M. Hughe Frayne
Conoco (U.K.) Ltd.
London
Development of Conoco (U.K.) Ltd.'s Quadrant 44 plans in the North Sea took its initial step in late 1993 with installation of remotely operated platform and pipelines for Caister and Murdoch fields (OGJ, Aug. 17, 1992. p. 52; May 22, 1993, p. 31).
Conoco operates the Murdoch field which it owns (54.5%) with ARCO British Ltd. (34%) and Total Oil Marine plc. (11.5%).
Total operates the Caister field which it owns (49%) with Canadian-Oxy North Sea Petroleum Ltd. (30%) and Lasmo (ULX) Ltd. (21%).
The Theddlethorpe terminal on the eastern coast of England receives the gas and is jointly owned by Conoco (U.K.) Ltd. (operator) and British Petroleum Co. plc.
FIRST PRODUCTION
The Caister-Murdoch system has been developed to produce natural-gas reserves from the Caister and Murdoch fields in the southern part of the U.K. Continental Shelf.
The development consists of an unmanned wellhead platform located at each field, a series of pipelines, and onshore reception facilities. The project phase of the development started with detail design in September 1991 and finished with the production of gas on Oct. 2, 1993.
Caister and Murdoch are the first gas fields to be produced from a relatively undeveloped section of the southern North Sea designated as Quadrant 44 (Fig. 1).
In addition, they represent the first production of gas from Carboniferous reservoirs. This gas, in contrast to commonly produced Rotliegendes gas, contains up to 3 vol % CO2.
The reduced quality, together with the fields' distance from any existing infrastructure, presented special challenges to the development.
Caister also produces gas from Bunter formations. This gas has a lower pressure than Carboniferous gas and contains 15% nitrogen compared to Carboniferous' CO2.
The combined reserves of the two fields are approximately 620 bcf of gas and 5.7 million bbl of condensate. A peak production rate of more than 300 MMscfd is expected.
MINIMUM PLATFORMS
The production facilities center on two "not normally manned" minimum-facility platforms.
The primary roles of the platforms are to provide support for the wellheads and trees, to house equipment to match the wellhead pressure to pipeline pressure, and to allow accurate testing of the wells individually to ascertain gas and liquid flowrates.
The fluid streams from each reservoir are also metered at the respective platforms in order to allocate total production rates from the individual reservoirs.
Murdoch is the gathering platform which receives gas from Caister via an 11-km, 16-in. interfield pipeline. Here, Caister gas is commingled with the Murdoch stream for transmission to the onshore terminal at Theddlethorpe on the Lincolnshire coast through the 185-km (115 mile), 26-in. export pipeline.
Platform designers focused on reducing the extent of the facilities and the maintenance requirements. This was achieved by keeping the systems and equipment simple and by making the process design as robust as possible with appropriate equipment sparing and redundancy only where absolutely essential.
Care was also taken with the selection of materials for equipment, pipework, and the structure. The technique of "value engineering" was used wherein each item of equipment was subject to a review of its purpose, specification, and necessity.
In particular, critical review of such schematic drawings as piping and instrumentation diagrams eliminated several valves and instruments. Continuous involvement of operations personnel also ensured that feedback from similar existing facilities was built into the design.
The platform layout is governed by two primary requirements:
- Segregation of the process and other hazardous equipment from the personnel areas including the protected muster point and local control room
- Accessibility of all well slots by several classes of jack up drilling rig without the need for the rig, once in position, to move (Fig. 2).
Both platforms are similar in layout, with the wellbay to the north and the muster point to the south behind a full width blast wall.
The deck area of Murdoch is larger than Caister to accommodate the automatic sphere launcher for the 26-in. export pipeline. There is also space allocated on Murdoch for additional risers and associated pipework to be installed should other gas fields be discovered in the area and their gas transported through the Caister-Murdoch system.
Metered gas from the Caister platform flows through to Murdoch where it is manifolded into the export pipeline. Consideration had to be given in the design of Murdoch to let Caister flow while Murdoch is shut in for maintenance or operational reasons.
POWER REQUIREMENTS
Power generation consists of three 45 kw diesel-powered units housed in an environmental enclosure.
During unmanned operation, a single unit can provide enough power for all the platform systems. Should the operating unit fail, a second generator will start automatically to allow production to continue uninterrupted.
When the platform is visited, the electrical load increases because of the need for lighting, heating, and other human amenities.
To accommodate this increase, the second generator is started. The third unit remains at all times as an emergency Generator.
In addition, battery, power is available which provides 1 hr full load supply to the uninterruptible system. Fuel for the generators and the crane is brought to the platform by supply boat.
A storage tank with 120 days' capacity has been installed to reduce periods between resupply.
The vent system is designed to handle either full-flow cold venting, ignited relief from one well, or ignited emergency blowdown.
It is a standard system with a collection header, knock-out drum, vent tip, and snuffing package.
The vent is continuously purged with process gas. The knock-out drum has been combined with the closed drain drum, and liquids collected in this vessel are drained into a sump from which they are manually blown back into the production manifold with gas pressure.
The system has the advantage of eliminating any drain pumps and their attendant maintenance.
Regular launching of spheres through the 26-in. export pipeline is an essential part of the pipeline operating procedure and corrosion-control program. It also reduces the liquid buildup in the line to manageable proportions so that when the line is sphered, the volume of liquid arriving at the terminal does not exceed the capacity of the slugcatcher.
To reduce manning requirements, an automatic sphere launcher was designed and installed on Murdoch. The launcher can be preloaded with up to seven spheres which can be launched remotely from the shore terminal's control room.
A command is sent to the platform where the equipment cycles automatically through the launching sequence. To date, the system has proven reliable and contributed significantly to reducing the frequency of visits to the platform.
COMMUNICATIONS, CONTROL
The distance of the two platforms from the nearest land (100 miles) and from any other platforms dictates that the normal "line of sight" communication system common elsewhere in the southern North Sea cannot be effectively used.
The project considered both a fiber optic cable and use of earth satellites for a communication route. A satellite system was adopted on economic grounds.
Once this selection was made, the problem of system reliability was addressed.
In order to provide sufficient redundancy, each platform communicates with a separate earth satellite. The platforms can then communicate with each other through a line-of-sight microwave link.
In this way, both platforms use the other platform's communication system as a backup, resulting in a fully triangulated fail-safe system.
Control of the offshore facilities is from the Theddlethorpe control room. All the platform functions can be operated from shore with the exception of the initial setting up of the test separator.
Another feature of the development, dictated by the distance of the new facilities from any other platform, was the installation of an automatic weather station. This provides details of the local weather at both platforms to learn helicopter pilots of prevailing conditions.
General weather information and that received from other platforms were considered inadequate for planning helicopter flights to the Caister and Murdoch facilities.
PIPELINE SYSTEM
The 26-in. export pipeline between Murdoch and the onshore terminal at Theddlethorpe carries gas and produced liquids.
The export line size was selected to accommodate the flow from Caister and Murdoch and also from other fields in the area up to a maximum capacity of 750 MMscfd with offshore compression.
In addition, there is a 4-in. line laid beside the gas line which carries the reclaimed methanol mixed with corrosion inhibitor from the terminal to Murdoch and on to Caister.
Other than in the immediate vicinity of the platform arid at the shore approach, the lines are laid parallel some 10 m apart. This separate lay was undertaken in contrast to the more common "piggy back" lay so that the methanol line could be trenched at least 1 in deep to protect it from fishing activity.
While the trunkline is generally conventional in design, one feature is worth special note. Above the highwater line, the route passes through a wide area of sand dunes.
These dunes, as well at being the primary sea defense for the low lying land behind, are also home to much wildlife of special interest.
Previous landfalls in the area have required large excavations across the dunes. This has disturbed wildlife habitats which are taking man), years to reestablish.
For the Caister-Murdoch system, it was decided to force a narrow tunnel through the dunes through which both lines would be pulled. Sections of 1,350-mm concrete pipe were jacked from a pit dug behind the dunes along a predetermined course towards the seaward side.
Once a continuous concrete tunnel was formed, both pipes were pulled through and grouted in place. This method was cost effective when compared with the normal trench and lay, and the work was completed with very little disturbance to the environment.
The interfield line between the two platforms is 16-in. diameter and the associated methanol line 3 in. As with the export line, these two lines were laid adjacent to each other for most of their lengths to achieve adequate burial.
CORROSION CONTROL
If left untreated, CO2 combined with free water in the gas stream creates the potential for corrosion within the system.
Experience has shown that corrosion inhabiter can be successfully carried in the methanol which is itself used for hydrate suppression. The amine-based inhibitor is injected into the methanol stream onshore and pumped to the platforms by pumps located at the terminal.
The methanol and inhabiter mixture is then injected at pressure into the production manifolds and other pipework on each platform.
Specially designed injection-rate control devices are used which ensure that the correct quantity of fluid reaches each part of the process equipment.
Pipework upstream of the manifolds together with the Christmas trees and production tubing are made of corrosion-resistant material so that then, need not be treated.
The corrosion inhabiter coats on all the steel surfaces which are exposed to the fluid stream and in so doing will prevent corrosion indefinitely. Within the surface pipework and the interfield pipeline, the fluid velocities are such that coating will take place unaided.
For the 26-in. line, however, in which flowrates and hence fluid velocities can be low, the necessary coating is ensured by liquid slugs regularly being pushed through the line with spheres.
HOOK-UP, COMMISSIONING
One of the initial goals of the project team was to accomplish as much of the commissioning of the platforms onshore as was reasonable, practicable.
Mechanical completion of all platform systems was completed on Jan. 15, 1943, for Murdoch and by Jan. 31 on Caister. This gave 10 weeks of dedicated commissioning time which meant that all outstanding items were completed.
In fact, work that had been identified in the offshore scope (including the erection of temporary access scaffolding) was completed at the fabrication yard which further reduced offshore work.
The final total of manhours worked offshore for both platforms was 33,000. This was achieved without the use of a support accommodation unit with small construction crews being shuttled each day, to the platforms by helicopter.
A noteworthy aspect of the commissioning program centered on the satellite communication system.
While at the fabrication site, the large satellite dishes were moved from their permanent locations onto temporary supports which allowed them to be directed at their appropriate satellite.
The system could then be fully function-tested onshore with the control of the platform systems being effected from Theddlethorpe. The tests proved valuable to the project because many problems with the equipment were identified and corrected before the platforms went offshore.
As a result, communications for both platforms have worked without interruption since their installation in April 1993.
INSTALLATION
Installation of both platforms was carried out with HeereMac's semisubmersible crane barge DB 101. The two jackets were brought out to the field on one barge, the decks on a second, and the piles on a third.
At both locations, seabed levelling work had been carried out around the templates before the heavy lift vessel arrived. This preparation ensured that both jackets were within the level tolerance as soon as they were placed on the seabed.
The Caister jacket was installed first starting on Mar. 27, 1993. The jacket was located over the well template with a system of docking pins and bumper guides.
Once the jacket rested on the seabed, its position was found to be within tolerance both directionally and vertically. The structure was therefore piled immediately and grouted.
The heavy lift vessel then moved to Murdoch where the jacket and deck were installed. Once the jacket was in place, the jacket top was prepared and the deck positioned and welded down.
During welding, the heavy lift vessel installed secondary items such as the seawater caissons, and the platform helideck was made ready for use. The power generation was started, and the platform accepted by the certifying authority for standalone operation.
Finally the DB 101 moved back to Caister and installed the deck. The installation of Murdoch took 8 days, Caister 9 days.
So that gas production could start as soon as possible after installation of the offshore facilities, all wells at both fields had been predrilled.
Two 10-slot well templates were fabricated in late 1991 and installed with the drilling rigs at the start of each predrilling operation. Drilling started at Murdoch in November 1991 and at Caister in April 1992.
At Caister, the template was placed over an existing exploration well. Five additional wells were drilled and temporarily abandoned at Murdoch, six at Caister.
The drilling operation was completed at both fields well in time for platform installation in April 1993. Once the installation was completed, the rigs were positioned over each platform to start the well tieback and completion (Fig. 3).
ONSHORE PROCESSING
Gas flowing from the Caister and Murdoch fields enters the Theddlethorpe terminal (Fig. 4) into a dedicated slug catcher. The 8,000-bbl slugcatcher allows the system to produce at a range of flowrates with a maximum sphering frequency of twice a week.
Wet gas from the slug catcher is metered then blended with high-quality gas coming from other fields. The total gas stream is further processed in the existing dew point facilities before being discharged through sales-gas meters into the U.K gas-pipeline system.
The condensate is stabilized in the existing stabilization trains and then, through storage, is pumped through a pipeline to the Conoco refinery at Killingholme on the River Humber.
The methanol is purified in a distillation column, passed through storage, then pumped back offshore through the 4-in. pipeline.
The methanol can be pumped at a pressure of more than 5,000 psi to allow it to be injected directly into the christmas trees, a procedure sometimes required at production startup.
Any water dropping out from the distillation process is purified before being disposed of (Fig. 5).
BLENDING MANAGEMENT
As described previously, the Carboniferous and Bunter gas is of insufficient quality for direct sale. The gas stream is therefore mixed with higher calorific gas from other fields coming into the terminal to produce an acceptable blend.
It is essential that on no occasion is offspec gas delivered by the terminal. To this end a "blending management system" was designed and installed.
This computer-based package, through information received from offshore and the test results of gas and nitrogen chromatographs installed within the terminal, computes the predicted Wobbe index of the sales gas.
(Editor's note: The Wobbe index number is the ratio between calorific value and the square root of the gas density relative to air under ambient conditions.)
Depending upon the predicted sales-gas quality, the system adjusts the Caister-Murdoch flow to achieve the required quality.
A secondary system has also been installed which directly measures the quality of the gas leaving the blender. Again, if this measurement is below the required Wobbe index the export valves are automatically closed.
The project was undertaken on a fast track from the beginning of detail design in September 1991 to first gas on Oct. 2, 1993.
