Lessons Learned From Template Installation in Harsh Environments T.Jacobsen, T.Næss Subsea 7 1. ABSTRACT Subsea templates are traditionally transported to the relevant installation site either on the deck of a crane vessel or a barge, depending on their size and shape. In both cases the template has to be lifted off from deck and lowered through the splash zone. Another possible way of transporting and installing a subsea template is to perform a submerged tow through the moonpool of an offshore service vessel. It may be argued that such an operation will have a wider operational window than traditional methods since offshore lifts are eliminated. However transit time during towing may reduce the profitability of this method. This paper outlines the two methods for template installation, and states typical installation criteria based on model tests and empirical data from full scale-measurements. Based on model tests, a numerical hydrodynamic model is made of both methods and the typical design criteria prior to the offshore operation are established. Also lessons learned from the offshore operations are outlined and novel structures for improving deck-handling are introduced. 2. INTRODUCTION The marine industry is growing, and the tendency of going deeper and developing new cost efficient installation concepts does not seem to end. Further development in the oil- and gas industry requires more complex subsea technology and new cost efficient methods for transportation & installation of subsea modules. This also requires detailed planning of marine operations with emphasis on equipment design, and safe installation criteria. Traditional methods for installation of subsea templates involve offshore lifts and hence the possibility of pendulum motions in air and snatch loads in lifting slings and lifting wire. In recent years, more advanced calculation methods and vessel equipment have been developed to increase installation criteria and ensure safe marine operations. However, marine contractors are still strongly dependent on weather and vessel limitations. This dependency can be reduced by quantifying the nature of the limiting factors for installation and development of novel installation assisting structures.
3. HARSH WEATHER INSTALLATION WINDOWS Marine operations may be delayed due to environmental conditions exceeding prescribed operational levels leading to a possible increase in the duration of the operations. Marine operations which must be completed without break are called weather critical operations. Otherwise they are termed non-critical. A template installation done by a cranevessel has usually an operation reference period of 12 hours. On marginal lifts using conventional crane vessels, a 300Te template is overboarded in significant sea states of 2m or below. On some fields in the Norwegian sea, a weather window with Hs<2 for 12 hours can be challenging, and usually offshore operations are limited to the summer month of May to August. This can be seen on the figure below showing characteristic duration, including waiting time in order to perform marine operations limited by a significant wave height of 2.0m for 12 hours. Figure 1 Characteristic durations, Hs=2[m] for 12 hours. Courtesy of Statoil The figures show duration characteristics for completing a critical operation including waiting time. Duration is measured from the day the operation is ready for launching. The day of launching is assumed to be an arbitrary day within the relevant month.
Figure 2 Characteristic durations, Hs=3[m] for 12 hours. Courtesy of Statoil As seen from Figure 1 & Figure 2, increasing the operational window from Hs=2m to Hs=3m may increase the operability window by a factor of 3-4 during winter months in the Norwegian sea. This is very beneficial for operator and contractors seeking to maximizing vessel utilization for field development and minimizing waiting on weather. Hence a modest increase in structure over boarding criteria, can greatly influence vessel utilization. 4. MEASURED LOADS DURING TEMPLATE OVERBOARDING OPERATIONS Since the introduction of active heave compensated crane vessels with knuckle boom cranes of 400Te, several successful installation operations of 300 subsea templates have been completed. Due to the high utilization of vessel equipment, the large geometrical dimensions of the structure, several engineering hours are used to ensure a safe and efficient installation operation. Particular interest is made in arranging a safe deck handling phase of the operation, where the template is controlled in air by the use of tugger wires routed from deck mounted winches. Vessel roll and pitch are important for the in-air behaviour of the template. Crossing of the splash zone is also an important phase of the operation where quasistatic buoyancy forces and dynamic slamming forces impose large variation in the cranewire tension. Due to the large geometrical dimensions and shape of the suction anchors, large quantities of entrapped air and entrapped water can be challenging to quantify as an input into deployment analyses and may set operational restrictions. This is further discussed in chapter 5. As a basis of comparison, the operational weather & maximum dynamic amplification factor (DAF) recorded during offshore operations of 9 template installation are presented in Table 1 below. All environmental data and crane loggings are taken in the period from 2008 to 2012 by the vessel Skandi Acergy.
Environment DAF Structure H S T P Deck handling Splash zone Submerged Gjøa ITS 0.9 8 1.05 1 0.9 Tyrihans ITS 2.1 9 1.05 1.16 1.32 Lavrans ITS 1 10 1.01 1 1.1 Visund Sør ITS 1.6 9 1.05 1.05 0.9 Hyme - - 1.05 1 0.9 Vigdis NE - - 1.01 1.11 - Skuld R & P ITS 1.7 13 1.01 1.07 0.9 Visund Nord ITS 1.3-1.07 - Table 1 Maximum recorded DAF during offshore operations As seen in Table 1 the maximum recorded DAF (related to weight in air) seems to occur independently of operation phase. This is also the DAF of importance for the crane utilization. However it should be noted that during crossing of the splash zone there is a quasistatic force occurring from buoyancy and entrapped air in the suction anchors acting upwards. Hence, the dynamics during splash zone crossing are greater than during deck handling, but not necessarily the cranewire tension. Also during the submerged phase of the operation a typical template has 50 Te of buoyancy from the displaced water volume of the structure hence, reducing the crane wire tension and DAF. It should be noted that this buoyancy is very dependent on hatch/air opening and lowering velocity and amplitude. If the air evacuation is restricted, large buoyancy forces may act giving risk of slack in the crane wire. Figure 3 ITS deployment operations The results from Table 1 show that the maximum registered DAF during the offshore operations are relative low. Also the registered environments during the operations are low, showing that most template overboarding operations are done in significant seastates (Hs) lower than 2m. Feedback from offshore operations on conventional monohull vessel is that the limit for conventional deck handling by the use tugger winches are approximately 1-2 degree of roll and pitch. By establishing a numerical panel model of a typical construction vessel with a 400Te construction vessel as seen from Figure 4, for a typical representative loading condition and wave spectrum, it can be seen from Figure 4 that these limits are often exceeded for longer wave periods. Hence in order for efficient deck handling with the current technology of using tugger winches to restrain the load, the seastate for safe deck handling on these type of operations are usually limited to Hs<2m. This is again
indicated in Table 1 where it shows that most installation operations are done seastates below 2m significant wave height. Often the vessel needs to wait on weather to get these safe overboarding criteria. The design criteria for overboarding operations are further evaluated in section 6. 5 4 max roll max pitch Hs=2.0m Max angled [deg] 3 2 1 0-1 4 6 8 10 12 14 16 18 Peak Period Figure 4 Maximum vessel rotations Hs=2.0[m] 5. SUBMERGED TOWING Another possible way of transporting a subsea template is to perform a submerged tow through the moonpool of an offshore service vessel. It is argued that such an operation will have a wider operational window than traditional methods since all offshore lifts are eliminated and will accordingly be more cost efficient. A submerged towing operation of a heavy structure also enables maximum utilization of the crane capacity on board the vessel, which ensures a safe installation process. During a submerged towing operation, the dynamic behaviour of the template and tow arrangement will depend upon the hydrodynamic loads, which act on the components of the system. The magnitude of these forces will affect the deflection angle of the towing wire in the moonpool and influence the probability that slack in the wire will occur. Accordingly, the forces will also set the operational limits both regarding permissible sea states and towing velocity. Hence, it is important that the computational models and procedures are validated before such an operation is carried out. The tow- and installation concept outlined within this paper is designed to allow a monohull construction vessel to pick up the template from its wet-store location, transit to the field with the template suspended through the vessel s moonpool and install the templates at the location in a single vessel operation. A winch system is used for the pick-up of the templates from the wet-store location and to set down the templates at the field. During subsea transportation the template is suspended from static hang-off wires connected to two trunnions on the hang-off
tower. During tow the winch system is unloaded and the subsea sheave connecting the winch to the template is disconnected and secured in the moonpool. During set down the template will be lowered to seabed using the winch system. The winch system is not heave compensated, but a Cranemaster is included for overload-protection during landing of the templates. The method consists of the following operations: Wet-store of template Pick up and hang-off Tow to field Transfer load to heavy lift winch system Landing of subsea template within the installation criteria Figure 5 displays some of the stages of the operation. Figure 5 Stages of operation - Wet Store, Pick up and Hang-off, Tow to field and Installation of Subsea template Figure 6 shows a schematic picture of the winch system. The system comprises of a 300te winch (located outside the figure) with an 88mm winch wire. The wire runs from the winch, over the fairlead, down through the moonpool and around the subsea running sheave block. Further the wire runs back up again to the Cranemaster which is connected to the hang-off tower. The template itself is supported by four slings, all connected to a delta plate. The static hang-off rigging used during tow of the templates is also displayed in Figure 6. The hang-off grommets (not taking any load in the figure) are connected at the hang-off trunnions and to the delta plate.
A fibre-strop is used between the running sheave block and the delta plate. The strop was connected / disconnected two times for each template and was chosen to ensure easy ROV handling. Between each connection the strop was brought up on deck for visual inspection. The method was successfully used for tow and installation of the four Tyrihans structures in May/June 2007 ref /12/. Previously the Yttergryta Integrated Satellite Structure (ISS) was installed using the same concept in January 2007. Further the Tordis ISS (March 2007), the Heidrun Template (January 2006), the Mikkel Host Protection Structures (2003) and the Tune A SPS (November 2005) have all been towed and installed in the North Sea by Subsea 7. Figure 6 Winch system Challenges related the tow and installation concept might be divided into the following three categories: Geographic o Harsh environmental conditions with a significant seastate of 11.5m with one year return period o Tow distance and fatigue o Water depth for installation - vertical resonance Template properties o Massive weight compared to installation vessel o Large hydrodynamic loads due to suction anchors Operational o Heavy rigging o Complex ROV operations o Non heave compensated system
5.1 NUMERICAL SIMULATION OF TOWING OPERATION Based on lessons learned during actual towing operations, and model tests from ref /5/, a strip-theory model is made in the time-domain simulation software SIMO developed by Marintek. Figure 7 Submerged towing of subsea template An experimental investigation is performed to simulate towing operations for different environmental conditions. The main response quantity of interest for the experimental investigation is the tension in the main wire connected to the template rigging. Dynamic tension forces in the towing wire are generated by an oscillator and they are measured by a force ring. In order to simulate a towing operation, the forward speed of the towing carriage is used to represent the horizontal translation of the vessel and a vertical oscillator is used to model the vertical translation of the vessel (i.e. heave motion) in regular waves for head sea. Figure 8 Submerged towing experiment in the longitudinal direction Findings from the experiment conclude that the dynamics occurring in the towing wire are large for swell dominated sea. The hang-off structure and the lifting points on the structure need to be properly designed for fatigue loads in order to ensure a safe towing operation. For a moonpool tow, clashing frequency between the towing wire and the moonpool edges increases drastically with increasing towing speed. In general typical towing speeds are 3 [knots] Ref./11/. Hence submerged towing can be an efficient method for subsea equipment installation when the availability of offshore construction vessels is limited, but restrictions to transit conditions apply.
6. OVERBOARDING ASSISTMENT AIDS As mentioned in the previous chapters, the limiting criteria for installation operations are often deck handling. To improve overboarding criteria, several novel concepts have been developed in the industry. Examples of such structures are as follows: Tugger wires Deck mounted winches with steel wires have been used for several decades for load control of subsea structures. Using deck mounted winches, pendulum motions can be stopped and rotations on the deployed object can be implemented to ensure safe loading. Challenges related to the use of tugger-winches are to control potential snap loads and also synchronizing operator movements during the lift. Also the wire angle need to be as horizontal and snag-free as possible which in reality often is hard to achieve. Figure 9 Tugger wires Static pivot point The pivot point load control arrangement was pioneered on the Skandi Acergy in 2008. The principle is that the load is controlled laterally & longitudinally during overboarding. Swings, momentum & hence large impact forces are not allowed to develop as the load is kept under control using a system of tuggers. The concept has been successfully used on both spools and template structures, but it should be noted that not all lifts are suitable for the pivot point due to the geometry of the loads, obstacles on deck, crane SWL and radius and rigging and hook height. Also the best utilization of the pivot point is when the load is kept on it for as much of the lift as possible allowing for the greatest angle of rotation. Static Pivot point Figure 10 Static pivot point
A lift with a pivot point is planned for the pivot point located in the optimum position but is still a compromise due to the following: o The geometry of the load can interfere with the crane or other fixed obstacles on deck o The arc that the crane hook has to follow is limited by SWL of the crane at the varying radii o The arc is also limited by available hook height o Limitations as to suitable locations on the load where an interface to the pivot can be achieved Hence the effect of these can be: o The lift is exposed to a greater portion of the slew/rotation operation only controlled by deck winches o The lift cannot be done using a pivot point at all so the entire deployment operation is done using a series of deck winches o Worst case is that without suitable control, the lift cannot be performed at all Moving pivot point Based on lessons learned and the need for controlling the rotation of the load for as much of the overboarding as possible, a more flexible version of the static pivot point has been developed to be used on a wide variety of projects. The movable pivot point offers a far more flexible approach in applying the principles of load control. The concept is based on a modular track layout where the pivot point is allowed to be skidded on the vessel deck to increase the rotation control. Figure 11 Moving pivot point The modular track layout consists of straight and curved sections assembled to suit the overboarding schematic of the considered structure. A snag-free trolley with a suitable pivot point is mounted on it and is driven along the during the lift in operation in coordination with small incremental crane moves and adjustment of deck tugger winches. The trolley system is made suitable for spools & large structures typically with suction cans.
Universal load assistant (ULA) As indicated above, several technology initiatives to enhance operations in challenging sea states have been performed in the last years. One of the latest one is a crane lifting aid designed to improve deck handling safety during lifting and with the potential to significantly extend winter-weather operations in harsh water like the Norwegian Sea. The Universal Launch Assistant (ULA) is a load-controlling mechanical arrangement attached to the crane pedestal of existing vessels and controlled by the crane operator. Its telescopic arms swivel around the crane pedestal, and give firmer and safer handling of horizontal inertia loads including templates, manifolds and associated spools. The ULA replaces the use of winches with holdback wires which are conventionally used to control horizontal loads, and therefore enhances on-board safety as well as significantly improving the utilisation of vessels involved in construction work in challenging weather conditions. Figure 12 Universal Load Assistant
7. CONCLUSION Installation of subsea structure, lifting operations In recent years larger and heavier subsea structures are being installed in harsher environment than before and this requires advanced and more sophisticated tools to model and analyse the lifting operation to perform safe and optimum installation engineering. Subsea 7 has successfully completed many such installations in the last couple of years and the lessons learnt from those projects are: It is concluded that the SIMO provides an excellent basis for the lifting analysis by being versatile to model complex lifting operations in the time domain. The forces occurring during deployment of templates are often limited since there is a large buoyancy force acting upward reduces the total dynamic hook load and DAF during splash zone crossing. The limiting criteria for deploying most large structures are deck handling and vessel kinematics. Tow solution The Tyrihans project installed four subsea templates in the Haltenbanken using a small monohull construction vessel. For this case the installation cost was significantly lower than the cost of using a heavy lift vessel. The campaign was completed in a safe manner with all critical operations performed subsea by ROV and engineered ensuring ROV friendly solutions. The following are the key conclusions and lessons learnt: No manual handling of heavy rigging offshore All heavy lifts were performed inshore in sheltered waters Extremely limited exposure to personnel Cost-effective solution Ensures availability of vessels Limited use of sophisticated cranes and crane modes subject to higher risk of technical / software failures The applied tow configuration was stable without any excessive motions. Further the performance and lessons learnt confirm that this method is suitable for installation of these kinds of structures. By using this method the number of suitable vessels available on the marked is increased dramatically. Subsea 7 has applied for a patent based on the described concept, and the method is currently patent pending. Further advances in overboarding assistance As it is pointed out in this paper, the limiting phase for many marine operations is deck handling and not the limiting force criteria related to crane capacity during deployment. Several novel structures to improve deck handling have been introduced in the industry, but these are often limited to specific structure geometries and dimensions.
8. ACKNOWLEDGMENTS The authors thank Daniel Karunakaran and Kenneth Aarset for technical advice while preparing this paper. Subsea 7 is acknowledged for permission to publish the paper. This paper reflects the opinion of the authors and does not imply endorsement by the company to which acknowledgments are made. 9. REFERENCES Ref. /1/ DNV-RP-H103 Modelling and analysis of Marine Operations Ref. /2/ DNV-RP-C205 Environmental conditions and environmental loads Ref. /3/ Faltinsen, O. M., Sea Loads on Ships and Offshore Structures Ref. /4/ Statoil, Dompap and Fossekall fields, Metocen Design Basis Ref. /5/ MARINTEK, Hydrodynamic data for the GJØA ITS installation. Forced oscillation model tests of ITS, MARINTEK Report No. 550147.00.02 Ref. /6/ Øritsland, O, et al A summary of Subsea Module hydrodynamic data MRINTEK Report No. 511110.05 Ref. /7/ Torsethaugen, K. And S. Haver Simplified Double Peak Spectral Model for Ocean Waves Proceedings of the Fourteenth International Offshore and Polar Engineering Conference, Toulon, France, May 23-28, 2004 Ref. /8/ NORSOK N-003 Section 6.2.2.3. Ref. /9/ M.J. Tucker & E.G. Pitt Elsevier Waves in ocean engineering Ref. /10/ Jacobsen, Næss, Karunakaran Comparison with full scale measurements for lifting operations Ref. /11/ Jacobsen, Leira Numerical and experimental studies of submerged towing of a subsea template Elsevier Ocean Engineering Ref. /12/ K. Aarset, A.Sarkar, D.Karunakaran Lessons learnt from lifting operations and towing of heavy structures in North sea