A subsea tree is deployed from a compensated winch on a semisubmersible. (Images courtesy of FMC Technologies Inc.)

As operators continue to develop oil and gas fields in deep water, the transit-on-wire (TOW) method of installing subsea equipment, such as trees, offers an alternative to traditional solutions that require mobile offshore drilling units (MODU). The adaptation of well proven technologies and techniques has been successfully applied to projects on a global basis, demonstrating that this approach provides operational, financial, and safety advantages.

The TOW principle can be applied to several subsea system components, but the installation of subsea trees in this manner provides operators with the highest value-added benefits. From an operational, financial, and safety standpoint, the TOW system eliminates the need for running a completion riser from a MODU, which is time-consuming, costly, and labor-intensive.

This technology also removes the requirement for a well control package to be stacked up, tested, and run. Instead, the tree is tested dockside prior to transit offshore. In addition, the TOW principle simplifies the scheduling of field equipment installation since a MODU is no longer part of the project’s critical path.

TOW installation vessel selection

When selecting an anchor-handling vessel or construction vessel for TOW installations, there are several key technical aspects to evaluate. The vessel crane must be suitably rated for lifting and maneuvering the equipment while on deck. The deck layout should also be well planned so that the crane has the necessary range and capacity to position all equipment in the desired location while not exceeding deck loading capacities.

Since TOW operations enable sequential installations of subsea equipment, thorough consideration must be given to the deployment sequence to ensure that all equipment is stored on deck in the appropriate configuration. Equipment tie-down points for sea fastening also need to be carefully selected and appropriately sized to ensure safety.

Typically, equipment can be deployed over the side, over the stern, or through the moonpool of the installation vessel. In all scenarios, the physical layout of the vessel must be evaluated to ensure that no structures will affect deployment.

Heave compensation systems

The first heave compensation landing system (HCLS) was patented by Shell in 1993. HCLS is now commonly used offshore to install not only subsea trees but many other types of subsea equipment such as tubing heads, choke change-out tools, external tree caps, and pressure caps.

There are several types of heave compensation systems available that vary in terms of capability and cost. Selecting the appropriate system requires analysis of the key project parameters, specifically water depth and sea-state conditions of the installation waters. The most common heave compensation systems include HCLS, compensated crane, compensated winch, compensated anchor-handler subsea installation method (CASIM), or the vessel’s modular handling system.

Once the heave compensation system is selected, the rigging attaching the lift wire to the equipment being deployed must be methodically installed and operated to ensure safe deployment and to mitigate potential risks. The placement of slings, sling length, shackle size, and swivel location are factors that need to be considered when planning the TOW installation. It is particularly important to minimize the number of rigging components required and to ensure a thorough analysis of the system loads and dynamics for all planned installations is conducted.

Tooling requirements

To implement a successful TOW installation, additional subsea tools are generally required to assist with managing mechanical interfaces and/or hydraulic interface connection points, e.g. hydraulic flying leads (HFL).

Installing a subsea tree typically requires mechanical tools with hydraulically operated bull-nose stabs for guidance during land-out. There are other tree installation/handling tools that can be used. For instance, some projects have specialty handling tools that provide an eccentric lift capability that compensates for the imbalance of the tree system.

It is necessary to prevent seawater ingress into the system from exposed low-pressure (LP) and high-pressure (HP) couplers to maintain integrity of electro-hydraulic systems on the subsea equipment such as the subsea control module (SCM) on a tree,. Potential issues like this are addressed by installing specialty subsea tooling designed to protect both the mechanical interface and the hydraulic system. A multiquick-connect (MQC) compensation tool is commonly used for such purposes. This is an assembly that is mounted to the subsea tree with an MQC connection prior to deployment. During the TOW installation, this tool provides positive compensation to LP and HP hydraulic circuits via a set of accumulators filled with preservation fluids.

In a traditional MODU deployment of a subsea tree, an integrated workover control system (IWOCS) would be required to vent the fluid from hydraulic lines back to the surface. This venting process prevents hydraulic locking of connection points. When the TOW method is used to install a subsea tree, it is not necessary to use an IWOCS.

Instead, MQC assemblies can be designed and installed to the tree prior to deployment to allow for venting of specific couplers such as connector unlock functions, seal test lines, and chemical injection lines.

Technology enablers

As subsea systems have evolved over the last several decades, there have been technology advancements that have enabled the TOW approach to be applied to increasingly complex systems.

Examples of these advancements include the elimination of the “U-loop” philosophy of tree designs in which control tubing was routed to “loop” through equipment, such as a tree cap, before reaching its final destination at the hydraulic valve or connector. The introduction of the MQC eliminated this indirect routing of control fluid and enabled direct access to tree hydraulic functions, which meant that installations became less complex as there was no longer a need for complex tree installation tools.

In addition to the advances in subsea tree technology, advances in completions technology contributed by implementing a hydraulically activated full-bore isolation valve (FBIV) instead of a tubing plug. Wells could be directly unloaded to a host facility without wireline runs that would normally require a riser system in place.

Although these technologies enable installation of subsea trees using the TOW method, there are some limitations. The inability to perform flowback operations to a rig from a completion riser means that the first time the well is flowed, it will be to the host facility. In a case where the desired results are not achieved, intervention with a MODU would be necessary.

The FBIV could also fail to open when control hydraulics are applied. This also necessitates a MODU or light well intervention package from a vessel.

Field-tested

FMC Technologies has worked with several international operators and installation contractors to facilitate the development of subsea fields with TOW installations. Projects have now been completed in five major oil and gas producing regions: Asia-Pacific, Brazil, Norway, North America, and West Africa.

Industry adoption of TOW installations is gaining momentum, and the potential for achieving significant cost reductions using this technique will help improve the viability of project economics.