STANDARDIZED WELLHEADS PROVEN ECONOMICAL FOR SUBSEA OPERATIONS

Carlos Cavalcanti Moreira, Cezar A. Silva Paulo
Petrleo Brasileiro SA
Rio de Janeiro

A standardization program for subsea wellheads and completion equipment has made development of Brazil's offshore fields more economical and efficient.

Petrleo Brasileiro SA, or Petrobrs, began a program to standardize subsea equipment because of its extensive operational experience offshore, the large number of subsea wells to be drilled and completed in its new fields, and the potential economical benefits in dealing with common rigs, tools, risers, wellheads, trees, and spare parts for more than 200 wells.

The standardization program defined functional and performance requirements, such as those normally found in American Petroleum Institute (API) specifications, and some critical geometry and dimensions, mainly of interfaces.

Offshore Brazil, hundreds of wells have been drilled, more than 180 wet trees have been installed in water depths from 70 to 781 in, 12 floating production systems have been installed, 14 fixed platforms have been installed, and considerable mileage of flow lines and umbilicals have been laid. These experiences offshore in the Campos basin have allowed Petrobrs to evaluate the pros and cons of standardized subsea equipment.

The simple conclusion was that standardization has considerable benefits.

The resulting operational flexibility associated with the use of field-proven equipment and procedures saves rig time and can reduce production loss during workovers. Additionally, investments can be rationalized economically by installing part of the completion equipment at the end of the drilling job and then delaying purchase and installation of the christmas tree and the flow lines until installation of the production platform.

Savings are also realized from the reduction in the number of spare parts and tools. Moreover, the savings related to improved operations exceed considerably those from equipment acquisition and storage. Thus, the greatest benefit is the operational flexibility.

Nonstandardized equipment, however, serves as an open door to continuous modifications of equipment design. Design changes are welcome when the equipment does not satisfy the customer's requirements, but once the equipment is properly tested and proven, unnecessary proliferation of slightly differing designs needs to be avoided.

Any standardization program should be conducted with the cooperation of all involved parties: operators, equipment suppliers, and service companies. Cooperation helps achieve a high degree of standardization for efficient, reliable, and economical subsea exploitation.

INITIAL STANDARDS

The first significant interface adopted as a standard by Petrobrs in the early 1980s was the subsea wellhead external profile and the sealing area for VX/AX rings. At the time, the various rigs contracted to drill in Brazilian waters had blowout presenters (BOPS) with H4 connectors of different sizes: 21 in., 18 in., 16 in., and 13_ in. The range of sizes led to the use of different wellhead systems and Christmas trees connectors.

Petrobrs then defined the 18-in. H4 profile as the standard external profile for all high pressure housings, and the BOPs were equipped with appropriate connectors.

More recently, this standard had to be changed, in accordance with the type of drilling system used: 18-in. H4 profile for drilling systems with guide posts (installed by moored vessels) and 16-in. H4 profile for guidelineless drilling systems (installed by dynamically positioned rigs). The reason for the different systems is that most dynamically positioned units have been equipped with 16-in. wellheads and stacks, because of variable deck load and drilling system limitations.

The following other items were standardized during the 1980s:

• The top of guideposts, which eliminated the need for a variety of remote connectors

• The internal profiles for wire line set plugs in the tubing hanger and Christmas tree reentry mandrel for both production and annulus bores

• The hub, clamp type, and seal ring for flexible flow line end-fittings and Christmas tree flow loops

• The tops of tubing hanger running tools and tree/tree cap running tools terminating as a completion riser pin to fit Petrobrs' completion riser

• The christmas tree configuration (Fig. 1)

• The hydraulic plate on all running tools to fit the same workover hose termination

• The preparation machined on tubing hangers to assemble the electrical connector for downhole pressure and temperature transducers

• The lay-away method for installation of diverless trees and flow lines instead of the diverless pull-in technique (Fig. 2)

• The use of blind tree caps with metal-to-metal seals on annulus and production bores

• A surface-controlled subsurface safety valve (Scssv), with two hydraulic parts, that can be actuated by either of the two independent hydraulic control lines

• Tubing hanger oriented in such a way that the tubing-hanger running-tool orienting key (BOP connector slot) and tree flow lines outlets always form a 135 angle (This standard eliminated the inconvenience of having a different orientation for each manufacturer, which caused confusion and operational problems in landing trees that did not fit misoriented hangers.)

• The guidelineless drilling base, including detailed temporary and permanent guide bases and 30-in. housing, prepared to receive the wellhead system of any supplier.

All these standard items have worked quite well.

More recently, Petrobrs has defined three types of remotely operated vehicle (ROV) interfaces used on Christmas trees: an intervention fixture for mechanical override of hydraulic-actuated valves, a fixture for control line small isolation valves, and a receptacle for hydraulic hot line stab (Fig. 3).

Also, a deepwater drilling base that does not require the use of two regular guide bases, temporary and permanent, was developed. The single guide base (BUP system) is landed with a vertically jetted conductor pipe, ensuring a reliable foundation on the sea bottom (Fig. 4).

SUBSEA PROGRAM

With the lessons learned from the previous operational experiences and with motivation to drill and complete economically the more than 200 subsea wells planned for the Albacora, Marlim, and Barracuda fields, Petrobrs standardized the interfaces for wet Christmas trees.

The main objective was to allow an already-completed subsea well to be equipped with a tree from any qualified manufacturer. Thus, the christmas tree interfaces with the tubing hanger, the wellhead, the flow lines, and the umbilical had to be determined.

PHILOSOPHY

The standardization philosophy aimed for operational flexibility: the ability to interchange equipment manufactured on different occasions and even by different suppliers. The standardized dimensions were only those related to the boundaries (that is, just the necessary dimensions for the ability to match and fit main components).

Actually, this standardization work is merely a definition of preparations and a definition of profiles (wellhead external profile, top of guideposts profile, tubing hanger top profile). The method is similar to defining orifices, bolt circles, thickness, and ring groove dimensions for a flanged connection.

No equipment as a whole, like wellhead connectors, hydraulically actuated gate valves, tubing hangers, running tools, or even guideline remote connectors, is standardized. Each subsea equipment manufacturer must have its own engineering and must search for technological development and enhancement of its products.

Another consideration was how to define the new interfaces to be standardized. Previously, an adequate interface used by a manufacturer was chosen among several field-proven and available types. This process sometimes led to inconveniences because some manufacturers became dissatisfied by not having their profiles chosen and by having to adapt their products to fit the profile designed by a competitor.

Some manufacturers, however, were not interested in negotiating release of their profile details to competitors. To avoid these problems encountered previously, Petrobrs decided to design the new standardized interfaces with input from subsea equipment suppliers.

In defining interfaces, most difficulties were related to existing systems. The primary objective was future subsea projects rather than existing projects. Thus, the ability to interchange with existing equipment and tools was not a main concern.

The dimensional standardization of interfaces is considered the hardware standardization. The "soft" part also had to be considered: equipment requirements, terms and conditions, required documentation, and testing.

The optimum situation was not to change these items continuously al: each new purchase order. So, these items were also carefully analyzed and stated as standard on the revised material requisition, the basic document for acquisition of wet christmas trees.

IMPLEMENTATION

The standardization work was carried out by Petrobrs with the cooperation of local manufacturers (namely ABB Vetco Gray (Brazil) SA, CBV Industria Mecnica SA, Conforja Equipetrol SA, and Equipamentos Villares SA, the suppliers of ABB Vetco Gray, FMC, National Oilwell, and Cameron Iron Works products, respectively).

After several months of discussions, technical specifications for standardization of wet Christmas trees were issued. These specifications covered the fabrication of the trees for standard service to be delivered from mid-1993 on.

Two technical specifications were written: one for diver-assisted trees and another for diverless, guidelineless wet Christmas trees.

DIVER-ASSISTED TREES

For the diver-assisted trees, only two interfaces had to be defined:

• Tree x tubing hanger-Tubing hanger pocket diameters, bore spacing, height from wellhead top surface, and preparation to match tree orienting bushing (The " x " denotes the interface between the two pieces of equipment.)

• Tree control lines x umbilical-Standard hydraulic plate.

The following interfaces had been defined previously: tree flow loops x flow lines, tree connector x wellhead, and ROV intervention fixtures.

Fig. 5 shows the standardized interfaces for diver-assisted standard-service (5,000 psi, 4 in. x 2 in., noncorrosive service) Christmas trees.

DIVERLESS TREES

Petrobrs adopted the lay-away type as the standard diverless, guidelineless Christmas tree. In this system, the flow lines can be laid attached to either the tree or production adapter base to eliminate using the vulnerable pull-in operations (Fig. 2). The production adapter base is set and locked to the wellhead. The adapter base has preparations to have the tubing hanger, the Christmas tree, and the flow line hub further set and locked in. The production adapter base preparation for the flow line hub is a cradle that gives a known datum for interface between tree and flow line hub. Thus, the simultaneous vertical connections of Christmas connector to the tubing head and Christmas tree lateral flow line connector to the flow line hub is guaranteed.

This system was conceived primarily for first-end connection with the flow lines lowered to sea bottom connected to the tree or production adapter base, requiring the simultaneous operation of the lay vessel and the rig. Petrobrs developed and successfully tested the vertical connection method, in which the flow line hub attached to the flow lines is abandoned near the well and supported by an abandonment structure.

Afterward, the flow line hub is captured ba, the tree or adapter base running tool (that lifts the flow lines and moves towards the well) to be lowered and locked in the production adapter base. No modification was required on the tree or flow line hub to allow these operations.

Four interfaces were defined for the diverless, guidelineless wet Christmas trees:

• Tree x tubing hanger-The same interface adopted for diver-assisted trees

• Tree x production adapter base-Production adapter base guide funnel, Y slots, and tubing head external profile

• Tubing hanger x production adapter base-orientation helix and internal locking profile

• Tree x flow line hub-External hub profile (11 in., 5,000 psi API 16A hub), sealing areas and bore spacing for production, annulus access and control lines orifices, and guide pins holes.

Fig. 6 shows the standardized interfaces for the diverless, guidelineless Christmas trees.

The trees and production adapter bases are required to be manufactured with the aid of jigs to guarantee that the ideal ability to interchange equipment will be achieved in practice. These jigs serve for both construction and final inspection (Fig. 7).

ECONOMICS

To evaluate the economical benefits of the standardization program, Petrobrs projected two scenarios: One based on the completion schedule of the Albacora field and another based on an actual workover on a well in the Marlim field.

• Albacora field

In the Phase II development of the Albacora field, the production facilities platform will be installed about 2 years after drilling begins. Drilling began in early 1993 with completions (installation of only the production adapter base and tubing hanger) to begin in early 1994.

The Christmas trees with the flow lines will be installed in early 1995, as soon as possible to the installation of the production facilities. With the standardization of subsea equipment interfaces, the Christmas tree could be bought in two parts: The first part consisting of the tubing hanger, the production adapter base, and the flow fine hub, and the second consisting of the Christmas tree and tree cap.

Because the Christmas tree and the tree cap represent about 65% of the total price of the subsea equipment, the gross investment should be delayed as much as possible to coincide with the platform installation. For this case in the Albacora field, the investment in Christmas trees decreases about 20% because of the timing.

• Marlim field

After more than 1 year of production, one of the wells in the Marlim field developed Christmas tree problems. The retrieved Christmas tree could not be repaired on the rig. The well had to be plugged, and 26 days were spent repairing the Christmas tree onshore.

This well was producing about 700 cu m/d (4,500 b/d). If the standardization program had been implemented previously for this well, it would have been possible to install an available tree from any other manufacturer as a substitute for the damaged tree, saving about $2.3 million in oil production and other costs.

ACKNOWLEDGMENT

The authors thank Petrleo Brasileiro SA for support and permission to publish this article. The authors extend a special thanks to Jos Formigli in the production department, Roberto Paschoalin in the drifting department, and Danilo Oliveira in the R&D center for their comments and assistance.

BIBLIOGRAPHY

1. "Standardization for Guidelineless Wet Christmas Tree-Technical Specification," Petrobrs, Depro-Cenpes, ET-3500.00-1516-273-PPC-016.

2. "Standardization for Diver Assisted Wet Christmas Tree Technical Specification," Petrobrs Depro-Cenpes, ET-3500.00-1516-273-PPC-017.

3. Moreira, J.R.F., and Viegas, A.F., "Guidelineless Completions Offshore Brazil," piper No. 5975, presented at the Offshore Technology), Conference, Houston, May 1989.

4. Hopper, H.P., "The Potential of Subsea Standardization: Approaches and Advantages," presented at the seminar Subsea Standardization: Trend for the 1990s, Aberdeen, June 5, 1989.

5. De Meyer, T., "Shell/BP/Texaco Proposal for a Common Riser Interface," presented at the seminar Subsea Standardization: Trend for the 1990s, Aberdeen, June 5, 1989.

6. Nagle, F.J.M., Mendonca, J.E., Costa, L.A.G., and Capllonch, R.W., "Vertical Connecting System for Flexible Pipes," paper No. 7260, Offshore Technology Conference, Houston, May 1993.