Impacts from Partial Removal of Decommissioned Oil and Gas Platforms on Fish Biomass and Production on the Remaining Platform Structure and Surrounding Shell Mounds

<div><p>When oil and gas platforms become obsolete they go through a decommissioning process. This may include partial removal (from the surface to 26 m depth) or complete removal of the platform structure. While complete removal would likely eliminate most of the existing fish biomass and associated secondary production, we find that the potential impacts of partial removal would likely be limited on all but one platform off the coast of California. On average 80% of fish biomass and 86% of secondary fish production would be retained after partial removal, with above 90% retention expected for both metrics on many platforms. Partial removal would likely result in the loss of fish biomass and production for species typically found residing in the shallow portions of the platform structure. However, these fishes generally represent a small proportion of the fishes associated with these platforms. More characteristic of platform fauna are the primarily deeper-dwelling rockfishes (genus <i>Sebastes</i>). “Shell mounds” are biogenic reefs that surround some of these platforms resulting from an accumulation of mollusk shells that have fallen from the shallow areas of the platforms mostly above the depth of partial removal. We found that shell mounds are moderately productive fish habitats, similar to or greater than natural rocky reefs in the region at comparable depths. The complexity and areal extent of these biogenic habitats, and the associated fish biomass and production, will likely be reduced after either partial or complete platform removal. Habitat augmentation by placing the partially removed platform superstructure or some other additional habitat enrichment material (e.g., rock boulders) on the seafloor adjacent to the base of partially removed platforms provides additional options to enhance fish production, potentially mitigating reductions in shell mound habitat.</p></div

Annual somatic production per individual observed by size class.

<p>The values presented here are the product of the annual growth in weight and annual survivorship (see reference [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135812#pone.0135812.ref022" target="_blank">22</a>] for more detail) and plotted for each species that contributed at least 1% of Total Production on any platform (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135812#pone.0135812.s002" target="_blank">S2 Table</a>). Values were plotted over the size classes that a species was observed and rockfishes, <i>Sebastes spp</i>. were plotted with dashed lines. We also identify the curves for the two species observed that have the highest individual production rates, and for <i>Sebastes hopkinsi</i> which was used as the proxy for unidentified rockfishes because it was the most common species and its relatively low annual production rate per individual would result in a relatively conservative production estimate. Note that while growth in length according to the von Bertalanffy growth equation is highest at the smallest size, production here is maximized at intermediate lengths due to the exponential increase with weight-at-length and low survival rates at small sizes. Also, production goes to 0 when fishes grow larger than the mean asymptotic length predicted by the von Bertalanffy growth function [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135812#pone.0135812.ref022" target="_blank">22</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135812#pone.0135812.ref030" target="_blank">30</a>].</p

Map of the study area.

<p>The 16 oil and gas platforms (filled circles) used in the study were surveyed for at least 5 (up to 15) years between 1995 and 2011.</p

(a) Standing Stock Biomass (SSB) and (b) Total Production with SE error bars for complete platforms (white bars), partially removed platforms (gray bars), and for the entire shell mounds associated with some platforms (black bars).

<p>Gray bars represent the predicted overall biomass or Total Production that will be retained on the remaining platform structure after partial removal. While the fate of shell mound habitats after partial removal is currently unknown, the black bars represent a potential additional reduction in SSB and production if they were totally lost. Note that both a shell mound surface area estimate and associated fish survey data (permitting overall SSB and Total Production estimates) were only available for the five platforms with black bars shown. It should not be assumed that shell mounds are not present around some platforms because estimates are not provided here. Platforms are ordered from south to north (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135812#pone.0135812.g002" target="_blank">Fig 2</a>).</p

(a) The relationship between seafloor depth and platform submerged surface area, and the relationships between platform submerged surface area and (b) Log₁₀ complete platform standing stock biomass (SSB) or (c) Log₁₀ complete platform Total Production.

<p>Depth was significantly related to platform surface area [Surface Area (m²) = 531 * Seafloor Depth (m)-14464; R² = 0.93; p-value < 0.001]. There was no significant linear relationship between platform surface area and complete platform Log₁₀ SSB (R² = 0.09; p-value = 0.141), nor between platform surface area and complete platform Log₁₀ Total Production (R² = 0.19; p-value = 0.053).</p

Reefing options for decommissioned oil and gas platforms.

<p>After all wells are permanently sealed, decommissioning may encompass one of four alternatives for the platform [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135812#pone.0135812.ref002" target="_blank">2</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135812#pone.0135812.ref004" target="_blank">4</a>]: (a. Complete Removal) explosives are detonated to sever the well conductors, pilings, and support legs 5 m below the seafloor and the structure is towed to shore and scrapped, (a. Tow-And-Place) the severed structure is towed to a designated reef location and placed on the seafloor, (b. Partial Removal) the well conductors, pilings, and support legs are mechanically cut off, often at 26 m depth, and then optionally placed back on the seafloor as additional reef habitat, (c. Toppling) explosives are detonated to sever the conductors in the middle and pilings and support legs on three sides of the platform at the seafloor and the whole structure is bent over to remain in a horizontal orientation on the seafloor.</p