Boskalis Offshore had one of its busiest seasons last year on the Norwegian continental shelf. The company's two fallpipe vessels D.P.F.V. Sandpiper and D.P.F.V. Seahorse dumped a record-breaking 1.5 million tons of rock in the North Sea and Norwegian Sea for the protection of pipelines, umbilicals, and structures.
The main client Statoil accounted for 90% of the total volume and this work was executed under a frame agreement. The main focus of the Statoil work was on the Åsgard Field in the Norwegian Sea, which consumed over 50% of the rock volume employed. This field, which has one of the world's largest subsea installations, came on stream in 2000.
Rock dumping benefits
Rock dumping is considered one of the most secure methods for protection of subsea installations. It not only provides supports for installations, but it also covers these structures to protect them from damage arising from fishing activities or anchors, and also against upheaval buckling after installation.
Another important application of the rock dumping technique is the insulation of flow lines with insulation sand. The cost reduction is such that the investment is returned almost immediately after placement of the insulation sand on the flow line.
Boskalis Offshore operates the two vessels cited above. Both have a carrying capacity of over 17,000 tons and are fully DP 2 Class, enabling them to operate in the direct vicinity of platforms.
Their normal working water depth in the Norwegian and North Seas is 300-450 meters, at the same time maintaining a vertical accuracy of at least 20 cm. However, working depths up to 1,500 meters are within their capabilities. The survey capabilities of both vessels extend to a level where they can compete with the dedicated survey ships operating in the North Sea. This allows the vessels to execute dumping operations including pre-survey and post-survey work, entirely by themselves.
The dumping spread consists of a long tube connected to the vessel at the top and equipped with a remotely operated vehicle (ROV) at the bottom of the fall pipe. The opening of the fall pipe can be moved in all directions using the thrusters on the ROV. This enables the operator to maintain position above a berm. A separate free flying ROV is used for survey purposes and, if necessary, for monitoring of the dumping process.
The configuration of the Åsgard Field includes 16 seabed templates linked to the main floating production units (Åsgard A, Åsgard B, Åsgard C). The total length of the flow lines exceeds 300 km. Many of these lines needed to be protected with rock.
The seabed in this part of the Norwegian Sea challenges both the engineer and the rock dump vessels, partly because of the iceberg scours. Pipelines and umbilicals crossing these scours must be supported by rock before installation and covered with rock after installation to ensure an adequate protection against trawling activities and upheaval buckling.
The flow lines connecting the J-template and Åsgard B in particular cross several iceberg scours and therefore needed extensive and meticulous rock dumping. A total number of 115 berms were placed on the J-101 and J-102 flow lines, running from the J-template to Åsgard B, with a total volume of 150,000 tons. Some of the berms required a counterfill to stabilize the berm itself.
This volume was placed on the seabed in a period of 50 days (including loading, sailing and weather delays), achieving an average production of 2,800 tons per day. Despite the many berms and the tight schedule, high accuracy was achieved to comply with the design requirements.
Rock dump operations under the Statoil Frame Agreement continue for two more years. Boskalis Offshore expects further advances in the rock dumping technique, making it a strong instrument in subsea developments.