Xmas Tree Research Articles

Taking the Pulse of Subsea Trees Design towards Deepwater Application

The challenges facing the subsea production system are becoming increasingly complex with hydrocarbon production moves into deep and even ultra-deep waters. As the most critical equipments, subsea trees need to be designed synchronously against the increasing technology gaps. This paper highlights the common considerations for subsea trees design, especially the horizontal Xmas tree, towards future deepwater application, and the main contents consists of structure design combined with subsea seal technology and the important thermal design in terms of insulation materials selection and application, numerical simulation. Meanwhile, some suggestions and application cases are also included in this paper.

Rigless well intervention and trees on wire from a DPII vessel: a case study

The Vincent field development is typical subsea development with 11 subsea wells required to be tied back to a central production facility. In 350–400 m of water all subsea operations would be diver-less and the well costs associated with using a semi-submersible rig would be a major element of the total field cost. Sequencing construction vessel and drill rig activities, and aligning this sequence with availability of suitable vessels is often a major challenge on such projects. This was accentuated on this development by the shortage of suitable vessels. In addition to this, multiple mobilisations—potentially incorporating lengthy, expensive transits from Singapore—drove a desire to minimise the use of specialist vessels. The challenges in reducing well costs by using rigless operations are both technical and commercial. Technically, the operations and proposed equipment must be matched to the capabilities and functionality of the vessel. Commercially, sufficient work needs to be identified to justify the one-off costs of preparing and mobilising the vessel and equipment, preparing the operations program, and training and familiarising the personnel. In this instance, the operator identified sufficient work to make the vessel mobilisation and equipment preparation worthwhile while not committing to a long-term deployment. Recognising the multi-role capability of the vessel permitted additional work on nearby assets. These removed the need to mobilise additional vessels to the field and improved vessel’s economy in well intervention mode. Other factors that ensured value was gained from the rigless approach included: the rig schedule being de-coupled from product delivery; the drilling program with limited drilling assets was accelerated, protecting first oil; and, the need to re-configure or modify a rig for Xmas tree (XT) handling was removed. Initial preparations for the planned operations began in the latter part of 2006. The main activities were to: confirm the specification of the subsea lubricator system and its deployment system; undertake a detailed hazard and risk assessment process on all operations; undertake an operability study for crane and deployment system; complete the modifications to the vessel for its intervention role; and, prepare detailed well programs and procedures for equipment operation. The design and fabrication of project-specific interface equipment between the subsea lubricator and the subsea Xmas trees, and specification of the required well services equipment such as wireline, pumping and fluid handling services were also required. As this represented the first time such operations had been undertaken in the region by the contractor or by the operator, a large number of supporting systems were required to be developed, adopted and managed, most notably: well control and barrier philosophies; emergency shutdown and disconnection (ESD) philosophy; project management processes; and, personnel training and familiarisation programs.

Intelligent subsea control

The production control system provides control of all functions of the subsea production system. Conventionally, subsea functions include operation and control of: down hole, safety valves, subsea chokes, production valves mounted on the x-mas tree and utility functions such as monitoring of fluid characteristics, pressure leakage, valve positions, etc. The subsea system comprises a wellhead, valve tree (x-mas tree) equipment, pipelines, structures and piping systems among others. In many cases a number of wellheads have to be controlled from a single location. The control system provides operation of valves and chokes on subsea completions, templates, manifolds and pipelines. The design of a control system must also provide a means for safe shutdown on failure of equipments or loss of electrical/hydraulic control from the topside (a platform or floating facility) and other safety features that automatically prevent dangerous events. Presently there are no tools for early fault detection and diagnosis in the subsea control systems area. Troubleshooting and fault finding are normally performed with procedures when the fault is already affecting the system performance and, in most cases finding the root cause of the fault is not straight forward. This fact affects subsea operations and does not provide any support for maintenance plans. Intelligent supervision, fault detection and diagnosis techniques successfully applied to other oil industry related processes are perfectly applicable to subsea technology.

Sand Jet Perforating Revisited

Summary Though sand jet perforating is not a new technique, it is one that has been almost forgotten, the last SPE paper on the subject being published in 1972. Unlike explosive perforating, which is literally a "one shot" process, sand jet perforating uses a high velocity jet of abrasive fluid to cut through the casing, cement and deep into the formation, enabling pressure, pumping time and other parameters to be varied to maximize penetration. Sand jets can penetrate much deeper than explosives, and offer a cost-efficient, safer and better-targeted alternative to hydraulic fracturing to bypassing deep near-wellbore damage. This paper is based mainly on experience in Lithuania, where, in 1995, joint venture oil companies first started field operations to complete development of small oil fields found in the west of the country (see Fig. 1) during the Soviet era, but which had been considered as too small to develop for the Soviet Union, with giant oil fields to the east. Wells, some of which had produced on test at over one thousand barrels per day, had been left with heavy mud across open perforations, often for more than a decade. When it proved impossible to get these wells to flow again using (western) tubing conveyed explosive perforators (TCPs), sand jetting was used, as has been the almost universal practice in Lithuania. The first well sand jet perforated by a joint venture company, which had yielded less than one barrel per day with (western) TCPs, gave over 900 barrels per day when perforated with sand jets. Subsequently, one of the best producers in Lithuania, which had already been sand jetted once, was reperforated using more advanced techniques. Coiled tubing was run through the xmas tree and completion to enable the well to be sand jet perforated, with oil as the carrier fluid, underbalanced, with the well flowing throughout, resulting in a doubling of production to 800 BOPD. Though sand jet perforating is, at least theoretically, available from the main pumping contractors, it is almost unknown outside North America and the former Soviet Union, the main technical references (Refs. 1–5) being over 30 yr old. Sand jet perforating does, however, provide a cost-effective means of passing deep formation damage and should form part of the armory of any practicing petroleum engineer. This paper aims to remind engineers of this, review the technology and suggest appropriate applications.

METAL SEAL WELLHEAD COMPLETION FOR THE ROLLER AND SKATE DEVELOPMENT

A complete metal sealing surface wellhead and Xmas tree has been designed and installed on the Roller and Skate production facilities. Two sizes of production tree have been utilised: a 5-1/8" tree for Roller and a 4-1/16" for Skate. The production and injection fluids favoured the use of metal seal wellheads. The design facilitated easy and quick makeup with time saving in several areas over a conventional standard elastomer wellhead. The design also provides long term metal sealing assurance. Based on research and to the knowledge of the writer, these completions are the first complete all-metal sealing surface wellheads installed in Australia.

Dry Subsea Production Systems: Garoupa-a Case History

Introduction Subsea one-atmosphere chambers for early petroleum production schemes or for wells beyond the reach of platforms have been in commercial use for several years. Such chambers are onstream in the Gulf of Mexico and offshore Brazil for Shell, Union Oil, Tenneco and Petrobras. This paper begins with a brief description of equipment experience with dry production system hardware supplied by Can Ocean Resources Ltd. (formerly Lockheed Petroleum Services) and installed in the Gulf of Mexico, and an outline of the impact on one-atmosphere technology for deepwater applications. The major and concluding part of the paper describes the Garoupa production system installed offshore Brazil for Petrobras. This system is one of the most ambitious undertakings of its kind and, therefore, a noteworthy case history of dry, subsea production technology. An over-all system description, key hardware descriptions and installation of the major subsea components are included in the paper. System commissioning and start-up progress to the end of February 1980 is described and some of the problems encountered, their solution and their impact on the project schedule are discussed. Introduction Every dry, one-atmosphere chamber has two basic functions in common: to isolate the equipment it contains from the subsea environment, and to allow trained oilfield technicians access to that equipment for installation and subsequent servicing. The first chamber (Fig. 1) was installed to produce oil from a subsea well in 114 m (375 ft) in the Gulf of Mexico by Shell Oil Company. At that time (mid 1972), it was the deepest producing subsea completion in the world. This single satellite well is still in production. A flow line carries well production to a nearby platform in shallower water. The one-atmosphere wellhead, called a wellhead cellar (WHC), encloses the Xmas tree, the hydraulic control package, and includes a flow-line connection port: A typical connector port is shown in Figure 2 during land tests for fitting the flow-line termination (bullnose) into it. A one-atmosphere servicing system (Fig. 3), consisting of a tethered diving bell, known as a service capsule, and surface support vessel (SSV), provided for personnel transfer to subsea chambers. The service capsule is rated for 365-m (1,200-ft) water depths and enabled work to be performed subsea on a 12-hour shift to set the Xmas tree, connect the flow line, and conduct final equipment preparation and checkout before well production start-up. Shell Manifold Center System In August 1975, Shell Oil Company successfully installed, in the Gulf of Mexico, an encapsulated subsea manifold center (MC) designed and developed by CanOcean. Figure 4, illustrates the part played by the MC in the subsea scheme. The 1-tonne MC (Fig. 5), with a chamber size 9.2 m (30 ft) long and 3.7 m (12 ft) in diameter, was designed to handle production from three wells in the Eugene Island Block 330 field. After installation in 74 m (240 ft) of water, and connections by flow line to satellite subsea wells and the adjacent platform, production commenced in August 1976.

A GUIDELINELESS TREE AND A FLOWLINE CONNECTION SYSTEM FOR DEEPWATER PRODUCTION

The advent of dynamically-positioned drillships and guidelineless drilling in the early 1970's has extended the search for hydrocarbons into waters which are too deep for conventional mooring. To meet the economic requirements to produce discovery wells in this type of exploratory drilling, a single vessel is used to drill and complete a subsea well and to lay the flowlines without the use of guidelines or intervention by divers.Flexible flowlines are reeled on the deck of the drillship and laid away from an extension of the wellhead prior to the installation of the tubing hanger and xmas tree. Each piece of equipment is aligned and oriented when it is landed. The xmas tree is equipped for the use of TFL pumpdown tools in maintaining the well while producing. In case of damage to a flowline, replacement is made by recovering the flowling guide funnel and wellhead extension.

A wind-profile index for canopy flow

Canopy wind profiles can often be represented by an exponential function. The associated attenuation index,a, is found to be proportional to [(Flexibility)(Leaf Area)(Density)]1/3. Leastsquare values of the index have been calculated for wind profiles in about a dozen natural and artificial canopies which included oats, wheat, corn, rice, sunflowers, larch trees, citrus trees, Xmas trees, plastic strips, wooden pegs and bushel baskets. It is found that canopy flow is a function of canopy density, element flexibility, and height and that the behaviour of artificial canopy elements is compatible with that of natural vegetation. The same calculations also show that the attenuation coefficient: (a) is not a universal constant, (b) is however, rather limited in range (∼-0.3 to 3.0), (c) varies with stage of growth, and (d) increases as density and flexibility increase. A compilation ofa-values for several canopies reveals that lowa-values correspond to sparsely arrayed rigid elements while higha-values correspond to densely arrayed and flexible elements. Finally, lowa-values appear to be relatively independent of wind speed, while higha-values tend to increase as wind speeds increase.