Proceedings of the ASME 28th International Conference on Ocean, Offshore and Arctic Engineering OMAE2009 May 31 - June 5, 2009, Honolulu, Hawaii

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1 Proceedings of the ASME 28th International Conference on Ocean, Offshore and Arctic Engineering OMAE2009 May 31 - June 5, 2009, Honolulu, Hawaii OMAE OFFSHORE PIPELAY OPERATIONS FROM A CONTROL PERSPECTIVE Gullik A. Jensen 1,2, Morten Breivik 2 and Thor I. Fossen 1,2 1 Deartment of Engineering Cybernetics 2 Centre for Shis and Ocean Structures Norwegian University of Science and Technology NO-7491 Trondheim, Norway {gullik.jensen@itk.ntnu.no, morten.breivik@ieee.org, fossen@ieee.org} ABSTRACT This aer considers the roblem of installing offshore ielines by means of ielay vessels from a control ersective. Secifically, the aer is divided into two main arts, where the first art gives a thorough introduction to the fundamentals of ielay oerations and the challenges and ossibilities associated with such oerations. The second art then suggests how ielay oerations can be further automated by augmenting the manually-oerated dynamic ositioning (DP) systems of today with sohisticated guidance functionality that may imrove oerational recision and safety by relying on advanced mathematical models. INTRODUCTION Offshore ielines are essential for the transortation of oil and gas, and range from small diameter interfield flowlines to large diameter exort lines for transortation from offshore installations to shore and for transortation across oceans between countries. In articular, the use of subsea ielines reresents a safer and more environmentally friendly alternative than surface transortation by tankers [1]. Currently, efforts are being made to locate and retrieve oil and gas resources in increasingly deeer waters, and offshore ieline technology is constantly develoed to kee u with these advances. Hence, the main drives in the offshore ieline industry today concerns installing ies at large water deths, and in articular the develoment of large gas gathering and distribution systems throughout the world, such as, e.g., the Langeled and Indeendence Trail ielines. Dedicated ielay vessels are used to install offshore ielines, and the first generation of these vessels consisted of flat-bottomed barges that were oerated in shallow waters close to shore. A barge was held in lace by an anchor-mooring system laced in a sread formation by anchor handling vessels. The osition and orientation of this barge could then be ket to avoid buckling and kinking of the ie, and the barge moved forward by controlling the anchor-line winches. As ieline diameters and water deths increased, the barges were relaced by large semi-submersibles to rovide stable working conditions in more hostile environments such as the North Sea. However, ositioning by mooring has several disadvantages [2]. The mooring lines radiate from the ielay vessel in a full circle of directions, such that in congested areas it becomes difficult to lace the anchors while keeing clear of existing structures, including the already installed ie. Dee waters also limit the recision of the ositioning due to the mechanical flexibility of the mooring system. Finally, the initial ositioning and subsequent relocation of anchors is cumbersome, time consuming, and sensitive to weather conditions and sea states. Due to such mooring-related constraints, the advantages of dynamic ositioning (DP) for osition control of ielay vessels was early recognized. A DP vessel has a considerably shorter start-u time, and has the ability to abandon and recover ielines quickly. The motion control becomes indeendent of water deth and can oerate in congested areas and close to latforms. There is also less mechanical downtime comared to an anchor vessel because the wear and tear on an anchor system is more intense. 1 Coyright c 2009 by ASME

2 Reference Figure 1. Controller Inut Feedback Loo System Outut CLOSED-LOOP CONTROL THROUGH FEEDBACK. The first DP-based ielay vessel was the Allseas Lorelay, which was introduced in 1985 for deewater alications [3, 4]. The vessel was shi-shaed, with an aft stinger and large iecarrying caability. It quickly roved very successful, articularly in the rough North Sea, and exeriences gained from the Lorelay was used to construct the largest ielay vessel currently in oeration, the Allseas Solitaire [5]. However, two factors have historically worked against the emloyment of DP technology, namely the reliability of the system, since system failure otentially causes severe damages to both ieline and vessel, as well as the ower required to balance the tension from the ie. Still, desite the initial reluctance to use DP systems, all newly commissioned ielay vessels today are dynamically ositioned. To ensure reliability, these vessels are usually configured to IMO Equiment Class 3, with full redundancy in all comonents. In ractice, a DP oerator (DPO) manually controls the DP system based on exerience and know-how, using secific features that have been develoed to aid in ielay oerations [6]. Such features include tension comensation, ie-ull (i.e., move forward one ie length at a time), and tracking of vessel ath, where the dynamics of the ie and environmental loads are comensated for by using a crab angle and track offset. However, the technology imlemented in commercial DP systems, including ielay-secific extensions, is classified and not easily accessible for academic researchers. Consequently, this aer attemts to give a convenient introduction to and overview of offshore ielay oerations which is suited for control researchers. The aer also looks at the ossibility of automating such oerations beyond the manual DPbased control rocedures of today since it seems lausible that control systems can be used to further imrove the erformance and rofit of ielay oerations, as they have for several other industries, including the rocess industry, aerosace industry, and others [7]. Secifically, the aer is divided into two main sections, where the first section deals with ielay fundamentals, while the second section deals with motion control issues. The aer also rovides an extensive literature list to aid in further research on automated offshore ielay oerations. Exerienced ieline engineers will be familiar with arts of the resented material. The main idea behind the concet of automatic control is to automatically force the value of a secific system outut variable to a desired value, as illustrated in Figure 1. This desired value is tyically called a setoint or a reference. The outut of the system, which reresents the interesting system behavior from an oerational oint of view, is a result of the internal system dynamics and our ability to maniulate the system through certain inuts. As already mentioned, the move toward an alication of automatic control technology started when anchor systems were relaced by DP systems for the dynamic ositioning of vessels involved in deewater oerations [8]. In the future, increasingly advanced control features can be introduced through the develoment of sohisticated guidance systems that reduce the need for manual DPO control to imrove recision, seed and safety of the oeration. By alying available ie-monitoring sensors and commercial-off-the-shelf (COTS) DP systems, new functionality can be imlemented without additional hardware costs. PIPELAY FUNDAMENTALS This section deals mainly with identifying the inuts, states and oututs relevant for control in a ielay oeration, and starts with an introduction to the construction of offshore ielines. Over the receding decades, develoment of offshore ieline technology has mainly been reorted at conferences such as the Offshore Technology Conference (OTC) [9], International Conference on Offshore Mechanics and Arctic Engineering (OMAE) [10], and Offshore Pieline Technology (OPT). However, during recent years, the field has become more accessible by the ublication of several textbooks, which are artly comlementary, artly overlaing. Non-technical aroaches are taken in [11] and [12], while [2] and [13] rovide useful introductions to technical asects. A comrehensive overview of design methods aimed at both new and exerienced ieline engineers are rovided in [14] and [15], while detailed mechanical design methods are found in [16]. Pieline Construction Pielaying encomass installation methods whereby the ie string is welded together from ie joints onboard a ielay vessel as it is installed on the seabed [15]. Historically, for ielay in shallow waters the ie string was jointed horizontally on the oen deck of the barge and extended down to the seabed in an S-shaed curve, commonly referred to as S-lay, see Figure 3. Tensioners gri the ie string to facilitate construction during the ielay, and are used to control the seed with which the ie is extended from the vessel, called the ay-out seed. The uer art of the ie string is suorted by a submerged oen frame structure, called a stinger, which controls its curvature. The main advantage with the S-lay method is that it enables a long firing line with arallel workstations for assembly of ie joints, field coating and non-destructive testing, making the method fast and 2 Coyright c 2009 by ASME

3 Guidance System Online Target Generator Environment Velocity Assignment Algorithm Existing Pielay Technology Inut Selector Position and Velocity Reference Dynamic Positioning System Control Law Control Allocation Pielay Vessel and Pieline Measurement System Sensors Observers States Human Oerator Figure 2. PIPELAY CONTROL SYSTEM ARCHITECTURE. economical, articularly for long ielines. For large deths and ie diameters, the ie strains over the stinger may exceed the standard allowable strain levels, and the steel ie may be deformed. To overcome this, the J-lay method was develoed [17]. In the J-lay method, the ie is extended on a near-vertical ram, the J-lay tower, which eliminates the stinger and overbend strains altogether. Semi-submersibles are frequently used to facilitate J-lay due to their high stability, which is required for the tall J-lay tower. The main drawback with the method is that this tower only facilitates one workstation, making the J-lay method inherently slower than the S-lay method, which is the rice ayed for ie installation at greater deths. These installation methods are thus comlementary [18]. However, the S-lay method has recently been develoed to also handle deewater installation by the construction of very large dynamically ositioned vessels with fixed stingers that suort the ie string down to a near-vertical lift-off oint. This stinger configuration has shown to reduce the tension in the ie comared to the traditional S-lay method. Furthermore, ieline engineers argue that it will be safe to relax the standard strain level of the design in order to reach greater deths and handle even heavier ielines [19]. Reeling is an installation method suitable for cables, umbilicals and flexible ies which generally have small diameters. The method is also suitable for small-diameter rigid ies (u to 16 inches). For reeling alications, the ie is constructed onshore in a controlled factory environment and sooled onto the reel. The ie is then ayed out by a reeling vessel with the reel mounted either horizontally or vertically. Since the ie is deformed on the reel it must be mechanically straightened out during the installation, which takes either the S-lay or J-lay aroach. Deending on the installation method used, ie roerties, water deth and weather conditions, the length of ie laid in a day varies from kilometers for J-lay and u to 5 kilometers for S-lay. A tyical rate for reeling is 14 kilometers er day. More detailed overviews of installation methods can be found in [2, 13, 14, 15], and details on ielay vessels and equiment can be found online at [20, 21, 22]. Note that so-called towing methods are not considered here. The actual installation rocess is always overseen by an engineer, who oerates the vessel together with the catain. The engineer relies on a team of three to four eole, where each team member has a dedicated task of monitoring or oerating equiment. A tyical task breakdown may be: 1. Navigation (Surveyor) 2. Vessel maneuvering (DPO) 3. Tensioner/reeling/carousel oerator 4. ROV oerator. The ositioning of the vessel is done by a DP system, where reference ositions are manually rovided such that the vessel moves in a discrete fashion between consecutive references. The distance between these reference ositions may vary in length, but the ielay vessel will tyically move in stes equal to the length of a ie extension, which is limited by the length of the firing line. However, a reeling shi tyically moves continuously since it can reel out its ie continuously. Pieline Installation A ieline develoment roject is tyically divided into three hases: design, installation, and commissioning and oeration. In the design hase, the ieline ath and the ie roerties are determined. The ath consists of straight and curved sections on the seabed which the ie must be laced along, and is documented through detailed layout drawings, alignment sheets and bathymetry mas. The ieline design hase also includes ieline sizing (diameter and wall thickness) and material 3 Coyright c 2009 by ASME

4 z ROV Touchdown Point x Overbend Stinger Ti Inflection Point td U > 0 Sagbend Seabed tm Firing Line Tensioners Stinger u > 0 of classification might instead be to consider the significant loads on the ie in each region. In shallow water, the bending stiffness is significant, and static comutational results on bending and stress loads are good aroximations of the true loads. For S-lay, the dearture angle at the lift-off oint will be in the vicinity of 30 from the horizontal for a water deth of 100 m. Relatively small vessel movements have large imacts on the ie configuration and stresses for such deths. In dee water, the effect of the bending stiffness becomes insignificant with resect to the axial loads and dynamic effects such as vortex induced vibrations (VIV), and the dynamic behavior of the ie must also be considered. In ultra-dee waters, ies are made very solid to withstand the external ressure at the seabed. These heavy and long ies cause an extreme vertical tension for the tensioners to handle. For S-lay, the dearture angle becomes close to vertical in order to kee the tension within reasonable limits, also known as Stee S-lay. Figure 3. S-LAY PIPE CONSTRUCTION SCHEMATIC. grade selection based on analyses of stress, hydrodynamic stability, san, thermal insulation, corrosion and stability coating, as well as riser secification. The choice of installation method can influence the design arameters of the ieline. In the literature, the term installation covers all the activities following the fabrication of the ie joints until the ieline is ready for commissioning and oeration. However, in this aer, the term installation will be taken to mean the ositioning of the ie on the seabed only. From a control ersective, the S- lay and J-lay methods are similar, and the following definition of ieline installation will be alied in the remainder of this aer: Definition 1. Pieline Installation (Pielay) Pieline installation is defined as the oeration of ositioning a ieline on the seabed from a surface vessel. The ie is extended from the vessel at a ay-out seed U (t) 0, and at a dearture angle α [0, π/2, defined relative to the mean sea surface. For the S-lay method, α = 0, and for J-lay, α π/2. For the on-site ie-assembly methods, the ay-out seed U (t) will switch between U (t) = 0 when the ie is being constructed and U (t) > 0 when the newly constructed ie is ayed out. Reeling allows a constant ay-out seed. Water Deth Three regions of water deths are usually considered, referred to as shallow, dee and ultra-dee waters. The numerical values covered by each region are constantly increased as ielay oerations move to increasingly deeer waters. A better method Pielay System and States The total ielay system to be controlled consists of the combination of the ielay vessel and the ieline, susended freely in the water down to the touchdown oint at the seabed. Following [23], the vessel is considered to be a rigid body, whose kinetics can be reresented in 6 degrees of freedom (DOFs) as M ν + C (ν) ν + D (ν) ν + g (η) = τ, (1) where the state vectors of generalized osition and velocity are η = [x, y, z, φ, θ, ψ] T R 3 S 3, (2) ν = [u, v, w,, q, r] T R 6. (3) The matrices M, C and D reresent inertia, Coriolis and daming effects resectively, g is a vector of restoring forces and moments, while τ reresents control inuts, environmental disturbances, and ieline effects. The model (1) is a convenient reresentation which catures the main vessel-ocean-ie effects, and has been adoted as a standard for vessel control design and analysis uroses. The ie is considered to be an elastic body, which is described by its configuration. The ie configuration is the comlete secification of the location and orientation of every oint of the ie. Let the line of centroids of the ie be reresented by the smooth curve ϕ : [0, L] R 3, where L is the total length of the undeformed ie, and a centroid is the geometric center of a cross-section of the ie. For the material variable S [0, L], the osition of any oint on the line of centroid is given by ϕ (S) 4 Coyright c 2009 by ASME

5 and the orientation of the cross-section at ϕ (S) relative to an inertial frame is given by the rotation matrix R (S). The set of all configurations is then called the configuration sace, which is given by C { (ϕ, R) S [0, L] R 3 SO(3) }. (4) Let Ĉ C be the set of all ie configurations for which the ie structural integrity is guaranteed. It is then required that the configuration stays in Ĉ at all times during the ielay oeration. While the geometry of the line of centroids is given by ϕ, R is required to reresent ie extension, shearing, bending and twist, which is used to derive stress and strain in the ie. For imlementation in a comuter, the continuous model must be discretized, e.g., by the finite element method. Details on mathematical ie models are beyond the scoe of this aer, and the reader is encouraged to consult [24] and the references therein. The common standards used for ieline design emloy a design ractice based on so-called limit states [25]. All relevant failure modes for a ie are formulated as limit states, which can be organized on a vectorial form by λ. The safe region for the values of the limit states ˆλ is conservatively comuted in the design hase. If the limit state values stay within their safe region during installation, the configuration is guaranteed to stay within Ĉ, λ i ˆλ i i [1... n] (ϕ, R) Ĉ. (5) where n is the number of limit states. Following the DNV OS- F101 [25], limit states are classified into one of the four categories: 1. Serviceability Limit State (SLS) 2. Ultimate Limit State (ULS) 3. Fatigue Limit State (FLS) 4. Accidental Limit State (ALS), where the limit state values are derived through simlified design formulas. These formulas use ie roerties and ielay arameters as their inut. Details on the limit states are beyond the scoe of this aer. Other standards and recommended ractices are found in API RP 1111 [26], ISO [27] and BS 8010 [28]. If the vessel DP system cannot kee the configuration in Ĉ due to environmental or other conditions, the ie may buckle or collase. To avoid this from haening, the ie is lowered to the seabed in an abandonment rocedure for rotection. When the conditions have imroved, the ie is retrieved and the oeration continued. Pieline Configuration The configuration of the ie is determined by the ie roerties, the ielay arameters and the installation method. The ie roerties refer to structural roerties of the ie such as the ie diameter, submerged unit weight, bending stiffness (EI) and axial stiffness (EA). These roerties are determined in the design hase and cannot be changed during the installation oeration. The ielay arameters refer to the conditions under which the ielay takes lace, and the most imortant arameters include: 1. Boundary conditions 2. Axial lay-tension 3. Pie dearture angle from the stinger 4. Water deth 5. Stinger curvature radius 6. Stinger roller ositions 7. Environmental loads. The ie extends beyond the freely susended configuration on both sides, reresented by the touchdown oint td and the osition of the last tension machine along the firing line tm, see Figure 3. The lower boundary condition is given by the osition and orientation at the touchdown oint, while the uer boundary condition is given by the osition and orientation of the ielay vessel. The lay-tension T is governed by the difference in td and tm, and the length of the ie in between. For the static lanar case, this situation can be well reresented by the catenary equation. Also, since ielaying is a low-seed alication, catenary considerations may give good lay-tension aroximations. The stinger curvature radius and the ositioning of the rollers are determined in the design hase and remains fixed during the oeration. The environmental loads are reresented by 1 st and 2 nd order waves loads as well as current loads, but wave excitation forces only affect the ie near the surface, tyically down to 20 meters deth. Different current rofiles will give different configurations. For more on sea loads, see [29]. Geometrically, three distinct sections of the configuration must be considered [30, 31, 19], namely the sagbend, the overbend, and the stinger ti. The sagbend is defined as the ie section that extends from the touchdown oint to the stinger ti or inflection oint. In the sagbend, the static load effect is governed by the tension, ie submerged weight, external ressure and bending stiffness. The equilibrium configuration is load-controlled since there are no hysical boundaries for the deformations that the ieline can exerience, so that the configuration in the sagbend is essentially the same for every deewater installation method. This shae is known as the catenary and is well known in the literature [32]. The overbend is defined as the ie section from the tension equiment over the stinger and to the stinger ti or inflection oint. The stinger suorts the ie on rollers saced out 5 Coyright c 2009 by ASME

6 along its length. The roller contacts are monolateral and can be considered a boundary condition to the achievable ossible configuration of the ieline. From the third roller counted from the ti and u, the ie is dislacement-controlled [30]. Note that the local loads on the stinger do not roagate beyond the inflection oint, and also that the J-lay method does not have an overbend region. The stinger ti or intermediate region is defined as the ie section that extends from the third-last stinger roller and down to the inflection oint. For the stinger ti, the main concern is the dynamic load effect caused by the contact of the ie with the last roller, which can cause very high and uncontrolled dynamic bending loads. Consequently, the ie liftoff from the stinger is usually required to occur before the last roller. Available Measurements The condition of the ie is closely monitored during the ielay by a measurement system that rovides state measurements from filtering of sensor data and from state estimators. The instrumentation varies with different vessels based on size, oerational deth and installation method, and the following measurements are considered here: 1. Vessel osition and velocity 2. Touchdown osition 3. Axial tension 4. Dearture angle 5. Roller ressure 6. Free-san ie length 7. Touchdown distance 8. Water deth 9. Environmental loads (current, wind, waves). Vessel osition and velocity - The vessel osition may be obtained with high accuracy from a number of available osition reference systems, including the global ositioning system (GPS), hydroacoustic osition reference (HPR) and microwave osition reference systems [8]. Since ielaying is done over long distances, using GPS with corrections (DGPS) is very common and rovides an accuracy of u to 0.1 meters. GPS also rovides vessel velocity measurements. Touchdown osition - A remotely oerated vehicle (ROV) is used to hover over the touchdown oint and rovide a visual image. It is not easy to define the exact touchdown oint since the seabed is uneven and enetrable, so the osition of the touchdown oint td is commonly assumed to coincide with the ROV osition. Multile ROVs may also be used to monitor other arts of the ie in the water. Axial tension - The urose of alying tension to the ieline through tension machines is to control the curvature of the sagbend and the moment at the stinger ti through suorting the submerged weight of the susended art of the ie. The tension exerted on the tensioners from the ie deends on the ie roerties and configuration. Dearture angle - The dearture angle of the ie leaving the stinger is estimated from the contact force on the rollers and/or closed-circuit television (CCTV) equiment on the stinger. An equivalent term is lift-off angle. Roller ressure - Rollers are saced out along the stinger, suorting the overbend and reducing the strain. The rollers are equied with ressure cells to measure the contact force from the ie. Theses measurements are used to ensure that the ie follows the stinger smoothly, and good ractice indicates that the ie should lift off from the stinger before the last roller. CCTV equiment may also be used for this urose. Free-san ie length - Since each link joint is numbered sequentially, visual identification of the joints at the touchdown oint and at the vessel means that the length of the susended ie L can be found. Touchdown distance - The horizontal distance between tm and td, also known as the lay-back distance. Water deth - Measurements of the water deth are readily acquired from acoustic sensors located on the ielay vessel. At the touchdown oint, the ROV will measure its deth using ressure sensors. Environmental loads - The wind will influence the vessel osition, but the direction and seed of the wind is easily measured and comensated for by the DP system. Currents and secondorder wave loads can also be accounted for by the DP system, but loads on the ie cannot be avoided. Nor can first-order wave effects, which make the vessel roll, itch and heave. Actuators for Tension Control Several asects are considered when a desired ie tension is determined. The axial tension in the ie has a vertical and a horizontal comonent. The vertical comonent is dictated by the water deth and the unit weight, and is assively comensated for by the ielay vessel restoring forces. The horizontal tension is left to active control by the motion control system, and to find the otimal tension, several economical and ractical factors must be considered. There are several reasons to kee low horizontal tension at the touchdown oint. Low tension will reduce free sans and also allows for shorter radii of the curved segments, which will reduce the need for seabed rearations. The ROV used for monitoring the touchdown oint has a limited range, and low horizontal tension will move the touchdown oint closer to the ielay vessel so that the ROV can be oerated from this vessel. Reduced tension also reduces the fuel consumtion of the surface vessel. The residual tension, which is the tension left in the ie after installation, is also reduced with reduced lay tension, and should be as small as ossible. However, too little tension will cause the ie to buckle. A grah of tension vs. cost is shown in Figure 4. 6 Coyright c 2009 by ASME

7 Cost Buckling occurs Excessive ie fatigue Too little tension Otimal region Otimal tension Figure 4. Too much tension TENSION VS. COST. Fuel cost and seabed rearations Tension imact of the environmental loads. Tug-boat suort may occasionally be used to kee osition if the thruster caacity becomes insufficient. Pay-out seed - The tensioners hold on to the ie with rolling tracks that allow ie ay out while maintaining tension on the ie. The ay-out seed U is considered ositive when the ie is ayed out, and its maximum value is limited by the mechanical roerties of the tensioners. Stinger configuration - The stinger configuration can be oerated by hydraulics or cranes, but once the stinger configuration has been fixed it cannot be changed easily. However, for cables and light ies, it seems ossible to alter the stinger configuration during oeration by using hydraulics or cranes. Still, this method is currently not in use, and it is therefore not reasonable to consider the stinger configuration as an active actuator, but rather as a rigid suort structure for the ie, designed in advance. The stinger of the Allseas vessel Solitaire is described in [34] and [35]. The horizontal tension T h acting on the ie can be considered as a nonlinear sring where the dislacement is given by the touchdown distance, and the sring coefficient k is a function of the touchdown distance and other ie roerties and arameters, such that T h = k tm td. (6) A quasi-static method based on the natural catenary equation [32] is commonly used to comute this tension. The change in tension is then roortional to u U, when the x-axis of the vessel-fixed BODY frame is aligned with the line of tensioners. The actuators (control inuts) for controlling T h are thus the osition and velocity of the vessel, as well as the seed at which the ie is extended from the vessel, i.e., the ay-out seed. In addition, the stinger configuration can be used to modify the tension if it can be dynamically controlled. Consequently, the actuators discussed in the following are: 1. Vessel osition and velocity 2. Pay-out seed 3. Stinger configuration. Vessel osition and velocity - These variables have a direct imact on the ie tension and thus the osition of the touchdown oint td. Dynamically ositioned ielay vessels kee their osition exclusively by active use of thrusters, and fully actuated vessels are required for recision ositioning in surge, sway and yaw. Background on DP systems is found in [8], [23] and [33], while a DP system reference manual can give a hands-on introduction [6]. The ielay vessel must also kee a secific heading, and is not at liberty to freely weathervane in order to reduce the MOTION CONTROL ISSUES A vehicle motion control system can be concetualized to involve at least three levels of control in a hierarchical structure as illustrated in Figure 5 [36]. The highest control level, termed the guidance level, is resonsible for rescribing vessel guidance commands to solve the ielay roblem. The intermediate level then encomass COTS DP controllers, which give commands through a control allocation algorithm to the low-level actuator controllers that maniulate the actual vessel roellers and thrusters. An objective of this aer is to motivate the design of guidance systems that can relace the human oerator at the highest control level. Such guidance systems can concetually be divided into an online target generator and a velocity assignment algorithm, see Figure 2. While the target generator comutes the virtual target oint on the sea surface which the ielay vessel must track for the touchdown oint to follow the seabed reference ath, the velocity assignment algorithm comutes the associated reference velocity. Both reference osition and velocity are subsequently fed to a COTS DP system. At least two distinct aroaches can be taken in the guidance system design: 1. In a rotocol-based aroach, the system tries to imitate a human oerator based on a set of rules obtained from quantifying the oerational rocedure which the oerator follows. The system is then imlemented as a flow diagram, which is simle to imlement, but which at best will erform the task only as well as the human oerator. 2. In a model-based aroach, the system comutes the guidance signals based on mathematical models of the ielay system. The great advantage of a comuter-based controller 7 Coyright c 2009 by ASME

8 Signals and Signal Processing Figure 5. Human Oerator Guidance Control DP Control Guidance System Control Allocation Actuator Control Pielay Vessel and Pieline MOTION CONTROL HIERARCHY. High Level Control Intermediate Level Control Low Level Control Environment is the available comutational ower, which allows for comutation of decision data based on real-time measurements, thus yielding more accurate results than data based on test scenarios. The erformance of a model-based guidance system can thus otentially exceed that of a human oerator. Advanced ieline models are available, such as the nonlinear dynamic finite strain model resented in [37]. However, simler models such as the robot-equation-based model resented in [38] are better suited for model-based feedback control in real-time. In the accomanying aer [24], mathematical models used for ielay design and simulation are considered for control alications. Note that in this scenario the human oerator is still a art of the system, but now only left to monitor the oeration. The Pielay Problem The overall objective of a ielay oeration is to osition a ieline along a desired seabed ath from a start oint to an end oint while simultaneously ensuring ieline integrity. All motion controllers try to achieve a control objective, and for ieline installation the main control objective can be stated as: Definition 2. The Pielay Problem The ielay roblem is defined as the task of laying a ieline along a re-secified reference ath on the seabed through active motion control of a ielay vessel. The structural integrity of the ie must be guaranteed at all times during the oeration. The rimary objective of the ielay roblem is thus to osition the touchdown oint as close as ossible to the reference ath on the seabed. A secondary objective can then be to move the touchdown oint at a desired seed along this ath. Thus, the touchdown oint faces a so-called ath-maneuvering roblem, see [36] and [39]. These two objectives must be satisfied such that the structural integrity of the ie is ensured. Hence, the surface vessel must move such that both the ath-maneuvering objective of td and the ie structural integrity is satisfied simultaneously. The ielay vessel DP system must then track an instantaneously comuted reference value for tm to achieve both objectives, and thus satisfy a target-tracking motion control objective [36]. This objective corresonds to motion control scenarios where the target motion is not known ariori, which is the case for our alication since the target is dynamically comuted based on td, tm, U, environmental conditions, and a desired tension. Hence, the surface vessel must achieve target tracking for tm such that td satisfies its ath-maneuvering objective. An analogous alication can be found in [40], where a DP-controlled semi-submersible must chase a virtual setoint on the sea surface to minimize bending stresses on a riser extending from the vessel to a stationary installation on the seabed. The ielay roblem can thus be seen as a dynamic version of this setoint chasing alication. Note that the ielay roblem definition is indeendent of ie diameter, water deth, installation method, dearture angle, and whether an automatic controller is used or not. Subsea Path Let the subsea ath P consist of a finite set of n straight-line segments connecting n 1 wayoints, each with an associated turn radius r i, i {1,..., n}, organized in a wayoint table. The location of each wayoint is given by the osition vector i R 3, stated relative to an inertial Earth-fixed reference frame. The turn radii ensure a smooth transition between consecutive line segments, and the ath P is thus imlicitly arameterized as a set of connected straight and curved segments, see Figure 6. Only one segment is in active use at a time. Let D then define the art of the seabed which is located within a distance δ > 0 of P, coinciding with the breadth of the seabed rearations, done, e.g., by trenching, dredging, or rock duming. Deending on the seabed conditions, this corridor has a tyical width of 1-10 meters. During ielay, the goal is to have td D, and for td / D, a fault condition has occurred which must be corrected. A switching between the nominal and some fault-recovery controllers must then be erformed to ensure that the situation is roerly handled such that the ie can be laced within D again. The field of fault-tolerant control deals with such scenarios [41]. Vessel Path Reresenting the vessel osition by tm, a nominal vessel ath V can be found by urely kinematic considerations, which is valid under the following assumtions: Dynamic effects can 8 Coyright c 2009 by ASME

9 x I x I i-1 y y Wayoint I r i i Pielay Corridor Turning Radius Figure 6. SUBSEA PATH PARAMETERIZATION. I td td tm Dynamic Target Area r i+1 i+1 Subsea Path Vessel Path P i+2 where td (ϖ) now deends on the articular ath and χ td (ϖ) is the tangential direction of the curved subsea ath at ϖ, see Figure 7. Vessel Target Unfortunately, the kinematically comuted vessel aths associated with (7) and (10) do not suffice in a ractical ielay oeration. They only rovide aroximations that are valid for ideal oerating conditions, including nice weather and minimum current. In ractice, the vessel must track a dynamic target located at a distance from the ideal location in order for td to be within D while simultaneously ensuring ieline structural integrity, see Figure 7. Today, this target is manually comuted by a DPO based on revious exerience through various offset arameters available in the DP system. A future goal is thus to comute this target dynamically by using mathematical models and real-time measurements to be able to achieve an even greater degree of accuracy and effectiveness in the ielay oeration. Figure 7. SUBSEA PATH AND CORRESPONDING VESSEL PATH. be neglected due to low seeds, no environmental loads, and the water deth is constant for each segment, see Figure 7. Let the osition of a oint on P be reresented by td (ϖ) R 2, where ϖ R is a scalar variable. For straight-line segments (zero curvature), the desired vessel osition can then be comuted by where tm (ϖ) = td (ϖ) + R (α i ) [ tm td d 0 ], (7) CONCLUSIONS This aer has considered offshore ielay oerations from a control ersective. Secifically, the first art of the aer has given a thorough introduction to the fundamentals of ielay oerations and the challenges and ossibilities associated with such oerations. The second art then suggested how ielay oerations can be further automated by augmenting the manuallyoerated dynamic ositioning (DP) systems of today with sohisticated guidance functionality that may imrove oerational recision and safety, as well as enabling ielay vessels to take on the challenges of installing ielines in even deeer waters and more hostile environments, by relying on advanced mathematical models. and R (α i ) = td (ϖ) = [ ] cos αi sin α i sin α i cos α i (8) [ ] xi + ϖ cos α i, (9) y i + ϖ sin α i ACKNOWLEDGMENT This work has been suorted by the Norwegian Research Council (NFR) through the Centre for Shis and Ocean Structures (CeSOS) at the Norwegian University of Science and Technology (NTNU) and through the strategic university rogram (SUP) on Comutational Methods in Nonlinear Motion Control (CMinMC). where α i is the orientation of the straight-line subsea ath and tm td d is the desired touchdown distance corresonding to a desired ieline tension. For subsea segments with varying curvature, the vessel osition can be comuted by tm (ϖ) = td (ϖ) + R (χ td (ϖ)) [ tm td d 0 ], (10) REFERENCES [1] Collier, G. P., and Rosier, P., Environmental imact of submarine ieline installation. Journal of Offshore Technology, 3(2), [2] Palmer, A. C., and King, R. A., Subsea Pieline Engineering, 2nd ed. PennWell Books. [3] Anonymous, From car ferry to ielaying vessel. Diesel & Gas Turbine Worldwide, 19(1), Coyright c 2009 by ASME

10 [4] Anonymous, New electrical concet for DP thruster control. Ocean Industry, 22(2), [5] Heerema, E. P., Major deewater ielay vessel starts work in North Sea. Oil and Gas Journal, 96(18), [6] KONGSBERG MARITIME AS, Kongsberg K-Pos DP Dynamic Positioning System. Reort no /B. [7] Åström, K. J., Automatic control: A ersective. Colloquium on Automatic Control, Vol. 215 of Lecture Notes in Control and Information Sciences. Sringer Berlin / Heidelberg, [8] Bray, D., Dynamic Positioning, 2nd ed., Vol. 9 of Oilfield Seamanshi. Oilfield Publications Limited. [9] Offshore Technology Conference. htt:// [10] OMAE. htt:// [11] Leffer, W. L., Pattarozzi, R., and Sterling, G., Deewater Petroleum Exloration & Production: A Nontechnical Guide. PennWell Books. [12] Miesner, T. O., and Leffler, W. L., Oil & Gas Pielines in Nontechnical Language. PennWell Books. [13] Guo, B., Song, S., Chacko, J., and Ghalambor, A., Offshore Pielines. Gulf Professional Publishing. [14] Bai, Y., and Bai, Q., Subsea Pielines and Risers. Elsevier. [15] Braestru, M. W., Andersen, J. B., Andersen, L. W., Bryndum, M. B., Christensen, C. J., and Rishy, N., Design and Installation of Marine Pielines. Blackwell Science Ltd. [16] Kyriakides, S., and Corona, E., Mechanics of Offshore Pielines, Volume 1: Buckling and Collase, 1 ed. Elsevier. [17] Wilkins, J. R., From S-lay to J-lay. In Proceedings of the 13th Int. Conference on Offshore Mechanics and Arctic Engineering, New York, NY, USA. [18] Perinet, D., and Frazer, I., J-lay and Stee S-lay: Comlementary tools for ultradee water. In Proceedings of the 39th Offshore Technology Conference. OTC [19] Perinet, D., and Frazer, I., Strain criteria for dee water ie laying oerations. In Proceedings of the 40th Offshore Technology Conference. OTC [20] The Acergy Grou. htt:// [21] Allseas. htt:// [22] Subsea7. htt:// [23] Fossen, T. I., Marine Control Systems: Guidance, Navigation, and Control of Shis, Rigs and Underwater Vehicles, 1st ed. Marine Cybernetics, Trondheim Norway. [24] Jensen, G. A., and Fossen, T. I., Mathematical models for model-based control in offshore ielay oerations. In Proceedings of the 28th OMAE, Honolulu, Hawaii, USA. [25] DNV, Offshore Standard DNV-OS-F101 Submarine Pieline Systems. [26] API, API RP Design, Construction, Oeration, and Maintenance of Offshore Hydrocarbon Pielines. [27] ISO, ISO 13623:2000 Petroleum and natural gas industries Pieline transortation systems. [28] BSI, BS Code of Practice for Pieline - Part 3. Pieline Subsea: Design, Construction and Installation. [29] Faltinsen, O. M., Sea Loads on Shis and Offshore Structures. Cambridge University Press. [30] Callegari, M., Canella, F., Titti, F. M., Bruschi, R., Torselletti, E., and Vitali, L., Concurrent design of an active automated system for the control of stinger/ie reaction forces of a marine ielaying system. In Proceedings of the Int. Worksho on Harbour, Maritime & Multimodal Logistics Modelling and Simulation, Portofino, Italy. [31] Torselletti, E., Bruschi, R., Vitali, L., and Marchesani, F., Lay challenges in dee waters: Technologies and criteria. In Proceedings of the 2nd Int. Deewater Pieline Technology Conf., New Orleans, Louisiana, USA. [32] Irvine, H. M., Cable Structures. The MIT Press. [33] Faÿ, H., Dynamic Positioning Systems: Princiles, Design and Alications. Éditions Techni. [34] Heerema, E., Recent achievements and resent trends in deewater ie-lay systems. In Proceedings of the 37th Offshore Technology Conference. OTC [35] Steenhuis, A. L. J., van Norden, T., Regelink, J., and Krutzen, M., Modifications to the ielay vessel Solitaire for the Indeendence trail roject. In Proceedings of the 39th Offshore Technology Conference. OTC [36] Breivik, M., and Fossen, T. I., Guidance laws for lanar motion control. In Proceedings of the 47th IEEE Conference on Decision and Control, Cancun, Mexico. [37] Jensen, G. A., Modeling and control of offshore ielay oerations. PhD thesis, Norwegian University of Science and Technology. [38] Jensen, G. A., Transeth, A. A., Nguyen, T. D., and Fossen, T. I., Modelling and control of offshore marine ieline during ielay. In Proceedings of the 17th IFAC World Congress, Seoul, Korea. [39] Skjetne, R., Fossen, T. I., and Kokotović, P. V., Robust outut maneuvering for a class of nonlinear systems. Automatica, 40(3), [40] Sørensen, A. J., Leira, B., Strand, J. P., and Larsen, C. M., Otimal setoint chasing in dynamic ositioning of dee-water drilling and intervention vessels. International Journal of Robust and Nonlinear Control, 11(13), [41] Blanke, M., Kinnaert, M., Lunze, J., and Staroswiecki, M., Diagnosis and Fault-Tolerant Control, 2nd ed. Sringer. 10 Coyright c 2009 by ASME