J-TUBE SEALS

When a riser is brought up from the sea bed to the surface it is often carried in a tube for protection purposes. This tube is called a J tube. The name "J tube" derives from the curve at the bottom of the tube to guide and accept the riser during pull in.

The inside of the tube is difficult to protect with paint systems so often a seal is put into the bottom of the tube and then the annulus between the pipe and tube is filled with corrosion-inhibited water. It is important to keep the corrosion inhibitor in the annulus so the seal at the bottom performs an important job.

There are two main types of seal on the market at present, they are the clearance fit seal and the interference fit seal. The clearance fit seal typically has a 4-6mm clearance on the J tube inner diameter. Once the riser has been pulled in then the seal is assembled around the pipe (most J tube seals are produced with split lines) and the through bolts are tightened. This activates the seal.

If depth or legislation does not permit divers to be in the water, the interference fit seal can be employed. An exploded view of this type of seal is pictured right, Fig 1. Typically between 2-4mm interference is used to create the seal and powerful back clamps are put in place to stop the seal from moving down the riser as it is pulled in.

It has been demonstrated both on test and offshore that both types of seals can withstand pressures up to 7 Bar depending on annulus size and service conditions. A seal is shown during installation in Fig 2. Typical seal materials are Polyurethane or Polychloroprene and can be supplied in hardness values from 40 Shore A to 60 Shore D.

Dunlaw Engineering Ltd. has been involved in the design and manufacture of J tube seals for a number of years. Both clearance fit and interference fit seals can be provided to suit the needs of clients. Ranging from seals for small diameter cables to large seals for flexible pipe and also caisson seals, a quality, cost effective solution to this type of sealing problem offshore can be provided.

WATERSTOPS

Waterstop seals have been produced by Dunlaw Engineering Ltd for a number of years and for several key projects. Their design has been developed through rigorous testing from which a thorough understanding of their operation has been obtained.

Waterstop seals are used in pipe-in-pipe systems where a need exists to stop flow of water along the annulus between the two pipes. This annulus will usually be filled with an insulating material which must be kept dry to function correctly. The flow in question would occur in a case where the outer "carrier" line is breached such that the annulus transfers from a dry case to a hydrostatic pressure equated to the water depth of the pipe-in-pipe assembly. The required number of waterstops will be spaced accordingly along the pipeline and this spacing will usually be controlled by the design analysis which would determine over what amount of the overall line it would be acceptable to experience a drop in the insulation performance of the pipe-in-pipe assembly.

Both EPDM and Viton have been used for the waterstop material and more recently Silicone Rubber. It has been found that Silicone compound can provide much better performance at elevated temperatures. This can often be an important factor as the operating temperature, due to the temperature of the main production line, can be up to 160°C. This can also have a large affect due to the loads under which it will be subjected. The seal design follows the same principle as J-Tube seals that elastomers can be placed under high levels of deformation providing they are given space into which they can flow and move.

The axial compression for the waterstops is provided by using an interlocking clamp arrangement which will provide the radial expansion of the ring against the pipe walls.

Fig 1: Exploded View of Common J-Tube Seal Components

Fig 2: J-Tube Seal During Installation

Waterstop Seal