Hot-tapping Subsea Pipelines

Development of new remotely operated technology extends possibilities for deepwater repair and tie-in.

Anew technique for remote hot-tapping into subsea pipelines has been developed which can be used without the aid of divers. The technique involves the installation of a valve module and hot-tapping equipment remotely operated from a support vessel, and provides a cost-effective option for tie-in of new pipeline systems to existing infrastructure, without turndown of production.

The development project has so far built and qualified the equipment required for hot-tapping into pre-installed hot-tap tees, and ongoing efforts are concentrating on developing techniques for retro-fit hot-tap tees on unprepared pipelines. The equipment is designed to operate in water depths down to 2,000 meters, and forpipeline pressure of 250 bars.

The goal here is to describe the technique, the functionality of the equipment, and the testing and operational characteristics of the system now available for commercial use.

Hot-Tap Technology

hot-tap technology has been used for many years onshore, particularly in process plant applications, to connect branch pipelines into production pipeline systems without halting production. The majority of hot-taps are based on welding the branch pipe to the pipeline, and then tapping by means of hydraulically operated drilling machines.

In the North Sea, so far less than 10 hot-taps have been carried out on subsea pipelines, of which two were on the Statpipe system. These hot-taps have provided very cost-effective solutions, but are based on an approach using divers, and is thus limited to water depths where diving may be applied. Typical layout of a hot-tap connection is shown in Figure 1.

Over the last few years, installed pipelines have been prepared for future connections by pre-installed tee-assemblies, of which some require hot-tapping to allow product flow. Some of these pre-installed tees and future planned tees are at water depths which will require diverless technology. Statoil therefore launched a development project to establish technology and tooling systems for remote hot-tapping, such that the pre-investments done may be utilized within realistic time frames.

The development project is split in two main phases:

• Phase 1 is to develop tooling systems required for remote-controlled hot-tapping into prepared pipelines via pre-installed tees.
•  Phase 2 is to develop technology and equipment for hot-tapping into unprepared pipelines.

The interface between Phases 1 and 2 is defined such that equipment developed in Phase 1 will be used together with the systems developed in Phase 2.

All new tooling systems developed are incorporated as part of the Pipeline Repair System (PRS) operated by Statoil on behalf of a consortium of pipeline operators in the North Sea. The system is being designed to provide repair and construction support for oil and gas pipelines on and from the Norwegian Continental Shelf. The PRS is a comprehensive suit of subsea pipeline construction and repair tools, from isolation plugs and cleaning tools, to large manipulation and installation frames, and welding habitat enclosures.

The repair methods range from applying support clamps to weakened sections, to cutting away damaged sections and replacing them with new pipe. In the latter case, the new pipe is joined to the old by eithermechanical connections or hyperbaric welding. The introduction of remote hot-tapping extends the possibilities both for repair scenarios and for tie-in of new pipeline systems to existing infrastructure.

Subsea Technique

The basic principle of hot-tapping is to establish a new branch pipeline connection to an existing pipeline while the mother pipeline is under full pressure. The method involves connecting the branch pipe, including a valve, to the mother pipeline, usually by means of welding or a mechanical clamp connection, and thereafter cutting a hole in the pipe wall by a cutting machine, which is then connected to the valve, and can undertake fullpipeline pressure. After cutting is finished, the cutting head is retracted, the valve is closed, and the cutting machine disconnected. The pipe branch may now be extended by spools and tie-in to a new pipeline in a normal way. This methodology has been shown to be very cost-effective compared to alternative methods, including shutdown and tie-in at ambient pressure conditions.

So far, such hot-taps have been performed using divers to weld on the branch pipe to the mother pipeline, and all installations and cutting operations have been completed by divers. The cutting machines are basically marinized versions of standard onshore hot-tap cutting machines, and have not been designed for subsea applications, or for being operated by relatively unskilled divers. The method is diver-intensive, requiring diver welders to be trained and qualified in advance for the substantial welding job to be undertaken, as well as being trained on operating the cutting machine itself.1 Although this has been a cost-attractive alternative to date, the new remote system will represent a substantial cost reduction; and, even more important, extend the capability of this method beyond water depths limited by divers (in the North Sea about 200 meters).

The rule of thumb for this type of hot-tap connection is that the branch pipe maybe up to half the diameter of the mother pipeline. This is to avoid extensive re-enforcement by either welding or mechanical clamping systems. For gas systems, this often does not represent any restriction, since a short section of reduced diameter has limited pressure loss effect, and gas velocity is less critical. For liquid systems, capacity limitation could be a problem. For these situations, however, two or more hot-taps, or other fullbore solutions, could be an alternative.

The new remote hot-tap technology uses the same principles as standard systems; however, all operations are remote-operated from a surface support vessel. The subsea equipment is installed by the support vessel crane, assisted by an ROV for guidance and surveillance. The equipment functions are controlled via an electric-hydraulic control system, which is connected by an umbilical to the support vessel. This type of operation and control system is well known and established in many other applications, but is being used for the first time ever for this type of operation.

Remote System

The remote hot-tapping system developed and qualification tested in Phase 1 is already providing equipment for hot-tapping in pre-installed hot-tap tees, which exist in some pipeline systems (Figure 2). The hot-tap tee is a pre-made tee-piece, welded into the pipeline during construction, and laid from the pipelay vessel. The tee-piece will normally have some mechanical connector interface with an installed blind cap, and have a curved or flat membrane inside the tee-branch to provide the pressure barrier (as the mother pipe wall).

These hot-tap tees are often installed in areas where future pipeline connections are expected, but not yet decided upon, and they represent a low-cost preparation for future connections without reducing the pipelineintegrity. A potential challenge related to such hot-tap tees are rotation from vertical position, and as much as 30 degrees have been experienced during pipeline installation.

The remote hot-tap system consists of the following main modules:

• Installation frame, PRS Pipeline Intervention Frame (PIF)
• Isolation valve module
• hot-tap cutting tool
• Flushing tool
• hot-tap spool
• Power and control system.

All new equipment is designed, built and operated in accordance with standard specifications developed for PRS pool equipment, in order to interface the common power and control system and operational philosophy. The result of this philosophy is that it is only the new equipment and its special application that is new and unverified; all common systems and the operational philosophy are field-proven from other applications, and about 20 years of experience.

The features of the new system compared with the older diver-operated systems are primarily:

• Higher specifications in general
• Designed in accordance with latest rules/standards/codes and modern design tools
• Three or more barriers on all dynamic seals
• Online monitoring of all barriers and pressurised zones
• Using a reliable, field-proven control system:
• Software and mechanical/hydraulic inter-locks preventing operator error
• Remote operation, avoiding the need for local intervention by humans
• 2-4 levels of redundancy, depending of functions criticality.
• Accurate system for metrology and alignment:
• Safe mating of and closing of connections
• Safe, efficient and cost saving operations.

The hot-tap system is optimized for branch pipe connections about 12-in. in diameter, as this is regarded as the most likely dimension to be used. This allows the remote tooling systems to be kept fairly limited in size and weight, allowing a greater variety of support vessels to be utilized. Smaller and larger branch pipes could be used, but in the currently built equipment, the cut-hole diameter is limited to 400 mm. The hot-tap cutting machine itself is developed and delivered by ClearWell Subsea Ltd., while the installation systems and power and control systems are developed as part of the PRS portfolio. Several suppliers have been delivering components and sub-systems.

Key design data for the remote hot-tap system is:

• Mother pipe size: 8-in. to 42-in.
• Maximum water depth: 2,000 msw
• Maximum pipeline pressure during tapping operation: 250 bar
• Temperature range: -20°C to + 60°C
• Cut hole: 150 mm to 400 mm
• Maximum height on valve stack: 3,000 mm (injector stroke)
• Maximum feed stroke: 220 mm (cutting)
• Maximum rotation on preinstalled tee: 30 degrees from vertical
• Control system: PRS-IPCON by ISOTEK Electronics Ltd.

Testing and Qualification

Development and qualification of such a new tooling system is regarded as "new technology," and hence follows the Statoil corporate requirements and work processes for qualification of new technology. This qualification process is in principal based on the DNV Recommended Practice for "Qualification Procedures for New Technology." 2 The principal of the approach in this standard is to categorize the new systems and the associated sub-systems by a technology assessment, and for each sub-system, identify failure modes and associated risk ranking.

Based on the qualification basis, the selected qualification methods, relevant qualification program, acceptance criteria and test results, the final reliability assessment results in a documented qualification status. In order to ensure the correctness of this process, as well as introduce third-party verification, DNV was engaged to supervise and verify the entire qualification process from concept selection to qualification testing and trials. As part of this process, the development project achieved phased statements issued by DNV:

• Statement of Feasibility
• Statement of Endorsement
• Statement of Fitness for Service.

The program was divided into the following sub-activities:

• Integration and handling trials
• Un-pressurized cutting trials
• Pressurised cutting trials
• Shallow water testing
• Procedure development and verification
• Operator training.

Remote Installation

Phase 2 of the project is currently ongoing with a planned completion in late 2006. The goal of this project is to establish methods, design and associated tools for remote installation of retrofit hot-tap tees, i.e., allowing tie-ins of new pipelines to existing pipeline systems without pre-installed hot-tap tees. This allows optimized locations for the hottap tie-in, reduces pre-investments in hot-tap tees, and may also be applied as a remediation method in emergency situations involving ice plugs, hydrate plugs or other blockage.

The challenge is to establish a branch pipe connection which has the structural integrity and the required pressure barrier capacity for the lifetime of the system. Two different approaches have been followed:

• Mechanical tee-clamps using elastomer seals for pressure containment
• Remote mechanical tee-clamp with hyperbaric welded seal (Figure 3).

The mechanical clamps using elastomer seals are commercially available on the market, however, so far predominately only in diverinstalled designs. Beside requiring divers, the elastomer seals in these clamps may not have the reliability nor durability needed for deepwater operations. Thus, the project aims to qualify this type of product for applications where divers may be used and where thepipeline system allows shorter lifetime and operational specifications compatible with the elastomer materials.

The main focus is development of a novel design which combines the use of a remotely installed mechanicalclamp providing all structural integrity, as well as interfaces towards the isolation valve module and the hot-tap cutting tool. The design would also include a "seal weld" made by remote-operated hyperbaric welding inside the branch pipe (Figure 4). The intention of the seal weld is not to provide structural capacity as such, but to make a highly reliable seal ("metal-tometal") to at least the same quality standard as traditional welded hot-taps are making today. This new development is a further extension of the PRS technology recently developed for remote-controlled pipeline repair using the welded sleeve technique.3

The welding is based on hyperbaric GMA welding which, in previous research and development programs, has proved feasible down to 2,500 msw. The welded-sleeve application1 and now the hot-tap tee seal weld are all developed for the PRS pool, and are using the same power and control systems as well as supporting facilities for filler wire feed, gas cleaning and drying, pre-heat and other service functions. Installation and operational procedures are almost identical, while the welding module to sit on top of the hot-tap branch pipe is dedicated for that application.

Since this design is novel, no international design codes describes such an application. Hence, the project has undertaken a substantial task again related to qualification of the technology. The basis for the design is DNV Offshore Standard for SubmarinePipelines,5 and where applicable, the DNV Recommended Practice for mechanical couplings' is applied. As described above, DNV will also be contracted to supervise and verify the whole qualification process.

sumber : Berge, J O; Apeland, K E; T I Næss; Ahlen, C H. "Hot-tapping Subsea Pipelines". 26 Januari 2014. http://search.proquest.com/docview/214452043?accountid=31562