What is a subsea tree?
Subsea production systems are a network of equipment used to extract oil and gas from the seabed.
Subsea trees are critical to this process as they are positioned over the well head to control the flow from the well. Subsea trees, similar to their land or topside namesakes are a series of valves which are used to perform various tasks such as testing and regulating the flow of liquids and gases to and from the well.
A number of factors such as geographical location, reservoir studies, pressure and expected flow rates will help determine the most feasible configuration of the subsea production system. This will decide the specification requirements of the subsea trees and how the resource will be channelled to either an on or offshore production or storage facility.
Subsea or Christmas trees; learn more about where this name comes from in our blog have been used by operators globally since the 1950s in various sizes and weights. Depending on the complexity of the valves and controls, bore size and pressure requirement these heavy pieces of equipment can weigh over 70 tonnes. However, manufactures are striving to utilise the latest technologies to reduce this footprint wherever possible. Learn more about BakerHughes lightweight contender in this video.
What are the Different Types of Subsea Trees?
Following a successful drilling campaign, subsea trees are fixed over the wellhead to monitor and control the flow of extracted resource whether that be oil or gas.
Subsea trees tend to be classified in two categories – horizontal and vertical. This relates to the placement or positioning of the series of valve bodies. These valves will be both manual and actuated with hydraulic and pneumatic functions. The quantity and configuration being adapted to suit the field development and demanding challenges of extraction.
They can vary in size, shape and configuration but the typical bore sizes are 5” and 7” with the following key components;
- Tree body
- Tree valves
- Tree piping
- Protection frame
- Subsea control module
- Production Choke
- Tree connector
- ROV Panel
In horizontal trees, the tubing hanger is in the tree body. These are usually smaller in size and manufactures opt to standardise to 5” which can help control the pricing. However, higher pressure subsea trees with 7” or greater bores will cost significantly more. Horizontal trees are designed to save rig time, reduce capital expenditure and increase safety. They eliminate the need to remove the tree to recomplete the well making the horizontal tree a good choice for subsea completions that require frequent replacement.
However they may not be the best solutions for well where inventions are anticipated, this would lend itself more to the vertical option. Vertical trees are often seen as the more conventional subsea tree. Their configuration sees the tubing hanger being suspended in the wellhead. Learn more about vertical tree installation below…
Pressure ratings are 5, 10 and 15KSI and this links to the water depth of the well. 5-10ksi is commonly used in water depths greater than 1000m
Few companies can design and supply 15ksi trees, therefore driving a higher price tag.
Tree type, pressure rating, bore size are the main cost-driving factors
Who manufactures subsea trees?
As explained, there are various components that make up a subsea tree and these are manufactured all over the world. When it comes to the design and assembly of these assets there are 5 main suppliers who bring together all required components and deliver to the end-user – global operators;
- Aker Solutions
Technology is continually evolving to the challenges of subsea extraction in more remote areas of the world. This coupled with the ever-pressing requirement for cost reductions see all subsea tree manufactures seeking standardisation of their equipment. Looking to provide “off-the-shelf” type equipment with the required ad-ons specific to the project demands.
This standardisation, modular drive allows for manufacturing efficiencies, reduced lead times and the all important cost savings to the end user. Typical lead time is ~ 1 year.
How much does a subsea tree cost?
As you might expect, this can vary dramatically depending on the specification;
- Type of tree – vertical or horizontal
- Bore size
- Number of valves / controls
- Pressure rating requirements
Market conditions must also be taken into consideration. Efficiency drives to reduce the overall cost and lead times of this type of equipment are always at the forefront of technology developments. Standardisation by vendors does help gain efficiencies however the potential add ons can see assemblies ranging from 1-6 MUST.
Another added cost is signage. Your subsea tree will need appropriate signage and this can add to the overall cost. At Aquasign, we specialise in Subsea Tree signage, and you can find out more about it and the costs in our subsea signage guide.
What signage is used on subsea trees?
Markings or subsea signage is used on every aspect of a subsea tree for many purposes including;
- Orientation / navigation
- Positioning / operating
The various valves, pipework, inlets etc all need to be identified. The valve markings are usually classified as critical markings and must be produced in a material suitable for long term subsea applications.
There will be certification plates on various pieces of equipment, this will ensure each piece of equipment has been correctly tested and certified for us. The details of which recorded for future reference and traceability.
Installation and hook up is often completed by an ROV. Clear navigation is essential and appropriate subsea markings help the ROV navigate around the equipment. Some guide markings that are there to aid the installation of the equipment are deemed little critical or long term and therefore tend to be painted on or produced on cheaper materials such as vinyl.
It is vital that the status of the valves or controls are clearly visible for obvious safety reasons. Traditional valve open/closed idents are used for this purpose or scale indicates for choke valves (insert photos)
Why is Aquasign used on subsea trees?
Each and every element of a subsea tree is intricately designed to cope with the required pressure, flow rates etc. However, the equipment does still need to be maintained.
Once the installation and hook up is complete various maintenance and repairs are scheduled on the hardware – these items have to function is extremely harsh conditions put under extreme pressure therefore annual programs are often agreed upon.
To help aid such intervention it is vital that the frame, ROV panels, connectors, controls and valves are appropriately identified. This can reduce time subsea and improve the overall safety of the operations.
Aquasign is a unique non-toxic anti-fouling material which has been used on hundreds of subsea projects worldwide. The natural marine growth protection ensures that the markings will remain visible to the divers or ROVs throughout the life span of the equipment.
Typically supplied as individual markers or subsea labels Aquasign clearly identifies specific valve idents as well as their operational positioning. For shallow water projects where marine growth can cause significant impacts to subsea operations and interventions, Aquasign can be supplied as a complete panel. These can be installed over the main ROV panel ensuring completed anti-fouling protection. This removes the need for the expensive subsea cleaning of the area prior to completing any of the necessary maintenance work.
Typical signage on subsea trees
The flexibility of Aquasign’s design means that it can be applied to all types of equipment within the subsea tree.
Whilst there are limited industry standards specific to subsea identification, typical text heights are provided below:
- Orientation (tree name) 100-150mm
- Identification (valve no.s) 50-150mm
- Positioning (open/closed) 15-100mm
Learn about the design features of our subsea markers by visiting our design page.
What fixings are best for subsea tree applications?
The most common fixing specification is our KISS self-adhesive range. This can easily be applied to both flat and tubular substrates.
Bolts can be included for a mechanical fixing, which is useful for the larger ROV panels.
Aquasign markers can also be encased within a Shield™ frame for additional protection in high intervention areas.
Where there is limited space Aquasign can be attached using a tag system.
Options for every application – contact us to discuss your project needs
Can you replace signage on a subsea tree on the sea bed?
Yes! It is obviously best to get these things right the first time round, it is always easier and far more cost effective to complete any task on the surface as opposed to subsea. We would always recommend Aquasign be installed on your hardware prior to be deployed however there are situations where markings may have been missed, incorrectly installed or damaged.
Our solution here is our retrofit range – Aquasign subsea markers are encased within polycarbonate Shields which can then be deployed by divers or ROV and attached to the subsea tree using either subsea magnets or adhesive.
There are limitations on the adhesive application – this is geared towards shallow water operations completed by divers.
Our subsea magnet range offers simple installation by both diver and ROV even at ultra deep water depths.
If you need help with your subsea signage, don’t hesitate to contact us on +44 (0) 1224 897060 or send us a message directly.