Page 39

‘Boskalis looks back on a turbulent year, with continued pressure on margins and volume.’

(1852 km) from each flowline route centre. The material was deposited in a controlled fashion through the bottom doors, with the trailer sailing at around 2 m/s and the material deposited over a length of approximately 600 m in order to avoid localized bathymetric reductions. In total, 28 hopper loads (260,000 m³) of soil were dredged. Limitations

As the flowlines tied back to an existing platform, over 10% of the dredging scope was within the vicinity of existing seabed infrastructure and within the 500 m zone of the platform. The Van Nes line also crossed three telecoms cables. During the design phase it was agreed that dredge-off and drift-off situations had to be observed to minimise the risk of damage to existing infrastructure. This meant that the current would take the vessel away from the hazard and the vessel would be dredging away from it also. Whilst this limited working windows, effective planning allowed the vessel to work continuously without downtime. The thorough preparation and selection of the most suitable equipment in combination with best practice and performing in-field tests resulted in an extremely high level of accuracy. Ultimately a precision of approximately 25cm was achieved in the critical areas, with tolerances 50%

tighter than those commonly accepted within the industry. The total duration of the offshore phase was 25 days including 3 days of mobilisation. This phase of the project was completed on time and within budget despite the highest significant wave heights recorded at the nearby metrological station Europlatform since 1983. The total production time (dredging/dumping) was 11.6 days with 3.2 days of survey operations. Baan: “By making use of the lessons learned and several developments arising out of the experience obtained on this project we believe we can make this high accuracy available to clients on future projects requiring pre-installation seabed interventions.” Post Lay Jet Trenching

Post lay jet-trenching was performed approximately 2.5 months after completion of pre-sweeping. While the jet trencher did experience local high values of pitch and roll, associated with the as-dredged seabed profile, the flowlines were successfully trenched in two passes approximately four weeks after completion of pipelay activities. In some localized cases, free span remedial trenching was required prior to first pass trenching. The free spans were reduced in height by trenching away from the feature, removing or lowering the support. One 500 m section of megarippled seabed on the Van Ghent flowline

route was not pre-swept. Along this section the jet-trencher, as one might expect, recorded the highest values of pitch and roll. In addition, the longest and highest free spans were noted along this section following pipelay. Seabed Regeneration

In December 2011, a multibeam survey was performed along of one of the flowline routes before application of rock installation for UHB mitigation. This afforded the opportunity to observe the regeneration of the dredged seabed features after five months. Figure 9 shows a sand wave and a series of megaripples, whilst the footprint of the dredged corridor is still clearly visible the regeneration of megaripples is evident. It is interesting to note that the megaripples appear to be reforming parallel with the dredged corridor. Van den Berg (2007) illustrates that sand waves recover rapidly with the majority of the dredged feature recovering within 1 year and almost fully recovered within 4 years. Future Developments

Boskalis is currently developing a new technique for reaching higher levels of accuracy, together with software development to allow ‘autodredging’ of non-linear profiles and undulating topography. In addition, Boskalis continues developing new techniques to achieve greater levels of accuracy specifically for projects where precision dredging is required.



Offshore Holland 3 | 2012  

Offshore Holland is een hoogwaardig, Engelstalig relatie- en promotiemagazine voor de internationale olie- en gasindustrie. Het doel van het...

Offshore Holland 3 | 2012  

Offshore Holland is een hoogwaardig, Engelstalig relatie- en promotiemagazine voor de internationale olie- en gasindustrie. Het doel van het...