New subsea field developments, or modifications to existing fields, require extensive effort in geophysical surveys to properly engineer pipeline and cable routes. This can potentially involve different campaigns of individually surveyed routes, followed by prolonged cycles of data review and engineering decisions.
Recent survey operations by Ocean Infinity in Australia have demonstrated the benefit of using multiple AUVs deployed from a single vessel to accelerate pipeline engineering decision making during a single campaign, improving efficiency and reducing costs at the early phases of field development design.
Collection of marine geophysical data in high-resolution can be a costly effort, particularly in deep water. Traditional planning involves scoping the first survey campaign along the most probable route, often based on limited existing seafloor data. A significant risk arises if unsuitable seabed conditions are discovered during the survey, as further campaigns will be required to confirm the safest route. Such a linear approach to survey design is inefficient, both in terms of costs for additional campaign mobilizations and delays to time-sensitive engineering tasks.
If geophysical data could be collected in broad strokes, at sufficient resolution, project stakeholders would benefit by evaluating multiple route options into the development field during the same survey campaign. Seafloor data collected would not only have immediate value to the sponsoring project but could be used in future field developments such as subsea tie-backs.
Over the past 15 years, AUV (Autonomous Underwater Vehicle) geophysical survey methods have been employed successfully on subsea development projects and offer many advantages over older, deep-towed survey methods. Recently, a survey completed by Ocean Infinity for Woodside using a single-vessel, multiple AUV configuration has demonstrated the ability to survey large areas of seabed at high-resolution to simplify route evaluation and save cost compared to single-AUV operations.
Obviously, operating multiple AUVs from a single vessel presents technical challenges and requires a larger vessel, which can be costly. Accordingly, the approach must leverage multiple survey assets from a single ship to maximize the survey production of the platform. The efficiency in survey production is made possible through the combination of 115-meter DP2 vessel, advanced sensor technology, multiple AUVs, Unmanned Surface Vessels (USVs), ROVs and innovative onboard data processing techniques.
Below we examine a recent multi-AUV survey to highlight the efficiencies over traditional single-AUV survey methods providing great value to stakeholders.
In June 2018, Ocean Infinity completed a survey campaign for Woodside Energy Ltd. (WEL) at the Scarborough Field off the coast of Western Australia. Familiar challenges were faced by the client leading up to the survey — a time-sensitive engineering phase and several route options over a large escarpment expected to have complex bathymetry.
-Figure 1. Scarborough Field Location including Routing Corridor
In total, the geophysical campaign aimed to survey four main areas from the outset (Table 1), with corridor widths ranging from 1 km to 6 km and lengths ranging from 9 km to 235 km. The project water depths extended from 70 meters to 1450 meters at the new subsea development field (Figure 1)
-Table 1 Initial project SOW and routes
The project was executed by deploying up to six Kongsberg HUGIN-6000 AUVs simultaneously in pre-planned missions. As the pipeline route was decided a single AUV was deployed along high resolution “low-pass” mission to collect additional data along the final route.
Ocean Infinity operated the AUVs nearly exclusively in autonomous mode – meaning they were not constantly supervised at all times be the host survey vessel. This precluded the need to deploy USVs while still maintaining the positioning specification within tolerance. The autonomous mode has been developed and well tested by Ocean Infinity but was not without challenges, particularly in shallow water (<100 m) environments containing dynamic thermal layers and variable currents.
Each AUV’s EM2040 multi-beam echosounder collected high resolution bathymetry (~1-2 meters) co-registered with Edgetech 2205 side scan sonar and subbottom profile data. Side scan frequency settings were varied between SSL (75kHz), SSH (230kHz), and SSX (410kHz) resulting in resolutions of 50 cm for the SSX setting.
As the multi-AUV survey campaign was underway, in near real-time, Ocean Infinity delivered 1-meter to 2-meter resolution XYZ bathymetric data to client pipeline engineers onshore via satellite. This allowed instant evaluation of critical areas while the survey mission was still in progress. (Figure 2)
Figure 2. Wide-area bathymetric data allowed evaluation of safe routing in near real-time.
Processed bathymetric data was generally completed and transmitted to shore ahead of schedule, with some XYZ gridded datasets being made available in 13 hours, with an average delivery time of 20 hours per set.
Data transmitted to the client pipeline engineers onshore allowed immediate decisions regarding route feasibility, and the vessel was redirected as required to develop alternative routes.
Figure 3. Alternative routing based on initial findings conducted during the same campaign.
Subsequent route deviations and flexible route adjustments were surveyed with 2 or 4 AUVs at a time, the quantity being determined by the most efficient number required to cover the given area. Compared to a single-AUV operation, route deviations of this magnitude would likely have required an additional survey campaign (Figure 3).
The project was executed successfully and resulted in a number of industry firsts, including near real-time route selection by client pipeline engineers based onshore. During the AUV survey, the vessel was able multi-task collecting ROV video of seabed anomalies, velocity measurements and inspection of existing pipelines.
As the routes were modified during the course of the project, the actual area covered varied slightly from the initial planned design. The following table shows the results of the final survey coverage.
Table 2: Completed SoW, Routes & Details
In total, nearly 6,800 line kilometers of seabed were surveyed in 19.8 operational days, with a depth ranging from 70 meters to 1450 meters. The operation averaged 342 line kilometers of coverage per day. Due to the sheer size and type of vessel, QHSE systems and safety culture all operations were completed without downtime due to weather and with zero HSE incidents.
From a survey production standpoint, a single-vessel single-AUV survey would have taken nearly 70 days to complete, excluding weather, and well beyond the typical endurance of a standard single-AUV survey vessel. The production efficiency compared to a multi-AUV survey in this project is nearly 4:1. This efficiency not only benefits the project schedule, but provides similar ratio reduction of: carbon emissions, produced onboard waste, garbage and man hours.
Cost comparisons to a single-vessel single-AUV will consider the advantages such as:
We conclude the multi-AUV concept has been shown to better facilitate pipeline route selection during a single survey campaign.
It is expected future subsea geophysical surveys will reduce costs by using a similar multi-AUV approach to subsea development projects.