Advancing the art of
subsea inspection
Shell deploys
autonomous underwater vehicles to inspect assets offshore Nigeria
Steve Keedwell
Shell Companies in Nigeria (SCiN)
Shallow-water
autonomous underwater vehicles (AUVs) are a valuable tool for asset inspection,
providing benefits for customers, operators, and contractors alike. However,
the technology has not been widely used in the oil and gas industry for asset
inspection in shallow water. Working together with partners, Shell Petroleum
Development Co. (SPDC) has achieved several notable firsts in utilizing
shallow-water AUVs offshore Nigeria, including the first survey under the hull
of an operational FPSO vessel.
In a notable first
for the industry in West Africa, SPDC, together with partners, has utilized
shallow-water AUVs off the Nigerian coast to survey and inspect subsea assets.
These include production systems, such as three platforms and infield
pipelines/flowlines located in the EA field, the Sea Eagle FPSO,
as well as the Offshore Gas Gathering System (OGGS), a 260-km (~160 mi)
pipeline spanning the edge of the Niger Delta.
Due to the
challenging near-shore security situation, the deployment of a slow-moving
traditional survey vessel towing sensors was risk-assessed as unacceptable. The
assets to be surveyed are located only some 20-40 km (~12-25 mi) off the Niger
Delta coastline, in water depths of 15-40 m (~50-130 ft). OGGS pipeline and EA
field surveys had previously been delayed due to security concerns. In order to
ensure safe offshore operations, regular surveys are required to assess the
integrity of subsea assets as well as evaluate any detailed inspection or
maintenance needs that may have emerged.
Shell has a track
record of adapting new technology to reduce the HSSE exposure of personnel. So
it came as no surprise that, with a growing need to gather data on the offshore
assets, SPDC’s Geomatics Team was tasked to review alternative survey options.
This led to the selection and deployment of the REMUS 100 AUV operated and
managed by Fugro Survey Nigeria Ltd. (FSNL), and supported by Fugro Survey
Africa (Pty) Ltd. (FSAPL).
Fugro Chance Inc.,
part of the Fugro group, provided two REMUS 100 (Remote Environmental Measuring
Units) systems with associated equipment, with FSNL and FSAPL providing project
management and personnel. SPDC provided the overall project management,
in-country logistics and vessels to execute the work.
The REMUS 100 is a
compact, portable, and lightweight (37 kg in air) AUV with an operating depth
of 100 m (~330 ft) that can be deployed either from a vessel of opportunity or
the quayside. A specially designed rake is used to launch and recover the AUV.
An additional tool adapted for the project was a “dog leash” to secure the nose
of the AUV and to control its entry to the rake. When safely in the rake, the
AUV was manually lifted back onboard the vessel. The AUV can be fitted with a
range of sensors and Inertial Navigation Systems (INS).
Immediate
results
To complete the
survey operations in Nigeria, two AUVs were operated simultaneously. Inspection
surveys were managed from a main operations vessel that maintained a safe
distance from the Niger Delta coastline. Launch and recovery of the AUVs,
carried out by SPDC personnel, was undertaken by a smaller and faster vessel.
Careful planning around logistics, operations, and continual risk assessment
was important to minimize security risks and personnel exposure.
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Work scope in the EA field offshore Nigeria.
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The ability of the
AUV to survey within a few meters of platforms and facilities provided
additional value. First, it avoids the HSSE exposure of the traditional survey
method of vessels making close passes to structures. Second, the time to
complete a 600-m (~2,000-ft) survey centred on a structure is reduced, as the
AUV turns significantly faster than a vessel equipped with a standard towed
array of sensors (which requires approximately 30 minutes to turn as opposed to
the AUV turning time of 20 seconds). Also, the time required to mobilize the
AUV on a vessel is significantly shorter than to install a survey package of a
side-scan sonar and winch on a traditional vessel. Ultimately, a high-quality
dataset was acquired more quickly, more safely, and at reduced cost.
Pushing
boundaries
To monitor the AUV
during the mission, an operational practice was established to receive iridium
calls (via satellite) from the AUV, at the control center on board the
operations vessel. On receiving the iridium message from the AUV, the position
would be plotted and an update on the estimated time to complete the survey
established.
As confidence grew
in the performance of the REMUS AUV, more complex missions were conducted,
including combined long baseline (LBL) and “dead reckoning” missions. An LBL
array uses seabed transponders placed at known locations on the seabed with
baselines that can be several kilometers in length. The position of each
transponder is uploaded into the AUV navigation software and, during the
mission, the AUV navigates by calculating its position relative to each of the
transponders, which are set to transmit when interrogated by the AUV.
The process of “dead
reckoning” is used to determine the current position based upon a previously
known position fix, and advancing that position based upon measuring speed over
elapsed time and course deviation. The REMUS AUV is fitted with a range of
sensors – Acoustic Doppler Current Profiler (ADCP), Doppler Velocity Log (DVL),
conductivity, temperature and pressure sensors – and uses internal software to
update its position based on the sensor inputs received for navigation. An L1
GPS antenna with iridium satellite communication, was also installed to send
updates on the AUV’s health, and to derive its sea-surface position. The AUV
sensor payload can be modified to include a dual high-resolution side scan
sonar (900 kHz), video camera, and INS.
Using this hybrid
approach, the purpose of these missions was to acquire surveys around the structures
and decrease the number of infill pipeline/flowline surveys that would be
required on completion of the platform surveys.
During these
missions, there were a number of notable firsts:
- Utilization of a shallow water AUV in Nigeria
- Multiple platform surveys completed in Nigeria by an
AUV
- Dual launch and operation of AUVs in Nigeria
- Survey under the hull of an operational FPSO in
Nigeria by an AUV.
Great
potential
The primary
challenge was to conduct an important survey in a volatile area with many security
challenges, and to do so in line with Shell’s principle of zero harm to people
and the environment. The use of AUVs enabled SPDC to achieve this goal, but
also to leverage technical and commercial benefits for surveying seabed assets
in shelf and near-shore environments.
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AUV recovery using the specially designed “rake.”
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There is potential
for using this method to conduct offshore surveys in high-risk locations. These
include risks of security, unexploded ordnance or mine surveys, requiring a low
profile and presence on the actual site. Further opportunities include
environmentally sensitive areas such as coral reefs, where minimal impact is
desired. The possibility of acquiring high-quality datasets with a reduced
footprint is in the early stages. The surveys conducted by SPDC and partners
have effectively proven the value of this method from cost, quality, and safety
perspectives.
Other scenarios
could include launching multiple AUVs from one survey vessel, which could also
conduct survey operations. For example, during annual pipeline inspection
surveys, the vessel could target pipeline crossings, leaving the AUVs to
complete other tasks, and carry out passes close to structures. AUVs have also
been deployed in shallow water by Shell’s Geomatics Team in the Netherlands at
the Ameland platforms.
Recommendations
Opportunities to
leverage benefits of shallow-water AUV operations in debris, inspection, and
general seabed surveys, plus operational recommendations, include the
following:
- AUVs can operate in sea conditions that would
preclude standard towed system operations. Depending on water depth and
prevailing weather conditions, a heave effect on side-scan sonar records
could impact data quality. This would need to be assessed on a
project-by-project basis.
- For deployment of AUVs from smaller vessels, a team
of three to four people (depending on operational hours) is recommended to
manage the AUV in the field – party chief, electronics engineer, and
online surveyor (plus one if required). Good pre-planning of logistics
(e.g. fueling) would enable extended field operations. It is recommended
that the data be checked in the field for integrity before transfer
onshore for processing. This reduces the number of personnel in the field,
hence HSSE exposure.
- Launch and recovery of the AUV needs to be improved.
These activities were managed by modifying the rake to contain two
supporting ropes on either side. In addition, the head of the AUV was
snared with a dog catcher on a long pole to bring it under control. The
person who catches the AUV guides it into the rake. AUV recovery time was
reduced to less than 15 minutes in a well-managed and safe manner.
- The REMUS AUV GPS antenna would benefit from
enhancement from L1 to L1/L2, the ability to receive differential
corrections and for dual operation (currently the iridium and GPS share
same components and do not operate simultaneously).
- Installation of a screen visible on the deck for the
AUV launch team to view their location.
- Ensure that there are sufficient connecting cable
lengths for the AUV power and VIP interface to keep laptops away from the
open deck.
- Strobe lights are recommended when searching for the
AUV in poor light conditions.
- Addition of two handles on the exterior casing for
lifting the unit.
The use of shallow
water AUVs for subsea asset inspection have been positive, and for shelf and
nearshore operators, the AUV is cost-effective. Other possible scenarios could
include multiple AUV launches from one vessel, which could also conduct survey
operations, e.g., during annual pipeline inspection surveys, the vessel could
target pipeline crossings, leaving the AUVs to complete other tasks such as
close passes to structures. Under-ice surveying is another possibility, since
technology is being developed to provide the operator with the ability to
track, monitor, command, and interact with the AUV remotely while it is under
way, and share information with all interested parties. Two more possibilities
are surveys in high-risk locations (unexploded ordnance/mine surveys) that
require a low profile and surveys in environmentally sensitive areas, such as
coral reefs.
04/01/2011
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