Pipelaying for the Statoil-operated Mariner field on the UK Continental Shelf (UKCS) started Monday, June 8. Around 40 kilometres of pipelines for gas, oil and diluent will be installed on the seabed from pipelay vessel Seven Navica.
The Mariner field is developed with a production, drilling and quarters platform (PDQ) and only dry wellheads. All wells are to be drilled through well slots on the platform. Despite this, the field development includes an extensive subsea installation scope.
“This year we will have more than 200 vessel days on nine different vessels,” says Jon Aksel Brynildsen, Project Manager for SURF (subsea, umbilicals, risers & flowlines) on the Mariner project.
In addition come guard vessels, of which the first was mobilised from Peterhead on June 3. Statoil will have a total of six vessels in rotation doing guard vessel duties to support offshore installation activities.
The turn-key (EPCI) contract for the SURF scope on Mariner was awarded to Subsea 7 in 2013. In addition, a contract for the fiber optic cable was awarded to Tampnet.
Gas import pipeline
The pipeline and riser scope for Mariner includes two infield pipelines, two flexible risers and a gas import pipeline.
The infield pipelines connect Mariner A – the PDQ – to Mariner B, the floating storage unit (FSU). Both the 10 inch oil export pipeline and the 6 inch diluent import line are approximately three kilometers long. Flexible risers will connect the infield pipelines to the FSU via two riser bases.
Gas will be imported to the Mariner A platform for power generation. The gas import pipeline will tie into the existing Vesterled pipeline that runs from the Norwegian Heimdal installation to the St. Fergus gas terminal near Peterhead in Scotland.
Earlier this spring the seabed was examined and large rocks that blocked the path of the pipeline were removed.
When the pipelaying is complete, other installation vessels will be used for flooding, trenching and rockdumping.
A new pipeline end manifold (PLEM) will be installed approximately 33 kilometres east of Mariner. This manifold will be the tie-in point for the gas import pipeline and also provide tie-in options for possible future projects in the area. A subsea safety isolation valve (SSIV) will be installed close to the Mariner A-platform.
According to Statoil, The PLEM is the largest steel structure to be installed as part of the subsea scope. It will be 16 metres long, 14 metres wide, 4 metres high and weigh 130 tonnes.
The PLEM, the subsea safety isolation valve and two riser bases are currently under construction. The PLEM and the riserbases will be installed in August. All tie-in spools and glass reinforced plastic covers will be wet stored as the last operation in the 2015 campaign.
The subsea work will pick up again in 2016, for installation of the subsea safety isolation valve, flexible risers, tie-ins and pre-commissioning. Finally, the communication cable and control cable will be installed once the platform deck is in place.