ADC study: Multi-purpose vessels - evaluation of the demand for intervention vessels on deepwater fields offshore Angola

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

Rapid evolution in the development of offshore deepwater fields has encouraged Sonangol and operators of the Angolan deepwater blocks to carry out this MPV study, which is highlighted from the well construction/risk track.

Following the development of the fields Girassol and Kuito, offshore projects in the deepwater blocks offshore Angola have followed on swiftly: Dalia (block 17 - operator Total Angola), Kizomba A and B (block 15 - operator Esso Angola), Greater Plutonio (block 18 - operator BP Angola) have already started up or will in the near future be in production. Others are at the study stage, for example Kizomba C or the development of ultra deep block 31 (operator BP).

Most of these developments have followed similar schemes based on an FPSO linked to a subsea production system. In the coming years we will see a strong level of activity in the operations of installation, commissioning and start-up and consequently of inspection, maintenance and repair of subsea equipment and lines.

Sonangol and operators of the deepwater blocks asked ADC to carry out a study of the requirement for intervention vessels to service and support the installed subsea equipment.

ADC, a consortium of Sonangol, Pride Foramer, and Doris Engineering, was created to execute engineering studies related to the development of deepwater zones offshore Angola. The studies are financed by the operators: BP Angola, CABGOC (Cabinda Gulf Oil Corp. - Chevron Texaco), Esso Angola, Sonangol P&P, and Total Angola.

The work of the study team and the chosen investigation paths are regularly reviewed with the operators through a system of document reviews. The study’s direction is reviewed through steering group meetings between all parties.

The objectives of the MPV study were:

• Evaluation of the intervention needs on subsea production system (SPS) equipment and the umbilical, flowline, and riser (UFR) systems as field equipment is phased in, additional production is tied back, and during operation over the life of the field
• Evaluation of the demand for well intervention work
• Definition of the minimum functionality and characteristics required for intervention vessels to carry out the work and drawing up of functional specifications
• Review and evaluation of the current vessel market
• Analysis of the market demand and the alternative commercial strategies that could be used in Angola.

The demands for intervention were analyzed for each block. Detailed analysis on a field-by-field basis was available to each operator, but the study presented data for each block and the whole Angolan deepwater area.

Intervention work and market aspects

For SPS/UFR interventions 60 tasks were identified based on typical subsea architecture. Each task has an associated duration, taking into account the water depth, and a frequency of intervention, based on reliability statistics - the failure rate was considered as constant over time. These tasks were associated with the means necessary (lift capacity, type of ROV, deck area required, etc.) and grouped in classes as shown below:

• IM0: modest-sized fast winch to raise and lower small loads, work class remote operated vehicle (WROV)
• IM1: lifting capacity of 10 tons at 2,000 m, WROV
• IM2: lifting capacity of 35 tons at 2,000 m, WROV
• IM3: lifting capacity of 130 tons at 2,000 m, WROV
• IM4: lifting capacity of 250 tons at 2,000 m, WROV, modular deck mounted equipment.

The study showed that the demand over all the blocks for all the vessels added together grew to approximately four vessels in 2009 with a split of 2.5 IM0, 0.8 IM1, and 0.5 IM2. A very low demand for inspection, maintenance and repair activity was forecast for the vessels of class IM3 and IM4, which correspond to the larger of the construction vessels currently in service. For this reason classes IM3 and IM4 were dropped from the later stages of the study. The corresponding tasks, if and when they arose, would be executed by vessels of opportunity such as installation vessels on sites or by the heavy-lift capabilities of vessels involved in well drilling or workover.

For well intervention, the definition of intervention tasks and their durations took into account the types of completion and the tree type-vertical or horizontal. The intervention frequencies - all types of intervention considered together - are of the order of four years for producers and are in the range nine to 12 years for injectors.

Four categories of intervention methods have been identified to execute 30 tasks linked to the operations of:

• Well maintenance (equipment breakdown)
• Flow assurance
• Reservoir management.

Intervention means were grouped as:

• H class-capable of heavy well intervention and having a limited drilling capacity (replacement of Xmas trees, changing completions, side-tracking)
• M class-medium level intervention capacity (wireline, slickline, coiled tubing, replacement of conventional Xmas trees).

The estimated demand for the vessel types M and H were, respectively, lower than one and two vessels in 2009 for all blocks summed together. Forecasting the arrival of this type of unit on the Angolan region where there are a number of drilling units already in activity therefore poses certain 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 both the H and M type work.
• For the other blocks, sharing the use of a vessel of type M could be commercially interesting - if an agreement could be worked out between all parties and put in place - the cost of this type of vessel would imply a day rate which would be very competitive when compared with an H type or a standard drilling rig.
• The H type is estimated to have a construction cost saving of 15% with respect to a drilling unit operating in the same conditions; it is therefore vulnerable to predatory competition from the drilling units already in the region.