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TSHD Oranje

The modern state of the art TSHD Oranje was selected to execute the works. The Oranje is a mid-sized TSHD of 156 m in length and featuring two standard 4.5 m dragheads. Each draghead is powered by a 3,000 kW dredge pump and has jetting nozzles capable of delivering 13 bar. The hopper has a capacity of 15,961 m³ and a carrying capacity of approximately 27,000 Te. As with the vast majority of dredging vessels, the Oranje is not DP rated but has a DT (Dynamic Tracking) system for navigation. Therefore the dredging corridor is manually navigated by the bridge crew. Simulator

Survey Preparation

A major part of the work scope was surveying. This comprised three components: ■

As part of the project preparation the project scope was replicated on the hopper simulator at Boskalis’ head office training facilities in Papendrecht to ensure that the seabed tolerances for the project could be met. The project team and key crew members together with the project teams from Dana Petroleum and Subsea 7 participated in several test trials. The results of these trials provided detailed insight what could be expected as best result and led to operational recommendations for the execution of the work. These trials also highlighted potential risks that were further discussed during the project hazard identification risk assessment. Boudewijn Baan: “For the simulation two representative 1 km sections were produced for each flowline. These sections were input into our hopper simulator at our training centre where experienced draghead operators dredged the profiles. These trials resulted in a number of operational

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recommendations and proved that the sand waves and megaripples could be smoothed to the required accuracy.”

2012

Pre-Survey – This was carried out to confirm the expected migration rate and to ensure no objects had been dropped along the pre-swept corridor; Intermediate Surveys – To check dredging progress and refine the dredging procedure as required; Post-Survey – For verification of compliance with the contract.

To complete these operations efficiently a number of survey improvements were made to the TSHD vessel. Boskalis Offshore designed and installed a comprehensive survey spread comprising MBES, SSS, GAPS USBL and OBSROV. Due to the size of corridor required for the Medway pipelines only one draghead would be used for dredging. This allowed the spare dredge pipe to be fitted with a GAPS USBL transponder and SSS fish. An OBSROV was also fitted at this side of the vessel. This was the first time that Boskalis had installed an ROV on a TSHD and support was provided from sister company SMIT Subsea Europe. The MBES was housed in the service frame of the dredge pipe in a fabricated housing.

dredging progress, identifying and investigating targets found during intermediate progress surveys, and finally, performing the as-built survey. In addition to the SSS and ROV, a mesh was also fitted over the draghead to ensure smaller items, which would not be detected by SSS, did not enter the hopper. Operation

After the as-found survey was completed and processed, dredging began in sea states with a significant wave height of around 2-2.5 m. To maintain dredging accuracy the datum was set 1 m above the design profile to allow the dredging works could start. Bulk dredging was performed to remove the crests of the sand waves. This was achieved using a ‘saw-dredge’ motion where the TSHD dredged in a forward direction, lifted the draghead and reversed before starting the next run. This process was repeated until the corridor width had been dredged and the seabed had been lowered to the required level. The megaripples were dredged in a ‘lazy-s’ pattern where the TSHD trailed the draghead between the lateral extents of the pre-swept corridor. This is required because if the trailer dredges along straight lines, the draghead has a tendency to fall back into its previous track and a deep trench is created. Disposal

Installing the survey equipment allowed the TSHD to work completely independently, performing the as-found survey, checking its

In accordance with the license to develop the field, the dredged material was deposited along two designated dump lines, offset one nautical mile

Offshore Holland 3 | 2012  

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