties, where many ships are expected to move across the pipeline. One of the possible protection methods is the use of a concrete mattress. The dropped anchor hazard to a pipeline is characterized by the impact of energy, which will mostly be absorbed by the internal strain energy of the pipeline. The safety criteria for a pipeline subjected to a dropped anchor are given by DNV RP-F107 for the maximum dent depth and DNV OS-F101 for the maximum stress. A finite element analysis using ANSYS software was performed to obtain the maximum equivalent stress and dent depth for a pipeline. The effects of the length and height of a concrete mattress in redistributing the impact energy were obtained.

Innovations in a submarine piggyback pipeline project in the East China Sea.

Submarine pipelines are widely used to transport offshore oil and gas. Bundling service pipes and cables with these pipelines can cut costs but such ‘piggyback’ installations can lead to problems with torsion and buckling, particularly at crossings. This paper reports on an innovative anti-rotation device that was successfully implemented in a piggyback pipeline project in the East China Sea. A three-layer concrete mattress together with secondary pipe protection devices was developed for crossings, and advanced engineering analysis was conducted to evaluate risks from shallow gas eruptions.

Root-Cause Analysis of Offshore Pipeline Failures

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 188717, “Offshore-Pipeline-Failure Root-Cause Analysis,” by Emmanuel Iyeh, SPE, Addax Petroleum, and Grant Adams, JEE Subsea, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed. This paper evaluates potential causes of failure for nine pipelines operating in shallow waters (8 to 14 m) in the Gulf of Guinea. The authors develop an analytical method to identify root causes and provide recommendations for pipeline design and placement. Methodology Once the subject pipelines had been identified, field visits and failed-section analyses were conducted. Pipeline properties and conditions are provided in Tables 1 through 5 of the complete paper. Mechanical-failure, hydrodynamic, and third-party-interference assessments were performed. Design-stability assessment was reviewed and gaps identified. A further stability assessment of the pipelines was performed in accordance with Det Norske Veritas- Germanischer Lloyd Recommended Practice (DNVGL-RP) F109. The authors then developed the best remedial option by use of a scoring matrix based on weighting with respect to business drivers. For a new tie-in spool, a subsea flexible spool can be used. This approach has been used safely and successfully to accommodate instability. This approach may be adopted for pipelines, with the following caveats: An unbonded subsea flexible spool verified for use in a multiphase system should be used. A flexible spool should be combined with a rigid riser. The flexible spool does not represent a solution to pipeline instability but rather a method to prevent failure of the riser caused by a mobile pipeline. The pipeline may continue to move over time, and the flexible spool may eventually enter tension and fail. An unstable pipeline is also at significant risk of fatigue if instability continues. A flexible spool could be used with a concrete mattress. This will allow a degree of movement (e.g., if the mattresses take some time to settle into the seabed). The position of the flexible tie-in can also be monitored and, in the event that the pipeline continues to move after mattresses have been installed, will give warning that the pipeline is still unstable. Analysis Results The majority of the studied failures occurred between the months of May and October, the rainy season in the Gulf of Guinea. This season features the region’s most extreme weather, likely contributing to the failures. Most failures involved pipeline movement or riser kinks.

Wave run-up and rundown on ACB Mats under granular and geotextile filters’ condition

ABSTRACTArticulated concrete block mattress revetment (ACB Mats) is an appropriate revetment for shore protection and breakwater. ACB Mats act as an integrated and flexible revetment against waves. There is a necessary need of investigation due to the lack of exact ACB Mat design relations in the estimation of hydrodynamic processes including wave run-up and run-down parameters. In the present study, using proper laboratory equipment, run-up and run-down processes were investigated under irregular waves and also granular and geotextile filters’ conditions. In this study, wave run-up and run-down estimation relations in ACB Mat revetment with the open area were explored for the first time. As the obtained results showed, relative maximum wave run-up and rundown are desirable for all conditions. Using a geotextile layer under ABC Mat caused a 14% increase in relative wave run-up values compared to granular filter. Further, in the run-down process, geotextile filter caused a 40% decrease in relative wave run-down values. The intensive decrease in run-down value occurred due to the outflow rate of the water from inside the filter with delay during water attack toward downward, which can be effective on structure stability significantly and should be considered in designs.

Experimental Investigation into the Effects of Reduced Vertical Centre of Gravity of an Articulated Concrete Mattress in Current View

An articulated concrete mattress model has been utilised to investigate the effects of reduced vertical centre of gravity on the stability of a 400 series block. Experimental testing was conducted at the AMC CWC, Beauty Point. To determine the effects that a reduced centre of gravity has on stability, the 3 by 3 articulated concrete mattress model was subject to pure uniform current flow. The subsequent forces were analysed with a six degree of freedom underwater force sensor. In order to gain a range of real world scenarios, the experimental model was tested at six flow angles ranging from -15 degrees through to 60 degrees, at 15 degree increments. Additionally, five fluid velocities starting at 0.6 m/s through to 1.4 m/s, at 0.2 m/s increments were investigated. These results explain how the inversion of a 400 series block increases its hydrodynamic coefficients and subsequently decreases its stability performance in current flow. Ultimately, this study provides experimental information for the installation of 400 series articulated concrete mattresses in the inverted orientation.

FERC pipeline decommissioning cost in the U.S. Gulf of Mexico, 1995–2015

At the end of its useful life or because of a catastrophic event such as a hurricane, offshore pipelines are decommissioned, which normally involves cleaning the line by pigging or flushing, cutting the pipeline endpoints, and then plugging and burying each endpoint below the seabed or covering with a concrete mattress. The vast majority of decommissioned pipeline in the U.S. Gulf of Mexico is abandoned-in-place if the pipeline does not constitute a hazard to navigation, commercial fishing, or unduly interferes with other uses of the seabed. Using a unique data source from the Federal Energy Regulation Commission, decommissioning cost estimates for 28 gas export pipelines in the shallow water U.S. Gulf of Mexico between 1995 and 2015 are evaluated. The average inflation-adjusted pipeline decommissioning cost was $301,000 per mile ($187,000/km) and $47 per cubic foot ($1660 per cubic meter). Hurricane damaged and leaking pipelines are about three to four times more expensive on a unit cost basis than undamaged and non-leaking lines. No time trends or scale economies were observed. Primary cost drivers are identified and discussed.

철근의 영향과 앵커 충돌각도를 고려한 유연콘크리트 매트리스의 손상평가

A flexible concrete mattress (FCM) is a structural system for protecting submarine power or communication cables under various load types. To evaluate its of protection performance, a numerical analysis of an FCM under an anchor collision was performed. The explicit dynamics of the finite element analysis program ANSYS were used for the collision analysis. The influences of the steel reinforcement modeling and collision angle of the anchor on the collision behavior of the FCM were estimated. The FCM damage was evaluated based on the results of the numerical analysis considering the numerical modeling and collision environment.

Modeling of concrete mattress for shore protection by Plaxis 2D

Modeling of concrete mattress for shore protection by Plaxis 2D