Seafloor Mapping for Shallow Water Rock Dumping Operations
Iroquois Gas Transmission System's Eastchester Extension Project is a 24-inch gas pipeline running from Hunts Point, New York (East River), up to Northport, Long Island. The construction project posed many obstacles since the pipeline not only runs through some of the busiest waterways in the United States, but it also covers extremely varied geology over the course of the route. Add to these factors a seafloor littered with debris, cable crossings, and other pipelines, and proper seafloor mapping during construction becomes critical. Weeks Marine (Cranford, NJ) was entrusted with one of the trickiest portions of the project - dumping rock over the installed pipeline in order to ensure it was protected against anchor dragging, etc.

Proving the correct rock coverage had been met was essential not only from a design perspective, but also from a permitting stance. Weeks Marine was faced with four major obstacles:

1. A varying geology (from soft silt to rock) meant that each area would react differently to rock dumping operations, affecting both the method of dumping and the amount of rock required to achieve proper coverage. In some cases an entire hopper would be dropped on the pipe and disappear into the silt, leaving only 10-foot mud waves in its wake. Therefore seafloor conditions, and the effects of a specific rock dump, had to be monitored in close to real time. Furthermore, it was essential that proper coverage be proven prior to moving the rock dumping barge and re-anchoring. Otherwise, a large percentage of the pipe would have remained unprotected, and once discovered during a traditional post-survey, would have required the remobilization of the barge, resulting in a significant loss of time and money.
2. Since specifications were extremely stringent, Weeks required a visual confirmation of the pipe's location prior to dumping rock, and they were not allowed to rely solely on the as-laid coordinates. They also had to position the barge extremely carefully in certain areas to avoid hooking electric and telecom cables or damaging other pipelines.
3. The monitoring system had to achieve its goals while remaining within the project's budget constraints and being simple enough to operate by project engineers without specialized assistance.
4. A very tight timetable required data from the monitoring system to be rapidly available to the decision makers. Operations started in December of 2002 and were completed in June of 2003.

To solve these problems Weeks Marine designed a dedicated rock dumping barge, the Weeks 529, and hired Alpine Ocean Seismic Survey (Norwood, NJ), which had already performed the pre-engineering route surveys, and Geod Corporation (Newfoundland, NJ), to engineer a specialized positioning and bathymetric surveying system. Alpine was tasked with the seafloor mapping system, while Geod handled the surface positioning. Alpine and Geod were also charged with installing, testing and deploying the systems, as well as training Weeks Marine personnel on their use.

The heart of the seafloor mapping system was a Reson 9001 shallow water multi-beam system operating at 455Khz with a 90 degree swath coverage. The 9001 was installed with the backscatter option, which allowed it to be interfaced to Chesapeake Technologies' Sonarwiz software, giving the engineers a virtual sidescan sonar image of the seafloor in addition to detailed bathymetry. Coastal Oceanographics' Hypack Max software was used to acquire and process the multibeam data. Heave, pitch and role compensation was accomplished using a TSS DMS2-05 unit.

Trimble MS750 RTK GPS receivers, provided by Measutronics (Lakeland, Florida), were used to position the barge, rock hopper, multibeam transducer, and provide heading and real-time tide corrections. Trimble HYDROpro software was used to integrate all positioning and heading data and provide tug and anchor information to the barge manager.

One of Weeks Marine's primary design criteria for the seafloor mapping package was that it should be deployable from the rock dumping barge without the use of a separate dedicated survey vessel. This would allow for operational cost and time savings, increased ability to operate in inclement weather, and full integration of rock dumping and surveying operations. Alpine and Weeks marine worked together to design and construct a self-propelled trolley onto which the multibeam transducer, the GPS RTK antenna, and the heave, pitch and roll sensor were mounted. The remotely operated trolley traveled on a track down the side of the barge, allowing it to map the entire area of seafloor covered by the rock hopper on any given barge anchoring station.

Once the barge was anchored on location, the engineer would run a pre-dump survey to assess exact pipe location and seafloor conditions. The information was used by the barge manager and construction engineers to plan rock dumping operations for maximum efficiency. It also provided a baseline to determine the effect of each rock dump. This pre-survey operation took 15 minutes.

After each cycle of rock dumping, a post-survey was performed. The data was compared to the pre-dump survey to ascertain if proper rock cover had been achieved. This took about 30 minutes.

This process was repeated until rock cover specifications were met. The data was presented to the client and the section signed off as complete prior to moving the barge to the next location.

The effort, preparation and upfront design costs resulted in a large payoff when it came down to the efficiency, speed and effectiveness of the rock dumping operations. The final system was so integrated and easy to use that the positioning and seafloor mapping system could be operated by one engineer. The system aided Weeks in completing the work before permit deadlines were imposed. In addition, no post-surveying was required, since Weeks had collected all the data immediately upon completion of the dumping operations, providing the client and permitting agencies with all the information they required to sign off on the project.

Swath Bathymetry in One of UK's Oldest Ports
Fathoms Limited, a leading UK's inshore and coastal survey specialist, recently completed its second annual survey of the historic Bristol Docks using swath techniques. A new view of the ancient docks was achieved, with a better understanding of the hydrodynamic processes of narrow waterways.

Located at the head of the forbidding Avon Gorge that leads out to the Atlantic Ocean, Bristol's history as England's once premier commercial port goes back a very long way. For thirteenth century civil engineering, the ditch was a colossal feat: 2,500 feet long, 120 feet wide, and about 18 feet deep. This ditch still forms part of the harbor infrastructure and today is known as St Augustine's Reach. The annual survey of the historic Bristol Docks performed by Fathoms is to assess the rate of siltation in the dock areas and appraise the need for maintenance dredging. The docks are still active and popular with leisure craft as well as commercial vessels and a minimum dredged depth of 10 feet (3m) is the target.

In 2001, Fathoms convinced the city council to survey its docks using modern swath techniques rather than the traditional methods with which they were familiar. The first survey in 2001 was conducted using a Submetrix 117/234 kHz interferometric swath system. Fathoms' November 2002 survey was done with a 240kHz Reson SeaBat 8101 multibeam echosounder. With a sounding rate of some 3,000 sps, the 8101 measures discrete depths to provide precision mapping of complex underwater features. The Reson 8101 was installed on the Elliann, one of Fathoms' inshore survey launches, through the moonpool using a purpose-built frame.

For positioning, Fathoms used the Fugro OmniStar DGPS system. The horizontal framework for the city is the Ordnance Survey national grid, so it was necessary to apply geodetic corrections to the WGS84 solution provided by the DGPS to obtain OSGB36 coordinates.

Like so many old city dock areas, Bristol Docks is fringed by tall warehouses and modern developments and crisscrossed by road and pedestrian bridges. Maintaining navigation by DGPS in such areas is fraught with frustrations, as first a building and then a bridge would obscure the open sky to the satellites. To overcome these difficulties, a combination of auxiliary fixes and short DR was necessary. The use of a multibeam in a highly detailed area greatly assists this process and aids in resolving ambiguities. Fathoms has developed a procedure that maintains precision without sacrificing accuracy. Nevertheless, working in such difficult terrain requires skill and planning to minimize outages and maintain production rates.

Although the docks are open to navigation, their water level is referred to as "sill level," which is an offset from the British vertical land datum known as Ordnance Datum (Newlyn) rather than a chart datum. Further, the complex nature of water levels and docking facilities render a purely 'navigational' datum inappropriate for engineering works. Real-time tidal observations at two fixed gauges were logged by the Harbor Office and passed to Fathoms for post-processing in Ordnance Datum. Considerable differences in water type are encountered in the docks' area: both fresh river and surface water run-off enters the dock complex, as well as highly variable saline influxes from the River Avon coming up from the Bristol Channel. For measuring the velocity of sound corrections for the multibeam, a Valeport 602 direct reading CTD unit worked well in the challenging environment.

To complete the survey set up on Elliann, data acquisition was managed by a QPS QINSy integrated navigation package. The sounding grid and waterfall multibeam display allows a real-time presentation of the sounding profiles that are color-coded and greatly facilitate on-line QC.

Seven areas were surveyed at 1:1250 scale (to tie with Ordnance Survey land mapping) with the Reson multibeam. Each area comprised a narrow waterway with a maximum width of 75m and at the narrowest, 20m. Either side, tall retaining walls, some as high as 30m, quays and dockside buildings lined the waterways. The depths of the water ranged between six and 20 feet (2-6m), except in the locks where the water was often less than five feet (1.5m) deep. The bed of the docks was typically silt muds and clays with occasional hard areas where masonry and concrete was exposed.

The contoured charts and imagery from the SeaBat proved beyond doubt the added-value that high-quality and managed multibeam gives to harbor engineering and maintenance dredging planning. In comparing the results from the Caris package used for post-processing with previous single-beam echosounding surveys, the only conclusion was that multibeam/swath sounding of the docks provided a quality of data and presentational techniques far beyond previous methods. When processed and presented as color-banded mosaics, the multibeam data immediately highlighted the areas of accretion and enabled the engineers to better understand the regime of the docks. This alone will save a great deal of money when planning the maintenance dredging program for the port.

Directly comparing the 2001 and 2002 allowed development of a difference model. Areas of accretion and depletion year on year became evident and, in one area, almost exactly balanced - information that will enable the engineers to better redesign their works program.

From an analysis of the multibeam bathymetry, Fathoms' engineers applied dredging templates to the data to determine the volume amount of material to be removed to meet the city's minimum dredged channel requirements. Further, it was possible to calculate the siltation rate within the docks over a year period. The high definition of the multibeam imagery provides important clues as to where the natural and man-made topography is influencing the siltation process which, in turn, will lead to effecting remedial engineering.

In conclusion, multibeam can be used successfully in narrow and very shallow waterways. It does require the guiding hand of expert operators and places considerable demands on the skills of the professional hydrographer. Planning is critical, even more so than in open sea conditions - moorings and shipping are constant difficulties and designing the works program around vessel movements is a challenge. Obscuration of satellite transmissions (GPS and telecommunications) by buildings and bridges is also difficult.

Alternative position fixing methods have to augment these outages and, for these, Fathoms has developed a successful solution combining traditional fixation with image matching.

Oceanic Imaging Surveys Honolulu Harbors
Hawaii's Oceanic Imaging Consultants (OIC) recently got some interesting results while demonstrating its inland surveying capabilities for Honolulu Harbours.

OIC's Tom Reed says, "We had been pressing Harbours for years to take a look at our stuff, to no avail. We have, over the course of the years, collected reams of data, and surveyed the harbors in their entirety with sidescan, multibeam and interferometric sonars several times over. We recently also did a demo of our ability to map extreme bathymetric detail with our hull-mounted Reson 8101 SeaBat, during which we not only were able to validate and cross-check the results of previous, 100-foot spaced trackline data with our 6-inch resolution swath coverage, but we found and mapped a 16-foot square, 8-foot tall object on the bottom which had been completely overlooked by conventional single-beam surveying."

While the bathymetry data clearly indicated the presence of a large object in a location next to an active pier, OIC was not able to definitively identify the object. In other areas, they had unambiguously detected and mapped pipeline crossings, sail and powerboat wrecks, a 40-foot shipping container, and numerous other manmade objects. However, interpretation of this one mysterious anomaly proved difficult.

Reed picks up the tale: "As luck would have it, just as we were puzzling over the nature of this object, Brock Rosenthal of Ocean Innovations, San Diego, was in town to demonstrate his SeaBotix Little Benthic Vehicle (LBV). We arranged to have Brock bring his LBV and meet us at a pier-side demo we had scheduled with a few clients the following day."

After installing the LBV monitor and base unit on the survey vessel, the crew began working their way around the pilings in front of Honolulu's Aloha Tower and Piers 10 and 11. Using their GPS vessel position, and previously created DTMs of the harbor from SeaBat data, they drove the LBV to the location of the large mysterious anomaly. They dove on it, only to find a large, roughly 16-foot square, 8-foot tall mound of seabed. A butte!

"And a beaut of a butte, at that," says Reed. So much for the screw theory. "As looking at lumpy bits of harbor floor has admittely limited appeal for most of us, one of the clients had wandered up to the bow for some refreshments. A gust of breeze swept his complimentary OIC cap into the harbor. Our ROV operator galliantly surfaced the LBV, and went off in hot pursuit of the errant cap, which it successfully snagged and returned to our vessel. As the client bent to retrive his cap, his cell phone, which was in his breast pocket, fell into the harbor."

After a brief moment of stunned silence, many unprintable remarks ensued, during which the ROV operator brought the ROV straight down from its location alongside the vessel and began circling the bottom. Within seconds, the cell phone came into view. The client, who was up on the bow consoling himself with refreshments, was called to the operator's console, and shown his cell phone, sleeping with the fishes.

"Wow!" the client said in utter amazement and genuine astonishment. "You mean to tell me you can see underwater with this stuff?" "It's moments like those that make it all worth while," says OIC's Reed. "Image is everything. The job is simple: Make the client happy."

ROV Surveys by Subsea Vision
Subsea Vision (UK) provides ROV inspections, surveys, construction, and diving services. The company recently completed several surveys using Seaeye Falcon and VideoRay Pro II ROVs.

Southern Water (May 2003) - Subsea Vision performed a major video survey for one of the UK's largest water companies, Southern Water, just north of Southampton. The ROVs operated in an extensive 4km underground labyrinth of flooded tunnels and shafts - some unseen since their construction. The tunnels were cut out of the chalk in the late 1800s to provide pure water for the city of Southampton. The ROVs worked to vertical depths of 144 feet (44m), and completed excursions exceeding 340m.

The bulk of work was performed by the Seaeye Falcon ROV. Areas with limited access were surveyed by the Video Ray Pro II. Individual sectional reports and videos were compiled, recording depths, headings, lengths, profile and other specific details.

South West Water (February 2003) - Subsea did some ROV work for South West water, surveying draw off grills to assess levels of debris prior to installing a new scour valve grill at Meldon Reservoir, near Okehampton, Devon. The ROVs manipulator was used to guide and position the grill at a depth of 134 feet (41m) before the ROV cut the tag line holding the grill. The operation was completed ahead of schedule and without involving any risks to divers.

Dean & Dyball (June 2003) - Subsea Vision's Seaeye Falcon surveyed pipeline diffusers from a Power Station off the coast of West Dorset, the first time an ROV has performed the task.

In the past, divers had to negotiate strong currents and over 82-foot (25m) water depths to complete the annual survey. The depth restricted dive times due to decompression constraints, and tidal movements resulted in limited periods of slack water in which to carry out the survey and causing delays. In addition a decompression chamber was required on site due to the remote location of the area. All this cost time and money.

Chris Bryant, Managing Director for Subsea Vision, had surveyed the area in the past as a diver. "Using our Seaeye Falcon ROV proved valuable when working at depths for longer periods and in the strong currents typical of this area. The ROV was able to complete the project in less time and without interruptions, which was therefore cost effective and far safer than using a dive team."

The survey commenced with a general visual inspection of the pipeline diffusers, concrete gravity anchors, and anchor chains. The ROV then performed a contact cathodic potential survey on the diffuser stems. The ROV was finally relocated to inspect the inshore works, which included Larson Pile & Gabion basket surveys followed by a scour survey of the pre-cast concrete apron and concrete in-fill sections.

Peter Diving Services Surveys Pipeline River Crossings in Russia
Russia has a wide system of crosscountry gas and oil pipelines that support not only the internal country needs but also export supplies, playing an important role in the Russian economy. Most of these pipelines start in northern regions and cross multiple rivers and lakes along their way.

Monitoring the condition of these underwater pipeline river crossings is part of the government's program to maintain safe petroleum transportation. Therefore pipeline owners value accurate and objective survey information.

Over the last decade, Russian surveyors have worked to decrease inspection costs while increasing survey quality. Customers want objective data to detect all possible defects in time to plan repairs without wasting time and money.

Vitally Latartsev, General Director of Peter Diving Services, says, "Pipeline inspections used to be performed by multiple dive teams, who were forced to make most of the measurements manually. This method was expensive left much to be desired in terms of accuracy." Founded in 1994, Peter Diving Services turned to sonar and computer-based technologies for pipeline crossing surveys.

Peter Diving settled on an underwater acoustic system with sidescan sonar, scanning head sonar, and a navigation system. But for a Russian diving contractor, finding this equipment was difficult.

"I was fortunate enough to have an internship at the commercial diving company of my friend, Tom Eason, in Charleston, South Carolina, in 1995," says Latartsev. "Along with very valuable experience gained while diving with Tom, I also learned about sonar technologies. In 1996 we purchased an underwater acoustic system, the Simrad MS-900. Time has shown that we made the right choice."

Peter Diving Services regularly performs pipeline river-crossing surveys for the Gasprom Company. Technical inspection, or acceptance, includes the following aspects:

• The most critical aspect is the trench bend, which should correspond to that specified by the project; height increase is not allowed.
• The second critical aspect is the trench bottom width.
• Information about the condition of the trench bottom (i.e., the presence of foreign objects in it, including stones, drowned logs, or rough changes of terrain) is also very important. These obstacles can damage the pipe coating or even stop the laying procedure.
• Last, but not least, is determining the volume of ground removed from a trench, to know how much to pay the construction contractors.

Peter Diving Services was involved in a particularly difficult pipeline construction project in Siberia for a local gas company called Norilskgasprom. The temperature reached -30 ¡ C, the length of the crossing was about 7,500 feet, maximum water depth was 177 feet (54m),and the current rate was about 2m/sec. Diving operations were out of the question.

Peter Diving Services relied on acoustic control of the underwater pipeline laying operation.

"We located our equipment on a barge that moved along the pipeline route for four days and nights," said Latartsev. "Twice during that period our operators detected, just in time, a change of direction that could have led to the pipe head exiting the trench, which usually results in the whole job being scrapped and started over. But our acoustic control allowed us to take urgent measures, changing the traction vector immediately, and the pipe reached the opposite bank successfully."

Working in the harsh Russian climate, Latartsev and Peter Diving Services are sold on acoustic systems for inland surveying. "The scanning head is extremely effective for companies like ours," says Latartsev. "The reliability and accuracy of the Simrad MS-900 has increased quality for our customers, and the safety level for our staff."

Coastal Wind Farm Surveys
Two recent wind farm site investigations by Andrews Survey, for clients off the east coast of England, illustrate the advantages of different survey tools in different situations.

The first involved the rapid acquisition of data to establish the seabed features of the turbine site and the route of the power cable. Andrews carried out a subbottom profiler and sidescan sonar survey, supplemented by a near-surface geotechnical site investigation to the beach landfall. The survey was then extended to the electricity substation using land techniques.

Andrews' proven development of lightweight CPTs for marine cables allowed the route inspection to be carried out from a suitable small vessel, far faster than measurements from jack-ups or vessels required for larger CPTs. The use of smaller vessels also has advantages in obtaining greater coverage close to landfall where the water may be shallow.

Offshore, Andrews used their Mini-cone CPT with a 25kN 2cm2 cone, capable of 5m penetration. From the beach landfall to the substation they surveyed the route using a 200kN land CPT truck. For all wind farm surveys the different types of data - geophysical, hydrographic, and geotechnical - are integrated into a single easily-interpretable set of charts and report.

The second site survey required deeper penetration for a meteorological mast foundation design and a wider investigation across the entire turbine site. Here, in addition to near-surface geophysics, Andrews used a heavy-duty hydraulic vibrocorer from a 68m vessel of opportunity to produce 6m cores into the weathered chalk. The cores were subsequently analysed at Andrews' Yarmouth-based soils laboratory.

Andrews recently refitted a 55m flatback vessel, the Strilbas, for a wide range of nearshore and continental shelf survey work. The refit included dynamic positioning and subsea acoustic positioning. Andrews built a dedicated survey room for four operators to work simultaneously.