Multibeam survey remarks for the oil and gas industry

Fernanda Mattos Pinheiro COPPE/UFRJ & E3 Energy Environment Emergency

Rio de Janeiro, Brazil fernanda.pinheiro@e3-services.com

Nivaldo Gomes Junior E3 Energy Environment Emergency

Rio de Janeiro, Brazil nivaldo.junior@e3-services.com

Grant D. Jennings Teledyne Odom Hydrographic

Poway, CA, USA gjennings@teledyne.com

Adam Davis

Wish Software Limited Wiltshire, UK

adam@wishsoftware.com

Abstract — The oil and gas industry has a strong demand of subsea knowledge in order to determine seafloor characteristics, to provide secure installation of underwater structures, to check its conditions, among others. In this context, acoustic methods have been providing essential information. In the study case presented here, a multibeam system was used for pipeline inspection at two different water depths and the results obtained allowed the discussion on how acoustic changes on the signal can affect what is observed from the survey. The main objective is to emphasize the relevance of understanding the needs of the offshore industry, the capabilities of the equipment available and its limitations aiming to take the most of it.

Keywords— multibeam; challenges; pipeline survey; data processing; charting.

I. INTRODUCTION The ever-growing demand of subsea knowledge, where

acoustics takes an important role, is the base for the oil and gas industry development. Site surveys, route surveys as well as pipeline inspections require the use of geophysical tools such as multibeam, side scan sonars and sub-bottom profilers. These indirect systems combined with cameras and sediments samples can tell us a lot about the shallow subsea environment, guiding the decision makers through the most suitable subsea projects for that specific area of interest.

In this scenario technical knowledge is vital for the complete understanding of what is needed for the oil and gas companies as well as what can be delivered by the specific sensor that is being offered by the survey companies.

In the study case presented here, Teledyne Odom MB1 multibeam system was used for pipeline inspection, initially at shallow waters (5-20 meters water depth) then at intermediate waters (30-45 meters). The MB1 is a shallow water system that operates between 170-220 kHz with a maximum ping rate of 60 Hz. It has a maximum number of soundings of 512 with a maximum swath of 120°, vertical resolution of 3.6 cm and acoustic beam width of 4.5° by 3°. The latter has the major

influence on the area of the seafloor ensonified by each beam (beam footprint). The pipeline external diameter is 12’’ (about 30cm) and it was recently laid on the seafloor, being completely exposed on the major part of the route. Considering that the beam footprint increases with depth, it is expected that the visualization of the pipeline decreases, the deeper it gets.

Using Caris HIPS and SIPS for bathymetric processing and Fledermaus for visualization, some 3D images of the exposed pipelines were taken from the bathymetric dataset as well as longitudinal profile with 1m interval and cross profiles at user selectable intervals. This allowed a better understanding of the acoustic behavior of the system with depth. Alignment Sheets showed to be the best charting method where pipeline conditions over the seafloor can be easily comprehended. AutoChart is an AutoCAD extension that automates the production of these charts while allowing the user to create the alignment sheets straightforwardly.

Hence, the main objective of this paper is to emphasize the relevance of understanding the needs of the oil and gas industry, the capabilities of the equipment available and its limitations. The continuous development of survey equipment makes this area of knowledge very interesting where limits are always being beaten.

II. MULTIBEAM BASICS It’s well known that acoustic frequency is the parameter

which determines the range and sound penetration in sediments. The attenuation of the acoustic signal in the water is proportional to the frequency. Thus, higher frequencies shall be considered for use at shallower environments or close to the bottom.

In order to determine detailed seafloor morphology, a multibeam echo sounder system (MBES) can be used, being able to provide 100% ensonification of the seafloor, while determining, by a number of elements at the transducer, distance and range of the returning acoustic signal that was

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bounced off the seafloor or other objects. The attributes of an MBES system traditionally described by technical specifications are operating frequency, pulse length, beamwidth or beam spacing, number of beams and coverage. Recently, many of the systems show an impressive number of beams or soundings, typically 512, which makes minimum soundings per cell size requirements easy to meet. However, it exponentially increases the size of the datasets and the time required for managing and processing it. The maximum number of beams is something that should be analyzed before the survey, considering the required expectations of the data

that will be collected, the post-processing time and particularly if the system is capable of defining the features of interest, as some of them may appear smoother than they really are due to the water depth and the area ensonified by each beam.

Details of the specifications described above are discussed by [1] and [2].

III. STUDY CASE Fig. 1 illustrates the bathymetric surface at 4 locations

taken from the study case described at the introduction. A (6m)

B (12m)

C (40m) D (42m)

Fig. 1: Caris HIPS and SIPS bathymetric images showing bathy plan surface and 2D (distance x depth) graphs from the yellow rectangles drawn over the pipeline. A was taken at 6 meters, B at 12, C at 40 and D at 42 meters. The pipeline is hardly seen at last two images, although the backscatter image observed at D illustrates two pipelines and a mattress at the crossing.

Each location is from a different water depth aiming to describe the pipeline behavior. A decrease of the pipeline signal can be noticed from A to D. The pipe is completely observed at A, which was taken at 6 meters water depth in an area where possible changes in sedimentation rates at each side of the pipeline makes it even more evident. Area B was taken at 12 meters and the complete exposure of the 30cm pipeline is noticed. Then, two images where taken at intermediate waters, 40 (C) and 42 meters (D). In those cases, the pipeline is barely seen, corresponding to a slightly change in soundings depths that is hardly observed in the bathymetric surface. This occurs due to the increase of the area ensonified by each beam with the increase of depth. If what is expected from the survey is just the location of the pipeline, backscatter signal can be processed and used as shown at D, where a crossing between two pipelines is observed as well as the presence of a mattress, which have been installed to guarantee the integrity of the pipelines.

Fig. 2: Illustration of an alignment sheet main parts, where: A shows a plan view with pipeline positions; B shows the cross

profiles; and C shows the longitudinal profiles.

Alignment sheets result in an easier understanding of the scenario, where plan view maps of the route can be presented aligned to cross profiles and longitudinal profiles (Fig. 2).

IV. FINAL CONSIDERATIONS Through what has been exposed here, once there is a need

of surveying an area, it is strongly recommended that the contractee specifies the conditions that must be meet under the desired project, describing the water depth, the dimensions of the features to be analyzed, the resolution to be achieved as well as what is needed to be described, as free-spans for example. That’s when the technical support plays an important role in analyzing the proposals. Having the desired system (i.e. multibeam, sidescan, etc.) and reduced prices does not mean that the conditions will be achieved. Perhaps, the multibeam could have smaller footprints or work on another frequency, or an autonomous system like a ROV (remotely operated vehicle) or an AUV (autonomous underwater vehicle) should be used in order to get closer to the pipeline/seafloor to increase beam resolution. As there are a number of possibilities, every detail should be described, discussed and analyzed by the players in order to meet the expectations.

REFERENCES [1] L3 Communications SeaBeam Instruments, 2000. Multibeam Sonar

Theory of Operation. Available online at: http://www.mbari.org/data/ mbsystem/sonarfunction/SeaBeamMultibeamTheoryOperation.pdf

[2] US Army Corps of Engineers, 2004. Chapter 11 of the Hydrographic Engineer Manual: “Acoustic multibeam survey systems for deep-draft navigation projects”. 44p.