The dredging industry has undergone significant changes in the past decade. Traditional dredging operations such as construction and maintenance of ports and terminals, dredging of drainage systems and channels, and dredging of lakes and waterways are still an important part of the dredging industry, but with the continuous development of the economic and technological situation, dredging technology has continued to have new applications such as land blowing, offshore infrastructure, environmental protection projects, and mineral extraction, etc. The traditional dredging business has been overtaken by other businesses. At present, major dredging companies are striving to expand their extension fields and cultivate new economic growth points. With the increasing demand for the development of the marine engineering construction field and the successful application of dredging construction equipment and technology in this field, offshore services have attracted extensive attention from major dredging companies and have gradually become one of their main businesses.
First, the four major dredging companies Jan De Nul, Van Oord, Boskalis and Deme are the major dredging companies with significant influence in the field of dredging and they occupy 60 % of the free market in the global industry. The Big Four are pioneers in expanding their business areas and are major contractors in major marine engineering and dredging-related fields, in addition to their main business of dredging, by contracting and executing many large marine projects. The four major dredging companies have their own focus on offshore projects, and they all attach great importance to this business and strive to expand the niche market, with their turnover accounting for more than 10% of the total turnover. The offshore engineering business has now been separated into its own segment. Jan DeNul's offshore engineering services include trenching and backfilling, dredging of subsea pipelines, cofferdam installation and pipelaying operations, subsea rock throwing, solid ballast filling and scour protection of offshore structures, trenching of subsea production systems, construction of drilling islands, etc. Jan DeNul's share of offshore engineering was 8% of total turnover in 2010, compared to 9% from 2007 to 2009. The share of offshore projects was 9 %, 20 % and 18 % from 2007 to 2009, respectively. The rapid growth of the company's offshore projects in 2008 and 2009 was due to the projects in Dubai, while the economic crisis caused the Middle East market to shrink, which reduced its share of offshore projects, but it remained around 10% of the total turnover. In 2010, the company's oil and gas-related revenues (including the onshore component) accounted for 33% of its total turnover. Van Oord provides safe and reliable solutions to many well-known oil and gas operators, national oil companies, offshore installation vessel owners, consultants and other private clients, including single point mooring installations, subsea rock dumping, subsea laying operations, offshore section construction and gravity based infrastructure installations. Since 2009, the company has grown rapidly in offshore projects thanks to the acquisition of offshore wind power projects. Deme's offshore activities include offshore pipeline trenching and backfilling (including construction of tie-in sections), precision rock throwing services up to 2,000 m deep, directional drilling for pipeline crossings, underwater rock drilling and blasting, marine environmental soil surveys, offshore lifting and rescue, offshore wind farm construction and maintenance, etc. The company has a specialised pipe-laying and offshore section dredging maintenance business, whose turnover is stable at around 10 % of the total turnover, 11 % in 2009 and 10 % in 2010.
Secondly, the typical projects of the dredging industry extending to the marine engineering market are offshore or marine engineering with a wide scope, including marine oil and gas extraction, bridge construction, tunnel construction, marine mineral development, marine energy utilization, marine space utilization, marine biological farming and fishing, etc. The expansion business of the dredging industry to offshore engineering mainly includes offshore oil and gas extraction related projects (including pipeline laying and its related business, pit excavation, rock throwing maintenance, etc.), as well as tunnel construction, deep sea mining and other businesses.
2.1 Subsea pipeline laying Offshore oil and gas pipeline laying includes sub-businesses such as trench pre-excavation, pipe laying, backfilling and leveling, rock throwing protection, shore section dredging and related civil construction works, etc. Unlike the past when multiple companies shared the sub-projects, today's bidders usually package the whole project and let the winning bidder carry out the scheme design, construction design and project coordination. Large dredging companies with multifunctional operation capabilities have considerable advantages in such projects as they can mobilize in a unified manner, coordinate flexibly and improve construction efficiency. Large foreign dredging companies are very concerned about this kind of business and have invested heavily in developing various specialized technologies and equipment. As early as the 1990s, Jan De Nul undertook the NorFra pipeline laying project. The project required the laying of a high-pressure gas pipeline between the Draupner E platform on the Norwegian continental shelf and Dunkirk on the northern coast of France, with a diameter of 1,066.8 mm (42in) and a length of 835 km [2]. Jan De Nul was awarded a share of 80 000 000 USD for one of the joint contracts, including the following tasks: 1) Pre-sweeping of the approximately 230 km long submarine beach on the Dutch, Belgian and French continental shelves to meet the stresses, strains and bottom clearances within the permitted limits during the pipeline laying. The pre-sweeping work was mainly carried out by the large dredger "Gerardus Mercator". 2) Pre-dredging of the hard clay trench at the Westhinder Channel crossing on the Belgian continental shelf to ensure that the pipeline was protected by a layer of gravel of at least 1 m. The trench was excavated by the then ultra-large rake suction dredger "Gerardus Mercator" equipped with an extra heavy and wide rake head, and the gravel layer was backfilled by the "Cristoforo Colombo". 3) Excavation and backfilling operations as well as shore section, coastal section protection and other related civil construction operations.
2.2 Pit excavation Under sea ice conditions, the wellhead equipment and pipe manifolds are protected by placing them in a pit below the seafloor and a channel is dug between the bottom of the pit and the slope to accommodate pipelines and cables. The excavation of the pit can be done with a variety of dredging equipment. For the White Rose wellhead protection project in Canada, Tideway and Boskalis used a grab dredging method with a submerged grab and towing system on the rock thrower Seahorse, while Jan De Nul sent the large dredge suction dredger "2.2.1 Grab dredging method Grab dredging includes the use of a grab to excavate the pit and to excavate the area between the working site and the dumping site. The grab is actually a remote-controlled grab. The grab used is actually a combination of a remotely operated vehicle (ROV) and a grab, which is equipped with four thrusters for positioning and guiding the grab above the excavation point to ensure the levelness of the pit. The grapple used for this operation was specially manufactured for the project, taking into account the effect of weight and shape on the fill rate of the grapple, and was equipped with a special blade to increase the initial digging depth and efficiency of the operation. A special software was developed to operate the grab, taking into account factors such as motion compensation, winch load or speed settings, cable suspension shape, etc. The program automatically calculates the cable load, winch speed and load and checks if it exceeds the allowable range, and once the allowable value is exceeded, the program alerts and automatically corrects, and can be changed to manual control if needed. During the whole operation, the monitoring equipment updates the position of the grapple in real time, and the data it obtains is also used as input to the motion control system to guide the construction operation.
2.2.2 Dredging method of rake suction dredger Vasco S.A., a subsidiary of Jan De Nul Group, completed another foundation excavation of the White Rose project. The dredger was 200 m long and 36 m wide, and could operate in sea conditions with a wave height of 2.5 m and a cycle of 8 s. The dredger reached a depth of 135 m. The dredge reached a depth of 135 m, and in the later stages of the project, the actual dredging depth reached 165 m. The "Vasco da Gama" was equipped with a deep water suction pipe with a submerged pump and an 8 m wide, 50 t rake head to reduce the number of operations and to ensure the accuracy of the bottom of the pit with a levelness of 25 cm. The entire operation is monitored by the dredging control system on board, with real-time monitoring of the seabed using a multibeam bathymetry system installed in the dredge's moon pool and connected to the dredging control computer for continuous updates on the progress of the operation.
