SPECIAL REPORT: IMR vessels support operations off West Africa

May 2, 2011
Inspection, maintenance, and repair (IMR) vessels have provided operational support off West Africa for such work as installation of jumpers and the maintenance and installation of subsea trees.

Patrick Belenfant
Bourbon Subsea Services
Marseilles, France

Inspection, maintenance, and repair (IMR) vessels have provided operational support off West Africa for such work as installation of jumpers and the maintenance and installation of subsea trees.

The Bourbon Trieste is a DP2 multipurpose vessel with a single active heave compensated crane (Fig. 1).

In Bourbon's case, since 2004, the company has installed 215 subsea connections in deepwater blocks off West Africa using its own IMR vessels. It currently has four vessels operating off Angola and two off Nigeria. Figs. 2 and 3 show two of the vessels.

The Bourbon Oceanteam 101 is a DP2 multipurpose vessel with two active heave compensated cranes (Fig. 2).

All of these vessels are or have been on a long-term charter to major oil companies such as units of ExxonMobil Corp., Total SA, BP PLC, and Royal Dutch Shell PLC. Overall, Bourbon IMR vessels have connected 20% of the existing subsea wells in operation on deepwater blocks off West Africa.

Its first oil tie-in connection was off Angola in 2004 for an operating unit of ExxonMobil.

Jumper design variations

Companies usually design a well tie jumper during the well construction. The length, width, height, and weight vary depending on manifold installation tolerance and well location. Design of the jumper greatly affects the installation vessel. Fig. 3 shows the wide range of jumper weight and dimensions based on more than 200 well ties that Bourbon has performed off West Africa.

The figure illustrates the effect of the jumper design on a crane, and as such on the vessel dimension because of stability and associated power considerations.

Jumper design also affects oil well tie-in once the construction vessel has left the site. At this stage because all manifolds are in place, the only tolerance left remains the well locations, implying minimal possible variations for length and width.

For most deepwater fields, production will start with less than 50% of the wells connected, leaving the remaining to be connected during 2-3 years depending on the number of drilling rigs on site. For large subsea fields, the IMR vessel often remains permanently in the field to provide commissioning assistance.

In both cases, the jumper is the main sizing setting. Its installation determines the IMR crane capacity size, which can represent a large cost, and as such the jumper needs an optimized design.

Some oil companies challenged engineering leading to designs that have resulted in substantial cost reductions for post first oil. Cost of reengineering being obviously minimal vs. the chartering of a specific vessel.

Subsea trees

In most cases oil companies will connect new wells as soon as they are completed.

This leads to low flexibility on tie-in schedules. Two options are available to oil companies: spot vessel (if available) with high mobilization-demobilization and day rate, or long-term committed vessel.

While fields with 6-15 wells cannot support a full time IMR vessel, the larger subsea developments have coped with the costs associated with a permanent IMR vessel, even if the sole support cost of inspection duties may not be sufficient to support such cost.

The IMR vessels tend to be only partially busy with inspection work (constant over the year), and maintenance requirements (reducing as the field equipment becomes more reliable). This left a certain amount of time available for the IMR for other work and Bourbon in a joint effort with oil and gas operating companies developed a way for the vessels to install subsea trees. The joint effort led to development of an active-heave compensated crane for installing subsea trees.

By the end of 2010, Bourbon's IMR fleet had installed 65 subsea trees with associated guide bases in depths of between 800 m and 1,500 m.

Weight in the air of the trees varied between 38 and 54 tonnes, with or without the guide base. Running tool weight varied between 5 and 15 tonnes, leading to a lift weight of between 43 and 62 tonnes in air.

Cranes used had a 90-tonne safe working load capacity with active heave compensation and are capable of reaching a depth of 1,500 m. Based on the series of trees installed, Bourbon has developed a specific hydrodynamic load coefficient on trees. Conventional engineering may lead to a much higher theoretical tension for installation requiring more expensive material. Practical results and coefficients allow loads to be in line with Bourbon's model.

Fig. 4 shows the variation of tension at depth with and without active heave compensation.

Estimated savings of using an IMR vessel for the installation of trees is $2-2.5 million/tree.

Bourbon has installed trees either as a single unit or in batches depending on the operating company's schedule. A single tree installation takes about 18 hr with an IMR vessel from the removal of the conductor pipe cap until completion of tree locking at a depth of 1,500 m.

In the case of a batch of four trees in 1,450 m of water, the time was 24 hr for landing the trees and another 24 for locking the trees.

A drilling rig, on the other hand, typically requires 2-3 days for installing a subsea tree from the time of recovery of the drilling riser, lowering of the tree on drill pipe, to re-installation of the riser.

Because of the difference in the day rates of drilling rigs vs. IMR vessels, oil companies have gained substantial savings for this work.

The author

Patrick Belenfant is senior vice-president business management of Bourbon Subsea Services, Marseille, France. He has more than 20 years of worldwide experience in the oil and gas industry as a project manager and engineering manager. He joined Bourbon in 2001 and was instrumental in the creation and development of its subsea services activity and in supervising and managing operations of its worldwide fleet and remotely operated vehicles. Belenfant has an engineering degree from the French Mechanical School of Engineering.

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