Offshore

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OFFS H O R E W I N D A S K E D G E R A R D K R O E S E , COMPANY DIRECTOR OF SPECIALIST MARINE ENGINEERING CON T R A C T O R B L U E O F F S H O R E T O P R OV I DE US WITH A GUIDE TO THE TECHNICAL ASPECTS OF THE DIFF E R E N T S TA G E S I N T H E S U B S E A C A B L E INSTALLATION PROCESS.THE INSTALLATION OF SUBSEA CAB L E S H A S D E V E L O P E D S U B S TA N T I A L LY IN MANY RESPECTS DURING THE PAST 20 YEARS. STIL L , T H E I N S TA L L AT I O N O F S U B M A R I N E CABLES IS FAR FROM SIMPLE AND REQUIRES METICULOUS PLAN N I N G . T H E P R O J E C T T E A M H A S T O TAKE INTO ACCOUNT THE PROPERTIES OF THE CABLES, THE C H A R A C T E RI S T I C S O F T H E C A B L E R O UTE AND THE ABILITIES OF THE CABLE INSTALLATION EQU I P M E N T. W I T H R E S P E C T T O T H E C A B L E INSTALLATION EQUIPMENT, THIS REQUIRES CAREFUL SELE C T I O N A N D I N T E G R AT I O N A S S E M B LY OF BARGES, VESSELS, CREWS, AND AUXILIARY EQUIPMENT. THE D E V E L O P M E N T O F S U I TA B L E D E T E R M INED INSTALLATION MANAGEMENT DURING THE PROJECT PLAN N I N G I S H E R E B Y E S S E N T I A L . W E W I L L DISCUSS IN THIS ARTICLE THE MAIN PIECES OF INSTA L L AT I O N E Q U I P M E N T T O G E T H E R W I TH THE EVENTS DURING CABLE INSTALLATION

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SUBSEA CABLE INSTALLATION A TECHNICAL GUIDE Installation Equipment CABLE LAYING PLATFORMS

Cable Installation System Overview, courtesy of Blue Offshore/Caley Ocean Systems

others. Bollard pull is important when a cable plough is to be used for cable burial. There are few fully equipped and highly specialised cable lay vessels for large power cables, such as Nexans’ Skagerrak and Prysmian’s Julio Verne. There are many other vessel types that can be temporarily converted for cable laying purposes.

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The main piece of equipment is the cable laying platform with its installation equipment. Cable lay barges and vessels are available at all sizes and with all kind of equipment. Main factors for selection of a cable laying platform are load carrying capability, maneuverability properties, deck space for handling equipment, accommodation amongst

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Nexans’ Skagerrak and Prysmian’s Julio Verne, courtesy of marinetraffic.com and from public domain/Internet

POSITION KEEPING The classical method to keep the cable laying platform in position is through the use of anchor systems, whereby the barge is maneuvered between the holds of numerous anchors fanning out in all directions. The anchors are handled by independent anchor handling tugs (AHT) together with positioning software. The barge controls its position, speed, and heading by operating the anchor winches. Position keeping by anchor systems is a wellproven method. However, nowadays, various vessel use a computer controlled system called dynamic positioning (DP) to automatically maintain position and heading by using her own propulsion. Position reference sensors, combined with wind sensors, motion sensors and Gyrocompasses, provided information to the computer pertaining to the vessel´s position and the magnitude and direction of environmental forces affecting its position. DP has significant speed advantages over anchor systems, but the system does affect a vessel´s capability to work in very shallow water, such as near onshore landing points.

LOAD CARRYING CAPABILITY The cables are typically held in turntables (or carousels), which have a vertical axis around which the cable holding section rotates. Most turntables are being loaded in horizontal layers starting from the bottom layer. Turntables can be divided into an inner and outer partition in which two cables can be loaded independently. The dimensions of the turntable will be largely dependent on the cable properties and cable length, while the dimensions of the turntable will have an impact on the choice of barge and vessel. The export/shore connection cable is typically both the longest and heaviest cable to be installed in an offshore wind farm development. They can have lengths greater than 100km with an overall weight of 7,000t or more in weight. The infield cables are typically both shorter and lighter than the export cable.

Turntable and Carousel, courtesy of Caley Ocean Systems

CABLE TENSIONERS These linear machines are necessary for applying tension during laying. Linear machines are available as pairs of wheels, which can be opened and closed with controlled pressure to grip the cable, whereby the wheel drive is most often hydraulical. Linear machines are also available with belts instead of wheels, so called caterpillars. A good friction between the wheels and the cable surface is necessary without undue pressure on the cable. The need for sufficient power in the linear machines must not be underestimated. In addition, the synchronisation of linear machines, turntable and the barge must be orchestrated carefully. Cableways, rollers, laying and pick-up arms, and chutes are other important devices for cable installation, whereby the cable’s minimum bending radius under defined tension values have to be taken into account.

Remotely Operated Vehicle (ROV) The ROV is a submersible powered tool. ROV are highly complex systems and are available in all sizes which can be equipped with different manipulators (ROV arms) and tools, such as cameras, pick-up operations, sonar and survey operations. During cable installation the ROV is primarily used for touchdown monitoring and assistance during pull-in operations. It is important to make sufficient deck space available for the complete ROV system, comprising the vehicle and its parking spaces, launch system (crane or A-frame), umbilical drum, container for control room and supplies, and cabin for ROV operating crew.

CABLE LOADING There are two concepts for the supply of cables. The first is via pre-cut lengths on drums/reels. The second is in a large coil or on a turntable. For the loading of a long cable length, the cable lay vessel is moored directly at the cable manufacturing facility to load the cable in one piece. The cable will run from the cable factory over cable rollers to the vessel via a loading arm onto the turntable. The loading speed is between 3m-to-20m per minute depending upon cable size and equipment capacity.

ONSHORE LANDING The landing of the export/shore connection cable is one of the most critical operations. In most situations, the cable laying operation starts here. There are various methods for the cable landing, whereby the methodology to be used will depend on the shore conditions and equipment abilities. In some cases, there will be an open trench through the beach with the entrance point stabilised by cofferdams. In other cases, horizontal directional drilling is used, where a rig will drill a hole with a pre-determined curvature through the dike and under the beach into the sea. The hole is lined with steel or plastic pipes for the pull-in of cables. When all onshore preparations are completed, the cable lay vessel will approach the landing location as close as possible. The pull-in wire from the onshore winch is brought to the vessel for connection to the first-end of the cable. The cable can be paid out from the vessel with either floating devices attached to the cable or onto cable

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rollers depending on the location. Weather conditions, such as water currents and wind, can effect these operations. Various assisting boats can assist with keeping the cable in position, while the shore winch hauls in the cable to the beach and further to the onshore connection location. The floating devices are removed during this operation and the cable is anchored once the final destination onshore has been reached.

vessel, whereby the plough share cuts a narrow slit in the soil into which the cable is guided. The use of a cable plough can have high tow forces. In recent years, sophisticated vibration and water jets have been added to the design of the cable plough to support the ploughing process.

Cable plough, courtesy of IHC Merwede Engineering Business

Pull-in at offshore transformer platform/ wind turbine generators

CABLE PROTECTION Cable Trencher, courtesy of Pharos Offshore

The potential risks to a cable are internal risks and external risks caused by foreign objects like dragging anchors, dropped objects and fishing tackle. One can expect more shipping activity in shallow water areas and therefore the probability of emergency anchoring is therefore higher. In addition, the probability of cable damage by fishing activities in shallow water is also higher than in deeper water. Therefore, most cables are either buried or covered to a depth of 1-to-2m below the reference level for morphological stable seabeds. Larger burial depths are typically selected for moving seabeds. The most common protection method is trenching, which is the burial of the cable under the seabed. There are various methods available. Cable ploughing has been around for decades and is a brute force method. The cable ploughs is towed by the cable lay

Rock dumping is also well-known technology for protection of cables. Purpose-built vessels carry a shipload of rocks over the laid cable and discharge the load. On some vessels, the rocks are simply pushed over the side of the vessel (side dump). Modern rock-dumping vessels have highly sophisticated control and monitoring equipment, with the latest versions of rock dumping vessels able to perform these activities in 1,700-plus water depths. Thanks to Gerard Kroese Blue Offshore bv

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The landing of the cable at the offshore transformer platform or wind turbine generator is different to the shore landing. These structures typically have foundation fixed to the seafloor and the cable is therefore pulled into the structure via a J-tube. This J-tube runs from the seafloor to the top of the platform/wind turbine generator. The J-tube is open at the top and has a bell mouth at the bottom end in order to guide the cable into the J-tube. The cable lay vessel will approach the J-tube at a predetermined location. A pulling wire will be brought from the platform/WTGs winch through the J-tube to the cable lay vessel. The pulling wire is connected to the end of the cable for the pull-in. Once connected, the winch will pull the cable through the J-tube to the top of the platform/ WTG. There are several methodologies for the pull-in of the second cable end. One can lay the cable down on the seabed via a so-called Omega- or S-loop as close as possible to the J-tube bell mouth in order to reduce pull in forces. One can also use floating devices to float the cable into the J-tube. The methodology is very much dependent on water depths and soil conditions around the structure.

There are also various jetting methodologies, ranging from sledges and swords with water nozzles to a purpose-built ROV with own propulsion. These tools are based on fluidisation of the soil with the cable sinking into the slurry.