SPONSORS OF COATINGS AND CORROSION FEATURE
FOUNDATION CORROSION CONTROL
Most such structures are in relatively deep water, are either multi- leg and braced jackets or sub sea completions. This RP does not cover impressed current systems, it does not address the particular design issues of monopiles, it does not address the particular issues of inshore shallow water with high tidal ranges and high current flows. It DOES give default design parameters which do NOT apply to these particular conditions.
– CONCERNS FOR DEVELOPERS AND OPERATORS
Brian Wyatt, a Past President of the Institute of Corrosion and experienced designer of offshore cathodic protection systems, is critical of past and present corrosion control systems for offshore wind farm foundations.
CURRENT SITUATION Offshore wind farms in Rounds 1 and 2 have predominantly utilised monopile (MP) foundations for turbines and multi-legged jackets for the offshore sub-stations. Most have been provided with galvanic (sacrificial) anodes for cathodic protection of external immersed surfaces.
Aluminium alloy anodes cast onto steel tubular cores are welded to the lowest elevation of transition pieces (TPs) or distributed on jacket legs and braces; if the design is adequate these anodes will corrode in preference to the steel and low voltage dc current will flow from the anodes, through the sea water, to discharge on the steel. IF the magnitude of current received at the steel surface is sufficient it will overcome the natural corrosion current, the potential between the steel and the water will shift negative to meet an established criterion and corrosion will be prevented.
Most of the Round 1 & 2 monopile foundations have been designed with galvanic anode cathodic protection (CP) systems to RP B401, often by structural engineers of considerable competence in their own field, but no expertise in corrosion, applying the default parameters from the RP. Some Round 2 foundations have been designed with impressed current systems, again by engineers with little experience in designing impressed current systems for offshore (oil and gas) structures and sometimes making incredible claims regarding reliability and current distribution.
KEY ISSUES The key issues for wind farm monopile CP designs are...
• The significantly higher current demand in the tidal range and due to high tidal flows, particularly if there are suspended solids in the water
• The attenuation/distribution of current from anodes fitted only to TPs
The result is that CP systems designed to RP B401 default parameters with anodes mounted on the TP are unlikely to provide adequate protection to foundations in more than 15-20m water depth unless the MP is coated with a high performance protective coating. Claims that impressed current systems will provide better protection and distribute more current to greater depth from TP mounted anodes are untrue.
PERFORMANCE ASSESSMENTS Some such systems are being ‘performance assessed’, sometimes by their designers, by measuring the potential of the anodes on the TP. This is little less than fraudulent and does not assess the performance of the CP system; steel/ sea potentials need to be measured over the MP and in particular at the bed.
These surveys need to be undertaken with the measuring reference electrode immediately proximate to the steel MP surface, not dragged away by tidal flow, and with equipment properly calibrated on site and the data for the calibrations recorded and declared. If these surveys are undertaken at slack tide, as they often are with divers or ROVs, the measured performance will be misleadingly better than average as the CP current demand increases in direct proportion to the water flow rate past the steel surface.
RECOMMENDED PRACTICE DNV, the Classification Society for many developments, has for many years presented design guidance (Recommended Practice) for cathodic protection of offshore oil and gas facilities, the current version of which is DNV RP B401: 2010. This is a proven and well established design code intended, and defined as applicable to, ‘permanently installed offshore structures associated with the production of oil and gas.’
The internals of MPs and the TPs, typically below an “air tight” deck, have often been designed to have corrosion protection solely by anticipated depletion of oxygen by initial corrosion. This ignores the likely ingress of oxygen if (as is common) the cable J-tube seals do not fully prevent tidal driven sea water ingress and egress. It also ignores the risks of microbially influenced corrosion (MIC) which is a known threat in anaerobic conditions and established as reality in the offshore oil and gas experience.
72
www.windenergynetwork.co.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108