Managing deepwater intervention vessels offshore Angola can save costs

Deepwater blocks offshore Angola.

Study results indicate that managing requirements on an area basis, especially for high-spec units, could lead to significant cost savings for operators. This holds true because the forecast most blocks does not indicate that each block will require one vessel of each type.

Projects in the deepwater blocks offshore Angola are coming swiftly. Dalia (block 17 - operator Total Angola), Kizomba B and C (block 15 - operator Esso Angola), and Greater Plutonio (block 18 - operator BP Angola) already have started up or will soon achieve first production. Others, such as Kizomba C and development of the ultra deepwater block 31 operated by BP, are at the study stage.

Most of theses developments have followed similar schemes based on an FPSO linked to a subsea production system. In the coming years, this will require a strong level of activity in installation, commissioning, and startup. There will also be a need for inspection, maintenance, and repair. To examine the consequences of this evolution, Angolan national oil company Sonangol and operators of the deepwater blocks asked ADC to determine the need for intervention vessels to service and support the installed subsea equipment.

The objectives of the multi-purpose vessel study were to:

• Evaluate the intervention needs on subsea production system equipment and the umbilical, flowline, and riser (SPS/UFR) systems as field equipment is phased in, additional production is tied back, and during operation over the life of the field.
• Evaluate the demand for well intervention work
• Define the minimum functionality and characteristics required for the intervention vessels to conduct the work and to define specifications
• Review and evaluate the current vessel market
• Analyze the marked demand and the alternate commercial strategies that could be used in Angola.

An inventory of subsea equipment was developed taking into account the projects under way or under study for blocks 14, 15, 17, 18, and 31. Inventory was estimated for the years 2004-2014, taking into account all of the data made available by the operators. The latest first-oil date was 2009, and the inventory estimates are reasonable up to that date. In the following five years, estimates increased due to the inevitable materialization of other developments not yet identified.

An artist’s rendering of an SPS/UFR intervention vessel.

The inventory is based on available field architectures, on provisional forecast production curves, and on estimates of the number of wells, SPS equipment, and UFR lengths required to meet the forecast production.

The results show strong growth. The number of subsea wells increases by a factor of 4.5 between 2004 and 2009, the number of manifolds increases by three, and the total length of flowlines and umbilicals increases by 10 times.

For the SPS/UFR interventions, 60 tasks were identified based on typical subsea architecture. Each task has an associated duration, taking into account water depth and intervention frequency. To conduct the study, these tasks were associated with the means necessary (lift capacity, type of ROV, deck area required, etc.) and grouped into the following classes:

• IM0 - Modest size fast winch to handle small loads and work-class ROVs to do inspection, maintenance, valve operation, diagnosis of breakdowns, survey and maintenance of jumpers, and to assist in commissioning and startup
• IM1 - Lifting capacity of 9 metric tons (10 tons) at 2,000 m (6,562 ft) to replace small subsea equipment items, control modules, valves, connectors, etc., and for intervention on wells during connection/disconnection of christmas trees
• IM2 - Lifting capacity of 32 metric tons (35 tons) at 2,000 m (6,562 ft) for installation and replacement of jumpers, and installation of christmas trees
• IM3 - Lifting capacity of 118 metric tons (130 tons) at 2,000 m (6,562 ft) for installation and replacement of manifolds
• IM4 - Lifting capacity of 227 metric tons (250 tons) at 2,000 m (6,562 ft) for work-class ROVs, modular deck mounted equipment, and replacement of all types of UFR lines.

The base estimate of activity for each vessel was increased by a contingency allowance of 33% for downtime and 13-23% for transit time. Vessel provisions were assumed to come from shore bases via supply vessels.

The study showed that the demand for all vessels would grow to about four vessels in 2009, with a split of 2.5 IM0, 0.8 IM1, and 0.5 IM2. There was a loss in demand for inspection, maintenance, and repairs forecast for IM3 and IM4 vessels.

For this reason, IM3 and IM4 were dropped from later stages of the study. If the need for vessels of these classes should arise, it could be met using vessels of opportunity, such as on-site installation or drilling/workover vessels.

The study also resulted in the following observations: first, each block required intervention from all classes of vessels; and second, the demand per block was consistently low at less than one vessel (all classes summed) for all blocks but one. Since “more able” vessels can do the work of “less able” vessels, there will be a tendency to charter vessels corresponding to the highest demand capabilities frequently required (IM2).

This flexibility carries an estimated cost of $5 million per year per block. The cost could be an incentive for operators to federate demands for a vessel to allow them to use lower-cost vessels, particularly the IM0, for which the largest portion of the work is forecast. The daily cost of the IM0 is estimated at 40% of that for an IM2.

For well intervention, the act of defining tasks and durations took into account the types of completions and trees (vertical or horizontal). The intervention frequencies of all types are on the order of four years for producers and nine to 12 years for injectors. Some 30 tasks for intervention methods were linked to well maintenance, flow assurance, and reservoir management.

Renderings of well intervention vessel types.

Intervention means were grouped as follows:

• H class - Capable of heavy well intervention with limited drilling capacity, such as christmas tree replacement, changing completions, and side-tracking
• M class - Medium level intervention capacity, such as wireline or slickline, coiled tubing, and conventional christmas tree replacement
• L class - Light well intervention such as riserless work.

In the end, the study excluded L class because riserless well intervention is not yet considered to be a mature technology. All of the work that would have been classified as L was re-allocated to the M class.

The estimated demand for M vessels was less than one vessel, and the demand for H vessels was two vessels in 2009 for all blocks.

Forecasting the arrival of this type of unit in Angola posed difficulties. Only one Angolan block was forecast to reach a level with enough work to keep an intervention vessel fully occupied, and then only by summing the H and M type work.

For other blocks, sharing a type M vessel could be commercial if an agreement could be reached. The cost of this type of vessel would imply a day rate competitive with an H type or standard drilling rig. This option would lose its commercial interest if the light well, riserless intervention technologies became common because these technologies can be deployed from an IM2 vessel. Operations can be executed faster with no riser, and the IM2 support would be mobile and easily redeployable when not in use.

As for the H type, the unit is estimated to have a construction cost saving of 15% with respect to a drilling unit operating under the same conditions. So the H type unit is vulnerable to competition from drilling units already in the area. A long-term charter would be needed to give the vessel owner confidence to construct a type H unit, and the type H unit probably would get work allocated to both types M and L.

Vessel requirements

One objective of the study was to define the minimum characteristics for vessels corresponding to each intervention class. The following steps were taken for each class:

1. List the systems needed to execute the tasks identified in the intervention study
2. Define the capacities required for each system
3. Select and dimension the best options
4. Outline the design of the vessel required to support the systems.

Functional specifications were developed for each vessel class suitable to outline the requirements for a newbuild.

In parallel, a review of the existing fleet guided the definition of the vessels and evaluated the number of existing vessels that meet ADC specifications.

For SPS/UFR intervention vessels, the principal systems defined were as follows:

• Lifting capacity in deepwater
• ROV support
• Propulsion and dynamic positioning
• Cargo deck
• Accommodation and helideck.

Deepwater lifting operations (placing and retrieving seabed packages) were characterized by the weight of the lifting cable as a significant addition to the package weight and apparent load increased by dynamic factors. Forces include drag on the package and inertial forces, particularly during transition across the air/water interface, as well as the vertical movements induced on the lifting point by wave forces on the floating support. These influences excite the mass-spring system made up of the package suspended on the cable. System elasticity increased with depth. The natural frequencies of the system also change with depth, increasing the chances of exciting them at some stage during a lift.

To reduce the impact of these dynamic effects, ADC recommends use of a dynamic heave-compensation system, positioning of lifting points to minimize vertical excitation from vessel motions, and inclusion of the lifting cable mass in the system capacity definition with an appropriate dynamic amplification factor. There are other ways to mitigate these effects, such as using synthetic lifting cable or reducing the apparent weight of the package with the aid of buoyancy.

ROV support

In terms of ROV support, the principle concern is redundancy. Experience shows that these complex pieces of equipment have a relatively high downtime. The advisability of having a back-up ROV depends on criticality of the task and the cost of associated equipment mobilized. For example, installing production equipment is more critical than inspection.

Only the IM0 vessel has been presented with a single, low-payload ROV during routine operations.

The vessels are equipped to receive the ROV equipment on skids that are installed at a location favorable for launching and equipped with dedicated control rooms and workshops. The ROV is seldom owned by the vessel provider.

The criteria for dynamic positioning class recommended by ADC for use in Angola depend on the potential of a loss of position, as follows:

• Loss of time: DP1
• Damage to surface installations: DP2
• Damage to equipment being installed or subject to IMR: DP2
• Damage to producing subsea equipment: DP3.

A DP2 system is recommended for intervention vessels in Angola’s deepwater fields.

DP2 vessel propulsion and power generation architecture usually is one of two types:

1. “Classic” propulsion system with a shaft driven by a diesel motor complemented by electric driven thrusters, either tunnel mounted or retractable azimuth, at the bow and stern

2. Diesel-electric propulsion system through the main azimuth propulsion thrusters at the stern complemented by bow-mounted tunnel and/or retractable azimuth thrusters forward.

Even though the initial cost of a diesel-electric system is higher than the classic system for the same power level, it is better suited to DP mode because it consumes less fuel, is easier to maintain while operating, and allows more flexibility in optimizing vessel design.

During subsea intervention operations in relatively benign environments, there are no large demands on DP system power, unlike the demands in towing. The classic propulsion system is more robust and efficient at full power for towing. The propulsion system, DP, and power generation specific for the operational conditions have been specified for each vessel class.

The ability to accommodate a helicopter is not required for intervention operations because they never are far from production or drilling surface installations, which can provide personnel transfer facilities. Nonetheless, a helideck was recommended for the largest vessels because its integration would have no impact on vessel architecture.

The dimensioning principles applied by ADC to the layout for the SPS/UFR intervention vessels were as follows:

• The general arrangement follows the traditional offshore supply vessel layout, taking into account the working areas required and integrating a workable system and layout for load transfer. The benefits of a catamaran or small waterplane area twin hull vessel to reduce wave-induced movements also were considered, but were set aside because of the difficulty in integrating all of the demands in a tight space, their construction, and the design’s sensitivity to transverse movements of loads, such as in lifting
• Estimation of lightship weight based on ADC experience
• Naval architecture of the designs was checked using the normal criteria of freeboard (International Convention on Load Lines), intact stability including the various stages of lifting, and draft limits in the coastal supply bases of Angola.

An analysis of the hydrodynamic behaviors of the vessels evaluated the best location for placing lifting devices. The analysis also allowed ADC to specify the associated stroke and speed required in the heave compensation systems.

The existing fleet was reviewed using ADC specifications to find out the principal characteristics of the vessels and their capacities. Of the 125 vessels with the capacity to carry out subsea intervention, 82 were identified as having the potential of IM0, IM1, or IM2, allowing for some minor modifications to some of them. The distribution was 15 potential IM0; 38 potential IM1, of which six were oversized; and 31 potential IM2, of which 10 were oversized. The oversized vessels could be used in appropriate, specific commercial circumstances.

Size and layout of well intervention vessels is determined principally by the loads and areas needed for equipment and by operating philosophies that determine the layout of the different systems.

Dimensioning studies were done for both monohull and semisubmersible designs for classes M and H, along with the equivalent drilling rig of the same type to provide a benchmark. Comparisons based on the cost of construction favor the monohull layout. Monohulls have an advantage in transit speed. These considerations lead to favoring the monohull design for class M.

For the H type, which has longer intervention times, the disadvantage in loss of operations time drops to 2%. The semisubmersible design was recommended for the H type despite an estimated 10% higher construction cost. This choice was linked to the great flexibility offered by the hull design, ease of equipment layout for the deck, and potential to use the derrick or auxiliary crane for heavy lifting in deepwater (tasks of the SPS/UFR IM3).

There are few existing units designed for intervention on wells and workovers that meet ADC’s specifications. Semisubmersibles of theAmethyst type correspond to the H type if upgraded for all tasks in water depths to 2,000 m (6,562 ft). The semisubmersible Q4000 is the only M class vessel currently operating. Certain first-generation drilling vessels designed for more limited water depths could operate with an intervention riser in 2,000 m (6,562 ft) of water.

Evaluation of construction costs and day rates for SPS/UFR and well intervention vessels were conducted on the basis of specifications and dimensioning by ADC. These evaluations were compared with the economic data for existing vessels and appraisals by a shipbroker. The economic data then were used to make comparisons between alternative classes of vessels and potential intervention arrangements, and to generate commercial evaluations.

NOTE: We thank BP Angola, CABGOC (Cabinda Gulf Oil Corp. - Chevron Texaco), Esso Angola, Sonangol P&P, and Total Angola for their participation during the study and for permission to publish the paper.

ADC, a consortium of Sonangol, Pride Foramaer, and Doris Engineering, was created to execute engineering studies related to development of deepwater zones offshore Angola. The studies were financed by the operators.

Dr. Richard J.S. Harris, Gaspar C. da Silva Fernandes, Pierre Vanhaecke
ADC