SUBSEA PRODUCTION SYSTEMS 

Performance verification testing or qualification testing is done to defined procedures to qualify new or significantly modified product designs which have not already been qualified or are not already “field proven”.

Key parameters requiring consideration are the simulation of all loads, pressures, and operating conditions that the system will be subjected to during all phases of installation and operation.

Equipment or fixtures used to qualify designs using performance verification procedures should be representative of production models in terms of design, dimensions, and materials. If a product design undergoes any changes in fit, form, function or material, the manufacturer should document the impact of such changes on the performance of the product.

A design that undergoes a substantive change becomes a new design requiring re-testing. A substantive change is a change identified by the manufacturer, which affects the performance of the product in the intended service condition. This may include changes in fit, form, function or material. Typical examples of this are size or pressure rating changes.

A change in material may not require re-testing if the suitability of the new material can be substantiated by other means.

Hydrostatic pressure tests are acceptable for all equipment performance verification pressure tests to API 17D. End users often require and manufacturers may at their option substitute or add gas test for some or all of the required performance verification pressure tests. Hydrostatic and gas performance verification test procedures and acceptance criteria should meet the requirements set by the API specifications or better. Gas testing is generally specified for equipment that will be used for gas service.

API Specification 17D and 6A lists the equipment that must be subjected to repetitive hydrostatic pressure cycling tests to simulate start up and shut down pressure cycling which will occur in long-term field service. For these hydrostatic cycling tests, the equipment will be alternately pressurized to the full rated working pressure and then depressurized until the specified number of pressure cycles has been completed. No pressure holding period is required for each pressure cycle during the cycling phase of testing. A standard hydrostatic (or gas if applicable) test will be performed before and after the hydrostatic pressure cycling test.

The cycling tests are additionally subjected to controlled alternating heating and cooling. Repetitive temperature cycling tests simulate start up and shutdown temperature cycling which will occur in long-term field service. For temperature cycling tests, the equipment will be alternately heated and cooled to the upper and lower temperature extremes of its rated operating temperature. During temperature cycling, rated working pressure will be applied to the equipment at the temperature extremes with no leaks. Temperature cycling from room temperature to the lower temperature extreme plus cycling from room temperature to the upper temperature extreme may be substituted for temperature cycling directly between the two temperature extremes.

Performance verification tests at rated working pressure or greater are performed at test temperature equal to or less than the minimum rated operating temperature classification, and at a test temperature equal to or greater than the maximum rated operating temperature classification to confirm the performance of the equipment. As an alternative to testing, the manufacturer can provide other objective evidence, consistent with documented industry practice, that the equipment will meet performance requirements at both temperature extremes.

Not all types of components are identified in the mentioned API specifications and therefore interpretation of type of function is often used to define testing requirements for components. Typical examples of these components include many Horizontal Tree components such as internal tree caps and crown plugs.

Performance verification testing, Finite Element Analysis (FEA) or classical engineering analysis can be used to verify the manufacturers rated load capacities for API Specification 17D equipment. If testing is used to verify the design, the equipment should be loaded to the rated capacity at least three times during the test without deformation to the extent that any other performance requirement is not met. If FEA or engineering analysis is used, the analysis will be conducted using techniques and programs that comply with documented industry practice.

Life cycle/endurance testing, such as make-break tests on connectors and operational testing of valves, chokes, and actuators, is intended to evaluate long-term wear characteristics of the equipment tested. Such tests may be conducted at any temperature. API Specifications 6A and 17D list equipment that should be subjected to extended life cycle/endurance testing to simulate long-term field service. For these life cycle/endurance tests, the equipment will be subjected to operational cycles per manufacturer’s performance specifications (i.e., make up to full torque, break out, open/close under full rated working pressure).

Scaling may be used to verify the members of a product family. A product family is a group of products for which the design principles, physical configuration, and functional operation are the same, but which may be of differing size. The design stress levels in relation to material mechanical properties must be based on the same criteria for all members of the product family in order to verify designs via scaling. Testing of one size of a product family will verify products one nominal size larger and one nominal size smaller than the tested size. Testing of multiple product sizes also verifies two nominal sizes larger than the smallest item tested and two nominal sizes smaller than the largest item tested. The test product(s) may be used to qualify products of the same family having equal or lower pressure ratings.

The procedures used and the results of all performance verification tests used to qualify equipment to API Specifications must be documented. The documentation requirements for performance verification testing are laid out in the API specifications.

Factory Acceptance Testing, commonly referred to as FAT, is always performed on newly manufactured subsea equipment (including spares) to ensure that the individual components and assemblies meet the specified requirements and function correctly. FATs normally consist of tests of the integrity of the equipment, (e.g. pressure testing, electrical insulation resistance, coating adhesion, critical dimension survey, etc.), tests of the operation of each function of the equipment, and interface tests with directly interfacing equipment.

In order to ensure successful FAT and thereby assist with maintaining project schedules, subsea equipment is manufactured, inspected and tested in accordance with predefined quality procedures and quality plans. These procedures and plans define the levels and methods of inspection and testing that will be followed during the manufacturing process. A proficient quality system and well defined quality plan will identify defects and problems early in all stages of manufacture so that corrective actions can be invoked to ensure timely delivery of good equipment. This normally involves a substantial amount of planning, experience and skilled manpower to enhance the project execution. Risk assessments can assist a targeted approach based on criticality of equipment. If not done properly, a cumbersome and burdensome quality system can hinder rather than enhance it.

During manufacture, components should be dimensionally controlled to verify conformance with design drawings. Acceptable deviations will occur and should be recorded. An experienced engineer typically approves these after the quality assurance process identifies them.

Body hydrostatic tests are performed to reveal any structural flaws in components as early as it is safe to do so. Having passed this overload test, a level of confidence is established for the safe conduct of future tests carried out at the lower maximum working pressure.

For interface checks, jigs and dummies may be used where testing with actual interface components is not practical. It is, however, recommended that the actual equipment be used where feasible. Fit tests should be performed in such a way as to prove the guidance and orientation features of the components. Misalignment checks should consider stack-­up tolerance, stack-up elevation, horizontal plane, orientation, and angular alignment. Equipment with self-alignment features should intentionally be misaligned to verify its alignment capability.

Functional checks should include make-up, normal emergency release, reversibility, repeatability, and pressure integrity. The sequence and items to be tested would normally be individual components, running tools, subsystems, and the total system assembly.

Following FAT, groups of closely interfacing assemblies making up a system may be interface tested.

An EFAT of the Subsea Tree system could include an interface test of the tree, Tubing Hanger (for horizontal trees in particular), and the tree cap. An EFAT of the control system could include connecting and operating the topsides control system, HPU, several control modules and subsea instruments.

The purpose of the EFAT is to detect any problems within each system prior to interfacing with the overall SPS at the SIT.

The purpose of the System Integration Testing (SIT) is to simulate, as closely as possible, the installation and operation of a complete SPS to verify that equipment which must interface with each other fits and works together acceptably.

Additionally, it can be an excellent opportunity for training of offshore crews, including familiarization with equipment and procedures. This is an important factor during all integration test activities. This aspect is particularly influential in promoting competence, safety and efficiency during installation and operation activities. Reliable rig handling systems and trained personnel are of vital importance to the overall success of a subsea project during the subsea installation, well completion and production testing phases. SIT is an opportunity for the tests to contribute to the success by optimizing installation procedures and familiarizing offshore personnel with equipment and equipment handling to promote efficiency and safety in installation and operation of subsea production wells.

The SIT can be used to expose relevant equipment to abnormal situations which can occur during operation such as low hydraulic supply pressure, low voltage supply etc. The purpose is to reveal “system margins”.

Depending on the production system, there are many types of checks that should be performed. If possible, it is best to perform the test utilizing the actual subsea equipment and tools. If the possibility to perform full-scale testing does not exist, system performance should be demonstrated by verification analysis.

Tests should include simulations of field conditions for all phases or operations from installation through maintenance. Special tests may be needed for handling and transport, dynamic loading, and backup systems. The SIT may be appropriate to verify data on response time measurements, operating pressures, fluid volumes, fault finding, and operation of shutdown systems.

The different tests performed during integration testing should be used to check reliability, and should be used to demonstrate tolerance requirements and the correct functioning of the complete system. Detailed procedures for the integration tests should be prepared prior to starting the tests.

Figure 8.1 - A Typical Workover System SIT With The Subsea Tree

The subsea system should be subjected to the following activities during system integration testing:

If dummy structures are to be utilized during testing, a verification of the possible dummy structures should be performed to verify that the dummy structures are in compliance with the real structures.

The facilities on the integration test site should include test facilities with crane capacity for handling and stack-up of tree and manifold systems. The test facility should be clean and not disturbed by other activities.

A typical format for a subsea equipment integration testing procedure could include the following: Purpose/Objective, scope, requirements for fixtures/set-ups, facilities, equipment, personnel responsibilities, performance data, changes, acceptance criteria, and certification and reference information.

Outline commissioning procedures should be developed prior to establishment of the test procedures. Hence the end user requirements should be defined prior to developing the actual test procedures. The idea behind this requirement is to maximize applicable experience from one phase to the next. Hence experience gained during FAT is applicable for test activities during SIT and commissioning.

A schedule for the activities should be developed prior to the start of the integration test. Equipment logistics should be part of the schedule. The Operation and Maintenance Manuals (O&M’s) should be used as guidelines for establishing the test procedures. Test procedures should be signed off step by step during each test operation.

A daily log should be written for each test activity. Test findings should be briefly described in the log. A query system that should handle all test findings should be developed including procedures to rectify the findings. The manufacturer should arrange frequent status meetings with operator during the integration test phase.

The operator’s personnel should have access to all test facilities during testing. They may monitor or witness all tests and should have free access to the test results. Emphasis should be put on the requirement that the operator’s offshore-nominated personnel have access for complete insight into system functions, system operation and debugging methodology.

Photographic records can be of considerable value in future diagnostic work when the equipment is subsea. Comprehensive still photography and video records are recommended.

The test program should include an index of the test procedures, equipment-handling procedures and further identify facilities, equipment, materials and other items required for the program.

The manufacturer should develop and establish procedures and check lists necessary in order to verify that the requirements of the contract are met. The integration test procedures should be developed in such a manner that operational conditions can be simulated. All procedures for system integration tests should be reviewed and approved by the operator prior to start of the test. The test procedures should include defined acceptance criteria.

When thermal performance of SPS equipment is critical, a “cooldown” test may be specified to check the steady state temperature drop through the system, and the time taken for the equipment to cool from operating temperature to the hydrate formation temperature.

Typically the test is performed by submerging the equipment in water at ambient temperature and circulating hot water through the appropriate production path to simulate the operating scenario. Once the steady state temperature is reached, the equipment is shut in and the temperature of the fluid is measured over time.

While the cooldown criteria may be specified for certain conditions of production fluids temperature and pressure, the testing is often performed using water, and the results transposed by calculation.

The cooldown test also gives an opportunity to check the temperature sensor readings against the actual fluid temperature.

The purpose of the shallow water test is to ensure that the proposed running, ROV and tie-in tools can be operated subsea and that the installation method works. This test is used to validate the design and outline installation procedures in a subsea environment, check all ROV operations, train ROV operators and SPS assistance crews. The test is particularly useful for operations where buoyancy of plays a significant role (e.g. insulation caps installation).

The depth chosen for the shallow water test is typically such that the Work ROV accesses all working zones of all the SPS equipment underwater without having to surface.

The following are some items to consider when planning the commissioning and start-up of a subsea production system.