Monohull deployment cuts time and cost of Ormen Lange tree installations

Once a subsea gas well is drilled and cased, and before the well is completed, the hole has to be plugged deep below the seabed and the christmas tree placed at the wellhead. This is a large, heavy structure, comprising an assembly of piping, fittings, transmitters, and valves which control and monitor the well in production.

Christmas trees at Ormen Lange usually are run from a dynamically positioned drillship. The drillship lowers the tree, which is locked on to a Tree Running Tool (TRT) with a drillstring. Stands of drillstring are added until the tree is at the required depth. Once the tree is installed, the drillship completes the well, after which commissioning and production can commence through the tree to the production manifold.

At Ormen Lange, running each christmas tree from the DP drillship typically takes five to seven days for the complete operation.

Tree on wire

Subsea 7 has a long-term relationship with Shell Upstream International, Europe (formerly Shell E&P Europe), which embraces a variety of work from inspection, repair, and maintenance to light construction for Shell’s European subsea oil and gas infrastructure.

Shell approached Subsea 7 in October 2008 to investigate the possibility of installing a christmas tree from a monohull construction vessel, a technique now referred to as “tree on wire,” at Ormen Lange. Subsea 7 previously applied similar techniques on the Anadarko Independence Hub project in the Gulf of Mexico and also offshore South Africa on the PetroSA project, but never before on a live template and in the North Sea.

Tree on wire uses a less expensive asset, i.e. a construction vessel with its crane, to lower the tree from the deck of the vessel into location in the template. Monitoring the installation, tree orientation, connection, and testing is done using the construction vessel’s ROVs which removes the need for the TRT and its associated umbilical.

For Subsea 7, installing a 61-metric ton (67-ton) subsea structure is not out of the ordinary. In this case, the real issues were lowering the christmas trees into a “live” manifold 850 m (2,789 ft) deep in the Norwegian Sea, during winter, in an installation window with a maximum wave height of 5.8 m (19 ft) - using unique tooling to perform all the operations normally handled by a drilling rig, and with a very tight schedule of around five months from conception to completion.

Early project work began with a feasibility study and peer reviews. Following those, offshore trials were performed at sea and in a deepwater fjord. Subsea 7 had to ensure that its master/quality procedures were synchronized with those of other project sub-contractors and with those involved in ROV tooling operations at the company’s base in Kristiansund, western Norway.

Some of the challenges included:

• The design and build of an ROV-mounted Hydraulic Power Unit (HPU) skid which allowed functioning of the tree’s connectors as well as carrying out all necessary pressure testing
• Design and fabrication of the over-boarding system to allow the lift/deployment of the trees in the given weather conditions
• Identification and procurement of rigging suitable to handle the trees in the design conditions, yet also light enough to be handled easily by the deck crew on a moving vessel
• Specification and provision of an Emergency Quick Disconnect (EQD) of the crane from the tree, i.e. a part of the rigging that would allow the quick disconnection required for contingency activities.

Trials involved over-boarding and testing the lift of a full-scale dummy tree in realistic weather conditions (3 m, or 9.9 ft, wave height) and deploying to depth. This was followed by testing of the structure’s maneuverability, crane operation tests, and altering/controlling the heading of the tree using an ROV. All these tests helped Shell get technical endorsement for the project.

Initially, two tree on wire installations were planned. However, following the success of the first two tree deployments, two more were brought forward in the schedule for installation immediately afterwards.

Four new trees are successfully in place at Ormen Lange, installed without incident from the Subsea 7 construction vesselSkandi Seven using the tree on wire method.

This technique also has proven to be quicker and more flexible, reducing the normal time between installation and commissioning. Most importantly, it frees valuable time for the drillship, allowing it to continue drilling operations. This, in turn, leads to accelerated production, with more wells onstream earlier.

It is also cost effective - Shell has put a minimum cost saving of $4 million per tree installed using the tree on wire technique. To date, Subsea 7 has installed four trees for Shell using this method, with three planned next year, and four in 2011 – potentially, a total cost saving of around $44 million.

Future applications

Subsea 7 is exploring scenarios in which it recovers trees as well as deploy them – a realistic possibility which would increase the reliability of the operation in Ormen Lange. The technique could be extended to tree replacement tasks and tree recoveries in field decommissioning operations.

Facts about Ormen Lange

Ormen Lange, which came onstream in October 2007, is the largest natural gas field development on the Norwegian continental shelf. The field is 120 km (74.6 mi) northwest of Kristiansund, in water depths ranging from 800-1,100 m (2,625–3,609 ft). The reservoir is around 40 km (25 mi) long, 8 km (5 mi) wide, and about 3,000 m (9,842 ft) below sea level. Proven gas reserves are estimated at 397 bcm (14 tcf).

Gas from Ormen Lange meets up to 20% of Britain’s gas demand, and should continue doing so for the next 40 years. The field is being developed without conventional offshore platforms. Eventually, 24 subsea wellheads in four seabed templates on the ocean floor will connect directly by two 30-in. (76-cm) pipelines to an onshore process terminal at Nyhamna on Norway’s west coast.

After processing, gas is exported via Langeled, the world’s longest subsea export pipeline, around 1,200 km (746 mi) from Nyhamna to Easington in England. At present two of the subsea templates have been installed.