Riser Tensioning Systems

What a riser tensioner does

The top of the riser is a telescopic joint. Its inner barrel hangs from the rig under the drill floor and slides inside the outer barrel, which is the top of the riser. The tensioners attach to the tension ring on the outer barrel and carry the riser from there (Figure 1). As the rig heaves, the inner barrel slides and the tensioners stroke, while the riser below stays where it is.

Each tensioner is a hydraulic cylinder working against high-pressure gas. As the rig rises, the tensioner gives out stroke, pushes oil into its gas bottles and compresses the gas, so the tension rises a little. As the rig comes down, the gas pushes the oil back and the tension falls a little. The gas is the spring, and its volume decides how little the tension changes.

How much tension a riser needs

The riser has to be in tension along its whole length, or it can buckle. The tension at the top also keeps the angles at its flex joints small in current and when the rig drifts off the well. API RP 16Q sets the minimum with a balance at the tension ring, which it calls the slip ring:

Here W_s is the riser’s submerged weight and B_n the net lift of its buoyancy modules. The factors f_{wt} \ge 1.05 and f_{bt} \le 0.96 allow for weight tolerance and for buoyancy lost to compression and water absorption. A_i is the riser’s internal area, including its choke, kill and auxiliary lines, and d_m H_m and d_w H_w are the pressures of the mud column inside and the seawater column outside at the bottom of the riser.

The tensioners have to deliver more than that, because some of them can fail at once and a little is lost on the way to the ring:

with N tensioner lines or cylinders, n of them that can fail suddenly, and R_f, usually 0.90 to 0.95, for the fleet angle (how far the wire leans from vertical) and friction.

Worked example: a 1,500 m riser

1. The inputs. Illustrative round numbers: 1,500 m of riser with a submerged weight of 900 t and 800 t of buoyancy lift, and A_i = 0.22\ \text{m}^2 for the bore and its lines. The mud is 1.50 SG against 1.025 for seawater, with both columns taken as 1,500 m. Twelve tensioner lines, of which one dual unit (two lines) can fail suddenly, and R_f = 0.95.
2. At the tension ring. In tonnes, with the densities in t/m³, the weight term is 900 \cdot 1.05 = 945, the buoyancy term 800 \cdot 0.96 = 768 and the mud term 0.22 \cdot 1{,}500 \cdot (1.50 - 1.025) \approx 157, so
   T_{\text{SR,min}} = 945 - 768 + 157 \approx 334\ \text{t}
3. The tensioner setting.
   T_{\min} = 334 \cdot \frac{12}{0.95 \cdot 10} \approx 422\ \text{t}
   That is about 4.1 MN, or 930 kips.

The buoyancy carries most of the steel, and the tolerance factors alone add about 80 t: 5 % on 900 t and 4 % off 800 t. Water depth and mud weight drive the rest. The same riser in 3,000 m of water with 1.8 SG mud needs about 1,100 t. Deepwater drilling units carry about 1,400 to 1,800 t of tensioner capacity to cover that minimum, the rise in tension over the stroke, and the extra tension that current and offset call for.

A real stack-up takes the weights from the riser joints’ data sheets and checks the effective tension along the whole riser, not only at the top. The top of the band is limited too, by the riser, the telescopic joint and tension ring, and the wellhead and conductor.

Stroke and gas volume

The stroke has to cover the rig’s heave at the riser, the set-down as the rig drifts off the well, the tide and the riser’s own stretch, with margin at both ends so the tensioner never bottoms out. Deepwater units have about 15 m (50 ft) of travel.

How much the tension changes over that stroke depends on the gas. With a gas volume V_0 at mid-stroke and a piston area A, a stroke x from mid-stroke that compresses the gas gives

where \kappa runs from 1.0 for slow, isothermal compression to about 1.4 for fast, adiabatic compression of an ideal gas. At the 100 to 200 bar a tensioner works at, real nitrogen is stiffer still, so fast strokes need more gas. The larger the gas volume against the volume the stroke sweeps, the flatter the curve (Figure 2). Holding the tension within ±5 % over the full stroke, for example, takes a gas volume about 14 times the swept volume, which is why tensioner systems carry banks of large gas bottles.

The same gas spring gives the riser and its tensioners a vertical natural period, which is checked against the wave periods of the operation. It matters most when the riser is disconnected and hangs off the tensioners, with nothing at the bottom to hold it.

Wireline and direct-acting tensioners

Tensioner ropes

On a wireline system the ropes are what wears. Current units run lines of up to about 250 kips each. Every wave cycle bends the same few metres of rope over the sheaves, and salt air corrodes it, so rigs move the rope along on a schedule to spread the wear (slip and cut), inspect it, and replace it at the end of its life. The rope is specified to API Spec 9A, and its care and use follow API RP 9B. The idler sheaves are placed to keep the fleet angle small, which puts more of the rope tension into lifting the riser and makes the rope last longer.

Production risers

Top-tensioned production risers on tension-leg platforms and spars are held the same way, by hydro-pneumatic tensioners, or on some spars by buoyancy cans. There the tension is set for the life of the field rather than for a drilling campaign.

Design cases the tensioner must cover

1. A failed tensioner. The factor N/(N - n) makes the remaining tensioners hold the minimum tension if one fails, or a dual unit, depending on how the system is split. On a direct-acting system a failed cylinder can also pull down on the tension ring, and that load is added to the minimum.
2. Recoil after an emergency disconnect. When the lower marine riser package (LMRP) disconnects from the BOP in an emergency, the tensioners are still pulling with hundreds of tonnes on a riser that has lost its bottom connection, and the mud starts to drain out of it. The tension has to lift the LMRP clear of the BOP without it falling back onto the stack, but the riser must not shoot up into the rig, slacken the tensioner wires or collapse the telescopic joint. An anti-recoil system throttles the flow between the gas bottles and the cylinders, and a recoil analysis sets the tension and anti-recoil settings for each mud weight.
3. Stroke margin. Heave, set-down, tide and stretch must fit inside the stroke, with margin at both ends, in every sea state the rig stays connected in. If the rig loses position, the stroke left is one of the limits on how early the disconnect has to start.
4. The upper tension limit. Heavy mud raises the minimum towards what the riser, the tension ring, the wellhead and the tensioners themselves can take, and the rise in tension over the stroke has to stay under that limit too.

Backup tensioning and SIRIUS

A tensioner can go out of service for a seal failure, a rope change or maintenance. With one unit out, the others carry its share and the allowance for the next failure shrinks, which can force a lower mud weight or tighter operating limits until the unit is back. A backup tensioner takes the missing unit’s share while the primary is restored.

For scale, a backup that stands in for one dual wireline unit carries about 230 t, the load of two 250-kip lines.

SIRIUS, our general-purpose tensioner family in development, can serve as that backup: topside units with manually set damping, planned from 5 to 5,000 t with 1 to 12 m of stroke, for backup riser tensioning, workover and other constant-tension duties. The hardware behind SIRIUS is not new: our team has delivered long-stroke tensioners before, including a 500 t drilling tensioner with 8 m of stroke.

Riser tensioners and lift heave compensators are the same kind of machine, a cylinder working against a gas spring, judged in different ways. A tensioner is judged by how constant its tension stays over the stroke, and it runs for weeks. A lift compensator is judged by how little of the vessel’s motion reaches the load, over one lift. For compensation on a workover string or coiled tubing run from a crane, see coiled-tubing heave compensation.