Dana Petroleum Netherlands B.V. is the operator of the P11b De Ruyter field and the oil-producing platform located in the Dutch section of the North Sea. Several small oil and gas prospects lie within P11b and adjacent blocks. Dana Petroleum planned initially to develop two confirmed small oil and gas fields as part of the Dutch gas development strategy, with other fields being developed over time to increase gas production. The two initial wells Van Nes and Van Ghent, combined referred to as the Medway Development, were to be developed as subsea tie-back to the existing De Ruyter platform. Subsea 7 was awarded the Subsea EPIC contract for the development, consisting of trenching, installing, backfilling and commissioning of the tie-backs. Each route features a piggybacked system comprising a rigid 8-in (276 mm OD) flowline and a 134 mm OD EHC control umbilical. The total length of the products was around 12.5 km. The products were to be installed by Subsea 7 using the pipelay vessel Falcon.
Boudewijn Baan explains: “Mobile seabed features are frequently an issue for subsea construction projects in shallow seas such as those found in the southern North Sea. These seabed features are found in the form of sand banks, sand waves, megaripples and ripples and all migrate with time, each with a different order of magnitude. Typically sand waves have wavelengths of between 100 m and 800 m with their crests aligned almost perpendicular to the direction of the main tidal flow and can migrate up to 10 m per year. Megaripples are smaller features exhibiting wavelengths up to 25 m and heights up to 1 m. Typical migration rates for megaripples are on the order of days. Sand waves and megaripples give rise to large variations in water depths over relatively short areas of seabed. This can be especially problematic for pipelay activities where large free spans can be created, which can lead to stability issues and premature utilisation of the pipeline fatigue life. Similarly, seabed migration can create a unique set of problems for subsea construction.
Subsea 7 subcontracted Boskalis Offshore to execute the following seabed preparatory works:
Peak shaving to reduce the steepness of slopes in order to aid pipeline trenching;
Examples include; instability of foundations of subsea structures through erosion and scour; exposure and free span development of previously buried pipelines causing hazards to fishing activities; burial and exposure of hazardous foreign objects such as unexploded ordnance (UXO); excessive cable burial leading to heat gain; maintenance problems and difficulty tracking pipelines and cables when buried by more than 2.5 m.”
Complete route survey work at specified stages to confirm the seabed profile.
Conceptual Design of Pre-Sweeping “Once the route layout was finalized it was necessary to establish the
Pre-sweeping of a 30 meter wide corridor to modify existing seabed features with the aim of reducing freespans;
maximum allowable free span length by performing an on-bottom roughness (OBR) analysis to quantify the amount of seabed conditioning required,” Baan continued. “The analysis concluded that the critical freespan length was 16 m. Longer free spans would mean the flowlines were susceptible to vortex-induced vibration (VIV), induced by cross-flow current, reducing the fatigue life during the untrenched condition. With the as-found seabed topography the maximum expected free span length exceeded the allowable length by some 30 m.” A decision process was put into place with regard to the extent to which pre-sweeping should be performed. This led to the following three options: 1. Removal of all seabed features and protection of the products With a blanket of rock material; 2. Localised pre-sweeping to allow the products to be laid followed by the installation of a rock blanket; 3. Localised pre-sweeping to allow the products to be laid and jet-trenched.
Given the cohesionless nature of the seabed and the requirement to bury the products below seabed for upheaval buckling (UHB) mitigation and protection from fishing vessel activities, jet trenching was considered to be the most economical method. This introduced a second design consideration as the pipeline passes beneath the trencher during burial. This means that the maximum allowable free span
Published on Apr 16, 2012
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