The work reported in this paper details the design of a facility for the water permeability testing of subsea drill-string riser buoyancy modules. The facility has been built and commissioned and is unique in the UK The only other test facility of this sort exists at the South Western Research Institute at San Antonio in Texas, USA. Buoyancy modules are used to reduce the effective weight of a subsea drill-string riser so as to avoid buckling and vibration problems in service. This weight reduction increases the natural frequencies of the riser well above typical frequencies of excitation as a result of drilling. The geometry of the modules depends on the particular riser design but generally approximates to a long, semi-circular section, sleeve which is clamped round the length of the riser. Many modules are used in service to surround the whole length of the riser from the sea bed to the surface. Individual module lengths vary but tend to be around the 5 m mark. The most important module performance characteristics are the effective buoyancy force in water and the ability of the module to resist water ingress at the high hydrostatic pressures encountered in deep water. These pressures can reach up to 345 bar in the deepest water, and are therefore potentially capable of forcing water into the material of the module. The concept behind the testing facility is a computerized data logging system that detects and computes minute weight changes in sample sections of the buoyancy riser module due to the ingress of sea water at very high hydrostatic pressures. The system simulates the hydrostatic conditions found in deep water, and provides a specific printout of pressure and buoyancy quantities over test periods of up to 100 hours; the format of the printout is to the standard generally required by industry. The work was carried out in recognition of the need for a UK facility which would enable manufacturers of the module material, and also customers purchasing the modules, to have confidence that the required buoyancy performance of the modules can be maintained under typical subsea conditions—without necessarily incurring the very high costs of testing in the USA.