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Clean-burning and safe, natural gas is an environmentally friendly fuel. When natural gas is cooled to -160 degrees C (-320 degrees F), it turns into liquefied natural gas (LNG) and occupies around 600 times less space than it does as a gas. LNG can then be stored in tanks, pumped into large, specially built LNG tankers that can carry up to 265,000 cubic meters and shipped across oceans. Natural gas is increasingly the fuel of choice for a wide number of applications. In 1997, it replaced oil as the largest single source of primary energy in the United Kingdom.
With its rich North Sea reserves, the UK had been an exporter of natural gas while supply exceeded demand. In the past several years, however, the UK’s natural gas situation has been changing. Output from older fields in the North Sea diminished and while new fields are still being discovered, demand for gas is beginning to exceed the available supply. In fact, most experts now believe that by the end of the decade the UK will be importing around 50 percent of its gas requirements. That is why LNG is becoming an increasingly attractive option, offering the opportunity to bring natural gas cost-effectively by ship from more distant locations where pipelines would not be economic.
Today, as gas supplies in the North Sea continue to decrease, it is planned that LNG will be shipped to a number of import terminals around Great Britain by tanker, then connected to the UK gas distribution network. One such import terminal currently under construction in Milford Haven South Hook LNG will receive gas from the North Field reservoir located off the coast of Qatar in the Arabian Gulf.
The South Hook LNG import terminal will receive vessels twice as large as the largest crude oil tankers that regularly dock at refineries within Milford Haven, a safe deepwater anchorage and the fourth largest freight port in the UK. The South Hook LNG Terminal project is currently one of the largest civil engineering projects in the UK.
Existing Terminal
Located in Southwest Wales, the LNG terminal is being constructed on the site of the former Esso oil refinery, which was built in the late 1950s and de-commissioned in the late 1980s. Its geographical advantages and history of handling large vessels made it an ideal choice for docking LNG tankers as well as for LNG storage and regasification.
In November 2004, Belgium’s largest construction group, Besix, was awarded the Engineering, Procurement and Construction (EPC) contract for the marine works necessary to refurbish the 50-year-old oil receiving jetty and make it suitable to receive LNG carriers. The existing facility consisted of a 1.0 km (0.62 mi) long approach trestle and a 1.3 km (0.81 mi) berthing heading incorporating five loading platforms.
The project would be carried out as a joint venture with the UK’s Kier Construction Ltd. The jetty refurbishment scope of work included:
• Removal of redundant items and heavy demolition of the old berthing and mooring facilities, excluding Berth 3, Berth 5 and their associated structures;
• Repairs and refurbishment of 800 m (0.5 mi) of existing structures on the Jetty Approach (80 roadway spans supported on two crossheads beams and concrete piles foundations); and
• Construction of the new marine facilities including 370 steel piles (1066 mm or 42-inch diameter, 50 m or 164 ft length) to be driven, and 422 pieces of prefab concrete elements to be cast, placed and filled with an in situ reinforcement steel cage and in situ concrete.
Challenging Environment
Marine construction work encompasses a wide range of constraint and safety issues including constantly working high over sea water on the jetty, severe weather and challenging conditions, i.e., gusting winds, high tide range, poor visibility, etc. In addition, due to the site location within the Pembrokeshire Coast National Park boundaries and the jetty standing above a Special Area of Conservation containing a protected "red maerl" coral bed, the Besix-Kier JV team had to contend with a number of environmental issues. Any work would have to be carried out without polluting the marine environment. All these factors combined to make achieving a cost-effective and accurate survey a major challenge.
Optimum Methodology
The huge project highlighted Besix-Kier’s experience in complex marine works as well as their ability to quickly adapt work methods to comply with client requirements, design changes and onsite progress. The project’s survey component was headed by Besix-Kier JV Survey Engineer Denis Meremans, who integrated advanced survey equipment including robotic reflectorless total stations, RTK GPS systems and 3D scanning. The integrated approach increased safety, quality and productivity while reducing time, resources required and project cost.
Marine Positioning: GPS Finds its Place
At the project’s onset, Mereman’s crew established a permanently-fixed RTK GPS base station, consisting of a Trimble MS750 RTK GPS receiver, Trimble SiteNet 450 radio-modem and Trimble Zephyr Geodetic antenna onshore. A total station control survey was initially performed to establish control marks around the site. A GPS site calibration was then carried out with an RTK GPS rover backpack (Trimble 4700 and 5700 GPS receivers with internal radio). Crews surveyed the main control points with a good geometry to enclose the construction site. In the office, the site calibration was computed using Trimble Geomatics Office software, using the WGS84 lat/long measurements and local site Easting-Northing coordinates and applying a Helmert transformation with a scale factor defined to 1 to conserve construction scale. The results defined the local parameters for relating the RTK GPS measurements to the local control.
The RTK GPS solution is mainly used to position the four self-elevating platforms (SEP) in the correct location onsite (within 500 mm or 2-ft tolerance) to enable piles to be driven accurately (within 100 mm or 4-inch tolerance). Each jack-up movement and positioning is monitored with Trimble HYDROpro software through two Trimble RTK GPS rovers (Trimble MS750 and more recently Trimble DSM 232 receivers) receiving RTCM and/or CMR corrections from the base via radio-link. RTK GPS is also used to record the exact location of the jack-up legs, so the seabed can be monitored for environmental purposes. In total, 11 RTK GPS receivers and 7 radio-modems are currently running non-stop, demanding constant care and maintenance, to provide position information 24/7.
3D Piles Layout
"On this project the new jetty and berth piles are being built alongside the old ones," explained Meremans. "Old piles have to be cut 1 meter below the seabed level with the inner part remaining, and new piles have to be driven through with the risk of overlapping and clashing. That’s why our Design Department in Brussels requested an accurate existing pile layout right up front. They wanted to know the exact existing piles’ location, raking and orientation to later assess the structural engineering and design model for the new structures."
In Spring 2005, Meremans started surveying the berthing line piles with a reflectorless total station, shooting at the same level three points on both the top and bottom of each pile. This methodology proved fine for visible and vertical piles, but Meremans soon became aware of its limitations.
"Any obstructions such as handrails or scaffolding caused a problem and the high-tide periods were reducing the pile length visibility," Meremans said. "On some of the larger platfo
rms, such as Berths 1 (1620 m2 or 17,438 ft2) and 2 (1750 m2 or 18,837 ft2), I was confronted by a jungle of 128 and 164 piles. Some of them were hidden by others at sea level and some were impossible to recognize in the slab shadow. All of this reduced surveying productivity."
"Repeating the work would be difficult and costly," he continued. "Even at the modeling stage there were problems. In the field, we recorded the points with attributes and codes, and we generated two circles and the center point from the three points surveyed at upper and lower level, which was fine for vertical piles. However, when you cut a tilting pile with a horizontal plane, the section is no longer a circle but an ellipse. And we were also losing some accuracy on the pile’s geometry definition: location, diameter, raking and orientation."
Meremans had already suggested 3D scanning as a good alternative to conventional equipment for surveying any inaccessible piles. After a few months surveying, spending hours trying to catch single piles or points and sending their Design Department unusable ASCII (.XLS) and 2D-CAD data, Meremans contacted several manufacturers’ representatives to visit the site, understand their applications and requirements and perform a demo of their 3D scanning products.
"I was already familiar with the technology and I knew that surveyed point clouds would give a lot of information with very good accuracy," Meremans said. "However, three concerns remained: How could I extract the right information from thousands and thousands of points when two or three points would be sufficient to define a line, a plane or a volume? Would the software algorithms be strong, rapid and accurate enough to calculate and process all that data? Would the software be easy-to-use and user-friendly?"
Using a Trimble scanner, local dealer KOREC Group carried out a trial scan at the end of the jetty to test the equipment’s portability; a mooring "dolphin" at 60 m (200 ft) and a berth at 120 m (400 ft) was scanned to test the range, speed, accuracy, operability and usage of the scanner and field software. Back at the office the data was processed with Trimble RealWorks Survey software. Based on the results, Meremans assessed how best the scanner could be used on the South Hook LNG Terminal project.
"Within minutes of downloading we had registered and geo-referenced the point cloud and were able to utilize RealWorks’ cylinder-fitting routine to turn a mass of scanned pile points into CAD objects," he said. "Checks on the computed diameter of the cylinders confirmed the actual seven hundred millimeter (28-inch) size within just a few millimeters. In fact, the demo proved so successful we decided to purchase a unit and restarted the entire berthing line pile layout survey of 650 piles."
With unusually sunny, dry weather in December 2005, Besix-Kier survey crew was able to complete the 3D scan survey in 20 days. A full set of piles data was available 10 days after completion of the field survey. By mid-January, the entire set of point clouds was processed, with piles geo-referenced and modeled.
"We acquired better quality and a far greater quantity of data in one month with a 3D scanner campaign than in six months with conventional equipment," said Meremans. "In addition, we discovered 34 hidden piles that were previously undetected by the total station."
The scan data was then used by the Besix Design Department to calculate and assess the new piles and their design. Using the 3D-CAD geometries and "objects properties" spreadsheet exported from RealWorks Survey software, the Design Department was able to extend the pile’s shape to the rock layer, increase the new pile’s diameter to look for overlapping, avoid clashes and produce the final pile and structures layout.
3D As-built: Real or Virtual?
With demolitions, refurbishments and new constructions ongoing, the jetty approach was disconnected from the berthing line and the newly refurbished concrete access trestle became too far from the inaccessible new structures to carry out a conventional reflectorless survey. So the Besix-Kier survey team took the scanner by boat and set up on a SEP to perform a 3D scan.
Using Trimble PointScape field software Meremans selected four areas from two setup locations to scan, defining the distance range in the selected areas to be scanned to optimize time on site. Mereman’s crew scanned the hemisphere targets at four different known locations on the barge; these locations were previously monitored with the RTK GPS system and then surveyed by total station, again highlighting Besix-Kier’s use of integrated surveying.
After four hours spent on the barge, the field survey was finished with more than one million points acquired.
The following day, the crew downloaded the data in their office PC platform using Trimble RealWorks software. Both scans were merged together, geo-referencing the point clouds with the target coordinates’ registration. Using the same methodology they had used earlier, surveyors selected the piles’ point clouds with the segmentation tool and fit them with 3D CAD geometries. After four hours of processing, the 3D geometries and pile shape properties were exported to finally compare the as-built survey with the design layout. All piles were within the tolerances!
"It is impossible to imagine carrying out the same survey in a single day using conventional techniques and equipment without having access to the structures," Meremans said. "In fact, after the planning stage, it took six more months to complete the building sequences and get proper access to stable structures to even be able to set up a total station!"
According to Meremans, the project clearly identified 3D scanner benefits:
• Rapid data acquisition and vastly increased redundancy;
• Rapid 3D-modeling with existing software tools;
• Best definition (tilt, orientation, deviation);
• 2D or 3D visualization; and
• Flexible data management.
"The benefits of the 3D scanner have been clear — a quick return on investment, less time on site for our surveyors and consequently less exposure to surrounding activities and conditions, as well as increased precision and greater data confidence," said Meremans. "Today, we continue to use the Trimble scanner to scan existing structures as well as to provide the as-built of the new structures. The applications are multiple."
And those applications include:
• inaccessible shoreline rocks and cliffs;
• alignment and verticality of SEP Pauline legs extension (15m to 65m total length);
• part of non-destructive investigations to verify piles’ structural capacity (3D scans and sections of removed piles confirmed eccentricity and variation in wall thickness;
• 3D-scans and modeling to quantify deviation measurements after piles damaged or crashed.
Integrated Survey Engineering
On a project this complex, Meremans points out that different surveying tasks and applications require different approaches and techniques.
"It is no longer a case of doing simple topography we are talking about `integrated survey engineering’," he said. "I’m proud to provide training and guidance to the eight surveyors who work day or night with me, increasing their knowledge and improving their learning curve and I try to develop and keep an `integrated survey’ team spirit. In this way, the Besix-Kier survey team can achieve the best precision at the lowest cost by using a large range of survey equipment and advanced technologies. This approach improves field and processing safety, quality and cost-effectiveness, with a quick return on investment for the project and companies. In this project the survey technologies have been integrated together and backed up by confident survey s
uppliers and good technical support."
Author Note: Denis Meremans is a Survey Engineer for Besix-Kier JV. Besix-Kier is a member of the BESIX Group.
Steve McGowen is a California freelance writer specializing in surveying. Lucy Hamilton is Media Liason for the KOREC Group in the U.K.
A 1.912Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE