Vessel cranes cut the cost of deepwater tree installation

July 1, 2011

With 107 West African xmas tree jobs to its credit since 2006, Bourbon Subsea Services has built a substantial track record in that region for the maintenance and installation of deepwater trees using vessel cranes. SVP business management Patrick Belenfant discusses how the company is shaping up to meet the challenges of a post-Macondo world.

The Bourbon Subsea Services fleet – which currently consists of 17 multipurpose installation, maintenance and repair (IMR) vessels and 13 remotely operated vehicles – has readily adapted to the changing requirements of oil & gas clients in recent years. Working in live production fields and to the highest risk management standards, they have demonstrated time and again to supermajors and independents alike the cost-efficiency and time-saving advantages of deploying vessels rather than rigs for subsea tree installation and intervention.

Since it first deployed a vessel for post first oil support on Angola block 17 ten years ago, the company has foreseen the need for complementary activity between inspection, post first oil, and drilling activity support. Opex and capex strategies, which correspond to production management and drilling management, belong to two different worlds. Yet, Bourbon Subsea Services takes pride in having contributed to bringing the two closer together through asset sharing and cross-sector cooperation. Now the company is sending that message further afield as it expands its operations to the west Atlantic (Brazil, Mexico), Asia Pacific waters (Indonesia, Australia) and the Mediterranean Sea (Egypt).

The loss of the Deepwater Horizon semisubmersible rig in the Gulf of Mexico's Macondo field last year served to demonstrate drilling and ROV activity's connection to subsea interventions involving deployment of significant equipment using crane vessels.

Bourbon Subsea Services' IMR support vessels, besides providing firefighting and rescue capacity in addition to anti-pollution support, are providing deepwater block support for beacon and xmas tree installation. The current development of containment equipment post Macondo may lead to the necessity of deploying capping equipment by rig or crane support vessels. It needs to be deployed by one or the other to provide flexibility to oil companies, but capping equipment dimensions and weight will be limited by the rig cranes or by crane vessels.

Rig crane capacity has been deployed to handle xmas trees, although advancements will require larger and more expensive rigs. Depending on company strategy, such containment equipment could either be shared for worldwide application or kept per country. If shared, it will still have to be flown to the accident site and be within transportable dimensions and weight. Keeping it in-country, on the other hand, would lead to higher capex. The strategy has not yet been clearly defined by operators.

Should containment equipment surpass rig crane capacity, larger standby crane vessels will be required to be available in the area at any time, and will thus have an impact on field operating costs. Bourbon Subsea Services believes that containment equipment will be developed with the dimensioning and weight envelope of a xmas tree, emergency disconnect package (EDP), or lower riser package (LRP). The company has therefore developed a new series of vessels rated for 3000m water depth based on 150t active heave compensated (AHC) crane technology to deploy xmas trees or capping equipment in these water depths. This February the first such vessels deployed an EDP/LRP package with the associated lubricator to perform a subsea wireline intervention in 1350m water, thus demonstrating the feasibility of light work over package and possible deployment of preliminary containment packages in such water depths.

The crane is key
In addition to supporting ROVs, one of the main activities of IMR support vessels is to deploy subsea structures. Since lifting operations are the second most common cause of accidents offshore, Bourbon Subsea Services' decision to increase the number of its deepwater cranes to nearly 40 by the end of 2012 entailed an increase in the probability of accidents and breakdowns, an issue that needed to be resolved.

It was essential to have cranes built up according to classification society rules (BV, DNV, ABS) and according to I EN-13852 standards for offshore cranes, as well as FEM, with the feedback from Bourbon Subsea Services. In order to control the cranes' design, manufacture and testing for further integration, the company decided to purchase the cranes independently from the shipyard. At this first step, it was also essential to have the knowledge in-house to inspect and maintain the cranes in order to minimize the probability of accidents.

Bourbon Subsea Services' vessels are chartered with and without deck foreman, but always with the company's crane drivers to both ensure and maintain asset integrity. A specific crane team dedicated to crane inspection and maintenance is deployed with the company's vessels.

It is also essential for shipowners to be able to assess the risk of lifting operations performed by charterers. As a consequence, Bourbon Subsea Services has its own engineering lifting department dedicated to the development of lifting and installation procedures, monitoring and verification of charterer procedures, and participating actively in risk assessment.

Enhanced cooperation
Whereas IMR vessels had usually been employed to perform inspection & survey activities, the Bourbon Subsea Services strategy was to perform in addition the post first oil well tie-in. Working in close proximity to rigs has enhanced cooperation between drilling and production teams supporting each other.

While a drilling rig had to recover its riser in deepwater to deploy trees, having IMR support vessels with 90t AHC cranes for 1500m of water and 150t AHC crane for 3000m water depth on site facilitated the xmas tree deployment at marginal cost.

The 107 trees – from the four main subsea hardware suppliers – handled by Bourbon Subsea Services offshore West Africa to date have been deployed in deepwater for companies including Total, Shell, Chevron, BP, Hess and CNR.

The table (below) provides a summary of the range of weights and dimensions of the trees installed thus far. It shows that standard hydrodynamic load analysis provides very conservative results, far from actual recording. As a consequence Bourbon Subsea Services developed its own database record of hydrodynamic drag coefficients for such structures.

Depending on company drilling strategy, trees can be deployed in batches of up to four units, for batch drilling and completion, or as a single unit.

To provide guidelines for xmas tree installation, from the arrival of the vessel on-site until the vessel's departure, total time in 1400m water depth registers between 18 and 22 hours (inspection of guide base, removal of tree cap, cleaning, lowering of tree, landing and locking, installation of protective tree cap, recovery of crane and ancillary tooling). Lowering a batch of four xmas trees on the same cluster in 1450m of water can be done within 28 hours (an additional 30 hours with locking tests).

Xmas tree preservation, locking, and testing depend on the tree's supplier and design. Glycol and methanol may be injected into the tree and CP before landing, and some trees require the deployment of control injection umbilicals. Other trees require heavier running tools (15t) since tree locking mechanisms cannot be activated by the existing hydraulic circuit and an external stab plate. Some trees require hydraulic circuit pressure compensation and flushing prior to landing, and there are some that have pre-integrated accumulators.

As a consequence, locking and testing sequence can vary depending on the company and hardware supplier.

Vessels vs rigs
While maintenance vessels are supported by opex and rigs by capex, each could be seen as part of a different sphere working day-to-day in close proximity of one another to their mutual benefit. Bourbon Subsea Services' parent company Bourbon provides AHTS for drilling rig support and it was obviously necessary to communicate with all drilling, logistics, operating and subsea departments in order to optimize vessel usage and provision of expertise.

In deepwater environments, a drilling rig has to recover its riser to set up a xmas tree, which requires 2-3 days of rig time. The IMR vessel, if already mobilized, will do the same job in less than a day. As the daily rate of an IMR vessel lies in the range of 1/5th to 1/7th of the price of the rig, this implies savings in the order of $1-2 million per tree installation†. In a scenario requiring 40 subsea well architectures, this would mean savings of around $40-80 million.

The above figures relate only to tree installation. By adding the LBL array deployment support and calibration for rig, post well tie-in work, inspection and maintenance of subsea architecture, ROV backup support etc, it becomes more economical to have a full time IMR/crane vessel in the field. It could also provide firefighting standby rescue duty in addition to being used as a backup ROV for drilling rigs.

Safety first
What must prevail is the risk assessment prior to an operation. Bourbon Subsea Services' ROV and IMR fleet intervene in live production field environments, where there is no room for mistakes, surprises, or improper management of change. The pre-operation engineering and risk assessment allows for a thorough assessment of risk and an appraisal of the offshore team's competence.

Still, in the end, trust and confidence remain key elements to a successful operation. Should an accident occur, the captain is ultimately the person responsible for bringing a vessel and its crew back in a safe and protected environment. Thus, it is vital to have captain and crew fully integrated into the ROV and deck crews. While Bourbon Subsea Services only charters vessels, customers such as Total and Esso have since 2004 entrusted the company with providing full service for such vessels, including engineering risk assessment, management, ROV, and all crews onboard vessels from mariners to deck and positioning crews, thereby breaking down the barriers between mariners, ROV, and construction. OE

About the Author

Patrick Belenfant
, senior vice president business management of Bourbon Subsea Services, graduated from the French Mechanical School of Engineering (ECA M) in 1989 and has since accumulated more than 20 years' oil & gas industry experience worldwide as a project manager and engineering manager. He joined parent company Bourbon in 2001 and was instrumental in the creation and development of its subsea services arm, as well as supervising and managing the operations of Bourbon's worldwide fleet and ROV services.

†Calculation based on rig rate in the $500k-$700k range with 2-3 days for tree installation.

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