NATURAL GAS: OFFSHORE PRODUCTION & HANDLING Topic: Mooring System Lecture by, M.Aslam Abdullah VIT University. Dept. of Chemical Engineering,VIT University. 21 May 2018

Introduction Mooring System: The mooring system consists of freely hanging lines connecting the surface platform to anchors, or piles, on the seabed, positioned at some distance from the platform. “Often laid out symmetrically in plan view around the object in question” Dept. of Chemical Engineering,VIT University. 21 May 2018

Types of Mooring Lines: Steel-Linked chain Wire rope Synthetic fiber rope Dept. of Chemical Engineering,VIT University. 21 May 2018

Steel-Linked chain Wire rope The above two types of catenary lines are conventionally used for mooring floating platforms. Each of the lines forms a catenary shape, depending on an increase or decrease in line tension as it lifts off or settles on the seabed, to produce a restoring force as the surface platform is displaced by the environment. Thus a spread of lines generates a nonlinear restoring force to provide the station-keeping function. Dept. of Chemical Engineering,VIT University. 21 May 2018

This force increases with vessel horizontal offset and balances quasi-steady environmental loads on the surface platform. The equivalent restoring stiffness provided by the mooring is generally too small to influence wave frequency motions of the vessel significantly, although excitation by low-frequency drift forces can induce dynamic magnification in the platform horizontal motions and lead to high peak line tensions. The longitudinal and transverse motions of the mooring lines themselves can also influence the vessel response through line dynamics. Dept. of Chemical Engineering,VIT University. 21 May 2018

To operate in more water depths, the suspended weight of 3. Synthetic rope To operate in more water depths, the suspended weight of mooring lines becomes a prohibitive factor. In particular, steel chains become less attractive at great water depths. Recently, advances in taut synthetic fibre rope technology have been achieved offering alternatives for deep-water mooring. Mooring systems using taut fibre ropes have been designed and installed to reduce mooring line length, mean- and low-frequency platform offsets, fairlead tension and thus the total mooring cost. (Still a lot of R&D in progress) Dept. of Chemical Engineering,VIT University. 21 May 2018

Mooring system design philosophy: Mooring system design is a trade-off between making the system compliant enough to avoid excessive forces on the platform, and making it stiff enough to avoid difficulties, such as damage to drilling or production risers, caused by excessive offsets. Easier to achieve for moderate water depths, but becomes more difficult as the water depth increases. Dept. of Chemical Engineering,VIT University. 21 May 2018

Single Point Mooring (SPM): Excessive offsets are often observed due to the environmental factors on the mooring system. SPM have been developed to overcome this disadvantage. In this the lines are attached to the vessel at a single point. This connection point is located on the longitudinal centre line of the vessel. The vessel is then free to weathervane and hence reduce environmental loading caused by wind, current and waves. Dept. of Chemical Engineering,VIT University. 21 May 2018

Single Buoy Mooring (SBM): A typical early facility consisted of a buoy that serves as a mooring terminal. It is attached to the sea floor either by catenary lines, taut mooring lines or a rigid column. The vessel is moored to the buoy either by synthetic hawsers or by a rigid A-frame yoke. Turntable and fluid swivels on the buoy allow the vessel to weathervane, reducing the mooring loads. Dept. of Chemical Engineering,VIT University. 21 May 2018

In order to further reduce the environmental loading on the mooring system from the surface vessel in extreme conditions, dis- connectable turret mooring systems have also been developed. Here the connected system is designed to withstand a less harsh ocean environment, and to be disconnected whenever the sea state becomes too severe such as in typhoon areas. Dept. of Chemical Engineering,VIT University. 21 May 2018

Figure: Turret moorings a) Dis-connectable b) Permanent Dept. of Chemical Engineering,VIT University. 21 May 2018

Functional requirements for the mooring system: Offset limitations Lifetime before replacement Install-ability Positioning ability These requirements are determined by the function of the floater. Dept. of Chemical Engineering,VIT University. 21 May 2018

Comparison of typical MODU and FPS mooring requirements: Floating Production Design for 50-yr return period event. Anchors may fail in larger events. Designed for 100 yr return period events. Risers disconnected in storms Risers remain connected in storm Slack moorings in storm events to reduce line tensions Moorings are usually not slacked because of risk to the risers, and lack of marine operators on board Components designed for < 10 yr life Components designed for > 10 yr life Fatigue analysis not required Fatigue analysis required Life dynamics analysis not required Life dynamics analysis required Missing line load case not required Missing line load case required Dept. of Chemical Engineering,VIT University. 21 May 2018

Steel Chain or Wire Catenary lines: In the figure: Catenary mooring is deployed from point A on the submerged hull of a floating vessel to an anchor at B on the seabed. Some part between AB is resting on the seabed, & horizontal distance “a” is 5-20 times larger than the vertical dimension “b”. From a static point of view, the cable tension in the vicinity of points A is due to the total weight in sea water of the suspended line length. The progressive effect of line lift-off from the seabed due to the horizontal vessel movement from Al to A4 increases line tension in the vicinity of points A. This feature, coupled with the simultaneous decrease in line angle to the horizontal, causes the horizontal restoring force on the vessel to increase with vessel offset in a non-linear manner. Dept. of Chemical Engineering,VIT University. 21 May 2018

As we shift the mounting point from A1to A4 the catenary line laying/resting varies from a significant length at A1 to none at A4. From a static point of view, the cable tension in the vicinity of point A is due to the total weight in sea water of the suspended line length. The progressive effect of line lift-off from the seabed due to the horizontal vessel movement from Al to A4 increases line tension in the vicinity of point A. This feature, coupled with the simultaneous decrease in line angle to the horizontal, causes the horizontal restoring force on the vessel to increase with vessel offset in a non-linear manner. Dept. of Chemical Engineering,VIT University. 21 May 2018

Synthetic Lines: For deep-water applications, synthetic fibre lines can have significant advantages over a catenary chain or wire because they are considerably lighter, very flexible and can absorb imposed dynamic motions through extension without causing an excessive dynamic tension. Dept. of Chemical Engineering,VIT University. 21 May 2018

This, causes reduced mean- and low-frequency platform offsets, Additional advantages include the fact that there is reduced line length and seabed footprint, as depicted in the adjacent figure This, causes reduced mean- and low-frequency platform offsets, lower line tensions at the fairlead and smaller vertical load on the vessel. This reduction in vertical load can be important as it effectively increases the vessel useful payload. Dept. of Chemical Engineering,VIT University. 21 May 2018

The disadvantages in using synthetics are that their material and mechanical properties are more complex and not as well understood as the traditional rope. This leads to over conservative designs that strip them of some of their advantages. Furthermore, there is little in-service experience of these lines. In marine applications this has led to synthetic ropes subject to dynamic loads being designed with very large factors of safety. Dept. of Chemical Engineering,VIT University. 21 May 2018

Important properties of synthetic lines to considered in design: Stiffness Hysteresis and heat build up Fatigue Other issues Dept. of Chemical Engineering,VIT University. 21 May 2018

Stiffness: In a taut mooring system the restoring forces in surge, sway and heave are derived primarily from the line stretch. This mechanism of developing restoring forces mostly differs from the conventional steel catenary systems that develop restoring forces primarily through changes in the line catenary shape. This is made possible by the much lower modulus of elasticity of polyester compared to steel. The stretch characteristics of fibre ropes can extend from 1.2 to 20 times as much as steel, reducing induced wave and drift frequency forces. (Stiffness of line is a function of load & age) Dept. of Chemical Engineering,VIT University. 21 May 2018

Hysteresis and heat build up: The energy induced by cyclic loading is dissipated (hysteresis) in the form of heat. In addition, the chaffing of rope components against each other also produces heat. Cases are known in which the rope has become so hot that the polyester fibers have melted. This effect is of greater concern with larger diameters or with certain lay types because dissipation of the heat to the environment becomes more difficult. Dept. of Chemical Engineering,VIT University. 21 May 2018

Fatigue: The fatigue behavior of a rope at its termination is not good. In a termination, the rope is twisted (spliced) or compressed in the radial direction (barrel and spike or resin socket). The main reason for this decreased fatigue life is local axial compression. Although the rope as a whole is under tension, some components may go into compression, resulting in buckling and damage of the fibres. In a slack line this mechanism is more likely to be a problem than in a rope under tension. The phenomenon can appear at any position along the rope. Dept. of Chemical Engineering,VIT University. 21 May 2018

Other relevant issues: Issues to consider are that the strength of a polyester rope is about half that of a steel wire rope of equal diameter. Additionally the creep behavior is good but not negligible (about 1.5% elongation over 20 years). Furthermore, synthetic fibre ropes are sensitive to cutting by sharp objects and there have been reports of damage by fish bite. A number of rope types such as high modulus polyethylene (HMPE) are buoyant in sea water; other types weigh up to 10% of a steel wire rope of equal strength. Synthetic fibre lines used within taut moorings require the use of anchors that are designed to allow uplift at the seabed. Dept. of Chemical Engineering,VIT University. 21 May 2018

Loading Mechanism on Mooring System: Figure: Environmental forces acting on a moored vessel in head conditions and transverse motion of catenary mooring lines Dept. of Chemical Engineering,VIT University. 21 May 2018

Loading Mechanism on Mooring System: There are various loading mechanisms acting on a moored floating vessel as depicted in the previous figure are: For a specific weather condition, the excitation forces caused by current are usually assumed temporally constant, with spatial variation depending on the current profile and direction with depth. Wind loading is often taken as constant, at least, in initial design calculations, though gusting can produce slowly varying responses. Wave forces result in time-varying vessel motions in the 6 rigid body degrees of freedom of surge, sway, heave, roll, pitch and yaw. Wind gust forces can contribute to some of these motions as well. Dept. of Chemical Engineering,VIT University. 21 May 2018

Mooring System Design Static design Quasi Static Design Dynamic Design Dept. of Chemical Engineering,VIT University. 21 May 2018

Mooring Hardware Components The principle components of a mooring system may consists of: Chain, wire or rope or their combination Anchors or piles Fairleads, bending shoes or pad-eyes Winches, chain jacks or windlasses Power supplies Rigging (e.g. stoppers, blocks, shackles) Dept. of Chemical Engineering,VIT University. 21 May 2018

Chain, wire or rope or their combination: Properties are given by “Det Norske Veritas OS-E301” codes. Chain and wire make up the strength members for the mooring system. There are primary 2 chain constructions: Stud-Link Chain (studs provide stability to the link and facilitate laying down of chain while handling.) Stud-less Chain (removing stud reduces the weight per unit of strength and increases the chain fatigue life, at the expense of making the chain less convenient to handle.) Dept. of Chemical Engineering,VIT University. 21 May 2018

Chain is specified as nominal diameter of the link “D” The largest mooring chain manufactured to date is the 6.25 in. (159 mm) Stud-less chain for the Schiehallion FPSO in the North Atlantic (West of Shetlands). Dept. of Chemical Engineering,VIT University. 21 May 2018

Wire rope consists of individual wires wound in a helical pattern to form a “strand”. The pitch of the helix determines the flexibility and axial stiffness of the strand. Wire rope used for mooring can be multi-strand or single-strand construction. Stud-link chain and six-strand wire rope are the most common mooring components for MODUS and other “temporary” moorings. Dept. of Chemical Engineering,VIT University. 21 May 2018

Multi-strand ropes are favored for temporary applications because of Wire rope: Multi-strand ropes are favored for temporary applications because of their ease of handling. Six-strand rope is the most common type of multi-strand rope used offshore. Mooring line ropes typically consist of 12, 24, 37 or more wires per strand. The wires have staggered sizes to achieve higher strength Dept. of Chemical Engineering,VIT University. 21 May 2018

Figure: Wire rope construction Dept. of Chemical Engineering,VIT University. 21 May 2018

Common “classes” of multi-strand rope include (Myers, 1969): 6x7 class: 7 wires per strand, usually used for standing rigging. Poor flexibility and fatigue life, excellent abrasion resistance. Minimum drum diameter/rope diameter (D/d) = 42. 6x9 Class: 16 to 27 wires per strand. Good flexibility and fatigue life and abrasion resistance. Common in lifting and dredging. Minimum D/d = 26-33. 6x37 Class: 27 to 49 wires per strand. Excellent fatigue life and flexibility, but poor abrasion resistance. Minimum D/d = 16-26. Dept. of Chemical Engineering,VIT University. 21 May 2018

Multi-strand wire ropes may contain either a fibre or a metallic core. The core is important for support of the outer wires, especially on a drum, and in some applications to absorb shock loading. Fibre core (FC) ropes are not generally used for heavy duty marine applications. Metallic core ropes may be one of the two types: Independent Wire Rope Core (IWRC) Wire-Strand Core (WSC). IWRC is the most common core filling for heavy marine applications. Dept. of Chemical Engineering,VIT University. 21 May 2018

Anchors or piles: Anchors are basically of two types, relying either on self-weight or suction forces. The traditional embedment anchors, as shown in figure, are not normally designed for vertical force components. Holding power is related to anchor weight and type of seabed. Dept. of Chemical Engineering,VIT University. 21 May 2018

Figure: Drag Anchor Dept. of Chemical Engineering,VIT University. 21 May 2018

Figure: Suction anchor installation sequence Dept. of Chemical Engineering,VIT University. 21 May 2018

Figure: Deep water FPSO design using suction anchors Dept. of Chemical Engineering,VIT University. 21 May 2018

Turrets: The design of mono-hull turret structures used for single-point moorings in floating production systems must allow for large static and dynamic loading caused by the vessel motions in waves together with forces transmitted by the mooring system. The hull design in the turret region must reflect the fact that the amount of primary steel is reduced here with an appropriate increase in the stress concentration. Dept. of Chemical Engineering,VIT University. 21 May 2018

Mooring line tension and riser loading. Vessel yaw Careful selection of turret position is important because of its influence on: Mooring line tension and riser loading. Vessel yaw Rigid body oscillation in the horizontal plane Dept. of Chemical Engineering,VIT University. 21 May 2018

Mooring System Analysis: The mooring system is assessed in terms of three limit states based on the following criteria: Ensuring that individual mooring lines have suitable strength when subjected to forces caused by extreme environmental loads - ultimate limit state (ULS). Ensuring that the mooring system has suitable reserve capacity when one mooring line or one thruster has failed - accidental limit state (ALS). Ensuring that each mooring line has suitable reserve capacity when subject to cyclic loading - fatigue limit state (FLS). Dept. of Chemical Engineering,VIT University. 21 May 2018

Potential failure modes as given in standards: Hysteresis heating: lubricants and fillers can be included to reduce hotspots, creep rupture - in particular this is relevant to HMPE yarns, and the risks need careful evaluation. Tension: Tension fatigue-only limited data exist, indications being that fatigue resistance is higher than for steel wire ropes. Axial compression fatigue - on leeward lines during storms for example, prevented by maintaining a minimum tension on the rope. Particle ingress - causes strength loss by abrasion from water-borne material such as sand, prevented by using a suitable sheath and not allowing contact between the rope and seabed. Dept. of Chemical Engineering,VIT University. 21 May 2018

SPM (Single Point Mooring) SBM (Single Buoy Mooring) Topics to be referred: SPM (Single Point Mooring) SBM (Single Buoy Mooring) SALM (Single Anchor Legged Mooring) Tower Mooring Reference: S. K. Chakrabarti / “Handbook of Offshore Engineering” Vol- 1 & 2 / Elsevier-05 Dept. of Chemical Engineering,VIT University. 21 May 2018

Dept. of Chemical Engineering,VIT University. 21 May 2018

Thank you for your attention Dept. of Chemical Engineering,VIT University. 21 May 2018