Study Determines Environmentally Superior Decommissioning Options
Decommissioning decision makers often need to determine the best and most practicable options for pipelines, flowlines, and power cables, including leave-in-place strategies or removal. This study presents results from several evaluations using advanced methods to support the determination of options.
The decommissioning of offshore fields includes pipelines, flexible flowlines, umbilical lines, and other subsea infrastructure. Decommissioning decision-makers often need to determine the best and most-practicable options for these structures, including leave-in-place (LiP) strategies or removal. This study presents results from several evaluations of pipelines, flowlines, and power cables using proven methods such as net environmental benefit analysis (NEBA), human health and ecological risk assessment (HHERA), and comparative environmental assessment (CEA) to support the determination of environmentally superior decommissioning options.
Various pipeline decommissioning options were evaluated using several key methodologies in this study. A NEBA was completed through a detailed analysis of a group of hard-structure (i.e., steel) pipelines. Environmental conditions were collected from routine inspection videos and included pipeline and baseline ecological characteristics. Also performed was a HHERA, which used established methods to determine the risk of residual pipeline contents to ecological or human receptors. Human receptors included resident and recreational exposures to adults and children, site workers, and fishermen. Ecological receptors included marine invertebrates, fish, birds, and reptiles. Finally, a CEA was prepared specifically to evaluate flexible flowlines and umbilicals potentially left in place and was centered on thermoplastics. This assessment considered line materials, degradation times, and decomposition products in relation to potential environmental and toxicological effects. Specific methodologies for each assessment are described in detail in the complete paper.
Results and Discussion
NEBA. For each option (LiP and full removal), the NEBA calculated a 100-year recovery period. The pipeline supports a large pipeline benthic index (PBI = 12.0) because large numbers of both fish and invertebrates live in and on the pipeline compared with the reference or baseline condition (PBI = 1.8). Removal of the pipeline diminishes the PBI quickly to below baseline (PBI = 0), and during the following years recovery returns quickly to baseline conditions. However, the LiP option continues the robust PBI value through time. This is because LiP does not result in an impact to, or loss of, habitat. This productive value of the pipeline shows that the LiP option contributes greatly to the overall ecological value of the region during the expected life of the structure (e.g., 100 years).
HHERA. Data Analysis. Residual (i.e., post-flushing) constituents from the pipelines were modeled in the water column after a hypothetical line break following a 12-hour release time (Fig. 1). The release of constituents from the pipelines was modeled with the use of climate, ocean, and sea ice modeling (COSIM) to determine concentrations in the surrounding water column. The resultant exposure concentrations were evaluated for risk in the HHERA.
The ecological risk portion of the HHERA evaluated three media: water, sediment, and fish tissue. Risk to aquatic life was evaluated using water exposure-point concentrations compared with aquatic-life toxicity reference values. This value is defined as the dose above which relevant effects might occur following exposure. The hazard quotient (HQ) for each constituent was calculated. Values of the HQ greater than 1 were considered as showing risk, while HQ values equal to, or less than, 1 were indicative of no risk.
Risk to humans (children and adults) from the multiphase oil pipeline was evaluated by considering direct pathways of contamination. Noncancer and cancer risks were calculated using ingestion rates for adults and children, exposure frequency, exposure duration, and body weight. Dermal exposure to contaminants of potential concern in water was assumed to be negligible. To calculate risk, an assumption that all seafood consumed would be from the contaminated area was necessary.
HHERA. Results. Modeling results of the 12-hour release showed that the release plume was limited in spatial area and was ephemeral. As an example, Fig. 1 shows the modeled plume after 24 hours of initial release for polycyclic aromatic hydrocarbon (PAH) compounds. Only minor exceedances of a PAH toxicity benchmark were found and those persisted in the environment only for hours. Within 24 hours and after all remaining line content was released, all environmental PAH concentrations were below toxicity benchmarks (Fig. 1e).
Calculated HQ risk values for ecological risk for aquatic life were all less than 1, indicating insignificant risk to aquatic life receptors. No ecological risk was identified under chronic or acute exposure conditions. Similarly, human health risk was below all target thresholds.
CEA. Flexible Flowlines. Flexible flowlines are comprised of a blend of thermoplastics that offer technical and economic advantages over rigid pipes. Additionally, their long polymer chains and hydrophobic nature make them ideal for industrial processes in high-pressure environments. Their durability promotes long-term performance with minimal cracking and a slow degradation process. The direct toxicity of thermoplastic polymers is extremely low; most long chain molecules are thought to be biochemically inert because of their large molecular size.
Because thermoplastic bonds are strong and generally are difficult to cleave, they are slow to degrade naturally. Polymer breakdown in the flowlines can occur from both abiotic and biotic degradation. As the flowlines are exposed to the marine elements over hundreds of years, the plastics will eventually become brittle. This weakness in the chemical structure of the polymer will allow for microbial sources to use the carbon as a food source, thus initiating biodegradation.
Thermoplastic Properties. Polyethylene (PE). The degradation of PE thermoplastics is characterized by the splitting of polymer chains, a process referered to as chain scission. Additional factors contribute to biological degradation in seawater: the presence of degrading organisms, ultraviolet exposure, biofouling level, water temperature, oxygen content, nutrient content, species present, and surface roughness. Additionally, these organisms only are able to metabolize the polyethylene polymer chains once they have been shortened to 10–50 carbon molecules in length.
High Density Polyethylene (HDPE). These have a high molecular weight and degrade slowly because of their strong carbon bonds. HDPE is an attractive choice of industrial plastic because of its impact strength, chemical resistance, and overall stiffness. The external plastic layer of umbilicals is typically formulated from HDPE. The umbilicals are affected differently than the flowlines because of their location in the oil field and can be particularly sensitive to environmental stressors. Additionally, sea currents can cause lateral displacements that bend and put external pressure on the cables. Therefore, these break down at a different rate than flexible flowlines and likely will degrade faster.
Polyvinylidene Fluoride (PVDF). This is a thermoplastic polymer similar to polyethylene; however, it responds differently to abiotic and biotic factors. Comparable to PE, it is extremely stable and considered highly resistant to all forms of environmental degradation. However, it is presumed that none of the abiotic degrading factors are likely to have any significant effect on the long-term strength of the material. Given this, the only possible abiotic degradation would come in the form of mechanical overstress, which would result in cracks and weakness to the structure. The loss of structural integrity could eventually lead to biotic degradation; however, research into this subject is limited.
Effects of Thermoplastics on Marine Life. Biofouling communities on pipelines have been found to create their own habitat and therefore attract species that would otherwise not inhabit the area. While this development is often positive, it may pose an additional threat if the macro- and microplastics following degradation are held in the area. It is therefore possible that pelagic species may be attracted to the biofouling community because of their habitat complexity and food availability.
Benthic organisms are particularly sensitive to microplastics. High concentrations of microplastics in polychaetes have the potential to induce suppressed feeding activity; prolong gut residence times and inflammation; reduce energy reserves; and affect growth, reproduction, and survival. Not only may the micro- or nanoplastics prove detrimental to the polychaetes communities, but this effect would have far-reaching impacts because polychaetes play an important role in marine food chains.
Results from the various assessment methods conducted in this study provided a consensus that the LiP option is the preferred decommissioning strategy for both pipelines and flexible flowlines. The NEBA results indicated that hard-structure pipelines provided valuable ecological habitats and the development of fish resources. This was supported by HHERA analysis, which showed that residual risks of the release of pipeline components could be mitigated through flushing efficiency and result in insignificant ephemeral risks to aquatic organisms and human receptors. Components of flexible flowlines, including environmental thermoplastics, do not present a high level of toxicological risk within the LiP option. However, depending on the location of the line, removal may prove a prudent option. The overall timeline for degradation of thermoplastics also suggests that, by the time that abiotic degradation occurs (e.g., centuries) on the lines, most will be buried in seafloor sediments and removed from the possibility of environmental exposure.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 199209, “Determining Environmentally Superior Decommissioning Options for Hard and Flexible Pipelines,” by Paul Krause, SPE, and Juliane Baquiran, Environmental Resources Management, prepared for the 2019 SPE Symposium: Decommissioning and Abandonment. The paper has not been peer reviewed.