Keeping offshore vessels safely out of dry dock

Size: px

Start display at page:

Download "Keeping offshore vessels safely out of dry dock"

Noel Armstrong
5 years ago
Views:

OFFSHORE & MARINE TECHNOLOGY CORROSION PROTECTION & SURFACE TECHNOLOGY Underwater inspection of an offshore structure Keeping offshore vessels safely out of dry dock HULL PROTECTION Offshore vessels

1 OFFSHORE & MARINE TECHNOLOGY CORROSION PROTECTION & SURFACE TECHNOLOGY Underwater inspection of an offshore structure Keeping offshore vessels safely out of dry dock HULL PROTECTION Offshore vessels have special requirements for hull protection. Unlike traditional merchant vessels, they are not expected to dry-dock regularly. David Phillips, editor of the Journal of Ship Hull Performance, describes the options that operators and owners have for long-term protection of stationary offshore structures. Offshore oil and gas exploration and production vessels such as drillships, floating storage and offloading units (FSOs) and floating production, storage and offloading vessels (FPSOs) differ from bulk carriers, container ships, ferries and passenger vessels in that they are not expected to dry-dock frequently or routinely. Whereas very large crude carriers (VLCCs), container ships, cruise ships or naval vessels are expected to dry-dock at least once in a five-year period, an offshore vessel is designed and expected to stay on station without dry-docking for 20 years or more. Another major difference is that offshore vessels tend to be stationary. A submersible or semi-submersible oil rig sits on the ocean bed or is tethered to one spot for extended periods. A drillship, an FSO or an FPSO is likewise expected to be stationary or move very little for years or decades. Dry-docking such vessels is very time-consuming and expensive. Their daily earning capacity is very high and each day out of service can be considered a huge loss. The pro cedure for getting them into dry dock, serviced and out of dry dock can be very complicated, especially if thrusters have to be removed ahead of time, tanks have to be emptied and other preparatory actions taken. The dry-dock fees for an extended stay can mount up alarmingly. Additionally, there are other major factors that have to be considered when prematurely removing FPSOs from their location. Pulling an FPSO off an active well for drydocking could result in the vessel returning to a dry well. While there is great concern about propulsive fuel consumption on most ships, this is not true of offshore vessels since they are usually on station. A rough hull due to paint degradation and/or biofouling is not a great issue for a drillship. The weight of the biofouling can be more of a problem. But the integrity of the hull coating and whether or not fouling species can penetrate the coating with resulting corrosion are of grave concern. Hull protection, fouling control and underwater repair and maintenance of offshore vessels are becoming increasingly important as oil and gas exploration moves into deeper waters and dry-docking such vessels becomes less and less of an option. Corrosion inside and out Whereas a ship that is drydocked every three or five years can have its anticorrosive coating system inspected and repaired or replaced regularly, an offshore vessel must go for much longer periods without such work. The extremely corrosive nature of salt water and corrosive effects of various living organisms impinge mercilessly on the steel of the outside hull. The highly corrosive nature of the cargoes of such vessels can take a particularly severe toll on their tanks and the inside structure of the ship Thus the steel is under continual and forceful attack, inside and out, which can rapidly weaken and threaten the structural integrity of a vessel if steps are not taken to make sure that it is properly protected against such corrosion. So while corrosion protection is a vital issue for all ships and any metal structures immersed in or in contact with seawater, the case of offshore vessels and rigs is a particularly acute one. And the current requirements of the offshore oil and gas exploration and production industry in the 44 Ship & Offshore 2013 N o 2

second decade of the 21st century are particularly demanding. These requirements extend, to some degree, to the tankers and VLCCs that carry highly corrosive petroleum products.

Corrosion protection is just as important inside the ship as on the outside hull, the splash zone which is the area just above the waterline where the highest corrosive forces accumulate and the

A corrosion-prone industry The sea is one of the most corrosive environments on Earth. The chemicals stored or transported in the tanks of offshore vessels include some of the most corrosive of all.

There are a number of different causes of corrosion in the metals used in the hulls and tanks of offshore vessels.

2 second decade of the 21st century are particularly demanding. These requirements extend, to some degree, to the tankers and VLCCs that carry highly corrosive petroleum products. The tanks of all these vessels, particularly with the now required double hulls, are especially prone to severe corrosion. Corrosion protection is just as important inside the ship as on the outside hull, the splash zone which is the area just above the waterline where the highest corrosive forces accumulate and the superstructure of vessels and rigs subjected to the onslaught of salt spray, condensation, extreme heat and other factors. A corrosion-prone industry The sea is one of the most corrosive environments on Earth. The chemicals stored or transported in the tanks of offshore vessels include some of the most corrosive of all. The combination means that the tanks and hulls of many offshore vessels are subjected to highly corrosive elements, both inside and out. There are a number of different causes of corrosion in the metals used in the hulls and tanks of offshore vessels. The main methods currently employed to control this corrosion are: Design of hulls and tanks to reduce corrosion, Protective coatings (this divides into organic, metallic and plastic coatings), Cathodic protection systems (either sacrificial anodes or impressed current), The use of corrosion-resistant alloys, The use of corrosion inhibitors. Mild steel is the most common material used for hulls and tanks. Protective coatings along with cathodic protection are the methods most commonly in use today to combat corrosion. The biofouling factor Even though propulsive fuel efficiency may not be a major consideration when it comes to hull coatings on offshore vessels, biofouling itself is a serious problem. Fouling organisms such as barnacles, coral and other hard fouling are incredibly tough. They stick to hulls forcefully and aggressively. They are quite capable of penetrating the softer coatings such as biocidal antifouling or silicone or fluoropolymer foul-release coating systems. On stationary vessels they can add a great deal of weight to the hull, thus reducing the loading capacity of rigs or ships. This biofouling also forms a thick covering on the hull, making a visual class inspection impossible without removing the fouling first. All of these factors add up to the requirement to keep offshore vessels relatively free of biofouling, but for different reasons than those normally given for fouling control on ships. Non-indigenous species (NIS), for example, are not really a problem for stationary vessels since they cannot be accused of carrying fouling organisms from one environmental zone to another they stay in one place. The fouling they acquire while on station is local, and if it is cleaned off locally then it is simply a matter of returning the organisms to their own environment. Breakdown The most usual sequence of breakdown in corrosion protection with resulting serious loss of steel, structural fatigue and failure is: The coating starts to break down for one or more of a number of reasons, This then exposes the underlying steel to corrosion, The cathodic protection system is inadequate to prevent the resulting corrosion, The corrosion sources combine and compound to accelerate the corrosion process so that in some cases the corrosion proceeds amazingly rapidly. This sequence begins in most cases with a failure of the protective coating. Conventional anticorrosive coatings such as those used in biocidal antifouling and nonstick foul-release systems With nearly a century s marine paint and coating experience, Hempel can always provide the protection you need to keep business moving. We believe in working closely with you to understand your speci c challenges. This enables us to supply a solution that not only keeps your vessel in prime condition, but that also makes sound nancial sense. Take the 60-second test and discover the product that best suits your needs at antifouling.hempel.com Proven performance. Dependable value.

OFFSHORE & MARINE TECHNOLOGY CORROSION PROTECTION & SURFACE TECHNOLOGY may provide adequate corrosion protection for vessels that dry-dock every two to five years where fairly frequent repair or

The coatings are penetrated by fouling organisms so that the integrity of the coating is lost and the corrosion can set in on a large scale.

Breaking the cycle Corrosion has been a known factor in shipping since metal was first used for the hulls of ships. Research into its prevention began as soon as corrosion was detected as a threat.

A coating that does not fail is the simplest and most far-reaching answer to significantly reducing, if not entirely eliminating, the threat of corrosion on the hulls and in the tanks of offshore

3 OFFSHORE & MARINE TECHNOLOGY CORROSION PROTECTION & SURFACE TECHNOLOGY may provide adequate corrosion protection for vessels that dry-dock every two to five years where fairly frequent repair or replacement of these coatings is possible. However, they are not adequate when the repair/ replacement times are extended. The coatings are penetrated by fouling organisms so that the integrity of the coating is lost and the corrosion can set in on a large scale. Underwater cleaning damages the soft coatings, again destroying the integrity and opening the door to corrosion. Being soft, the coatings are prone to mechanical damage. Breaking the cycle Corrosion has been a known factor in shipping since metal was first used for the hulls of ships. Research into its prevention began as soon as corrosion was detected as a threat. And yet, despite some progress, the problem is still very much alive. The key to solving the corrosion problem on ships hulls and inside ships tanks lies in the choice of the coating. A coating that does not fail is the simplest and most far-reaching answer to significantly reducing, if not entirely eliminating, the threat of corrosion on the hulls and in the tanks of offshore vessels. What is needed for offshore vessels is an extremely tough, durable coating that will last the 20-plus years required without the need to dry-dock for repair or replacement. Additionally, the coating must permit frequent cleaning without the coating itself suffering damage or creating an environmental hazard. The most suitable coating available consists of fairly large glass flakes in a very tough, durable, resilient base. This coating is inert and does not leach toxic substances, biocides or oils. It has a very high bonding capability but is not brittle, so it can flex with the steel. Conventional biocidal antifouling paint or non-stick foulrelease coatings are not suitable for offshore vessels that must stay on site for ten or 20 years or even more without being dry-docked. These coatings are either designed to leach toxic substances so that the coating is depleted, or they require a rapid flow of water to wash off fouling organisms attempting to attach themselves. And in both cases, fouling organisms penetrate the coating system down to the steel, which opens the door to the cycle described above in the section Breakdown. With the right hard, inert coating, the rig, drillship or FPSO can be cleaned as needed using divers and industrial-grade cleaning equipment, removing the fouling so as to make class inspection of the plates and seams possible and lighten the load from fouling. But this type of coating will not be penetrated by the heaviest of fouling and the protection of the steel will remain intact for the life of the vessel. On-site maintenance and repair There is another factor in maintaining an offshore vessel in top working condition without having to resort to the dry dock, namely underwater repair and maintenance carried out by divers and technicians at the location of the vessel or rig. Typical repairs and maintenance consist of one or more of the following: Underwater inspections, Underwater cleaning, Cathodic protection (e.g. replacement of sacrificial anodes), Underwater welding, Structural repairs, Thruster removal, repair and replacement, Seal repairs, Propeller straightening or cropping, Propeller cleaning or polishing. This is not a complete list but gives an idea of the type of repair that may be required. Linear polishing... solved. Get the full story at

Some of these operations are quite complex and require specially trained and certified divers/technicians.

cofferdam repairs in the mid-1990s. Hard cofferdams with one atmosphere were first used for thruster repairs in the mid-1990s. Flexible hyperbaric cofferdams were introduced in the early 2000s.

Offshore vessels have special requirements for hull coating and maintenance The same company developed stern tube seal repairs using a special habitat around 2003/2004 and performed the first dry

All of these developments in underwater repair and maintenance technology (and research and development continues) make it possible to carry out operations on site that previously would have

Conclusion What all this adds up to is that it is perfectly feasible, using today s available technology, to maintain an offshore vessel on station, in good repair, corrosion-free and with

The necessary steps are: Correct choice of hull coating (a hard, non-toxic, longlasting cleanable coating), Standard application of the coating, preferably during construction but also at

4 Some of these operations are quite complex and require specially trained and certified divers/technicians. A series of technical developments introduced over the years by the international underwater technology company Hydrex have made it possible to carry out certain operations that could previously only be done in dry dock. For example, the 1980s introduction of cofferdams for hull repair, making it possible for divers to work underwater in dry-dock-like conditions, and later class approval for permanent underwater cofferdam repairs in the mid-1990s. Hard cofferdams with one atmosphere were first used for thruster repairs in the mid-1990s. Flexible hyperbaric cofferdams were introduced in the early 2000s. Propeller cropping machines came into use in the early 1990s, but a cold straightening machine for straightening bent propeller blades with the ship on site came into use around Offshore vessels have special requirements for hull coating and maintenance The same company developed stern tube seal repairs using a special habitat around 2003/2004 and performed the first dry bonding in a habitat in mid All of these developments in underwater repair and maintenance technology (and research and development continues) make it possible to carry out operations on site that previously would have required the burdensome, time-consuming and expensive dry-docking of the vessel. Conclusion What all this adds up to is that it is perfectly feasible, using today s available technology, to maintain an offshore vessel on station, in good repair, corrosion-free and with biofouling kept to a minimum for 20 or more years, which this industry sector requires. The necessary steps are: Correct choice of hull coating (a hard, non-toxic, longlasting cleanable coating), Standard application of the coating, preferably during construction but also at drydocking, Routine removal of fouling by divers with industrial-grade equipment, Regular inspection of the underwater hull and equipment, Any needed repair/maintenance carried out by qualified, competent, well-equipped divers/technicians. When one compares today s possibilities with those of just a couple of decades ago, the advances have been quite spectacular and just in time as onshore or easily accessed offshore oil and gas reserves are depleted and exploration and production move into deeper and deeper waters. A brand new biocidal antifouling featuring patented technology that delivers predictable long term performance. Reduced drag. Reduced fuel. Reduced emissions *. New This is no evolution... this is revolution. Use antifouling paints safely. Always read the label and product information before use. *compared to typical controlled depletion polymer (CDP).

OFFSHORE & MARINE TECHNOLOGY INNOVATIVE VESSELS Tough tasks prompt complex supply ship designs OFFSHORE SHIPPING Hardly any ship type exhibits more novel design approaches than offshore supply

But more often, serial designs are being modified for special purposes, writes shipbuilding expert Ralf Witthohn in the current issue of our series Innovative Vessels.

SHIP 7+ ICE CLASS IA SUPER ALM (SS) STRENGTHBOTTOM INWATERSURVEY Royal Niestern Sander Shipyard in Delfzijl, the Netherlands, has completed two OSVs for the Dutch group Royal Wagenborg meant to serve

5 OFFSHORE & MARINE TECHNOLOGY INNOVATIVE VESSELS Tough tasks prompt complex supply ship designs OFFSHORE SHIPPING Hardly any ship type exhibits more novel design approaches than offshore supply vessels (OSVs). Among the 250 currently on order worldwide, there are individual examples of sophisticated layouts. But more often, serial designs are being modified for special purposes, writes shipbuilding expert Ralf Witthohn in the current issue of our series Innovative Vessels. This catamaran design from Poland is also intended to work in shallow waters Photo: GSMDG TECHNICAL DATA Shallow-draught supply vessel Sanaborg Tonnage 1,520gt Deadweight 867t Length 68.2m Breadth 14m Draught 3.15m Propulsion Diesel-electric propulsion, two azimuth thrusters 2x1,750 kw Bollard pull 36t Classification Bureau Veritas I Hull Mach Tug Supply vessel Special service/icebreaker AUT-UMS (SS) CLEAN- SHIP 7+ ICE CLASS IA SUPER ALM (SS) STRENGTHBOTTOM INWATERSURVEY Royal Niestern Sander Shipyard in Delfzijl, the Netherlands, has completed two OSVs for the Dutch group Royal Wagenborg meant to serve year-round in the Caspian Sea. The Sanaborg and Serkeborg have been designed to the Bureau Veritas ice class 1A Super. The 68.2m-long and 14m-wide newbuildings have also been laid out as shallowwater vessels, with a maximum draught of 3.15m and an operational draught of 2.5m. Their design was modified further due to the conditions of their 3,600km transit via the inner- Russian canal system from St Petersburg to Astrakhan, which includes 14 locks, 36 bridges and eleven high-voltage lines restricting the maximum air draught to 17.65m. Compendium Marine Engineering Operation-Monitoring-Maintenance According to the German edition this book represents a compilation of marine engineering experience. It is based on the research of scientists and the reports of many field engineers all over the world. This book is mainly directed towards practising marine engineers, principally within the marine industry, towards ship operators, superintendents and surveyors but also towards those in training and research institutes as well as designers and consultants. 98,- (plus postage) Find out more about this compendium and order your copy at 48 Ship & Offshore 2013 N o 2

ECOLOCK LIFETIME CORROSION PROTECTION FOR OFFSHORE UNITS

ECOLOCK LIFETIME CORROSION PROTECTION FOR OFFSHORE UNITS The offshore vessel challenge Keeping a drillship, FPSO, FLNG or other offshore unit on location, in production and out of drydock for 20 or even