An approximate method based on simple free body diagrams and simple friction theory is demonstrated to relate mechanical advantage and efficiency in a marine hydraulic connector to coefficient of sliding friction and connector taper angles. An equation relating these variables is derived for one of three principal connectors now marketed. The method can readily be applied to the other two connectors, not necessarily with the same result. The equation derived is plotted and shows that rate of change of efficiency for the connector analyzed is highest at friction coefficients near zero. Changes of a few hundredths in friction coefficient in this range may alter connector clamping force by a factor of two or three. The same connector is instrumented with strain-gaged studs to experimentally determine how surface finish, lubricants and coatings affect connector mechanical advantage and efficiency. The initial experimental results indicate friction coefficients in the range predicted and suggest the importance of surface coatings and extreme pressure lubricants.
One of the most complex and critical devices used in the drilling and production of a subsea well is the marine hydraulic connector. The connector is the only method for remotely attaching a blowout preventer stack or production unit to an underwater wellhead. Hydraulic connectors also attach production risers to subsea production manifolds and anchor subsea manifolds to permanent bottom installations. In most applications the connector energizes a metal ring to seal cavity pressure. Figure 1 shows connectors above and below a blowout preventer stack in a typical drilling configuration.
Cross sections of marine hydraulic connectors marketed by three principal manufacturers are shown in Figures 2, 3 and 4. These pictures are reproduced from advertising materials. Although there are important differences, the designs are similar in that all three utilize sliding inclined planes to generate clamping force. Hydraulic force drives a sleeve (a) in Figure 2, having a gradual I.D. taper, downward to drive segments (b) or collets radially inward. A taper (or in the case of Figure 4, two tapers) on the segments or collets engages matching tapers on the hub O.D. The hub faces are squeezed together first deforming a metal ring to seal cavity pressure, and then generating a clamping force between the hub faces. The gradual I.D. taper on the sleeve will be referred to below as the a taper. The segment taper will be referred to as the ß taper. All three of the designs involve the sliding of steel surfaces under load; so it is reasonable to anticipate that the coefficient of sliding friction will playa role in the capability of these connectors to generate clamping force from hydraulic force.
The author intends to use the connector in Figure 2 to demonstrate a simple method for approximately relating connector mechanical advantage and efficiency with sliding friction coefficient and the angles ? and ß. Later on, test results are introduced to verify the approximate analysis.