Study identifies environmentally superior decommissioning options

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The decommissioning of offshore fields includes pipelines, flexible flow lines, umbilical lines and other subsea infrastructure. Decommissioning decision-makers often need to determine the best and most practical options for these structures, including hold-in-place (LiP) or exit strategies. This study presents the results of several assessments of pipelines, flowlines and power cables using proven methods such as Net Environmental Benefit Analysis (NEBA), Human Health Risk Assessment and environment (HHERA) and comparative environmental assessment (CEA) to support the determination of environmentally superior decommissioning options.

Methods

Various pipeline decommissioning options were evaluated using several key methodologies in this study. A NEBA was carried out through a detailed analysis of a group of hard-structured (i.e. steel) pipelines. Environmental conditions were gathered from routine inspection videos and included the ecological characteristics of the pipeline and base. An ERSHE was also performed, which used established methods to determine the risk of residual pipeline contents to ecological or human receptors. Human receptors included exposure of residents and recreational activities to adults and children, site workers, and fishermen. Ecological receptors included marine invertebrates, fish, birds and reptiles. Finally, an ACE was prepared specifically to assess flexible flowlines and umbilicals potentially left in place and focused on thermoplastics. This assessment considered pipe 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 full document.

Results and discussion

NEBA. For each option (LiP and complete deletion), the NEBA calculated a payback period of 100 years. The pipeline supports a large benthic index (PBI = 12.0) because large numbers of fish and invertebrates live in and on the pipeline relative to the baseline or baseline condition (PBI = 1.8 ). Withdrawal from the pipeline rapidly decreases the PBI below baseline (PBI = 0), and over the following years, recovery quickly returns to baseline conditions. However, the LiP option keeps the PBI value robust over time. Indeed, LiP has no impact on habitat or loss thereof. This productive value of the pipeline shows that the LiP option contributes greatly to the overall ecological value of the region over the expected life of the structure (eg 100 years).

HHERA. Data analysis. Residual (i.e. post-flush) constituents of pipelines were modeled in the water column after a hypothetical line break after a release time of 12 hours (Fig. 1). The release of pipeline constituents was modeled using Climate, Ocean and Sea Ice Modeling (COSIM) to determine concentrations in the surrounding water column. The resulting exposure concentrations were assessed for risk in the HHRA.

The ecological risk part of the EHSR evaluated three environments: water, sediment and fish tissue. The risk to aquatic life was assessed using the exposure point concentrations in water relative to the reference values ​​for toxicity to aquatic life. This value is defined as the dose above which relevant effects may occur after exposure. The risk quotient (HQ) for each constituent was calculated. HQ values ​​greater than 1 were considered indicative of risk, while HQ values ​​equal to or less than 1 were indicative of no risk.

The risk to humans (children and adults) from the multiphase pipeline was assessed taking into account the direct routes of contamination. Cancer and cancer risks were calculated using ingestion rates for adults and children, frequency of exposure, duration of exposure and body weight. Dermal exposure to contaminants of potential concern in water was considered negligible. To calculate the risk, it was necessary to assume that all seafood consumed would come from the contaminated area.

HHERA. Results. The 12 hour release modeling results showed that the release plume was spatially limited and ephemeral. For example, Fig. 1 shows the modeled plume after 24 hours of initial release for polycyclic aromatic hydrocarbon (PAH) compounds. Only minor exceedances of a toxicity benchmark for PAHs were observed and these persisted in the environment for only hours. Within 24 hours and after release of all contents of the remaining line, all concentrations of PAHs in the environment were below the toxicity thresholds (Figure 1e).

Fig. 1 — COSIM plume maps showing the 12 hour modeled PAH pipeline release. The red arrow indicates the PAH toxicity reference.

The calculated HQ risk values ​​for ecological risk to aquatic life were all less than 1, indicating insignificant risk to receptors to aquatic life. No ecological risk has been identified under conditions of chronic or acute exposure. Likewise, the risk to human health was below all target thresholds.

CEA. Flexible flow lines. Flexible flow pipes are made of a mixture of thermoplastics which offer technical and economic advantages over rigid pipes. In addition, 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 considered biochemically inert due to their large molecular size.

Since thermoplastic bonds are strong and generally difficult to cleave, they are slow to degrade naturally. Degradation of polymers in flowlines can result from abiotic and biotic degradation. As drains are exposed to the marine elements for hundreds of years, plastics will eventually become brittle. This weakness in the chemical structure of the polymer will allow 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 breaking of polymer chains, a process called chain scission. Additional factors contribute to the biological degradation of seawater: the presence of degrading organisms, exposure to ultraviolet rays, level of biological fouling, water temperature, oxygen content, nutrients, species present and surface roughness. In addition, these organisms are only able to metabolize polyethylene polymer chains once they have been shortened to 10 to 50 carbon molecules.

High density polyethylene (HDPE). These have a high molecular weight and degrade slowly due to their strong carbon bonds. HDPE is an attractive choice of industrial plastic because of its impact resistance, chemical resistance, and overall stiffness. The outer plastic layer of umbilicals is typically formulated from HDPE. Umbilicals are affected differently from flowlines due to their location in the oilfield and can be particularly sensitive to environmental stressors. In addition, sea currents can cause lateral movements which bend and exert external pressure on the cables. Therefore, these break down at a different rate than flexible pipes and will likely degrade faster.

Polyvinylidene fluoride (PVDF). It is a thermoplastic polymer similar to polyethylene; however, it reacts differently to abiotic and biotic factors. Comparable to PE, it is extremely stable and considered to be very resistant to all forms of environmental degradation. However, it is presumed that none of the abiotic degradation factors are likely to have a significant effect on the long term strength of the material. Taking this into account, the only possible abiotic degradation would be in the form of mechanical overvoltage, which would lead to cracks and structural weakness. The loss of structural integrity could eventually lead to biotic degradation; however, research on this topic is limited.

Effects of thermoplastics on marine life. It has been discovered that biofouling communities on pipelines create their own habitat and therefore attract species that otherwise would not inhabit the area. Although this development is often positive, it can constitute an additional threat if the macro and microplastics resulting from the degradation are retained in the area. It is therefore possible that pelagic species may be attracted to the biofouling community due to the complexity of their habitat and the availability of food.

Benthic organisms are particularly sensitive to microplastics. High concentrations of microplastics in polychaetes have the potential to induce suppressed food activity; prolong intestinal residence times and inflammation; reduce energy reserves; and affect growth, reproduction and survival. Not only can micro- or nanoplastics prove to be detrimental to polychaete communities, but this effect would have far-reaching impacts as polychaetes play an important role in marine food chains.

Conclusion

The results of the various assessment methods carried out in this study have led to a consensus that the LiP option is the preferred decommissioning strategy for pipelines and flexible pipes. NEBA results indicated that hard-structured pipelines provided valuable ecological habitats and development of fishery resources. This was corroborated by the HHERA analysis, which showed that the residual risks of release of components from the pipeline could be mitigated by the efficiency of the flushing and lead to insignificant ephemeral risks to aquatic organisms and human receptors. Flexible flow line components, including environmental thermoplastics, do not present a high level of toxicological risk in the LiP option. However, depending on the location of the row, deleting may be a prudent option. The overall timeline of thermoplastic degradation also suggests that by the time abiotic degradation occurs (e.g., centuries) on the lines, most will be buried in seabed sediments and removed from the possibility of exposure. environmental.


This article, written by JPT Technology Editor Chris Carpenter, contains article highlights EPS 199209, “Determining environmentally superior decommissioning options for rigid and flexible pipelines”, by Paul Krause, SPE, and Julienne Baquiran, Environmental Resources Management, prepared for the 2019 SPE Symposium: Decommissioning and Abandonment. The document has not been peer reviewed.


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