CUTTINGS HANDLING SYSTEM FOR WASTE MUD DISPOSAL

Use of traditional cuttings handling systems based on washing the cuttings. By cleaning the cuttings, the oil content in the cuttings can be brought down to approximately 100g oil content per. kg drilled cuttings, which is the average allowable discharge of oil per. well. This again means that the cleaned cuttings can be dumped to sea after cleaning is performed.

The new regulations states 0% oil content in the cuttings. The following two options are then given for disposal of oily cuttings:

  1. Platform installed cleaning system capable of cleaning cuttings to 0% oil content.
  2. Transport of cuttings to shore for cleaning/disposal.

Up to now, no proven cleaning equipment can meet design requirements to minimum weight, power and area for installation on new rig developments or rigs in operation. Due to this fact, contaminated drill cuttings have to be brought ashore for disposal.

cuttings handling system on drilling pit

This will be carried out by a PLC controlled transportation system, which transports the drill cuttings from the shale shakers to a container loading station for shipment to shore with supply boats.

PHILOSOPHY FOR USE OF OIL BASED MUD

The trend in the trade for drilling platform wells, is to limit the use of oil-based mud, by use of other mud types:

  1. Drilling with water based mud.
  2. Drilling with alternative, “nonpolluting” oils.

Water based mud poses operational limits set by high torque and low hole stability in highly deviated wells. Mud based on alternative oils has yet not proved to have the same qualities as conventional oil based mud. Hence conventional oil based muds are required for certain applications. In order to minimize the use of oil based mud, Norsk Hydro limits use of oil based mud to the following:

  1. In 17 -1/2 sections where the average hole inclination exceeds 60 degrees.
  2. In 12-1/2 sections where the average hole inclination exceeds 50 degrees.
  3. In 8 -1/2 sections (reservoir sections) no matter what the hole inclination is.

These guidelines are applicable for production drilling, and will change as both water based mud, and alternative oil based mud are developed.

DESIGN PHILOSOPHY

In order not to set too many limitations during detail design/development of the CUTTINGS HANDLING SYSTEM the guidelines were kept to a minimum.

However, the overall design basis for the system was to achieve a system with a high level of reliability and keeping the drilling down-time to a minimum. Additionally the design philosophy clearly stated following requirements:

  • Maintain the functional requirements to secure proper discharge (no sticking of cuttings) from the cuttings holding tank and the transporters.
  • Cuttings holding tank/buffer to be designed for minimum 1 hour drilling at full rate in the 17 -1/2 section.
  • A container loading station to be designed for filling and storage of minimum seven, 7 off containers with maximum total weight 0 tones each loaded.
  • Minimize and facilitate easy installation/hook up.
  • Easy maintenance of equipment.
  • Simple operation of system.

The design philosophy also identified the operating conditions.

The CUTTINGS HANDLING SYSTEM was designed to handle the amount of cuttings at maximum rate (including overflow of fluid from the shakers) which was given as 14.0m3 /h. The CUTTINGS HANDLING SYSTEM shall transport both water based and oil contaminated drill cuttings. Typical characteristics for this project are as follows.

Oil contaminated drill cuttings generally consists of crushed sedimentary rocks mixed with oil based drilling mud. The cuttings are separated after mud return from the well by means of vibrating screens (shale shakers). The screen sizes may typically vary from 80 microns to 200 microns opening.

The crushed sedimentary rock particles which may originate from layers of shale, sandstone and lime stone, including drilled cement, are of irregular shape and size distribution. Generally the maximum size will be below 25 mm length but may occasionally be larger (maximum 40 mm should be assumed if size is critical to the design).

Oil mud consists of a fluid phase of 80-90% base oil and 20-10% water. The base oil is a low aromatic oil with a specific gravity of 0.85. CaC12 is added to provide a salinity of 80.000 to 160.000 mg/l. Polymers are also added to achieve a fluid of thixotropic nature. Barite (BaS04 ) is added to increase the specific gravity ranging from 1.3 to 1.6.

Cuttings from drilling with water based mud consists of the same types of crushed sedimentary rocks as for oil contaminated cuttings. Physical properties of mixture is different in some respects due to lack of an oil phase. Consistency of cuttings in the holding tank if this fills up may in some cases be compared to wet clay. Particle size of water mud cuttings may be larger than for oil mud cuttings.

During certain drilling operations, i.e. when drilling out plugs and cement shoes the cuttings will contain aluminium swarfs and rubber pieces.

In order to protect the CUTTINGS HANDLING SYSTEM against oversize particles protection bars are installed across the top of the cuttings holding tank.

The conveyors are designed for continuous operation. The conveyors shall be capable of starting in a fully loaded condition. The screw conveyors must also be able to start with the cuttings holding tank totally filled with cuttings.

The chain of transport conveyors must be designed, so that no build up of cuttings may take place.

DETAIL ENGINEERING/DESIGN

Was carried out by the project engineering team which coordinated statutory and company requirements against the vendor of the CUTTINGS HANDLING SYSTEM who also performed the detail design.

The design, fabrication and testing of CUTTINGS HANDLING SYSTEM, equipment were required to be in accordance with following codes and standards:

Authority Codes/Standards
Det Norske Veritas Technical Notes for Fixed Offshore Installations Volume B.
Internat. Electro-technical Commission All relevant standards from IEC 79.
Internat. Electro-technical IEC 529 Classification of degree of protection Commission provided by enclosure.
American Petroleum Institute API BUC 13C Drilling fluid processing equipment.

Additional to statutory regulations and specifications was detail specifications given to area classification and ambient condition as following:

Area classification:

The CUTTINGS HANDLING SYSTEM to be located within Hazardous Area Zone 1 according to IEC publication 79-10 (1972), the IP model code of safe practice (Electrical fifth revised edition).

Ambient conditions:

Minimum – 7°C

Maximum + 40°C

General requirements:

The system was specified to comply with the project general specifications for instruments etc. These specifications stated detail requirements to all technical disciplines. However, additional general requirements for design, materials and fabrication was also given as:

Materials and equipment dimensioning was selected to ensure adequate safety factors under extreme loading conditions for all critical crossections of load bearing structures and pressure retaining components.

  • All surfaces in contact with the cuttings were 316 stainless steel to prevent corrosion, and facilitate cleaning.
  • All unprotected surfaces exposed to corrosive environment was made of corrosion resistant material.
  • All screws and bolts etc., shall be corrosion resistant material, suitable for the environment.
  • Materials of different galvanic potentials shall not be directly in contact with each other.
  • All el. motors was designed with overload protectors to prevent damage in case of an object falling into the conveyors. This was activated before any damage is done to the conveyor.
  • Use of aluminium was avoided.
  • Maximum noise was set to average of 83 dBA.

Pre-engineering and preparations for procurement of the CUTTINGS HANDLING SYSTEM took place when all statutory and company requirements was settled. Due to critical time limit for CUTTINGS HANDLING SYSTEM site need dates, the engineering and vendor selection then commenced simultaneously. Potential vendors were called in for questioning and discussions.

Prior to the inquiry, process and instrument ‘diagrams, process data sheets, special technical specification and supplementing documentation were revised to “Approval for Inquiry”.

SYSTEM PROCUREMENT AND FABRICATION

The bids were evaluated on cost, compliance, reliability, and vendors previous experience with similar equipment.

The Cuttings Container Gantry Crane (CCGC) was ordered separately.

The scope of CUTTINGS HANDLING SYSTEM supply included delivery of equipment as specified in diagram 3.

Arrangement & System

All supplied equipment is to be permanently installed in/on the modules MSO/M40/M20/M30, see figure 3 and 4. Screw conveyors, gate valves and diverter flap assembly was mounted together in the order mentioned, starting with the self cleaning Screw Conveyors flange mounted to the bottom of the cuttings holding tank. Flanges were designed to withstand the weight of the cuttings in the holding tank and weight of diverter flap assembly and conveyors.

The cuttings paddle conveyors installed on M40 and M20 were designed with hatches to match all well positions.

The last conveyor hooked up to the end of the M20 conveyor was designed with a slewing ring which allowed the conveyor to be turned ±15º for complete filling of containers.

The cuttings are dumped into standard 5 m3 containers The filling of containers stops automatically when the total weight of the container reaches 8 tonnes, this due to limitations for rig pedestal crane. The CCGC removes the full container and positions an empty container. The CUTTINGS HANDLING SYSTEM is started by the operator when the empty container is in position on the weighing table.

When a container is full the whole CUTTINGS HANDLING SYSTEM system stops. Cuttings are then collected in the holding tank until another empty container is brought into position and the system started.

The control panels allows the operator to control the CUTTINGS HANDLING SYSTEM from M50 and M30, with the M30 panel acting as master. Control panel M50 is mainly used when drilling with water based cuttings.

The diverter flap assembly which is flanged up below the cuttings holding tank gate valve diverts the cuttings to dump into the sea while drilling with water based cuttings.

The gate valve is arranged to enable filling the cuttings holding tank during stop of the CUTTINGS HANDLING SYSTEM and for maintenance of other equipment during drilling.

FACTORY ACCEPTANCE

Prior to shipment of the CUTTINGS HANDLING SYSTEM to the module fabrication yard, the components/systems were function tested and inspected. The system which required a Factory Acceptance Test at the vendors facility was set up as near as possible to the offshore operating conditions. The Factory Acceptance Test was performed at full rate with real oil based cuttings.

Prior to the test and inspection the vendor submitted comprehensive procedures, with detailed listing of all activities including acceptance criteria. The procedures were approved by the company prior to start up of the test and inspection.

Electrical motors were tested in all load conditions at rated frequency and voltage. This required a special power supply.

All technical and operational data was logged, including noise. The noise test results was logged to 53 dBA which is far below specified requirements.

The test results showed that the system screw conveyors performed at an average feed rate of 14.2 m3/h cuttings each with full cuttings holding tank.

Total feed both screws average 28.4 m3/h which is correctly below the feeding rate for transport paddle conveyors M40/M30/M20 which performed at a rate of 33.4 m3/h.

This 100% back up compared to maximum calculated return rate of cuttings 28.0 m3/h was required during emptying of full cuttings holding tank using both screw conveyors simultaneously.

The control panels were installed to check correct operation of the PLC and wiring against drivers. Shutdown, stop sequences of drivers and valves was tested. Simulating test against weighing station was also performed showing stop of CUTTINGS HANDLING SYSTEM when container full, 8.0 tones.

HOOK UP AND COMMISSIONING OF CRANES AND TRANSPORT SYSTEM

The hook up and the commissioning of the platform cuttings handling/transport system will be performed in two stages:

  1. Onshore commissioning.
  • Offshore commissioning.

The onshore commissioning will not include a full system test since the system is split in 4 modules which are fabricated on different yards. The system will however be run complete as possible on the module yards during the onshore commissioning. The following modules contains parts of the cuttings handling/ transport system:

  • M40/MSO – These modules contains the mud return/treatment plant, including shale shakers, desander, desilter and degasser. The cuttings transport system part of these modules includes the cuttings and north-south conveyor which will take the cuttings to the east-west conveyor mounted on the M20
  • M20 – The east-west cuttings conveyor will be mounted on the M20 wall between M20 and This conveyor is an offshore hook up item, and will not be commissioned onshore.
  • M30 – This module contains the north-south short conveyor with the container filling device, weighing station and gantry crane. The onshore commissioning will involve testing of all the above mentioned components.

The offshore commissioning will be a complete system test. No oil based mud is expected to be used on the platform, as the first wells are 8 predrilled wells to be brought on stream.

INTRODUCTION TO DRILLING PROGRAM

The drilling modules on Oseberg C (M30/M40/MSO/M60/M70) will be used for drilling the 17 (one spare slot) remaining wells on Oseberg C, in addition to the completion of the eight predrilled wells that have been drilled from a semi submersible prior to platform installation. All wells will be deviated to angles from approximately 30 deg., to the most extreme at 72 deg. 7000 m (23000 ft) total depth.

A Low Toxic Oil mud system will be used when required according to the philosophy. In all other sections a standard water based mud system will be used (KCl mud, gyp/polymer mud or similar).

During completion of the wells, brine will be used for balancing the well.

TOTAL VOLUME OF CUTTINGS EXPECTED WHILE DRILLING

The attached diagram 1, gives an indication of the amount of cuttings which is to be processed while drilling the 17 platform wells.

TOP SIDE HANDLING OF CUTTINGS

Operation of the cuttings handling/transport system will be performed by the drill-/deck crew on the platform. The system demands one person full time at the loading station on M30 for filling and moving the cuttings containers, when usin9 oil based mud in the large (17i”) sections with fast drilling. In the smaller sections, and as the drilling slows down, the cuttings loading station will be manned part time. The sequence of operation for the system will be as indicated in the attached diagram 2.

The cuttings handling/ transport system is PLC controlled to prevent overflow, overfill and maloperation of the system. It is therefore possible for one person, in addition to one roughneck at the shale shaker, to operate the complete system in a safe and efficient manner.

OPERATIONAL LIMITS FOR THE SYSTEM

There are three main operational limits for this method for disposal of oily cuttings compared to the traditional cuttings cleaning system.

  • Storage areas for full containers are limited on the platform. This may be improved by utilizing the tender support vessel as a storage area. This, however, depends on the accessibility of both platform pedestal cranes.
  • Pedestal cranes are needed for moving containers off M30. These cranes do not operate at higher wind speeds than 45 m/s, which means that the drilling operation will have to be stopped if oil based mud is used, the M30 storage is full and the wind speed is to high.
  • Supply boats must be available continually when drilling 17-1/2 sections.

It is clear from the above mentioned points that the platform will be more dependent on external factors than what is normal today. The extensive use of boats and cranes together with the limited storage area, will be a possible restriction during the drilling program on Oseberg C.

CONCLUSION

The CUTTINGS HANDLING SYSTEM introduced at a late stage involved major changes.

The good relationship between involved parties was a major contribution to the successfully design and installation of the CUTTINGS HANDLING SYSTEM.

Proper planning and definition of the scope of work, the design and fabrication was performed within minimum of time while maintaining compliance with environmental guidelines and governmental requirements.

The operator’s insistency on compliance with environmental guidelines successfully affected the operational organization to think and take action accordingly with minimum of time. Procedures for cuttings handling topside, offshore and transport to shore were established in parallel with design and fabrication of the CUTTINGS HANDLING SYSTEM.

Good project co-ordination ensured safe, cost effective, efficient, well organized platform rig installation and operation at an early stage avoiding future more expensive rig installations.

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