For years operators have reported accelerated rates of corrosion in wire rope and chain in warmer waters. Mooring design codes specify corrosion endurance of these components predominantly based on experience from the colder waters of the North Sea. The scope of the SCORCH JIP was to investigate and characterize corrosion of steel chain and wire rope moorings for a wide range of operating Floating Production Units (FPUs) and Floating Production Storage and Offtake vessels (FPSOs) in warm waters. A database was compiled of detailed corrosion measurements of in-service and retired mooring chains and wire ropes from about 30 FPUs operating in warm waters off Asia, Africa and the Americas. A set of standardized procedures were developed for forensic examinations of retrieved chain and wire rope, including photogrammetry and laboratory measurements that allowed 3D reconstruction and statistical analysis of corroded surfaces. The database was complemented by destructive tensile tests of a number of samples in order to correlate the observed degradation with residual breaking load. The SCORCH JIP also investigated the impact of sea temperature, water velocity, depth and oxygenation, steel grade and chain and wire rope construction through over 750 sample and full-scale tests spanning 3.5 years at sites around Australia. Additional laboratory tests were conducted into Microbiologically Influenced Corrosion (MIC) and the combined effect of corrosion and wear of mooring chains. The SCORCH JIP has produced a series of findings with long-lasting implications for the industry, including detailed investigations of factors in corrosion such MIC, chain pitting, chain wear, wire rope blocking compound efficacy, and the effect of environmental and operating conditions. The SCORCH JIP has produced tabulated predictions for corrosion in varying temperatures, at positions in the mooring line and for nutrient levels that could encourage MIC. The predictions are underpinned by a large number of field experiments and results from operating FPUs, and are supplemented by detailed operational guidance and recommendations for maximizing mooring corrosion endurance.

The prediction of Vortex-Induced Vibration (VIV) of cylinders under fluid flow conditions depends upon the eddy shedding frequency, conventionally described by the Strouhal Number. The most commonly cited relationship between Strouhal Number and Reynolds Number for circular cylinders was developed by Lienhard [1], whereby the Strouhal Number exhibits a consistent narrow band of about 0.2 (conventional across the sub-critical Re range), with a pronounced hump peaking at about 0.5 within the critical flow regime. The source data underlying this relationship is re-examined, wherein it was found to be predominantly associated with eddy shedding frequency about fixed or stationary cylinders. The pronounced hump appears to be an artefact of the measurement techniques employed by various investigators to detect eddy-shedding frequency in the wake of the cylinder. A variety of contemporary test data for elastically mounted cylinders, with freedom to oscillate under one degree of freedom (i.e. cross flow) and two degrees of freedom (i.e. cross flow and in-line) were evaluated and compared against the conventional Strouhal Number relationship. It is well established for VIV that the eddy shedding frequency will synchronise with the near resonant motions of a dynamically oscillating cylinder, such that the resultant bandwidth of lock-in exhibits a wider range of effective Strouhal Numbers than that reflected in the narrow-banded relationship about a mean of 0.2. However, whilst cylinders oscillating under one degree of freedom exhibit a mean Strouhal Number of 0.2 consistent with fixed/stationary cylinders, cylinders with two degrees of freedom exhibit a much lower mean Strouhal Number of around 0.14–0.15. Data supports the relationship that Strouhal Number does slightly diminish with increasing Reynolds Number. For oscillating cylinders, the bandwidth about the mean Strouhal Number value appears to remain largely consistent. For many practical structures in the marine environment subject to VIV excitation, such as long span, slender risers, mooring lines, pipeline spans, towed array sonar strings, and alike, the long flexible cylinders will respond in two degrees of freedom, where the identified difference in Strouhal Number is a significant aspect to be accounted for in the modelling of its dynamic behaviour.

Whilst mooring chain design practice has traditionally considered wear and corrosion together under a single allowance for material loss, the two phenomena are influenced by different factors whereby they may act synergistically to accelerate degradation rates or act completely independently. As chain wear has been attributed to both the acceleration of corrosion-related degradation and the cause of chain failures and early mooring repairs by itself, it was investigated as part of the SCORCH JIP. An extensive literature review and meta-analysis of wear research was carried out in order to determine a framework for assessing the wear of mooring chains. The literature review found that the phenomenon of wear in general and mooring chain wear in particularly was not well understood, with little agreement as to a general analytical model for wear prediction. The particular wear regimes under which mooring chain wear occurs was identified and a critical analysis of available data on mooring chain wear was carried out where it was determined that experimental conditions were likely to have been outside of the bounds of validity for what would be experienced in the field. A generic formulation for chain wear was presented, where the amount of wear is proportional to the work done at the interlink contact due to the relative sliding of the chain links. Supplementary to the literature review, a series of wear tests of full-scale mooring chain links were carried out under varying load conditions for different grades of chain. These tests were used to determine empirical coefficients for the prediction of chain wear. A model for wear prediction was presented based on the position of the chain in the mooring line, the tension in the line and the relative motion of the chain links. The evolution of the contact surface between chain links was modelled using 3D geometries of idealized chain links in order to determine a relationship between the volume of worn metallic area and the depth of wear, which can be more easily measured in-service.

A variety of contemporary test data for elastically mounted cylinders, with freedom to oscillate under one degree of freedom (i.e. cross flow) and two degrees-of-freedom (i.e. cross flow and in-line) were evaluated and compared against the conventional Strouhal Number relationship. It is well established for VIV that the eddy shedding frequency will synchronise with the near resonant motions of a dynamically oscillating cylinder, such that the resultant bandwidth of lock-in exhibits a wider range of effective Strouhal Numbers than that reflected in the narrow-banded relationship about a mean of 0.2. However, whilst cylinders oscillating under one degree of freedom exhibit a mean Strouhal Number of 0.2 consistent with fixed/stationary cylinders, cylinders with two degrees-of-freedom exhibit a much lower mean Strouhal Number of around 0.14-0.15.

Irregular seabed bathymetry around subsea pipelines can lead to the formation of pipeline free spans. When exposed to on-bottom currents these free spans can be subject to Vortex-Induced Vibration (VIV), with consequential effects on the fatigue life of the pipeline. Traditional VIV suppression technologies such as strakes and fairings present installation challenges and durability concerns due to the significant increase in overall diameter associated with the geometric profiles of strakes and fairings. Longitudinally Grooved Suppression (LGS) technology was developed from a concept stage through to field deployment on active drilling risers (Johnstone et. al., OMAE 2017) [1]. The low profile and VIV suppression abilities of LGS present an opportunity for a more effective and operationally beneficial VIV suppression solution for pipeline free spans. Based on existing Class guidance for assessing pipeline free spans, a simplified framework for assessing free spans with LGS under a response based approach is presented. The simplified assessment implied a suppression efficiency (reduction in vibration amplitude) of up to 80%. An alternative comparative analysis using a force based approach was also performed in SHEAR7 of a bare pipeline and a LGS-wrapped pipeline. The requirements for qualification of new VIV mitigation technologies are also addressed and an example of an actual field installation of the device is presented, on an existing pipeline free span with low seabed clearance.