100 Tonne Tension Cylinders for Offshore Client

Apex Hydraulics was approached by a large offshore drilling corporation to produce two 100-tonne tension cylinders for their equipment.

Deep sea risers

The fabrication was designed for deep sea drilling rigs, which use conductor pipes (also known as risers) to create a column between the rig and the seabed. These columns are cemented into the sea floor at the base and can be 100 metres long or more.

They cannot be allowed to stand up independently, because when waves pass, they would sway, breaking the cement and the base of the pipes.

The challenge

Hydraulic cylinders are required to provide tension on the riser, making it rigid and therefore resistant to buckling under its own weight or breaking with the lateral deflection of the wind and waves.  Apex were commissioned to manufacture these upward tension cylinders; something they have designed and manufactured in various styles for many years.

The cylinders were designed with a forked base clevis to suit the 100 tonne lifting eye on the underside of the rig, and a rod end clevis to suit a 100 tonne shackle. The base is attached to the rig structure, while the cylinder rod connects to the shackle and chain assembly joining the rig and cylinder to a positive grip tension ring on the riser. These cylinders provide an upward tension that can be adjusted according to the height and weight of the risers and can be monitored to provide a constant and stable upward tension.

Apex Hydraulics has produced 12 of these 100 tonne tension cylinders for the company before, across a series of three projects. Other similar projects have required fifty-tonne versions, with four of these working in unison to provide the same 200-tonne tension force. On this occasion, the space that usually holds the four cylinders had space for only two, meaning that the specifications all had to be scaled up to allow the cylinders to each provide double the amount of tension, at 100 tonnes each.

Apex quality

The client designed the Hydraulic Power Unit (HPU) which acts in both an active and passive way. The HPU strokes and tensions the cylinder to the correct position and then isolates the system into a passive closed loop. The HPU contains a number of bladder accumulators that act as a spring; allowing the cylinders to respond to any relative movements between the riser and the rig as well as the wind and waves. The cylinders are connected to the HPU with 3/4 inch bore male and female quick release coupling hoses. Once connected, this creates a passive heave system which helps to keep the tension on the conductor pipe.

The cylinders were manufactured by Apex Hydraulics using chrome plated stainless rods and 3 coats of offshore paint in order to provide the required durability for this equipment.

Seals were selected to suit the application and environment. The salt in the air and water offshore results in an extremely corrosive environment for any metal products, so resistant materials such as stainless steel, chrome plating and offshore paint for the cylinders and the large forked base clevis were all particularly vital.

To make the system suitable for the offshore environment, the client uses an environmentally friendly biodegradable water glycol subsea control fluid for the hydraulic cylinder. The cylinders were provided with full certification evidencing their conformity to guidelines governing the production of offshore cylinders.

The project was completed within an eight-week turnaround. The company are extremely pleased with the quality and service provided by Apex Hydraulics for this fourth job within 13 months.

Apex have manufactured 25, 50, 75 and 100 tonne tension cylinders as well as 100 and 150 tonne push cylinders all for the riser tension and conductor rig industry. For more information on BOP cylinders, riser tensioner or conductor cylinders or any other bespoke cylinder repair or manufacture, call Apex Hydraulics today on 01924 456788.