Fault monitoring and fault recovery control for position-moored vessels

Abstract

This paper addresses fault-tolerant control for position mooring of a shuttle or floating production storage and offloading vessels. A complete framework for fault diagnosis is presented. A loss of a sub-sea mooring line buoyancy element and line breakage are given particular attention, since such failures might cause high-risk abortion of an oil-loading operation. With significant drift forces from waves, non-Gaussian elements dominate forces and the residuals designed for fault diagnosis. Hypothesis testing is designed using dedicated change detection for the type of distribution encountered. A new position recovery algorithm is proposed as a means of fault accommodation in order to keep the mooring system in a safe state, despite faults. The position control is shown to be capable of accommodating serious failures and preventing breakage of a mooring line, or a loss of a buoyancy element, from causing subsequent failures. Properties of the detection and fault-tolerant control algorithms are demonstrated by high fidelity simulations.

Mooring system diagnosis and structural reliability control for position moored vessels

“…CUSUM based change detection is efficient in detecting a change of known magnitude and the CUSUM approach was employed in [18] on the position mooring application, and it was shown to give very convincing results for the known magnitude events, which are: loss of a buoy, or a line breakage. This paper uses the vector based GLR test, as the GLR has the ability to detect a change of unknown magnitude.…”

“…This section will summarize the properties of the fault-tolerant set-point-chasing control algorithm, first introduced in [18] and its properties will be investigated and documented from model tests where faults were imposed.…”

“…Around a working point, mooring-line tension T i can be expressed as a function of the distance, ∆r, and direction, β, to a mooring system neutral point where all tensions are equal in calm weather, see [4] and [18]:…”

“…This fault-handling control was based on thruster usage with the reference position maintained, also when accommodating the effects of line failure. An optimal set-point chasing algorithm was proposed in [18], where it was suggested to move the reference position to an optimal position where a tension-based cost function achieves a minimum value. This paper investigates diagnosis of several mooring system failures, including loss of underwater buoyancy elements, and the paper shows how diagnosis is combined with reliability-based faulttolerant control.…”

“…The structural reliability index was further proposed into a set-point chasing algorithm in [11] to reduce the risk of mooring line breakage in an extreme environmental situation and fault-tolerance aspects were also considered. This paper aims to combine the structural reliability index into the fault-accommodation strategy suggested in [18]. In the case of mooring line breakage, the fault-tolerant control helps to prevent the structural reliability index of the mooring line from a critical index.…”