A flexible pipe is a composite structure consisting of several layers. One of the layers is the fluid barrier, often known as the internal pressure sheath and in this context named the inner liner. In the end-fitting, all the layers are terminated in a way that allows the forces to be transferred from the flexible pipe to the connecting structure.
Termination of the inner liner is performed by use of an inner liner holder and a seal system. The inner liner holder prevents the inner liner from being pulled out of the end-fitting (due to temperature-induced axial forces).
In order to verify the integrity of the concept, the seal system is tested under cyclic pressure and temperature variations to simulate the service conditions of the flexible pipe. These tests are carried out in a purpose built autoclave. This article presents some results of a thermo-mechanical test of the NKT seal system, and a description of the facilities.
The principle of the seal system is to energize a number of ring joint gaskets to obtain sufficient contact pressure between the gaskets and the surrounding surfaces. By applying torque to the bolts connecting the casing and the sleeve, the casing is moved toward the sleeve.
Due to the geometry of the gaskets, the axial force from the casing is transferred into a contact pressure to the inner liner and the seal seats. The gaskets are sufficiently activated when a pre-defined deformation of the carcass is measured.
Seal loading
The seal system is subjected to different types of loading during the lifetime of the flexible pipe. During activation, the loading is mechanical while the loading during service is a combination of thermo-mechanical and physio-chemical. This article considers the thermo-mechanical loading. Working conditions for a flexible pipe may include significant temperature variations. Dependent on the environment and pipe content, the seal system may face temperatures in a range between approximately -20°C and 130°C.
The seal system consists of many different parameters to be considered in the design phase. These parameters can be categorized in three groups as geometry, material selection, and seal activation. Geometry includes the number of gaskets, gasket dimensions, surface roughness, and other items. For a seal system containing one gasket, there are in the order of 20 geometric parameters to define.
The seal system consists of a range of various materials. The gaskets are made of Teflon (PTFE) reinforced with carbon fibers. The steel surrounding the gaskets is carbon steel. The inner liner is made of a polymer material (high-density polyethylene (HDPE), polyamide (PA11 or PA12), or polyvinylidenefluoride (PVdF).
Activation of the gaskets is essential for the seal performance. It is necessary to apply a sufficient torque on the bolts connecting the casing and the steel sleeve to ensure the contact pressure is high enough. On the other hand, the contact pressure between the gaskets and the inner liner must be below the collapse resistance of the supporting carcass.
Failure mechanisms
There are several possible failure mechanisms of the seal system. The primary failure mode is simply loss of contact pressure between the gaskets and the inner liner and/or the gaskets and the steel casing.
Because of the threaded carcass, an external pressure may cause the carcass to expand axially, which will decrease the diameter and thus reduce the contact pressure. Therefore, it is important to fix the carcass axially (by inserting a carcass holder).
Thermal load changes the geometry of the seal system - the outer diameter of the inner liner decreases with temperature falls. If the gaskets are activated at a temperature above the service temperature, the contact pressure between the gaskets and the inner liner will decrease in service. If the content in a flexible pipe is under high pressure, it will improve the effectiveness of the seal and may compensate for the loss of contact pressure. But at low pressures, this effect will not exist. The conclusion is that the seal system is most sensitive to low pressures at low temperatures.
Moreover, focus is also put on the problem of shrinkage of the polymeric inner liner due to de-plasticization or swelling of some polymer materials and possible corrosion of the steels in the system.
Test procedure
An autoclave is built with the aim of testing the seal system of a typical flexible pipe used for oil transportation. The seal system of a flexible pipe test section can be tested in the autoclave. The gaskets are mounted on a pipe section containing a carcass with an extruded inner liner. The gaskets are activated similar to a full scale end-fitting, and the autoclave is filled with hydraulic oil.
The pressure is raised by use of a high pressure pump and decreased by letting out controlled volumes of oil from the autoclave. The temperature is regulated by circulating hot or cold oil through the helical coil.
The autoclave is able to test the seal system in a combination of temperature and pressure variation between 5°C and 130°C, and 0 MPa and 51.7 MPa. These limits are chosen with a 34.5 Mpa oil pipe with inner liner of PVDF in mind. The rates of both pressure and temperature can be controlled.
Before initiating the test, the seal system is mounted and activated. The carcass is axially fixed at both ends to prevent any movements. To reflect the service profile of a flexible pipe used for oil transportation, the test program contains a combination of pressure and temperature loading.
The seal system must be able to resist hydrostatic pressure test as described in API Spec. 17J. Therefore, the autoclave is pressurized to 1.5 times design pressure and kept constant for 24 hours. After the hydrostatic pressure test, the seal system is loaded under a combination of varying pressure and temperature. The number of load cycles, the rates, and the maximum and minimum values depend on the operational profile and can be varied from test to test.
Test of the seals
A thermo-mechanical test of the seal system used in the NKT end-fitting was conducted for qualification of a 34.5 MPa flexible pipe. Typical values for the rates of pressure and temperature are as follows
According to API Spec. 17J, the pressure loss during FAT must not exceed 4% of the pressure at the start of the test (2.06 MPa for a 34.5 MPa flexible pipe tested at 51.7 MPa).
An accompanying figure shows a pressure loss of approximately 1 MPa, which is less than 4%. The FAT test is therefore acceptable according to API Spec. 17J. After FAT the pressure is cycled between 1 MPa and 38 MPa (1.1 times the design pressure).
The pressure rates in this particular test were 18 MPa/hour during pressure rise and 200 MPa/hour during pressure decrease. The temperature cycles between 2°C and 93°C. The pressure is kept constant at a level of 8 MPa.
Using a purpose-built autoclave, the seal system of a 6-in. ID by 34.5 Mpa flexible qualification pipe has been tested. The seal system was subjected to a combination of pressure and temperature loads in ranges of 1.0-51.7 MPa and 2-90°C. The seal system showed excellent resistance to the variations in pressure and temperature. No leakage occurred during the test. The overall conclusion is that the seal system used in the NKT end-fitting is robust.
